MCQs on Electrical Machines

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01․ Which of the following voltage regulation method is accurate ?
Synchronous impedance method
MMf method
ZPF method
American standard association method

In Potier Triangle or ZPF method, voltage quantities are calculated on emf basis and mmf quantities are calculated on mmf basis. Therefore, ZPF method is most accurate among all these four methods.

02․ In which of the following, voltage regulation may be zero or negative
unity power factor
lagging power factor
all of the above

As the armature flux is in phase with the main field flux it is additive in nature and increases the net flux. Therefore the emf will be increasing with the load. Voltage regulation is zero or negative value on the leading power factor load or capacitive load as the voltage rises due to increase in load.

03․ For which of the following loads, armature reaction is magnetizing in nature?
none of the above

For capacitive load, the armature flux is in phase with the main field flux, it will add and increases the net flux in the air gap. This effect is known as magnetization. The induced emf will be increasing with the load. Therefore under leading load condition the flux is greater than the actual value.

04․ When pure inductive load is connected to the alternator, what is the effect of armature reaction?
Cross magnetization
Demagnetization
Magnetization
None of the above

When the load is pure inductive, the armature flux will entirely demagnetize the main flux. Therefore net flux in the air gap reduces with load and the main flux should be more than the actual under lagging loads.

05․ For pure resistive load what is the armature reaction?
cross magnetization
demagnetization
magnetization
all of the above

When the load is pure resistive alternator operates at UPF (unity power factor) and the armature flux is 90° lagging the main field flux and both are stationary in air gap. Under a pole half there will be a magnetization and other half of the same pole there would be demagnetization. Therefore net effect is only cross magnetization in nature for resistive load. However there is a slight demagnetization which can be negligible.

06․ To eliminate 5th harmonic in the induced voltage phase breadth should be
18°
36°
72°
30°

To eliminate 5th harmonic distribution factor Kdr should be zero. Distribution factor Kdr = sin (nrÎ±/2)/(n sin rÎ±/2) To eliminate harmonics in the induced voltages Kdr =0 sin (nrÎ±/2) = 0 nrÎ±/2 = 180Â° Phase breadth nÎ± = 360Â°/r To eliminate 5th harmonic, nÎ± = 360Â°/5 = 72Â°

07․ Armature reaction depends on which of the following?
both 1 and 2
none of the above

The effect of armature flux on main field flux distribution is called as armature reaction and its effect depends on magnitude of load current and power factor of load.

08․ To eliminate the 5th harmonic in the induced voltages their respective pitch factor angle should be
30°
18°
72°
36°

To eliminate the harmonics in the induced voltages their respective pitch factor should be made zero. Pitch factor Kp = cos(rÎ²/2) Where r is order of harmonic For Kp = 0 rÎ²/2 = 90Â° Î² = 180Â°/r To eliminate 5th harmonic, the angle should be Î² = 180°/5 = 36Â°

09․ Breadth factor is defined as the ratio of
emf with distributed winding to emf with concentrated winding.
emf with concentrated winding to emf with distributed winding.
both 1 and 2.
none of the above.

The winding is uniformly distributed in order to improve the shape of the waveform, due to which the induced emf is reduced. The amount of reduction due to distributed nature can be calculated using breadth factor also known as distribution factor. Breadth factor is defined as the ratio of emf with distributed winding to emf with concentrated winding. Breadth factor is always less than 1.

10․ Which of the DC Motor use flywheel?
Shunt motor.
Cumulatively compound motor.
Differently compared motor.
Series motor.

In case of cumulative compound motor, whenever a mechanical load is subjected to violent fluctuations of short duration, a flywheel is generally provided for carrying peaks and so to smooth out the load on the motor as well as to reduce peaks on power system. Without flywheel the motor would have to be much larger in order for the flywheel to alternately slow down while carrying the peak load and to accelerate again between peaks.

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