Which of the following is the primary function of a commutator in a DC machine?
Answer: B
The commutator reverses the direction of current in armature conductors as they move from one pole to another, ensuring unidirectional torque production.
Q.2Easy
The back EMF in a DC motor is given by the expression?
Answer: B
Back EMF in DC motor: Eb = PφZN/120 where P = poles, φ = flux, Z = conductors, N = speed in rpm.
Q.3Easy
In a three-phase induction motor, the rotating magnetic field is produced by:
Answer: B
Three-phase currents 120° apart create a rotating magnetic field that rotates at synchronous speed Ns = 120f/P.
Q.4Easy
The synchronous speed of a 4-pole, 50 Hz induction motor is:
Slip s = (Ns - Nr)/Ns, where Ns is synchronous speed and Nr is rotor speed.
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Q.6Medium
A DC shunt motor is running at no load. If the field winding is suddenly short-circuited, the motor will:
Answer: B
Flux reduces drastically when field is short-circuited. As back EMF reduces, armature current increases causing motor speed to increase dangerously.
Q.7Easy
In a transformer, the turns ratio k = N2/N1. If k > 1, the transformer is:
Answer: B
When secondary turns are greater than primary turns (k > 1), voltage is stepped up.
Q.8Medium
The efficiency of a transformer is maximum when:
Answer: A
Transformer efficiency is maximum when core loss (Pi) equals copper loss (Pcu), i.e., Pi = Pcu.
Q.9Medium
The starting current of a 3-phase induction motor is typically how many times the full-load current?
Answer: B
Induction motors draw 4-7 times full-load current during starting because rotor speed is zero and rotor resistance appears as reactance.
Q.10Medium
Which method is NOT used for starting a three-phase induction motor?
Answer: D
Standard starting methods are DOL, star-delta, autotransformer, and rotor resistance. Frequency variation is used for speed control, not starting.
Q.11Medium
A DC series motor is suitable for:
Answer: B
DC series motors have high starting torque (proportional to load current) and are used in electric vehicles, cranes, and hoists.
Q.12Medium
The power factor of an induction motor at full load is typically:
Answer: C
Induction motors operate at lagging power factor (0.85-0.95) due to reactive magnetizing current required for flux production.
Q.13Medium
What is the relationship between slip and rotor frequency in an induction motor?
Answer: A
Rotor frequency fr = slip × stator frequency = s × fs, where fs = 50 Hz for 50 Hz supply.
Q.14Medium
In a synchronous motor, the damper winding is used to:
Answer: B
Damper windings provide asynchronous starting torque to accelerate the rotor near synchronous speed before synchronous operation begins.
Q.15Medium
A 100 kVA transformer has 1000 primary turns and 100 secondary turns. The turns ratio and voltage ratio are:
Answer: A
Turns ratio k = N2/N1 = 1000100 = 0.1. Voltage ratio V1/V2 = N1/N2 = 110 or 10:1 (step-down).
Q.16Hard
The torque developed in a 3-phase induction motor is maximum at a slip of:
Answer: C
Maximum torque occurs at slip sm = R2/X2. This is crucial for motor design and performance analysis.
Q.17Hard
An 8-pole induction motor running at 720 rpm with 50 Hz supply has a slip of approximately:
Answer: A
Ns = 120×850 = 750 rpm. Slip s = (750-720)/750 = 75030 = 0.04 or 4%.
Q.18Hard
In a DC machine, armature reaction causes:
Answer: B
Armature reaction is caused by armature MMF which distorts the main magnetic field, reduces net flux, and causes commutation issues. Compensation is needed through commutating poles.
Q.19Hard
The equivalent circuit of an induction motor includes a series resistance R2/s in the rotor branch. The power dissipated in this resistance represents:
Answer: B
In equivalent circuit, voltage across R2/s is rotor voltage and current through it gives mechanical power = I2² × (R2/s - R2) = mechanical power developed.
Q.20Easy
A 3-phase induction motor has 4 poles and is connected to a 50 Hz supply. What is the synchronous speed?
