Electrical Engineering Interview Questions & Answers

Are you looking for electrical engineering interview questions and answers? An interviewer may ask questions related to electrical engineering subjects such as network theory, control system, signal and systems, electrical machines, power system, power electronics, electrical and electronic measurement, electromagnetic theory, power system protection, analog electronics, digital electronics, electrical energy utilization, electrical installation and wiring, electrical machine design, electric drive and microprocessors. We have collected and listed the best interview questions on electrical engineering. We have also categorized questions based on different subjects so that you can find questions and answers easily.

Best Electrical Engineering Interview Questions

Network Theory Interview Questions

Q. What is a node?

Answer. A node is a point in a network where two or more branches meet.

Q. What is meant by loop?

Answer. A loop is a closed path in a network formed by a number of connected branches.

Q. What is meant by mesh?

Answer. A mesh is a loop that contains no other loop within it.

Q. What is a network?

Answer. It is an interconnection of electrical elements that may or may not have closed paths.

Q. What is a circuit?

Answer. It is an interconnection of electrical elements but it has atleast one closed path and one independent source. All circuits are always a network but vice-versa is not always true.

Q. What should be the condition for a network to be a circuit?

Answer. For a network to be a circuit

  1. It must have atleast one closed path.
  2. It must have atleast one independent source.

Q. What is network analysis?

Answer. For a given network, with the existence of energy sources, finding the responses (current or voltage) is known as network analysis.

Q. What is network synthesis?

Answer. The construction of a new network based upon known input and output relation is known as network synthesis.

Q. What is Tellegen’s theorem?

Answer. Tellegen’s theorem states that the summation of instantaneous powers for the n-branches in an electric network is always zero.

Q. What is maximum power transfer theorem?

Answer. The maximum power transfer theorem states that a resistive load, served through a resistive network, will abstract maximum power when the load resistance value is the same as the resistance “viewed by the load as it looks back into the network”.

Q. What is superposition theorem?

Answer. The superposition theorem states that in a network of linear impedances containing more than one source, the response (current or voltage) at any point is algebraic/phasor sum of all responses (current or voltage) at that point if each source was considered separately and all other sources are replaced by their internal impedances.

Electrical Machines Interview Questions

Transformer Interview Questions

Q. What is the significance of vector grouping in Power Transformers?

Answer. Every power transformer has a vector group listed by its manufacturer. Fundamentally it tells you the information about how the windings are connected (delta or wyes) and the phase difference between the current and voltage. Example, DYN means Delta primary, Wye Secondary and the current is at 0 clock referred to the voltage.

Q. How many types of a cooling system it transformers?

Answer. The cooling systems used in the transformer are

  1. ONAN (oil natural,air natural)
  2. ONAF (oil natural,air forced)
  3. OFAF (oil forced,air forced)
  4. ODWF (oil direct,water forced)
  5. OFAN (oil forced,air forced)

Current Transformer Interview Questions

Q. What are current transformers?

Answer. Current transformer (CT) is a device that is used in conjunction with current measuring devices in order to step down the current from primary (main circuit) to secondary (the measuring device).

DC Machine Interview Questions

Q. A dummy coil is in the armature winding of a dc machine. Is the machine lap wound or wave wound?

Answer. Wave wound

Q. Air gap lengths under the main poles of a dc machine are less than those under the interpoles. Why?

Answer. The gaps under the interpoles are kept longer than that under the main poles (1.5 to 1.7 times the main gap) in order to avoid saturation in the interpoles.

Q. Commutator in a DC machine converts AC to DC or DC to AC quantity of the current?

Answer. In DC Generator the commutator converts from AC induced current produced in the armature windings is converted to DC and gives to the external load connected.

On the Other hand, in case of DC Motor when the supply was given is DC it is converted from DC to AC and provided to the armature windings so that in order to produce the rotational torque.

Thus Commutator in a DC machine converts both AC to DC and DC to AC.

Q. Differentiate between geometrical neutral axis and magnetic neutral axis.

Answer. The geometrical neutral axis is the axis between the magnetic poles and the magnetic neutral axis is along zero magnetic fields.

Q. Explain why a motor of smaller rating can be selected for a short time duty?

Answer. The short-time duty motor operates at a constant load for some specified periods which is then followed by a period of rest.

The period of the run (or load) is so short that the machine cannot attain its steady temperature rise while the period of rest is too long that the motor temperature drops to the ambient temperature. Thus a motor of smaller rating can be operated with larger power output for the maximum temperature rise in a short duration.

Q. For what type of dc machine lap winding is employed?

Answer. For high current and low voltage rating machine.

Q. For what type of dc machine, wave winding is employed?

Answer. For low current and high voltage rating machine.

Q. How can the direction of the stator revolving field be reversed?

Answer. The direction of the stator revolving field can be reversed by reversing any two of the three supply lines to the stator winding.

Q. How do the various losses occurring in a dc machine varies with the load?

Answer. The copper losses occurring in armature winding, series field winding, compensating winding and interpole winding, if provided, almost vary as the square of the load current while that occurring in dc shunt field winding remain practically constant. Rotational losses (iron and mechanical losses) are almost independent of load.

Q. In a dc machine flux per pole decreases with an increase of load. Justify the Statement.

Answer. In a dc machine, flux per pole decreases with the increase of load due to the demagnetization effect of armature reaction.

Q. In a dc machine without interpole to get improved commutation, whether the brush shift should be varied with a change in load or brush shift should be fixed?

Answer. Brush shift should be varied with the change in load in a dc machine so as to lie along MNP to provide improved commutation.

Q. In what terms are dc machines rated?

Answer. The dc machines are rated in terms of kW (kilowatt) output at a given speed and voltage.

Q. In what way compensating winding is connected to the armature?

Answer. The compensating winding is connected in series with the armature in a manner so that the direction of current through the compensating winding conductors in any pole face will be opposite to the direction of current through the adjacent armature conductors.

Q. Is the interpole AT required for a compensated dc machine more or less than of an uncompensated one?

Answer. Interpole AT required for a compensated dc machine is less than of an uncompensated one.

Q. Name the factor that opposes full reversal of current in a coil undergoing Commutation.

Answer. Self-inductance of the coil undergoing commutation.

Q. Which of the power losses occurring in electrical machines are influenced by the magnitude of flux, the load and the square of the load?

Answer. Core or iron losses are influenced by the magnitude of flux; copper losses (armature, series field, interpoles, compensating windings) are proportional to the square of the load while the brush-contact loss is proportional to the load.

Q. State with reason whether field test on two identical dc series machines is the regenerative method?

Answer. A field test is not regenerative test because the output of the generator is not feedback to the motor but is dissipated in a resistor.

Q. The outside of the machine should be cleaned frequently and the air passages are blown out regularly. Why?

Answer. Otherwise, dust deposit upon the external surface of a machine will act as a heat insulant and in the air, ducts will restrict the flow of cooling air and consequently, the temperature will rise.

Q. What are Eddy currents and where they are present in DC machine and how they are minimized?

Answer. Eddy currents are the circulating currents produced in the iron because of the alternating magnetic field. This eddy current will cause local circulating currents which produce flux and magnetic field which opposes the main magnetic field. This results in the loss of the machine.

Eddy currents are produced in the field poles and armature of the DC machine.

Eddy Currents are minimized by providing thin laminations separated by the insulating coating This lamination reduces the area for producing the circulating currents. Hence eddy current losses are minimized.

Q. What are the components of iron loss in a dc machine and in which part of the machine it occurs?

Answer. Iron loss consists of hysteresis and eddy current losses caused by changing flux densities in the iron core.

Hysteresis loss incurs in revolving armature and teeth of a dc machine.

Eddy current loss incurs in armature core and teeth of pole faces.

Q. What are the different losses in dc machine?

Answer. The different losses in dc machine are

  • Armature and field copper loss
  • Iron and magnetic losses (hysteresis and eddy current loss)
  • Mechanical losses
  • Stray-load losses

Q. What are the different types of armature windings commonly used in dc machines?

Answer. Lap and wave windings

Q. What are the different types of losses in a dc machine?

Answer. The different types of losses that occur in a dc machine are

  1. Copper or electrical losses
  2. Iron or magnetic losses
  3. Mechanical or friction and windage losses

Copper or electrical losses include power lost in heating the shunt field and also in heating the armature circuit, which includes armature winding, brush contacts, series field and interpole field.

Iron or magnetic losses include the hysteresis and eddy current losses which are present in any part of the machine that is constructed of iron and subjected to variations of magnetic flux.

Mechanical losses consist of power losses due to the friction of the bearings, air friction or windage, as it is called, caused by the motion of the moving parts through the surrounding medium, and the friction between brushes and commutator.

Q. What are the drawbacks of Brake test?

Answer. Some of the drawbacks of the brake test are

  • This test is performed with small motors only
  • Internal losses cannot be determined

Q. What is the factor that has to be considered while choosing the resistor in a DC machine?

Answer. While choosing the resistor for connecting in filed or armature circuit care should be taken that the resistor current rating is more than the full load current rating of the circuit. For example, if the rated field current is about 2 Amps. Then the resistor connecting in the field circuit should be chosen in such a manner that the current rating of the resistor is greater than 2 Amps.

Q. What are the no-load losses in a dc machine?

Answer. No-load losses in a dc machine consist of core loss or iron loss, windage loss and loss due to friction and bearing.

Q. What is the standard period for a short-time rating of a machine?

Answer. The Standard short-time ratings are 10, 30, 60 and 90 minutes.

Q. What are the two unwanted effects of armature reaction?

Answer. Demagnetization or weakening of the main field and cross-magnetization or distortion of the field.

Q. What do you understand by self-excitation mode of dc machine? Name two dc machines working in this mode.

Answer. The dc machine in which field winding(s) is/are self-excited by the current supplied by the machine itself is called the self-excitation mode of the machine. In such machines, the field coils are interconnected with the armature winding.

The self-excited machines may be

  1. dc series machine
  2. dc shunt machine
  3. compound wound

Q. What informations should the rating of a machine give?

Answer. The rating of an electrical rotating machine should include the output, voltage, speed and any other information that may be necessary for the proper operation of the machine.

Q. What is armature reaction?

Answer. Armature reaction is the effect of the magnetic field set up by the armature current on the distribution of flux under the main poles of a dc machine.

Q. What is bad Commutation?

Answer. A machine is said to have poor or bad commutation if there is sparking at the brushes and commutator surface. The bad commutation may be caused by mechanical or electrical conditions. The mechanical conditions include uneven commutator surface, non-uniform brush pressure, vibration of brushes in the brush holders etc. The electrical conditions include an increase in voltage between the commutator segments, an increase in the current density at the trailing edge of the brush etc.

Q. What is compensating winding?

Answer. A compensating winding is an auxiliary winding embedded in slots located on the faces of the main poles.

Q. What is the energy balance equation?

Answer. According to the principle of energy conservation, the energy input to the electro-mechanical energy conversion device is equal to the summation of the useful output energy, the energy converted into heat, and the change in the energy stored in the magnetic/electric field i.e.

Energy input from electrical source = Mechanical energy output + increase in energy stored in the magnetic/electric field + energy converted into heat.

Q. What is field flashing?

Answer. In a case when the dc machine fails to build up the voltage due to lack of residual magnetism, field coils are connected to the dc source (battery) for short while for magnetizing the field poles. Application of external source of direct current to the field of the dc machine is called field flashing.

Q. What is heating time constant of the machine?

Answer. The heating time constant may be defined as the time during which the machine will attain 63.2% of its final steady temperature.

Q. What is linear commutation?

Answer. Linear commutation is an ideal commutation and causes a change of current in a coil undergoing commutation from + Ic to – Ic uniformly.

Q. What is meant by commutation?

Answer. The process of reversal of current in the armature coils by means of brushes and commutator bars is called the commutation process.

Q. What is nature (shape) of the dc armature mmf waveform?

Answer. Triangular

Q. What is reactance emf in dc machines?

Answer. The self-induced emf in the coil undergoing commutation is called the reactance voltage.

Q. What is the best-suited material for commutator segments?

Answer. Silvered copper (extruded copper) containing about 0.05% silver.

Q. What is the Importance of the Commutator in DC Machine?

Answer. Commutator in the DC machine is the component which converts the ac induced current in the armature to the unidirectional current or dc current to the load circuit. In the DC Generator, due to the Electro-magnetic Induction principle, the current induced in the armature coils are alternative current in nature. The commutator of the DC machine contains segments separated by the insulation medium.

It happens that when the alternating current in the armature varies from one half cycle ( say positive half cycle) and enters into other half cycle ( say negative half cycle), at the same time the polarity of the commutator segments changes i.e, initially the commutator segment which is in contact with the positive polarity of the brush when the induced currents in the armature changes at that instant the segment of the commutator which is earlier in contact with the positive polarity brush will now be in contact with the negative polarity brush results in the same direction of current flow in the external circuit.

Q. What is the location of interpole in a dc machine?

Answer. Commutating poles are located midway between the main poles.

Q. What is the main factor which governs the size and rating of an electric machine?

Answer. “Temperature rise” is the main factor which governs the size and rating of an electric machine.

Q. What is the nature (shape) of air gap flux distribution in space at no load in dc machines?

Answer. Flat-topped

Q. What is the primary reason for making the coil span of a dc machine armature winding equal to a pole-pitch?

Answer. By making coil span of a dc armature winding equal to pole pitch, we have full-pitch winding, hence emf induced in the winding is maximum.

Q. What is the purpose of compensating winding in a dc machine?

Answer. The purpose of compensating winding is to neutralize the effect of armature reaction outside the influence of the interpoles and particularly to maintain uniform flux distribution under the faces of the main poles.

Q. What is the volt drop allowed for all brushes of each polarity for carbon or graphite brushes?

Answer. 1 V per brush.

Q. When does the maximum efficiency of a dc machine take place?

Answer. The maximum efficiency of a dc machine takes place when variable losses equal constant losses.

Q. When the armature of an electromagnet relay moves to close the air gap slowly, where does the energy come from for doing mechanical work?

Answer. In the slow movement of the armature to close the air gap, current I stay substantially constant, energy is drawn from the electrical source, half of the input energy is utilized in doing mechanical work and rest is stored in the magnetic field.

Q. When the armature of an electromagnetic relay moves to close the air gap instantaneously, where does the energy come from for doing mechanical work?

Answer. In an instantaneous movement of the armature to close the air gap flux linkages Ψ remains almost constant and there is no electrical input. The energy required in doing mechanical work comes from the field energy which is reduced by an equal amount.

Q. When the brushes in a dc machine are placed at the interpolar axis, is the armature mmf fully demagnetising, fully cross-magnetising or partly demagnetising and partly cross-magnetising?

Answer. Fully cross-magnetising.

Q. Which of the machine in Hopkinson’s test has iron loss less than in the other and why?

Answer. The machine operating as a motor will have iron loss less than that operating as a generator because the excitation of the motor will be lesser than that of the generator.

Q. Why are carbon brushes used in a dc machine?

Answer. The brushes are employed to collect current from the commutator and supply it to the external load circuit. The rate of wear caused to carbon brushes is the same as that of the commutator segment and therefore contact made is always smoother and sparkless. Carbon brushes also have self-lubricating property.

Q. Why are commutating poles provided in the construction of a large dc machine?

Answer. In a large dc machine armature reaction effect becomes very large and the commutation problem increases owing to large armature currents. Commutating poles are, therefore, provided to overcome these problems.

Q. Why are equalizer rings used in lap wound dc machines?

Answer. Equalizer rings are used in lap wound machines to avoid unequal distribution of current at the brushes thereby assist in achieving sparkless commutation.

Q. Why are the brushes staggered?

Answer. The brushes are staggered in order to prevent ridge formation on the surface of the commutator.

Q. Why are the interpoles of a dc machine tapered?

Answer. Interpoles are tapered in order to ensure that there is no satura­tion at the root of the pole at heavy overloads.

Q. Why brake test is performed with small machines only?

Answer. Testing of a large machine entails a considerable loss of energy. In fact, in the case of very large machines, there may be no facility for such a test.

Q. Why constant losses can be considered equal to input to a machine at no load?

Answer. Because at no-load variable losses (armature copper losses and losses in the series field winding, interpole winding and compensating winding, if any) are negligible owing to negligible armature current.

Q. Why do copper losses occur in a dc machine?

Answer. Copper losses occur in a dc machine due to the power consumed in forcing a current against the resistance of windings and connections.

Q. Why do we use a starter for dc machine?

Answer. At the time of starting, no back emf is offered by the dc machine, hence huge starting currents flow into the machine which damages the armature, in order to avoid the damage, a starter is employed. By using starter armature current is limited during starting the machine by providing external resistance and while starting this external resistance is removed in steps.

Q. Why energy storing capacity of a magnetic field is much larger than that of an electric field?

Answer. Energy storing capacity of the magnetic field is much larger than that of the electric field because the value of permittivity of free space is very small compared to the permeability of free space.

Q. Why fractional-pitch winding is preferred over full pitch winding?

Answer. To affect saving in the copper of end connections and to have improved commutation.

Q. Why in dc machines sometimes brushes arc shifted from the neutral axis by a small angle?

Answer. So as to lie along MNP to provide sparkless commutation.

Q. Why is the air-gap between the pole-pieces and the armature kept very small?

Answer. The air-gap between the pole-pieces and the armature is kept very small to reduce the reluctance of the magnetic path.

Q. Why it is necessary to use a large number of coils and commutator segments with the coils evenly distributed around the surface of the armature of a dc machine?

Answer. It is done so as to have practically constant unidirectional voltage at the brushes.

Q. Why machines having long core length are provided with radial ventilating ducts while machines having deep cores are provided with axial cooling?

Answer. Radial ventilating ducts provided with long core machines take away heat at a faster rate. Also in the case of deep cores axial ventilating ducts do. Otherwise, the central portion of the long core and inner portion of the deep core may attain a temperature higher than the permissible value.

Q. Why Swinburne’s test cannot be performed on dc series machines?

Answer. Because on no-load dc series motor will attain such a high speed that it will get damaged and secondly this test is only applicable to those machines in which flux and speed remain constant.

Q. Why Swinburne’s test is considered a convenient and economical method for testing of dc shunt machines?

Answer. Swinburne’s test is considered a convenient and economical method for testing of dc shunt machines because the power required to test a large dc machine by Swinburne’s test is quite small.

Q. Why the cooling time constant of a rotating machine is usually larger than its heating time constant?

Answer. The cooling time constant of a rotating machine is usually larger than its heating time constant owing to poor ventilation conditions when the machine cools.

Q. What are the conditions for parallel operation of an alternator?

Answer. The following conditions must be fulfilled before an incoming alternator can be put in parallel with the bus bars.

  1. The terminal voltage of the incoming alternator must be same as that of bus bar voltage.
  2. The frequency of incoming alternator must be same as that of bus bars.
  3. The phase of the incoming machine voltage must be same as that of bus bar voltage relative to the load.
  4. The phase sequence of the voltage of incoming alternator must be same as that of bus-bar voltage.

Q. Why two reaction theories are applied only to salient pole machines?

Answer. In the cylindrical rotor machine, the air gap is uniform and therefore, its reactance remains the same, irrespective of the spatial position of the rotor. But in case of salient pole machines, the air gap is not uniform and its reactance varies with the rotor position. Therefore the MMF of the armature is divided into two components

  1. direct axis component
  2. quadrature axis component

These facts form the basis of the two reaction theory applied to salient pole machines.

Q. What is a stepper motor?

Answer. A motor in which the rotor turns in discrete movements is called a stepper motor.

Q. Where is stepper motor used?

Answer. Stepper motors are used in industrial situations which call for precise positioning of an object or precise control of speed without having to resort to closed-loop feedback.

Q. What is a servomotor?

Answer. A servomotor is a mechanical positioner used very commonly in control circuits to position the system depending on the magnitude of the error.

Q. Why does an ac servomotor have high rotor resistance?

Answer. High rotor resistance of an ac servomotor makes the torque-speed characteristics linear for various armature voltages over a wide speed range, particularly near the zero speed.

Control System Interview Questions

Q. What is a System?

Answer. When a number of elements or components are connected in a sequence to perform a specific function, the group of elements that all constitute a System.

Q. What is Control System?

Answer. In a System the output and inputs are interrelated in such a manner that the output quantity or variable is controlled by input quantity, then such a system is called Control System. The output quantity is called a controlled variable or response and the input quantity is called a command signal or excitation.

Q. How Time response of the system is divided?

Answer. Time response of the system consists of two parts:

  1. Transient state response
  2. Steady-state response

Transient response of the system explains the response of the system when the input changes from one state to the other. Steady-state response of the system shows the response as the time t, approaches infinity.

Q. What are different types of Control Systems?

Answer. Two major types of Control Systems are

  1. Open-loop Control System
  2. Closed-loop Control Systems

Open-loop Control Systems:

The Open-loop Control System is one in which the Output Quantity has no effect on the Input Quantity. No feedback is present from the output quantity to the input quantity for correction.

Closed-Loop Control System:

The Closed-loop Control System is one in which the feedback is provided from the Output quantity to the input quantity for the correction so as to maintain the desired output of the system.

Q. What are Test signals and their significance?

Answer. The knowledge of the input signal is required to predict the response of the system. In most of the systems, input signals are not known ahead of the time and it is also difficult to express the input signals mathematically by simple equations. In such cases determining the performance of the system is not possible.

Test signals help in predicting the performance of the system as the input signals which we give are known hence we can see the output response of the system for a given input and can understand the behaviour of the control system. The commonly used test signals are the impulse, ramp, step signals and sinusoidal signals.

Q. What are the applications of sampled data systems?

Answer. Sampled data system finds applications in

  1. High-speed tinplate rolling mills using quantized data for control.
  2. Numerically controlled machine tool operations.
  3. Pulse controlled or digital controlled electric drives.
  4. Large complex systems using telemetry links based on pulse modulation (PM) translation of data.

Q. What are the basic components of the feedback control system?

Answer. Basic components of the feedback control system are process system (open loop system), feedback path element, error detector, and controller.

Q. What are the basic properties of Signal Flow Graph?

Answer. The basic properties of the signal flow graph are

  1. Signal Flow Graphs are applicable to linear systems.
  2. It consists of nodes and branches. A node is a point representing a variable or signal. A branch indicates the functional dependence of one signal on another.
  3. A node adds the signals of all incoming branches and transmits this sum to all outgoing branches.
  4. Signals travel along branches only in a marked direction and are multiplied by the gain of the branch.
  5. The algebraic equations must be in the form of cause and effect relationship.

Q. What are the Characteristics of Negative Feedback?

Answer. Negative Feedback in a Control System has the following Characteristics

  • Reduction in the gain at the expense of the better stability of the system
  • Rejection of disturbance signals in the system
  • Low Sensitivity to parameter variations
  • Accuracy in tracking the steady-state value

Q. What is the feedback in Control System?

Answer. The Feedback in Control System in one in which the output is sampled and a proportional signal is fed back to the input for automatic correction of the error (any change in desired output) for further processing to get back the desired output.

Q. What is Cut-off rate?

Answer. The slope of the log-magnitude curve near the cut-off frequency is called the cut-off rate. The cut-off rate indicates the ability of the system to distinguish between the signal and the noise.

Q. What is Gain Margin?

Answer. The Gain Margin is defined as the reciprocal of the magnitude of the open-loop transfer function at the phase crossover frequency. The gain margin indicates the amount by which the gain of the system can be increased without affecting the stability of the system.

Q. What is meant by overshoot?

Answer. Overshoot is the maximum difference between the transient and steady-state solutions for a unit step function input.

Q. What is Mechanical Translational System?

Answer. Model of a mechanical translational system can be obtained by using three basic elements Mass, Spring and Dash-pot.

  1. Weight the mechanical system is represented by mass and is assumed to be concentrated at the center of the body.
  2. The elastic deformation of the body can be represented by the spring.
  3. Friction existing in a mechanical system can be represented by dash-pot.

Q. What is Order of the system?

Answer. Order of the system is defined as the order of the differential equation governing the system. Order of the system can be determined from the transfer function of the system. Also, the order of the system helps in understanding the number of poles of the transfer function. For nth order system for a particular transfer function contains ‘n’ number of poles.

Q. What is Phase crossover frequency?

Answer. The frequency at which the phase of the open-loop transfer function is 180° is called the phase crossover frequency.

Q. What is Phase Margin?

Answer. The phase margin is the amount of additional phase lag at the gain crossover frequency required to bring the system to the verge of instability.

Q. What is Pole of the system?

Answer. Pole of a function F(s) is the value at which the function F(s) becomes infinite, where F(s) is a function of the complex variable s.

Q. What is Resonant Frequency?

Answer. The frequency at which resonant peak occurs is called the resonant frequency. Resonance frequency explains about the speed of the transient response.

Q. What is S-domain and its significance?

Answer. By taking Laplace transform for a differential equation in the time domain equations in S-domain can be obtained. L{F(t)}=F(s)

S domain is used for solving the time domain differential equations easily by applying the Laplace for the differential equations.

Q. What is Signal Flow Graph?

Answer. A Signal Flow Graph is a diagram that represents a set of simultaneous linear algebraic equations. By taking Laplace transform the time domain differential equations governing a control system can be transferred to a set of algebraic equations in s-domain. The signal Flow graph of the system can be constructed using these equations.

Q. What is the basic rule for Block Diagram Reduction Technique?

Answer. The rules of the Block Diagram reduction techniques are designed in such a manner that any modifications made in the diagram will not alter the input and output relation of the system.

Q. What is the effect of positive feedback on the stability of the system?

Answer. Positive feedback is not used generally in the control system because it increases the error signal and drives the system to instability. But positive feedbacks are used in minor loop control systems to amplify certain internal signals and parameters.

Q. What is the mathematical model of a control system?

Answer. Control system is a collection of physical elements connected together to serve an objective. The output and input relations of a various physical system are governed by differential equations. The mathematical model of a control system constitutes a set of differential equations. The response of the output of the system can be studied by solving the differential equations for various input conditions.

Q. What is Time Invariant system?

Answer. Time-Invariant System is one in which the input and output characteristics of the system does not change with time.

Q. What is Time response of the control system?

Answer. Time response of the control system is defined as the output of the closed-loop system as a function of time. Time response of the system can be obtained by solving the differential equations governing the system or time response of the system can also be obtained by the transfer function of the system.

Q. What is Transfer Function?

Answer. Transfer Function of a control system is defined as the ratio of Laplace transform of Output to the Laplace transform of the Input with zero initial conditions.

OR

A transfer function is defined as the Laplace transform of an Impulse response of the system with zero initial conditions.

Q. What is Zero of the system?

Answer. Zero of a function F(s) is a value of s at which the function F(s) becomes zero, where F(s) is a function of complex variable s.

Q. Where Servomechanism is used?

Answer. The servomechanism is used in a control system where the output is pertained to vary the mechanical position of a device.

A servomechanism is widely used in Governor value position control mechanism used in the power plants where the speed of the turbine is taken and processed using the transducers and final control element is brought as the mechanical movement of the value. Nowadays Governor value control is done with Electronic controls using power Thyristors. The servomechanism is also widely used in the robotic hand movements.

Q. Why Negative Feedback is preferred in the Control System?

Answer. The role of Feedback in the control system is to take the sampled output back to the input and compare the output signal with the input signal for error (deviation from the desired result). Negative Feedback results in the better stability of the system and rejects any disturbance signals and is less sensitive to the parameter variations. Hence in control systems, negative feedback is considered.

Power System Interview Questions

Power system is an important subject in the electrical engineering domain. We have collected the best power system questions with their answers so that you can crack your interview/viva voce exam.

Q. What are the Differences between Solid Conductor and Stranded Conductor?

Answer. Solid Conductor

It consists of a single piece of metal wire. It is cheap for manufacturing. Skin effect is higher in solid conductors as at higher frequencies current flow on the surface of the conductors results in the increase in the effective resistance. The main disadvantage of the solid wire is its more rigid property. It cannot be bent easily.

Stranded Conductor

Stranded wire consists of sub-conductors touch each other. It is costlier to manufacture compared to solid wire. For the given current carrying capacity the size of the stranded conductor is large compared to solid wire.

Different elements of strands can be wound together to get the transmission line of desired property (eg: ACSR conductor contains Aluminum and Steel stands wound together). Proximity and skin effect is reduced using stranded conductors.

Q. What is the Difference between Bundled Conductors and Composite Conductors?

Answer. Composite Conductors

In Composite conductors, sub-conductors touch each other. Composite conductors are typically stranded conductors. In Composite conductors, different elements are used (In ACSR conductors aluminium has the properties of lightweight, good conductivity and rustlessness and steel has the property of high tensile strength). Composite conductors are employed as they are flexible compared to a solid conductor. Composite conductors reduce proximity effect and also reduces skin effect up to a certain extent.

Bundled Conductors

Bundled conductors are employed in Extra High Voltage (EHV) transmission as at higher voltages corona effect is significant. In bundled conductors sub-conductors are placed as certain distance throughout the transmission lines. This reduces the corona discharge loss and interference with the communication lines nearby.

Q. What is corona?

Answer. When the voltage between conductors of an overhead line exceeds the disruptive critical voltage value, a hissing noise accompanied by a violet glow appears. This phenomenon is called the corona.

Q. How can the corona effect be minimised?

Answer. Corona effect can be minimised in a transmission line by using large-diameter conductors which may be accomplished by using hollow conductors with a hemp core, ACSR conductors or bundled conductors. Corona effect can also be reduced but to a limited extent, by increasing the spacing between the conductors.

Q. What are the adverse effects of corona?

Answer. The adverse effects of the corona are

  1. power loss but not very important except under abnormal weather conditions
  2. corrosion due to the production of ozone gas
  3. interference with neighbouring communication circuits

Q. What are the disadvantages of corona?

Answer. Some of the disadvantages of the corona are

  • Corona causes power loss
  • Corona causes unacceptable noise
  • Corona cause radio interference

Q. What are the various factors which affect corona loss?

Answer. The factors affecting corona loss are

  • system frequency
  • system voltage
  • air conductivity
  • air density
  • conductor diameter
  • conductor surface condition
  • atmospheric conditions
  • load current

Q. What are the various factors which affect corona?

Answer. The factors affecting corona are

  1. number of ions, size and charge per ion, mean free path depending on atmospheric conditions
  2. line voltage
  3. ratio of conductor spacing to conductor radius
  4. contour of the conductor surface
  5. state of the conductor surface

Q. What do you mean by dielectric strength of air?

Answer. The value of the potential gradient at which complete disruption of air occurs is called the dielectric strength of air.

Q. What is meant by disruptive critical voltage?

Answer. The disruptive critical voltage is defined as the minimum phase to neutral voltage at which corona occurs.

Q. What is visual critical voltage?

Answer. Visual critical voltage is defined as the minimum phase to neutral voltage at which glow appears all along the line conductors.

Q. What are the reasons of power crises in India?

Answer. The causes, which are responsible for power crises in India, are the sharp increase in demand, poor utilization of electrical equipment, high transmission losses, the delay in commissioning of power projects, erratic monsoons, shortage of coal, faulty planning and plant outages.

Q. What are the sources of energy that are utilized for the generation of electrical energy?

Answer. Various sources of energy for the generation of electrical energy are fuels (solid, liquid or gaseous), water and the atomic energy.

Q. Can an old steam power plant be an alternative to a pumped storage plant for peak load operations?

Answer. No, because it is a costly proposition and moreover it (steam power plant) needs large starting time and shutdown time.

Q. On what factors does the power output of a hydropower plant depend?

Answer. The power developed by a hydro plant depends upon the discharge and head and is directly proportional to their (discharge and head) product.

Q. On what factors does the selection of a water turbine depend?

Answer. The selection of a water turbine depends upon various factors such as working head, available discharge, speed, output and nature of the load.

Q. What are the advantages of small hydropower plants?

Answer. Small hydropower plants offer several advantages such as simple operation, reliability, minimal maintenance, quick construction, simple civil engineering and small financial requirements.

Q. What are the different types of turbines?

Answer. Turbines employed in hydroelectric power plants are Pelton wheel, Francis turbine and Kaplan turbine.

Q. What civil engineering works are necessary for a hydroelectric project?

Answer. The civil engineering works for a hydroelectric project include reservoir dam, forebay, spillway, intake structures, surge tank, penstock, tail race, etc.

Q. What for a dam is provided?

Answer. The functions of a dam in hydro-power plants are to raise the water surface of the stream to create an artificial head and also to provide the pondage, storage or the facility of diversion into conduits.

Q. What for pondage is provided?

Answer. In case a power plant is away from the storage reservoir, a small pond is provided near the power plant in order to meet the hourly changes in power demand.

Q. What for surge tank is provided?

Answer. A surge tank is provided to absorb sudden changes in water requirements and reduce water hammer and negative pressure in the penstock.

Q. What is a mass curve?

Answer. A mass curve is a curve which indicates the total volume of run-off in m3 up to a certain time. The mass curve is used in the determination of the capacity of the storage reservoir in hydro-projects.

Q. What is hydrograph?

Answer. A hydrograph is a graphical representation between discharge or flow with time.

Q. What is meant by catchment area?

Answer. The catchment area is the area, bounded by watersheds, which drains into a stream or river across which the dam has been built at a suitable place.

Electrical & Electronic Measurement Interview Questions

Q. State the principle of moving coil galvanometer.

Answer. The principle of moving coil galvanometer is based on the fact that when a current-carrying loop or coil is placed in the uniform magnetic field, it experiences a torque.

Q. What is a moving coil galvanometer?

Answer. Moving coil galvanometer is a device used to detect/measure a small electric current flowing in the electric circuit.

Q. What is the sensitivity of a galvanometer?

Answer. A galvanometer is said to be sensitive if a small current flowing through the coil of galvanometer produces a large deflection in it. There are two types of sensitivities in galvanometer i.e. current and voltage sensitivity.

Current Sensitivity

It is defined as the deflection produced in the galvanometer per unit current flowing through it.

Voltage Sensitivity

It is defined as the deflection produced in the galvanometer per unit voltage applied to it.

Q. What is Q-factor of the coil?

Answer. The ratio of the inductive reactance to the effective resistance of the coil is called the Q-factor of the coil.

Q. What is Q-meter?

Answer. The Q-meter is an instrument designed for measurement of Q-factor of a coil as well as for the measurement of electrical properties of coils and capacitors.

Q. On what principle does a Q-meter operate?

Answer. The Q-meter operates on the principle of series resonance i.e., under a resonant condition of an ac series circuit voltage across the capacitor is equal to the applied voltage times of Q of the circuit.

Q. What are the different parameters that can be measured using a Q-meter?

Answer. The Q-meter is used for measuring Q-factor, inductance, effective resistance, self-capacitance, bandwidth and capacitance.

Q. What are the factors which affect the measurement accuracy of Q-meter?

Answer. The factors affecting measurement accuracy are

  1. Distributed capacitance or self-capacitance of the coil
  2. Residual inductance of the instrument
  3. Conductance of voltmeter
  4. Shunt resistor of Q-meter

Q. Why the actual Q-factor of the coil is somewhat larger than the calculated Q-factor?

Answer. The calculated value of Q-factor is somewhat smaller than its actual value because Q-factor measurement includes the losses of the resonating capacitor, voltmeter and the shunt resistance Rsh.

Q. What is Cathode-ray Oscilloscope (CRO)?

Answer. A Cathode-ray Oscilloscope (CRO) is an electronic device with a Cathode ray tube (CRT) as its main component and other associated circuits consisting of a power supply unit, a saw-tooth wave generator, horizontal and vertical amplifiers.

Q. What are the different types of passive probes?

Answer. The passive oscilloscope probes are Direct probe, isolation and high impedance or 10:1 probe.

Q. For what a triggering circuit is provided in a CRO?

Answer. In a CRO, a triggering circuit is provided for synchronizing two types of deflections so that horizontal deflection starts at the same point of the input vertical signal each time it sweeps.

Q. For what an attenuator is used in CRO?

Answer. An attenuator is used to reduce the signal level by the desired amount. The ordinary resistance voltage divider is one of the simplest types of attenuators.

Q. For what electron gun assembly is provided in a CRT?

Answer. The main function of an electron gun assembly in a CRO is to provide a narrow and sharply focused electron beam which is accelerated towards the phosphor screen.

Q. For what vertical and horizontal plates are provided in a CRO?

Answer. Horizontal and vertical plates are provided between the electron gun and screen to deflect the beam according to the input signal.

Q. How is CRO superior to ordinary measuring instruments?

Answer. CRO is an electronic device that gives a graphical representation of alternating quantities under examination. The CRO gives very accurate measurements as it is free from the errors introduced by the moving parts. It is also free from damping mechanism and other inertia containing parts.

Q. How is focusing achieved?

Answer. Focusing is achieved by various sets of plates forming a sort of electronic lens. In general, there are three sets of plates. Voltages at anode 1 and 2 are kept fixed while it is variable on the third anode. By varying the voltage on the third anode, the spot may be focused.

Q. What are the applications of CRO?

Answer. The applications of CRO are given below

  1. Measurement of voltage and current
  2. Measurement of phase
  3. Measurement of frequency
  4. Comparison of two frequencies

CRO is a versatile instrument that can be used for display, measurement, waveform analysis and other phenomena in electrical and electronic circuits.

Q. What are the essential components of a CRT?

Answer. The essential components of a CRT are electron gun, focusing and accelerating anodes, horizontal and vertical deflection plates, and evacuated glass envelope with a phosphorescent screen.

Q. What are the limitations of oscilloscopes?

Answer. The limitations of oscilloscopes are

  • Loading effect
  • Hum and noise pickup
  • Oscilloscope errors (reading error, parallax error, calibration error, frequency-response error, loading error)

Q. What are the parameters that are affected by capacitive loading?

Answer. The parameters affected by capacitive loading are

  1. Rise and fall times
  2. Impedance Bandwidth
  3. Probe attenuation ratio

Q. What are various focusing techniques?

Answer. Two most commonly employed focusing techniques are

  1. Electrostatic focusing.
  2. Electromagnetic focusing

Q. What effects are caused by resistive loading?

Answer. Resistive loading causes attenuation of the amplitude, dc offset drift and change in circuit bias.

Q. What is a dual beam CRO?

Answer. A dual beam CRO has got two electron beams that are controlled independently. The constructional features are almost the same as in the case of single beam CRO.

Q. What is a dual trace oscilloscope?

Answer. Dual trace oscilloscope employs a single electron gun and provides two traces by switching the deflection plates from one input signal to another which implies that Y-channel is time shared by two signals. The display of the input is continuous and simultaneous to the eyes although it is sampled display.

Q. What is a Lissajous pattern?

Answer. A lissajous pattern is a pattern that results from applying periodic signals to the deflection plates of a CRO.

Q. What is an oscilloscope probe?

Answer. A probe is a means that connects the test circuit to the oscilloscope without altering, loading, or disturbing the test circuit.

Q. What is aquadag?

Answer. Coating of a conducting material, known as aquadag, is provided over the interior surface of CRT in order to accelerate the electron beam after passing between the deflection plates and to collect the electrons produced by secondary emission when the electron beam strikes the screen.

Q. What is Astigmatism control?

Answer. Astigmatism control is an adjustment that will provide sharp focus over the entire screen.

Q. What is graticule?

Answer. The graticule is a scale on transparent material that is fitted to the face of CRT for the purpose of measurement.

Q. What is meant by retrace time?

Answer. Retrace time is the time required by the electron beam to return to its original position on a CRT screen after being deflected to the right by a sawtooth waveform.

Q. What is meant by the deflection factor of a CRO?

Answer. The deflection factor of a CRO is the reciprocal of the deflection sensitivity.

Q. What is meant by the deflection sensitivity of a CRO?

Answer. The deflection sensitivity of a CRO is defined as the vertical deflection of the beam on the screen per unit deflecting voltage.

Q. What is sweep time?

Answer. Sweep time is time duration during which the beam is swept from left to right on the screen of a CRT by the linearly increasing sawtooth voltage.

Q. What is the attenuation factor of the probe?

Answer. The attenuation factor is the amount by which the probe reduces the amplitude of the signal. The typical attenuation factors for probes are 1,10 and 100.

Q. What is the difference between a dual trace and dual beam CRO?

Answer. Dual beam CRO uses two separate electron beams and deflection systems mounted in the same CRT to display two different electron beams simultaneously whereas the dual trace oscilloscope alternately displays two signals on one screen by time-sharing one electron gun and deflection system.

Dual beam CROs are not as versatile as the dual trace CROs because the dual trace CROs can operate in a number of modes.

Q. What is the difference between analog storage oscilloscope and digital storage oscilloscope?

Answer. Digital storage oscilloscope uses digital storage techniques whereas analog storage oscilloscope uses special CRT called the storage tube.

Q. What is the source of inductive loading and what is its effect on measurement?

Answer. The inductive loading is due to the inductance of the probe ground lead and it causes distortion of the signal under measurement.

Q. What type of loading effects are introduced in measurement due to probes?

Answer. Probe loading effects are Resistive loading effects, inductive loading effects and capacitive loading effects.

Q. Why capacitive loading becomes a matter of concern at high frequencies?

Answer. With the increasing signal frequencies or transition speeds, the capacitive load becomes predominant and, therefore, capacitive loading becomes a matter of concern. In particular, capacitive loading affects the rise and fall times on fast-transition waveforms and also the amplitudes of high-frequency components in waveforms.

Q. Why is it preferable not to use CRO in direct sunlight or in a brightly lighted room?

Answer. It is preferable not to use CRO in direct sunlight or in a brightly lighted room because this will allow a low-intensity setting.

Q. Why is the grid in a CRO provided with a hole in it?

Answer. The hole in a grid of a CRO is provided to allow passage for electrons through it and concentrate the beam of electrons along the axis of the tube.

Q. Is modulation required in telemetry? If so why?

Answer. Yes, modulation is required in telemetry. It is because the data available is in the form of current or voltage which is usually very weak for such transmission.

Q. What is multiplexing in the telemetry system?

Answer. Transmission of two or more messages at the same time over a single interconnecting link is called the multiplexing in telemetry system i.e. multiplexing is a method of transmitting more than one measurement over a single transmission channel.

Electrical Energy Utilization Interview Questions

Q. What is tariff?

Answer. If we talk about electric supply, the schedule or rates framed for the supply of electrical energy to different classes of consumers is called tarrif.

Q. Can a power factor be included in a tariff?

Answer. Yes, power factor is included in power factor tariffs such as kVA maximum demand tariff, kWh and kVARh tariff and sliding scale or average power factor tariff.

Q. What is two-part tariff?

Answer. In the two-part tariff, the total charge to be made to the customer is divided into two components namely fixed charge (proportional to connected load or maximum demand) and the running or operating charges (proportional to the units consumed).

Q. Which type of tariff encourages the consumers to keep the load factor and power factor high?

Answer. kVA maximum demand tariff encourages the consumers to keep the load factor and power factor high.

Q. Differentiate between lamp efficiency and specific consumption.

Answer. Lamp efficiency is the ratio of the luminous flux in lumens to the power input in watts while the specific consumption is the ratio of the power input in watts to the average candle power in candela.

Q. How is the polar curve useful to an illumination engineer?

Answer. The polar curve is used to determine the mhcp and mscp of a lamp. They are also used to determine the actual illumination of a surface by using the cp in that particular direction as read from the vertical polar curve in illumination calculations.

Q. How projectors are classified according to the beam spread?

Answer. Projectors, according to beam spread, are classified as narrow beam angle projectors (beam spread 12-25°); medium beam angle projectors (beam spread 25-40°) and wide beam angle projectors (beam spread 40-90°).

Q. What are the advantages of artificial lighting produced electrically?

Answer. Artificial lighting produced electrically have the advantages of cleanliness, ease of control, reliability, steady output as well as low cost.

Q. What are the two laws of illumination?

Answer. (i) Law of inverse squares (ii) Lambert’s cosine law.

Q. What for an integrating sphere is used?

Answer. Integrating sphere is a piece of apparatus which is commonly used for measurement of mscp.

Q. What is Angstrom unit (AU)?

Answer. Angstrom unit (AU) is the unit of wavelength of light and equals to 10-10 m.

Q. What is the difference between illumination and light?

Answer. Light is the cause and illumination is the result of that light on the surface on which it falls.

Q. What is the difference between maintenance factor and depreciation factor?

Answer. Maintenance factor is defined as the ratio of illumination under normal working conditions to the illumination when everything is perfectly clean. Depreciation factor is merely inverse of the maintenance factor.

Q. What is flood-lighting?

Answer. Flood-lighting means flooding of large surfaces with light from a powerful projector(s).

Q. What is the relation between candela (cd) and lumens per steradian?

Answer. Candela (cd) and lumens per steradian, both, are units of luminous intensity.

1 Candela = 1 lumen per steradian

Q. Why does the efficiency of a filament lamp increase with the increase in operating voltage?

Answer. The efficiency (lumens/watt) of a lamp increases with the increase in operating voltage owing to the increase in temperature and is proportional to the square of the operating voltage.

Q. Why is tungsten selected as the filament material?

Answer. Tungsten is the most commonly used metal for filament due to its high melting point (3,400°C), high resistivity, low-temperature coefficient (0.0051), low vapour pressure, being ductile and mechanically strong to withstand vibration during use.

Q. Why none of the practical types of the lamp provides uniformly distributed light in all directions?

Answer. Because of the unsymmetrical shape of the practical types of lamps.

Q. Why sodium vapour discharge lamps are not used for general lighting?

Answer. Sodium vapour discharge lamps are not used for general lighting because they have got drawback of colour discrimination.

Q. Why the glass globe of a filament lamp is completely evacuated?

Answer. The glass globe of a filament of an incandescent lamp is completely evacuated to prevent the oxidization and convection currents of the filament and also to prevent the temperature being lowered by radiation.

Q. Why thermal type starter for use in fluorescent tubes have become obsolete nowadays?

Answer. Because of more complicated construction, greater cost and greater power loss.

Q. What is welding?

Answer. Welding is a materials joining process that produces coalescence of materials by heating them to the welding temperature, with or without application of pressure or by the application of pressure alone, and with or without the use of filler metal.

Q. The location of the focal point of the beam w.r.t. the surface of the workpiece is very important in laser beam welding. Why?

Answer. Maximum penetration occurs when the beam is focused slightly below the surface. Penetration is less when the beam is focused on the surface or deep within the surface.

Q. What is the main drawback of the electron beam process?

Answer. The main drawback of the electron beam process is its high capital cost. The price of the equipment required is very high, and it is expensive to operate, due to the requirement of vacuum pumps. In addition, fit up must be precise and locating the parts w.r.t. the beam must be perfect.

Q. What is meant by welding electrode?

Answer. An electrode is a piece of wire or rod (of a metal or alloy), with or without flux covering, which carries the current for welding.

Q. What is resistance welding?

Answer. Resistance welding is a group of welding processes that produce coalescence of the faying surfaces with the heat obtained from resistance of the work to the flow of welding current in a circuit of which the work is a part and by the application of pressure.

Q. What is the difference between plastic welding and fusion welding?

Answer. In the plastic welding, also called the pressure welding, the pieces of metal to be joined are heated to a plastic state and then forced together by external pressure whereas in fusion welding, also called the non-pressure welding, the material at the joint is heated to a molten state and allowed to solidify. Former one includes forge welding, resistance welding, thermit welding and gas welding while the latter includes gas welding, arc welding and thermit welding without pressure.

Q. What is the fundamental difference between electric arc welding and resistance welding?

Answer. The resistance welding processes differ from arc welding in that pressure is used but filler metal or fluxes are not.

Q. What type of dc generator is used in electric-arc welding?

Answer. Differential compound wound dc generator.

Q. Why alternating current is found most suitable for resistance welding?

Answer. Alternating current is found most suitable for resistance welding as it can provide any desired combination of current and voltage by means of a transformer.

Q. At what power factor would you like to operate the electric arc furnace?

Answer. At a power factor of about 0.9, but not below 0.8 in any case.

Q. How the amount of heating is controlled in high-frequency eddy current heating?

Answer. The amount of heating is controlled by controlling the supply frequency and the flux density in high-frequency eddy current heating.

Q. How can the rate of dielectric heating be varied?

Answer. Rate of dielectric heating can be varied by varying either supply voltage or supply frequency.

Q. How is control of power affected in electric arc furnaces?

Answer. The power input to arc furnaces can be controlled by raising or lowering the electrodes or by varying the voltage applied to the arc furnace (by changing of tapping on supply transformer); preferably both voltage and electrode controls.

Q. How is the power input to submerged arc furnace controlled?

Answer. The power input to submerged arc furnace is controlled by the varying distance between electrodes or by varying the voltage applied to the electrodes.

Q. Name a few materials that are widely used as heating element materials.

Answer. The materials widely used as materials for heating elements are nickel-chromium (Nichrome) alloy, Ni-Cr-Fe alloy, nickel-copper alloy (Eureka or constantan), iron-chromium-aluminium alloy (Kanthal) and alloys containing iron, chromium, cobalt and aluminium.

Q. Name the types of electrodes used in arc furnaces.

Answer. The electrodes used in arc furnaces are of three types namely carbon, graphite and self-baking electrodes.

Q. What are the applications of high-frequency eddy current heating?

Answer. The applications of high-frequency eddy current heating are

  1. surface hardening
  2. annealing soldering
  3. welding
  4. drying of paints
  5. melting of precious metals
  6. sterilization of surgical instruments
  7. forging of bolt-heads and rivet heads

Q. What are the applications of infra-red heating?

Answer. Infra-red heating finds applications in

  1. paint stoving
  2. drying of radio-cabinets and wood furniture
  3. preheating of plastics prior to moulding
  4. softening of thermo-plastic sheets
  5. drying of pottery, piper, textiles etc. where moisture content is not large.

Q. What are the causes of failure of heating elements?

Answer. The chief causes of failure of heating elements are the formation of hot spots, oxidation and intermittency of operation, embrittlement due to grain growth, contamination and corrosion.

Q. What are the different modes of heat transfer?

Answer. There are three modes of heat transfer namely conduction, radiation and convection.

Q. What is an oven?

Answer. An oven is a low-temperature heating chamber with provision for ventilation.

Q. What properties are considered for selecting material for the heating element?

Answer. The properties to be considered in the selection of material for heating elements are resistivity, melting point, temperature coefficient of resistance, and freedom from oxidation.

Q. What type of supply is used for direct core type induction furnace in India?

Answer. Single-phase, 50 Hz, 600 V supply is used for direct core type induction furnaces.

Q. Why dielectric heating is employed only where other methods are impracticable or too slow?

Answer. Because of the very high cost of equipment required for dielectric heating.

Power System Protection Interview Questions

Q. What is a circuit breaker?

Answer. It is a mechanical device designed to close or open contact members, thus closing or opening an electrical circuit under normal or abnormal conditions.

Q. What are the different insulators employed in substations?

Answer. Some of the insulators employed in substations are:

  1. Pin insulators
  2. Post insulators
  3. Strain insulators
  4. Suspension insulators
  5. Hallow apparatus insulators
  6. Dead-end insulators
  7. Solid core insulators

Q. What is Basic Impulse Level (BIL)?

Answer. Basic Impulse Level (BIL) refers to the peak value of lightning impulse voltage withstand capability of the equipment.

Q. What is Creepage distance?

Answer. Creepage distance is the shortest distance between two conducting points along the surface of the insulating material.

Q. What is the neutral point earthing and its advantages?

Answer. In a substation, all the neutrals of rotating machines, transformers, busbars and other equipment are connected to ground. Some of the advantages are

  1. Earth fault protection is based on the method of neutral grounding
  2. The system voltage during earth faults depends on the neutral grounding. During line to ground faults, grounding of neutral helps to reduce the over-voltages which occur on the one healthy phases.

Q. What is the significance of Corona rings in substations?

Answer. It is observed that corona discharge occurs mostly at sharp corners, edges near the conductor fixing points. Corona rings (also called grading rings) are hallowed metallic rings having a large diameter and will have a smooth surface. These corona rings are placed suitably to metal clamps, joints to encircle the conductors surface. This has the advantage of shifting of high-stress point shifted from the conductor surface to corona ring surface. As the diameter and surface area of the ring is higher than that of the conductor. This helps in:

  1. Field distribution is made uniform.
  2. Surface stress is reduced to a much lower value.
  3. Corona discharge is eliminated.
  4. Dust deposition is also minimized.

Q. Name the different faults likely to occur in an overhead transmission line.

Answer. The different faults likely to occur in an overhead transmission line are

  1. Single-phase to ground
  2. Phase to phase
  3. Two phases to ground
  4. Phase to phase and third phase to ground
  5. All the three phases shorted
  6. All the three phases to ground.

Q. What are the basic requirements of a protection system?

Answer. The basic requirements of a protection system are essentially reliability and simplicity, selectivity and discrimination, sensitivity, speed of operation and economy.

Q. What are the sources of short-circuit power in a power system?

Answer. Mainly there are three sources viz synchronous generators, synchronous motors and synchronous condensers and induction motors, which can feed into the short-circuit.

Q. What is the difference between a short-circuit and an overload?

Answer. On the occurrence of a short-circuit, the voltage at the fault point is reduced to zero and the current of abnormally high magnitude flows through the network to the fault point. On the other hand, an overload means that load greater than the designed value have been imposed on the system. In case of an overload, the voltage at the overload point may become low, but cannot be zero. The under-voltage conditions may extend to some distance beyond the overload point in the remainder of the system. In case of an overload, the currents are high but are substantially lower than in the case of a short-circuit.

Q. What is meant by short-circuit?

Answer. When conductors of different phases come into contact with each other in a power line, power transformer, or any other circuit element, part of the impedance of the system is shunted out of the circuit, a large current flows in one or more phases, a short-circuit is said to have occurred.

Q. Which type of fault occurs more frequently in overhead lines?

Answer. Single-phase to ground (L-G) fault occurs more frequently in overhead lines.

Q. Which type of faults are taken into account while determining the rating of the circuit breaker?

Answer. The balanced three-phase fault, though its occurrence is quite rare (only about 5 per cent of the total), imposes a most severe duty on circuit breakers and is, therefore, used in the determination of the circuit breaker rating.

Q. Why current transformers are required in a protective relaying scheme?

Answer. The primary circuit currents which are of high magnitudes are to be reduced to values suitable for relay operation and current transformers are used for this purpose. In addition to this CTs provide insulation against the high voltage of the power circuit and protect the apparatus and the operating personnel from contact with the high voltages of the primary circuits.

Q. Why potential transformers are required in a protective relaying scheme?

Answer. It is not possible to connect the potential coils of the protective devices directly into the system in case of high voltage systems. It is, therefore, necessary to stepdown the voltage and also to insulate the protective equipment from the primary circuit (high voltage power circuit). This is achieved by employing potential (or voltage) transformers (PTs).

Q. Why the probability of the failure or occurrence of abnormal conditions is more on the power line?

Answer. The probability of the failure or occurrence of abnormal conditions is more on the power line. This can be explained by the fact that the power lines are widely branched, have greater length, operate under variable weather conditions and are subject to the action of atmospheric disturbances of electrical nature.

Energy Storage Interview Questions

Q. What is Energy Storage System?

Answer. The systems which receive energy with the objective of storing it electrically, chemically, electrochemically, mechanically or thermally and of making it available again for use at a later time are known as Energy Storage Systems. Energy storage is the incarceration of energy produced at one time for use at a later time. A device that stocks energy is sometimes called an accumulator or battery.

Q. What are the different types of energy storage systems?

Answer. The classification of energy storage systems (ESS) is determined by the use of energy in a specific form. ESS is classified as mechanical, electrochemical, chemical, and electrical storage.

Fig. Classification of energy storage systems (ESS) according to their energy formations

Q. What are the functions of the battery management system?

Answer. A battery management system (BMS) performs the following functions

  1. Data Acquisition
  2. Safety protection
  3. Predict the state of the battery
  4. Control battery charging and discharging
  5. Cell balancing
  6. Thermal management
  7. Battery status delivery and authentication to a user interface
  8. Communication links
  9. Enhance battery life

Analog Electronics Interview Questions

Q. An ideal p-n junction act as a bi-stable switch. Justify?

Answer. A diode is a two-terminal electronic component with asymmetric conductance, it has low (ideally zero) resistance to current flow in one direction and high (ideally infinite) resistance in the other. Apply voltage in one direction; it acts as an open circuit. Reverse the polarity of the voltage and it acts as a short circuit. If the P side is more negative than the N side, the depletion layer increases in thickness and permits no current to pass through it. and so it acts as a voltage-operated switch.

In forward bias, the diode is on means ‘1’, in reverse bias diode is off means ‘0′. So we are getting two states i.e. an ideal p-n junction act as a bi-stable switch.

Q. Can a transistor be used as clipper circuit? Justify.

Answer. Yes, a transistor can be used as a clipper circuit. The reason is that the transistor has two types of linearities. One linearity happens when the transistor passes from cut-in region to the active region. The other linearity occurs when the transistor passes from the active region to the saturation region. When an input signal passes through the transistor, across the boundary between the cut-in region and active region, or across the boundary between the active region and saturation region, a portion of the input signal waveform will be clipped off. A portion of the input waveform which keeps the transistor in the active region shall appear at the output without any distortion.

Q. How the bandwidth of an amplifier can be increased?

Answer. The bandwidth of an amplifier can be increased by configuring the amplifier in a negative feedback configuration.

Q. Mention the basic four sections of an operational amplifier circuit.

Answer. The basic four sections of an operational amplifier circuit are

  1. Input stage: Dual input, balanced output differential amplifier.
  2. Intermediate stage: Dual input, unbalanced output differential amplifier.
  3. Level shift stage: Emitter follower using a constant current source.
  4. Output stage: Complimentary symmetry push-pull amplifier.

Q. What is the bandwidth of an amplifier?

Answer. The bandwidth of an amplifier can be defined as the range of frequencies over which the gain of the amplifier remains almost constant within a certain number of decibels (usually 3 dB).

Q. What is an oscillator?

Answer. An oscillator may be defined as a circuit which generates a continuous, repeated, alternating waveform without any input. Oscillators basically convert unidirectional current flow from a DC source into an alternating waveform which is of the desired frequency, as decided by its circuit components.

Q. Every electronic oscillator can be considered to be an amplifier with infinite gain. Explain where you get the starting signal voltage from.

Answer. Due to the thermal energy, electrons are at random vibrations. These vibrations give rise to small potentials. All oscillators use positive feedback. The small voltage due to movement of electrons due to thermal energy is repeatedly amplified in the oscillator circuit and potential build-up occurs. This results in oscillations. To make these oscillations, the dc biasing voltage is essential. 

Q. Explain why positive feedback and not negative feedback is necessary to produce oscillations.

Answer. With negative feedback, feedback factor Aβ becomes less than unity, AβVi less than Vi and therefore output signal will die out.

With positive feedback, Aβ exceeds unity, AβVi exceeds Vi and oscillations are produced. That is why positive feedback is necessary to produce oscillations.

Q. How does Clapp oscillator circuit differ from Colpitt’s oscillator circuit in construction?

Answer. Clapp oscillator circuit is a refinement of the Colpitt’s oscillator circuit. The only difference is that the single inductor found in the tank circuit of the Colpitt’s oscillator is replaced by a Series L-C combination (inductor L in series with capacitor C).

Q. How does Hartley oscillator differ from Colpitt’s oscillator in construction?

Answer. Hartley oscillator circuit is similar to Colpitt’s oscillator except that phase-shift network consists of two inductors and one capacitor instead of two capacitors and one inductor.

Q. Sometimes a lamp is used in one of the resistance arms of a Wien bridge oscillator. Why?

Answer. A lamp is used in one of the resistance arms of a Wien bridge oscillator so as to provide negative feedback, in proportion to the output current, in the circuit and thus ensure constant output over a range of frequencies.

Q. What are Non-Sinusoidal Oscillators or Relaxation Oscillators?

Answer. Oscillators that produce output that has square, rectangular or sawtooth waveform or have an output which is of pulse shape are called Non- Sinusoidal Oscillators or Relaxation Oscillators.

OR

Non- Sinusoidal Oscillators also be defined as a circuit in which voltage or current changes abruptly from one value to another and which continues to oscillate between these two values as long as dc power is supplied to it.

Non- Sinusoidal Oscillators are classified as

  1. Sawtooth Generators
  2. Blocking Oscillators
  3. Multivibrators

Q. What is the Barkhausen criterion for Oscillator?

Answer. Barkhausen conditions for sustained oscillations are

  • the loop gain of the circuit must be ≥ 1
  • the phase shift around the circuit must be zero

Q. Why are R-C oscillators not used at RF frequencies?

Answer. R-C oscillators, when used at R-F frequencies, will require bulky and expensive low-value capacitors for phase shift circuits which is not practical and therefore not used.

Q. Why Clapp oscillator is preferred over the Colpitt’s oscillator?

Answer. In a Colpitt’s oscillator, the resonant frequency is affected by the transistor and stray capacitances but in a Clapp Oscillator, the transistor and stray capacitances have no effect on the oscillation frequency. Thus in the Clapp oscillator, the effect of transistor parameters on the operation of the circuit is eliminated and the frequency stability is improved. That is why Clapp oscillator is preferred over Colpitt’s oscillator.

Q. Why is Armstrong oscillator less common than the Clapp and Hartley oscillators?

Answer. Armstrong oscillators use transformers which occupy large space and are bulky and costly. Hence Armstrong oscillators are less common than other oscillators (Colpitts, Clapp 0r Hartley oscillators).

Q. Why is crystal oscillator used in communication transmitters and receivers?

Answer. Crystal oscillators are used in communication transmitters and receivers because of their greater frequency stability.

Q. Why is L-C oscillator not used at audio frequency?

Answer. L-C resonant circuits are impractical at audio-frequencies because components required in their construction for low- frequency operation are too bulky and heavy and expensive.

Q. Why we use two transistors in a Wien bridge oscillator?

Answer. Wien bridge oscillator employs two transistors so as to obtain 360° or 0° phase shift between output and input voltages.

(EMT) Electromagnetic field theory Interview Questions

Q. An electrostatic Field line is a continuous curve. That is, a field line cannot have sudden breaks. Why not?

Answer. An electrostatic field line is a continuous curve because a charge experiences a continuous force when traced in an electrostatic field. The field line cannot have sudden breaks because the charge moves continuously and does not jump from one point to the other.

Q. Explain why two electric field lines never cross each other at any point?

Answer. If two electric field lines cross each other at a point, then electric field intensity will show two directions at that point. This is not possible. Hence, two electric field lines never cross each other.

Electric Drive Interview Questions

Q. What is an electric drive?

Answer. An electric drive is defined as a form of equipment designed to convert electrical energy into mechanical energy and provide electrical control of this process.

Q. What is group drive?

Answer. Group drive means a drive in which a single electric motor drives a line shaft by means of which an entire group of working machines may be operated.

Q. Does a shaded-pole motor have large starting torque?

Answer. No, it has very little starting torque.

Q. Electric braking is accomplished in an induction motor by interchanging two stator terminals. Name the method.

Answer. Plugging or counter-current braking.

Q. For stopping the motor, the line switch should always be opened rather than throwing back the starting arm. Why?

Answer. If the starter arm is thrown back the field circuit is broken at the last contact button and owing to inductive nature of field, this will cause a hot arc and burn the contact.

Q. How can the speed of a linear induction motor be controlled?

Answer. The speed of a linear induction motor can be controlled by varying both frequency and voltage simultaneously.

Q. How does active torque differ from passive torque?

Answer. Active torques are due to either gravitational force or deformation in elastic bodies whereas passive torques are due to friction or due to shear and deformation in inelastic bodies. Active torque continues to act in the same direction even after the direction of the drive has been reversed while passive torque always opposes the motion, retarding the rotation of the driven machine.

Q. How is the choice of drive governed?

Answer. The choice of the drive is governed by the speed of driving and driven machines, convenience, space available, including clutching arrangement required and cost.

Q. How is speed control affected in a 3-phase ac series commutator motor?

Answer. In a 3-phase ac series commutator motor, the speed control is affected by moving all brushes simultaneously round the commutator by means of a hand-wheel.

Q. How is speed control affected in a 3-phase ac shunt commutator motor?

Answer. In a 3-phase as shunt commutator motor, the speed is adjusted by changing the tapping points on a transformer inserted between the supply mains and the brushes, which are in a fixed position.

Q. How plugging of a dc motor is done?

Answer. Plugging is obtained in a dc motor by reversing the supply terminals to the armature of the motor.

Q. How speed variation is affected in a repulsion motor?

Answer. Speed variation in a repulsion motor is affected either by varying the impressed voltage or by changing the position of brushes.

Q. How the power factor of a synchronous motor is changed keeping the shaft load undisturbed?

Answer. The power factor of a synchronous motor is changed keeping the shaft load undisturbed by varying the field excitation.

Q. If the number of poles on a motor is increased to lower the speed, how will the power factor be affected?

Answer. If the number of poles on a motor is increased to lower the speed, the power factor will be reduced.

Q. Is the maximum torque of a 3-phase induction motor depends on the rotor resistance?

Answer. No, maximum torque of a 3-phase induction motor is independent of rotor resistance.

Q. Is the torque developed by a single-phase induction motor at synchronous speed zero?

Answer. No. The torque developed by a single-phase induction motor at synchronous speed is negative.

Q. On what factors does the rate of rise of temperature depend?

Answer. The rate of temperature rise depends upon

  1. amount of heat produced and
  2. the amount of heat dissipated per °C rise of the surface of the machine

Q. State whether repulsion motor has series speed-torque characteristics, or shunt speed-torque characteristics.

Answer. Series speed-torque characteristics

Q. What are the main disadvantages of a cage motor?

Answer. The main disadvantages of a cage motor are

  • Low starting torque with large starting current
  • no possibility of speed control

Q. What is meant by load equalization?

Answer. The process of smoothing out of fluctuations in load is known as load equalization.

Q. What is meant by plugging?

Answer. Plugging means stopping of an electric motor by instantaneously reversing its torque till it stops.

Q. What is meant by two-quadrant operation of a dc motor?

Answer. The operation of an electric motor in forward motoring mode and in regenerating (or braking mode) is called the two-quadrant operation of the motor.

Q. What is multi-motor drive?

Answer. Multi-motor drive consists of several individual drives each of which serves to operate one of many working members or mechanisms in some production unit.

Q. What is the most economical method of electric braking?

Answer. Regenerative braking.

Q. What type of protection is provided in the starters meant for 3-phase induction motors?

Answer. Overload and under-voltage protection.

Q. What will be the effect on the direction of rotation of a dc motor if the supply terminals are reversed?

Answer. There is no effect on the direction of rotation of a dc motor. The motor will rotate in the same direction, as before the reversal of the line terminals.

Q. When it is required to control the induction motor speed during deceleration what kind of braking is preferable to employ?

Answer. DC dynamic braking is highly suitable for controlling the speed of an induction motor during deceleration by controlling the dc excitation.

Q. Which is the cheapest method of starting a 3-phase induction motor?

Answer. Direct-on-line (D-O-L) starting.

Q. Which method we must adopt to control the speed of a dc shunt motor above the base speed?

Answer. The field control method is used to control the speed of a dc shunt motor above the base speed.

Q. While controlling the speed of a dc shunt motor what should be done to achieve a constant torque drive?

Answer. To achieve a constant torque drive while controlling the speed of a dc shunt motor, the applied voltage should be maintained constant so as to maintain field strength constant.

Q. Why are induction motors called asynchronous motors?

Answer. Induction motors are called asynchronous motors because the rotor does not turn in synchronous with the rotating field developed by the stator currents.

Q. Why do you require starters for starting a 3-phase induction motor?

Answer. The starter is required in a 3-phase induction motor to limit the initial rush of current to a pre-determined value at the starting instant.

Q. Why is series motor preferred for traction purposes?

Answer. Series motor preferred for traction purposes because dc series motor develops large starting torque and slows down with the increase in load and so gets automatically relieved from heavy excessive load.

Q. Why should a dc series motor not be run without load?

Answer. On no-load, dc series motor would attain tremendously high speed and the motor may get damaged due to heavy centrifugal forces setup in rotating parts.

Q. Why single-phase induction motors are usually set on rubber spring mounts?

Answer. In a single-phase induction motor, pulsating torque is developed as a direct consequence of the pulsating power in a single-phase circuit.

Q. Why speed-torque (not torque-speed) characteristic curves are commonly used in utilization studies?

Answer. In application engineering, it is usually the load torque which is known and the speed at which the motor can drive this load is unknown and is to be determined. Thus the torque is independent variable and speed is a dependent variable. Consequently, speed-torque characteristic curves with torque as abscissa are commonly used in utilization studies.

Q. Why the cooling time constant of a rotating machine is usually larger than its heating time constant?

Answer. The cooling time constant of a rotating machine is usually larger than its heating time constant owing to poor ventilation conditions when the machine cools.

Q. Why the dc motors mostly gear motors?

Answer. The dc motors are mostly gear motors because the low-speed motors are much more expensive than high-speed motors of the same kW output rating.

Q. Why the V/f is kept constant while controlling the speed of a 3-phase induction motor?

Answer. The V/f  ratio is kept constant while controlling the speed of a 3-phase induction motor in order to maintain flux level constant.

In this article, we see the best electrical engineering interview questions and answers. If you any question, do ask in the comment below.

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