Previous gate question Topic Electrical Machines

Previous gate question Topic Electrical Machines

1. The slip of an induction motor normally does not depend on

(A) rotor speed (B) synchronous speed

(C) shaft torque (D) core-loss component

Ans: Option (D) is correct. ns = synchronous speed n = rotor speed Thus, slip depend on synchronous speed and the rotor speed. Also, torque increases with increasing slip up to a maximum value and then decreases. Slip does not depend on core/loss component

2. A 4 point starter is used to start and control the speed of a

(A) dc shunt motor with armature resistance control

(B) dc shunt motor with field weakening control

(C) dc series motor

(D) dc compound motor

ans: Option (A) is correct

3.A three phase, salient pole synchronous motor is connected to an infinite bus. It is operated at no load a normal excitation. The field excitation of the motor is first reduced to zero and then increased in reverse direction gradually. Then the armature current.

(A) Increases continuously

(B) First increases and then decreases steeply

(C) First decreases and then increases steeply

(D) Remains constant.

Ans: Option (B) is correct. The Back emf will go to zero when field is reduced, so Current input will be increased. But when Field increases (though in reverse direction) the back emf will cause the current to reduce.

4. A field excitation of 20 A in a certain alternator results in an armature current of 400 A in short circuit and a terminal voltage of 2000 V on open circuit. The magnitude of the internal voltage drop within the machine at a load current of 200 A is

(A) 1 V (B) 10 V

(C) 100 V (D) 1000 V

Ans Option (D) is correct. Given field excitation of = 20 A Armature current = 400 A Short circuit and terminal voltage = 200 V On open circuit, load current = 200 A So, Internal resistance = 2000/400 =5 Ω Internal vol. drop = 5x 200 = 1000 V

5. Distributed winding and short chording employed in AC machines will result in

(A) increase in emf and reduction in harmonics

(B) reduction in emf and increase in harmonics

(C) increase in both emf and harmonics

(D) reduction in both emf and harmonics

6. In a stepper motor, the detent torque means

(A) minimum of the static torque with the phase winding excited

(B) maximum of the static torque with the phase winding excited

(C) minimum of the static torque with the phase winding unexcited

(D) maximum of the static torque with the phase winding unexcited

Ans Option (D) is correct. Detent torque/Restraining toque: The residual magnetism in the permanent magnetic material produced. The detent torque is defined as the maximum load torque that can be applied to the shaft of an unexcited motor without causing continuous rotation. In case the motor is unexcited.

7. The speed of rotation of stator magnetic field with respect to rotor structure will be

(A) 90 rpm in the direction of rotation

(B) 90 rpm in the opposite direction of rotation

(C) 1500 rpm in the direction of rotation

(D) 1500 rpm in the opposite direction of rotation

Ans Option (A) is correct. Given 3-φ, 440 V, 50 Hz, 4-Pole slip ring motor Motor is coupled to 220 V Ns =1500 rpm , Relative speed = 1500 − 1410 = 90 rpm in the direction of rotation.

8.In a transformer, zero voltage regulation at full load is

(A) not possible

(B) possible at unity power factor load

(C) possible at leading power factor load

(D) possible at lagging power factor load 

Ans : Option (C) is correct. In transformer zero voltage regulation at full load gives leading power factor.

9. The dc motor, which can provide zero speed regulation at full load without any controller is

(A) series 

(B) shunt

(C) cumulative compound 

(D) differential compound

Option (B) is correct.

10. A three-phase synchronous motor connected to ac mains is running at full load and unity power factor. If its shaft load is reduced by half, with field current held constant, its new power factor will be

(A) unity

(B) leading

(C) lagging

(D) dependent on machine parameters

Ans : Option (B) is correct. When the 3-φ synchronous motor running at full load and unity power factor and shaft load is reduced half but field current is constant then it gives leading power factor

11. A three-phase, three-stack, variable reluctance step motor has 20 poles on each rotor and stator stack. The step angle of this step motor is

(A) 3c (B) 6c

(C) 9c (D) 18c 

Ans : Option (B) is correct. Given 3-φ, 3-stack Variable reluctance step motor has 20-poles Step angle = (360)/ (3x20)= 6

12. In transformers, which of the following statements is valid ?

(A) In an open circuit test, copper losses are obtained while in short circuit

test, core losses are obtained

(B) In an open circuit test, current is drawn at high power factor

(C) In a short circuit test, current is drawn at zero power factor

(D) In an open circuit test, current is drawn at low power factor

13.For a single phase capacitor start induction motor which of the following statements is valid ?

(A) The capacitor is used for power factor improvement

(B) The direction of rotation can be changed by reversing the main winding terminals

(C) The direction of rotation cannot be changed

(D) The direction of rotation can be changed by interchanging the supply terminals 

Ans: Option (B) is correct. A single-phase capacitor start induction motor. It has cage rotor and its stator has two windings. The two windings are displaced 90c in space. The direction of rotation can be changed by reversing the main winding terminals.

14. In a DC machine, which of the following statements is true ?

(A) Compensating winding is used for neutralizing armature reaction while

interpole winding is used for producing residual flux

(B) Compensating winding is used for neutralizing armature reaction while

interpole winding is used for improving commutation

(C) Compensating winding is used for improving commutation while

interpole winding is used for neutralizing armature reaction

(D) Compensation winding is used for improving commutation while

interpole winding is used for producing residual flux

Ans: Option (B) is correct. In DC motor, compensating winding is used for neutralizing armature reactance while interpole winding is used for improving commutation. Interpoles generate voltage necessary to neutralize the e.m.f of self induction in the armature coils undergoing commutation. Interpoles have a polarity opposite to that of main pole in the direction of rotation of armature.

15.A synchronous generator is feeding a zero power factor (lagging) load at

rated current. The armature reaction is

(A) magnetizing (B) demagnetizing

(C) cross-magnetizing (D) ineffective

Option (B) is correct. A synchronous generator is feeding a zero power factor(lagging) load at rated current then the armature reaction is demagnetizing.

16. In relation to the synchronous machines, which on of the following statements

is false ?

(A) In salient pole machines, the direct-axis synchronous reactance is greater

than the quadrature-axis synchronous reactance.

(B) The damper bars help the synchronous motor self start.

(C) Short circuit ratio is the ratio of the field current required to produces

the rated voltage on open circuit to the rated armature current.

(D) The V-cure of a synchronous motor represents the variation in the

armature current with field excitation, at a given output power.

Ans: Option (C) is correct. In synchronous machine, when the armature terminal are shorted the field current should first be decreased to zero before started the alternator. In open circuit the synchronous machine runs at rated synchronous speed. The field current is gradually increased in steps. The short circuit ratio is the ratio of field current required to produced the rated voltage on open to the rated armature current

17. For a given stepper motor, the following torque has the highest numerical value

(A) Detent torque (B) Pull-in torque

(C) Pull-out torque (D) Holding torque

Option (C) is correct. Stepper motor is rotated in steps, when the supply is connected then the torque is produced in it. The higher value of torque is pull out torque and less torque when the torque is pull in torque.

18. The following motor definitely has a permanent magnet rotor

(A) DC commutator motor (B) Brushless dc motor

(C) Stepper motor (D) Reluctance motor

MCQ 4.84 The type of single-phase induction motor having the highest power factor

at full load is

(A) shaded pole type (B) split-phase type

(C) capacitor-start type (D) capacitor-run type

Option (C) is correct. The stepper motor has the permanent magnet rotor and stator has made of windings, it’s connected to the supply

19.A single-phase induction motor with only the main winding excited would

exhibit the following response at synchronous speed

(A) Rotor current is zero

(B) Rotor current is non-zero and is at slip frequency

(C) Forward and backward rotaling fields are equal

(D) Forward rotating field is more than the backward rotating field

Ans: Option (D) is correct. Given that: 1-φ induction motor main winding excited then the rotating field of motor changes, the forward rotating field of motor is greater then the back ward rotating field.

20. The core flux of a practical transformer with a resistive load

(A) is strictly constant with load changes

(B) increases linearly with load

(C) increases as the square root of the load

(D) decreases with increased load

Option (A) is correct. We know that in case of practical transformer with resistive load, the core flux is strictly constant with variation of load.