Losses in a DC Machines

Q. What is meant by “energy loss” in a machine?

Answer. Whole of the input energy to the electrical machine is not converted into useful output energy but a part of the energy is converted into heat and is lost for the useful purpose of the machine. The energy converted into heat is called the energy loss.

Q. What is the basic power equation for a dynamo?

Answer. Pinput = Poutput + Plosses

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

(i) copper or electrical losses

(ii) iron or magnetic losses

(iii) 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 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 no-load losses in a dc machine?

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

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

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

Q. How do the various losses occurring in a dc machine vary 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. 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 brush-contact loss is proportional to the load.

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. On what factors do hysteresis and eddy current losses depend?

Answer. Hysteresis loss depends upon the peak magnitude of the flux density attained in a magnetic cycle, frequency of magnetic cycles, volume of magnetic material and type of magnetic material.

Eddy current loss depends also upon the thickness of magnetic material laminations in addition to factors mentioned above.

Q. Why CROS cannot be used in small rotating machines?

Answer. In small rotating machines it is difficult to correspond the axis of core with the rolling direction.

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

Answer. When variable losses equal constant losses.

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