1. DC machine windings are




... Answer is A)
DC machine windings are full pitched winding only because it offers maximum induced emf within the coil. No requirement of short pitching of coils like alternator.


2. If field current is decreased in shunt dc motor, the speed of the motor




... Answer is B)
As a shunt field current If decreases, flux also decreases and the speed rises as speed is inversely proportional to flux.


3. In Ward-Leonard system, the lower limit of the speed imposed by




... Answer is C)
In Ward-Leonard method of speed control, the lower limit of speed is imposed by residual magnetism of the generator.

4. Eddy current loss will depends on




... Answer is D)
eddy current loss will depend upon frequency, flux density and the area of the eddy current loop.

5. Which of the following are variable losses?




... Answer is B)
Armature copper loss is directly proportional to square of armature current. Therefore as the load varies it will also vary.

6. As compared to an induction motor, the air gap in a DC motor is




... Answer is D)
In case of DC motor a small air gap exists between the pole pieces and armature so that there will be no rubbing in the machine. However this gap is kept as small as possible, since larger the air gap greater is the mmf required to create the required flux. The air gap length is about 1.0 mm to 6 mm. But in case of rubbing against the stator, the air gap between the stator and rotor is made as small as practicable (0.3 to 0.5 mm in small machine and 1 mm to 1.5 mm in high power machine) so as to make air gap reluctance minimum.

7. The starting resistance of DC motor is generally




... Answer is B)
At the time of starting of DC motor, starting resistance is only the resistance of armature. Since the armature resistance of a motor is very small generally less than one ohm. But for protection purpose an extra resistance must be connected in series with the armature. This would limit the initial current to a safe value until the motor has build up with the stable speed and back emf.

8. DC series motors are used




... Answer is D)
DC series motor are variable speed motors, develop large starting torque. It slows down with the increase in load and so get automatically relieved from heavy excessive load.

9. In case of DC Shunt Motor electromagnetic torque developed is




... Answer is B)
In case of DC shunt motors the torque is proportional to only armature current because flux is practically constant in DC shunt motor.

10. Which type of motors are preferred in lifts?




... Answer is A)
DC compound motors are preferred for lifts. Shunt type commutator motors are preferred in case of single phase installation. The latest lift designs use 3-phase slip ring induction motors with variable frequency drive electronic controls. Some may also use a similar system with permanent magnet (brush less) DC Motors.

11. Which type motors are preferred for lathes?




... Answer is D)
Lathes machines requires uniform torque. Hence squirrel cage induction or DC shunt motors are preferred for lathes.

12. If an induction machine is run at above the synchronous speed it acts as




... Answer is A)
Induction motor acts as generator when its speed increases than the speed of its stator rotating magnetic field. If a prime mover is connected to the shaft and the stator is connected to ac supply, when it is rotated at a speed greater than the speed of rotating magnetic field, slip becomes negative(-1 < S < 0), torque becomes negative along with the rotor current and power. Therefore motor starts supplying current or power to the supply lines where it is connected.

13. Which type of motors uses slip rings?




... Answer is D)
Slip rings are used in induction motors to increase the starting torque. A suitable value of external resistance is added equally into the rotor 3-phase winding through stationary sliding contacts known as brushes which are placed on the slip rings. The motor is started with suitable external resistance which produce the high starting torque, when the motor accelerates or under running condition, the brushes are lifted and the slip rings are joined together in order to form a closed circuit through metal collar arrangement. Under running condition slip ring induction motor is equivalent to squirrel cage induction motor. In Synchronous motor the field winding is connected in series with the slip rings to connect with a DC exciter.

14. The basic function of a transformer is to change




... Answer is C)
The basic function of a transformer is step up or step down voltages without changing its supply frequency. Therefore the frequency of voltage at the secondary is equal to the primary side.

15. In a transformer zero voltage regulation is achieved at a load power factor which is




... Answer is C)
At leading power factor the voltage regulation can be negative or zero. This can be found from this equation % regulation = εxcosθ - εrsinθ

16. The constant losses in transformer is




... Answer is D)
Eddy current loss and hysteresis loss are almost independent of load, significantly depending on supply voltage and frequency. As the flux density or flux is constant for a given voltage and frequency, eddy current loss and hysteresis loss remain constant at any load. Therefore, these losses are called constant losses. Copper loss varies as the square of load current and called variable loss.

17. Open circuit test on transformer gives




... Answer is D)
The no load current drawn by the primary is around 2 % to 6 % of rated current. Therefore copper loss of primary is very low and can be neglected. As the copper loss is negligible, the watt-meter reading is considered as iron loss or constant loss i.e eddy current loss and hysteresis loss.

18. Transformer core is laminated to reduce




... Answer is A)
Transformer core is laminated to reduce eddy current loss. Each lamination will act as individual core to form a single core. This will increase the eddy resistance and reduce the eddy current. The size and volume of eddy current loops become smaller. Therefore, eddy current losses will be reduced when core is laminated.

19. Transformer oil is used as




... Answer is C)
Small transformers less than 25 KVA are naturally air cooled and insulation type. But in large transformers, transformer oil is used. It has two functions. 1. Cooling and 2. Insulating It should contain high dielectric strength.

20. stepped cores are used in transformers in order to reduce




... Answer is B)
As the ratings are increased stepped or multi-stepped cores are preferred, which improve mechanical stability, reduce the length of mean turn and result in copper savings besides increasing efficiency and improving regulation.