In a 3-phase squirrel cage induction motor, which parameter can be increased to improve the starting torque?
Answer: C
Starting torque in an induction motor is proportional to rotor current and rotor resistance. Increasing rotor resistance improves starting torque at the cost of full-load efficiency.
Q.42Medium
The slot leakage reactance in an induction motor depends on:
Answer: B
Slot leakage reactance is determined by the magnetic flux confined within the stator and rotor slots, hence depends on dimensions of both stator and rotor slots.
Q.43Medium
In a DC generator, armature reaction causes:
Answer: C
Armature reaction in DC generators creates a demagnetizing effect and shifts the neutral plane forward (in direction of rotation), both effects occurring simultaneously.
Q.44Medium
The power factor of a transformer at no-load is low because:
Answer: B
At no-load, the transformer draws mainly magnetizing current which lags the applied voltage by approximately 90°, resulting in low power factor (typically 0.1-0.2).
Q.45Medium
Which of the following losses in an induction motor is proportional to slip?
Answer: C
Rotor copper loss = sP_gap, where s is slip and P_gap is the air gap power. It is directly proportional to slip.
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Q.46Medium
In a separately excited DC motor, if flux is reduced to half while keeping the supply voltage constant, the armature current will:
Answer: C
Back EMF = KΦω. When Φ is halved, back EMF reduces, so armature current (Ia = (V - Eb)/Ra) increases approximately to double for the same load.
Q.47Medium
The regulation of a DC shunt generator is poor because:
Answer: B
In a DC shunt generator, as load increases, armature current increases, causing armature reaction that demagnetizes the field, reducing terminal voltage significantly.
Q.48Medium
In a 3-phase synchronous generator, the pitch factor (or coil span factor) accounts for:
Answer: B
Pitch factor (Kp) accounts for the reduction in EMF when the coil span is less than the pole pitch (full pitch), representing incomplete flux linkage.
Q.49Medium
In a transformer connected to a load with lagging power factor, the secondary current will:
Answer: B
If the load has lagging power factor, the secondary current lags the secondary voltage. The phase relationship between primary and secondary depends on the load power factor.
Q.50Medium
A 3-phase squirrel cage induction motor is running at 960 rpm when connected to a 50 Hz supply. If the supply frequency is suddenly changed to 60 Hz, what will be the approximate new speed of the motor, assuming the slip remains constant?
Answer: A
Synchronous speed Ns = 120f/P. When frequency changes from 50 Hz to 60 Hz with constant slip, new speed will be proportional to frequency. New speed = 960 × (5060) = 1152 rpm. This is because slip s = (Ns - N)/Ns remains approximately constant for the same load.
