21. The air-gap between the yoke and armature in a dc motor is kept small
- to achieve a stronger magnetic field.
- to avoid overheating of the machine.
- to avoid locking of the armature.
- to avoid transverse motion.
22. Which one of the following statement is correct for the ideal field excitation during starting in case of shunt and compound motors?
- Maximum because motor torque required to overcome friction and load torque.
- Maximum, because voltage should be built slowly.
- Maximum, because small excitation will lead to low operating speed during starting.
- It can be anything because it does not matter in case of these motors.
23. A Separately excited dc motor is started using a 8-phase ac/dc controlled rectifier using ‘Soft starting’. For limiting the starting current, it is required that firing angle should be
- Gradually increased from 0° to 180°
- Gradually reduced from 180° to 30°
- Fixed at 30°
- Gradually increased from 30° to 180°
24. A dc motor operated from a type A chopper is switched to type B chopper. How does the motor operate when type A and B are in operation respectively?
- Plugging and motoring
- Motoring and plugging
- Regenerative braking and motoring
- Motoring and regenerative braking
25. The armature MMF waveform of a dc machine is
- Rectangular
- Pulsating
- Triangular
- Sinusoidal
26. When is the mechanical power developed by a d.c. motor maximum?
- Back e.m.f. is equal to applied voltage
- Back e.m.f. is equal to zero
- Back e.m.f. is equal to half the applied voltage
- None of the above
27. Torque developed by the armature of a dc motor is proportional to which one of the following?
- (Back emf) × (Armature current)
- (Magnetic flux per pole) × (Armature current)
- (Back emf) × (Magnetic flux per pole)
- (Back emf)/(Magnetic flux per pole)
28. A 5 kW, 200 V dc shunt motor has armature resistance of 1 ohm and shunt field resistance of 100 ohm. At no load, the motor draws 6 A from 200 V supply and runs at 1000 rpm. What is the total copper loss of the machine?
- 400 W
- 16 W
- 36 W
- 416 W
29. The field flux method of speed control of dc shunt motor is generally used for which one of the following?
- Constant hp drive below the base speed
- Constant torque drive above the base speed
- Constant torque drive below the base speed
- Constant hp drive above the base speed
30. Neglecting all losses, how is the developed torque (T) of a dc separately excited motor, operating under constant terminal voltage, related to its output power (P)?
$\\a.\; T\propto\sqrt{P} \\ b. \; T\propto P \\ c. \; T^{2}\propto P^{3} \\ d. \; \text{T is independent of P}$