Test: Electrical Machines- 1 - Electrical Engineering (EE) MCQ

The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature. The series-connected compensating windings produce a magnetic field, which varies directly with armature current. Because the compensating windings are wound to produce a field that opposes the magnetic field of the armature, they tend to cancel the cross magnetizing effect of the armature magnetic field.

Interpoles are similar to the main field poles and located on the yoke between the main field poles. They have windings in series with the armature winding. Interpoles have the function of reducing the armature reaction effect in the commutating zone. They eliminate the need to shift the brush assembly.

As v/f ratio is constant.

If v increases, f also increases.

As frequency increases, core losses also increases.

We know that, 

As v and f both increases, flux remain constant and hence magnetizing current will remain constant.

At lagging zero electrical power factor, the armature current lags by 90° to induced emf in the armature. As the emf induced in the armature coil due to main field flux thus the emf leads the main field flux by 90°.

Armature flux and field flux act directly opposite to each other. Hence armature reaction of the alternator at lagging zero power factor is a purely demagnetising type. That means, armature flux directly weakens main field flux.

If the rotor slots are skewed then the effect of slot harmonics (parasitic torques) can be eliminated.

Since the skewed rotor slot is not parallel to start slot in an induction motor, there is more leakage reactance. As a result, induction motor has lower starting and maximum torques.

With skewed rotor bars, the revolving flux, in effect, encounters an air gap of uniform reluctance and this results in a uniform torque and quieter operation.

Torque developed in a series motor is,

As motor is working under magnetic saturated conditions, the flux is constant. Hence torque developed is directly proportional to armature current.

Iron losses of transformer are measured by conducting open circuit test. During open circuit test transformer works at very low power factor.

The low power factor wattmeter is the instrument that measures lower values of power factor accurately.

Hence low pf wattmeter is used to measure iron losses of transformer.

In lap winding, the number of parallel paths = number of brushes = number of poles

In wave winding, the number of parallel paths = number of brushes = 2

Given that, primary turns (N₁) = 1200

Primary voltage (V₁) = 240 V

Secondary voltage (V₂) = 360 V

The maximum efficiency of a transformer occurs at

kVA at maximum efficiency 

A split-phase motor has no capacitance in the auxiliary circuit. A phase shift with respect to the main current is achieved by using narrow conductors to achieve a high resistance to reactance ratio.

Increasing the resistance means that the auxiliary winding can only be used during starting, otherwise, it would overheat.

A split-phase motor has significantly lower torque at starting than any of the capacitor motors due to the reduced phase angle between main and starting winding currents.

Torques developed by a split phase motor is proportional to sine of angle between I_m and I_s

A fluctuating voltage supply is detrimental to a refrigerator motor, but not a ceiling fan motor because the refrigerator motor is enclosed in a sealed unit while the fan motor is open to the environment.

The direction of rotation of the fan motor can be corrected by following these steps:

1) Reversing connections to the terminals of the capacitor: The direction of rotation of the motor is dependent on the phase relationship between the main winding and the auxiliary winding. By reversing the connections to the terminals of the capacitor, the phase relationship can be changed, thus reversing the direction of rotation.

3) Reversing supply connection to the main winding: By reversing the supply connection to the main winding, the phase relationship between the main winding and the auxiliary winding can be altered, resulting in a change in the direction of rotation.

4) Reversing supply connection to the auxiliary circuit: Similar to reversing the supply connection to the main winding, reversing the supply connection to the auxiliary circuit also affects the phase relationship between the windings, leading to a change in the direction of rotation.

By performing steps 1, 3, and 4, the direction of rotation of the fan motor can be corrected.

For a given output and speed, a universal motor as compared to 220 V, 50 Hz supply will require less voltage at low frequency.

Universal motors are used mostly in household sewing machines. The size of the motor is around 100 to 150 watts.

At maximum torque, rotor resistance is equal to slip times of reactance.

At starting condition (s = 1), rotor resistance is equal to standstill reactance.

Synchronous compensator is a DC-excited synchronous motor, whose shaft is not connected to anything but spin freely. Its purpose is not to convert electric power to mechanical power or vice versa, but to adjust conditions on the electric power transmission grid. Its field is controlled by a voltage regulator to either generate or absorb reactive power as needed to adjust the grid's voltage or to improve power factor.

Metals like Iron provide a low reluctance path and the flux lines like to pass through the least reluctance path. The least reluctance path is along the pole axis where the air gap is minimum. So the flux lines will exert force on the rotor to align with the flux lines. This torque is created additional to the torque produced by the locking of excited poles with synchronously rotating magnetic field. This effect is absent in cylindrical rotor machine as the air gap is uniform.

Correct Answer :- C

Explanation : A synchronous motor may have a squirrel-cage winding embedded in its rotor, used to increase the motor starting torque and so decrease the time to accelerate to synchronous speed. The squirrel cage winding of a synchronous machine will generally be smaller than for an induction machine of similar rating. When the rotor is turning at the same speed as the stator's revolving magnetic field, no current is induced into the squirrel-cage windings and the windings will have no further effect on the operation of the synchronous motor at steady-state.

Alternators are classified into salient pole and non-salient pole machines.

In salient pole type of rotor consist of large number of projected poles (salient poles) mounted on a magnetic wheel. Typically number of salient poles are 4 to 60. Salient pole rotors have large diameter and shorter axial length. Flux distribution is relatively poor than non-salient pole rotor, hence the generated emf waveform is not as good as cylindrical rotor.

Non-salient pole rotors are cylindrical in shape having parallel slots on it to place rotor windings. Number of poles is usually 2 or 4. They are smaller in diameter but having longer axial length. Flux distribution is sinusoidal and hence gives better emf waveform.