MCQs on Electrical Machines

Page 76 of 114. Go to page
01․ A series motor is efficiently suitable for
high starting torque operation.
low starting torque operation.
constant speed operation.
none of these.

For avoiding sleep ring arrangement on alternator shaft and other complexity of design especially in case of large size alternator, it is recommended to have stationary armature winding and a rotating magnetic field.

02․ A single-winding single-phase motor has
low starting torque.
zero starting torque.
high starting torque.
starting torque equal to full-load torque.

When alternating voltage is applied across the single winding single phase motor, there will be a double revolving magnetic field which creates a pulsating magnetic field. Due to this pulsating magnetic field there will be no resultant torque in any direction of rotation hence net starting torque of this type of motor is zero.

03․ An ideal synchronous motor has no starting torque because
rotor is made up of salient poles.
rotor winding is highly reactive.
relative velocity between the stator and rotor mmf’s is zero.
relative velocity between stator and rotor mmf’s is not zero.

In an ideal synchronous motor, at steady state condition, the rotation of the shaft is synchronized with the frequency of the supply current by an external prime mover and the rotation period is exactly equal to an integral number of AC cycles. Synchronous motors contain electromagnets on the stator of the motor that create a magnetic field which rotates in time with the oscillations of the line current. The rotor turns in step with this field, at the same rate. Hence, both MMFs cancel with each other and there is no starting torque because of the relative velocity between the stator and rotor MMFs are zero.

04․ Armature reaction in a DC machine
is in direct opposition to the main poles.
is in the same direction as the main poles.
makes an angle of 90° with the main pole axis.
makes an angle with the main pole axis which is load dependent.

This can be explained by considering a two poles DC machine. Ideally, in two poles DC, machine brushes are placed at exact inter-poles positions. The armature conductors one side of brush axis or q - axis carry armature current in one direction and the armature conductors of another side of brush axis or q - axis carry armature current in opposite direction. If we draw the flux lines created by the armature current of both side's conductors we will find the armature flux passing through the armature core is exactly parallel to the brush axis or q - axis. Ideally, the main field flux passes through the armature core from the North Pole to the South Pole, exactly parallel to the pole axis or d - axis. As in two poles machine, the brush axis exactly perpendicular to the pole axis, it can be concluded that armature flux which is also termed as armature reaction flux crosses the main field flux or main pole axis at 90° angle.

05․ A differentially compound motor under high-over-load conditions will behave like a / an
series motor.
shunt motor.
AC synchronous motor.
cummulative compound motor.

A differentially compound DC motor has one shunt field and one series field and these two fields are oppositely directed. The motor operates on resultant field of these two fields. As we know that the series field current and armature current are same. Again armature current in that type of motor is directly proportional to the applied load on that. If the motor is highly overloaded the series field current increases to a high-value which produces a very strong series field in respect of shunt field hence the motor operates as series motor because series field in that situation dominates the shunt field.

06․ A 4-pole generator with 16 coil has a two layer lap winding. The pole pitch is

Here, the winding is 2 layer winding and it have 16 coils. So, the total number of conductor is 2 * 16 = 32. Hence, the pole pitch of a DC generator is the ratio of the total number of conductor to the number of poles \ Hence, the pole pitch is 8.

07․ If peak value of phase mmf is F max , then peak value of the rotating field caused by three phase is

The peak value of the resultant mmf is peak FPEAK = 1.5 FM. The value of depends on No. of turns/phase, winding current, No. of poles, and winding factor. The formula for the peak value of MMF is given below \

08․ For a fault at the terminal of a synchronous generator, the fault current is maximum for a
3 - phase to ground fault.
3 - phase fault.
line to line fault.
line to ground fault.

Most of the faults occur in an alternate, are line to ground fault. This type of faults is most saviour in an alternator because the phase winding conductors are placed in the slots of the steel stator and stator is always kept in ground potential.

09․ The slip of an induction motor normally does not depend on which of the following?
Synchronous speed.
Rotor speed.
Shaft torque.
Core loss component.

Slip is related to speed components. It is basically, the difference in the synchronous speed to the rotor speed. Hence, there is no relation with any losses which is associated with the induction motor.

10․ Compensating windings is placed in which part of a D.C motor?
On yoke in the pole faces.
On yoke in the inter pole gap.
On armature.
None of the above.

The purpose of using compensating winding in a DC machine to reduce the effect of armature flux as the armature flux distorts the main field flux. Hence the compensating winding should be placed as near as possible to the armature conductors. Armature conductors are placed in the slots on the periphery of the armature core. As the pole faces are nearest to the armature periphery, the conductors of compensating windings are generally placed in the slots on the pole faces.