Electrical Engg (EEE) - MCQ Practice Questions

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.

The rotating magnetic field produced by the 3-phase stator winding rotates at synchronous speed (Ns = 120f/P), independent of load or rotor speed.

At no-load with increased excitation, the motor operates at leading power factor. Armature current decreases because the motor absorbs less reactive power from the supply.

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.

Armature reaction in DC generators creates a demagnetizing effect and shifts the neutral plane forward (in direction of rotation), both effects occurring simultaneously.

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).

Rotor copper loss = sP_gap, where s is slip and P_gap is the air gap power. It is directly proportional to slip.

In an autotransformer, the turns ratio is N2/N1 = $\frac{400}{100}$ = $\frac{4}{1}$, so voltage transformation ratio is Vs/Vp = 1:4 for a step-up autotransformer.

Synchronous speed Ns = 120f/P = (120 × 50)/4 = 1500 rpm for a 4-pole motor at 50 Hz.

Cogging (or coggling) occurs when the number of stator and rotor slots are equal or have common factors, causing reluctance torque variations due to slot permeance interaction.

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.

The double-field revolving theory represents a single-phase AC winding's pulsating magnetic field as two counter-rotating fields, used to analyze single-phase induction motor behavior.

At no-load, the transformer draws mainly reactive (magnetizing) current, which lags the applied voltage by approximately 90°, giving a power factor of about 0.1-0.2.

In a DC shunt generator, as load increases, armature current increases, causing armature reaction that demagnetizes the field, reducing terminal voltage significantly.

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.

Shaded pole motors use a copper ring or shaded pole around part of the pole face to create a phase shift in the magnetic field, producing starting torque without additional components.

At constant V and reduced frequency, the air gap flux increases (Φ = V/f), but the rotor reactance decreases. However, the overall effect results in reduced starting and running torque.

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.

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 × ($\frac{60}{50}$) = 1152 rpm. This is because slip s = (Ns - N)/Ns remains approximately constant for the same load.

In a DC series motor, when unloaded: (1) Load torque decreases, so armature current reduces significantly since field current equals armature current in series connection. (2) With reduced current, voltage drop (Ia×Ra) decreases. (3) Back EMF increases (Eb = V - Ia×Ra). (4) Since N ∝ Eb/Φ and both Eb increases while Φ decreases (less field current), speed increases substantially. This is why series motors are unsuitable for no-load operation.