Govt. Exams
Entrance Exams
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Topics in Electrical Engg (EEE)
A 3-phase induction motor has 4 poles and is connected to a 50 Hz supply. What is the synchronous speed?
Correct Answer:
A. 1500 rpm
EXPLANATION
Synchronous speed Ns = 120f/P = 120×50/4 = 1500 rpm
In a transformer, the turns ratio k = N2/N1. If k > 1, the transformer is:
Correct Answer:
B. Step-up transformer
EXPLANATION
When secondary turns are greater than primary turns (k > 1), voltage is stepped up.
The slip of an induction motor is defined as:
Correct Answer:
A. (Ns - Nr)/Ns
EXPLANATION
Slip s = (Ns - Nr)/Ns, where Ns is synchronous speed and Nr is rotor speed.
The synchronous speed of a 4-pole, 50 Hz induction motor is:
Correct Answer:
A. 1500 rpm
EXPLANATION
Synchronous speed Ns = 120f/P = (120 × 50)/4 = 1500 rpm.
In a three-phase induction motor, the rotating magnetic field is produced by:
Correct Answer:
B. Three-phase stator currents with 120° phase difference
EXPLANATION
Three-phase currents 120° apart create a rotating magnetic field that rotates at synchronous speed Ns = 120f/P.
The back EMF in a DC motor is given by the expression?
Correct Answer:
B. E = PφZN/120
EXPLANATION
Back EMF in DC motor: Eb = PφZN/120 where P = poles, φ = flux, Z = conductors, N = speed in rpm.
Which of the following is the primary function of a commutator in a DC machine?
Correct Answer:
B. To reverse the direction of current in armature conductors
EXPLANATION
The commutator reverses the direction of current in armature conductors as they move from one pole to another, ensuring unidirectional torque production.
