MCQs on Electrical Machines

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01․ In a DC Shunt Motor if the load current increases keeping the terminal voltage constant, then flux will
remain constant.
any of the above.

In shunt motor because field is parallel to armature. If Vt is constant then Ish will also be a constant. Hence the flux remains constant.

02․ In DC series motor the speed is ______________ proportional to the armature current.
has no relation.
none of these.

In DC series motor, the series field and armature field carrying the same current, so the flux φ is produced by the armature current flowing in the field winding and φ is directly proportional to Ia. From emf equation, it can be stated that N α (1/φ) and so in this case N is inversely proportional to Ia.

03․ A DC compound motor has
only shunt field winding.
only series field winding.
both shunt and series field winding.
tertiary winding.

A compound motor has both series and shunt field winding that is why it is called compound motor. It may be a short shunt compound or long shunt compound depending upon the shunt field combination and differential or cumulative compound depending upon direction of field current.

04․ A DC motor with armature resistance of 0.5 ohms is connected directly to a 230 Volt supply. If the armature is stationary then the current flowing through armature is
340 A.
250 A.
460 A.
570 A.

As the armature is stationary so the back e.m.f Eb is zero. The initial armature current .

05․ Which of the following rotor requires damper winding?
Cylindrical rotor.
Salient pole rotor.
Both type rotor.
None of the above.

Damper winding is used to suppress the vibrations in the rotor. As cylindrical rotor has no projections and there is a closed portion to contain field winding which is distributed. Therefore it acts as a smooth rotor. It does not require any damper winding as the smooth rotor itself acts as good damper. The salient pole rotor contains projected poles and the field winding is a concentrated winding on the pole core. The rotor is not mechanically stable and does not offer high operating speeds because of the projected nature. Any mechanical instability of the rotor i.e. vibration or hunting should be suppressed. This is achieved by providing damper bars in the poles/rotors.

06․ For a DC Shunt Motor having armature resistance of 0.5 Ω and 2A armature current at no-load, when the armature current is changed to 20A at loaded condition the speed is 1000 r.p.m. What is the speed at no-load?
1037.5 r.p.m.
1130 r.p.m.
4575.4 r.p.m.
963.8 r.p.m.

07․ The armature of a DC motor is laminated to reduce
hysteresis loss.
eddy current loss.
copper loss.
friction and windage loss.

Thinner the lamination, the area of the loop current produced by the eddy current is minimized. Hence the I2R loss due to loop currents in the iron path is also reduced.

08․ What will be the e.m.f. generated by a 4 pole lap connected DC Generator rotating at 1500 r.p.m.? Its flux per pole is 0.4 mWb and total no of armature conductors is 1000.
10 volts.
20 volts.
30 volts.
40 volts.

Given N = 1500 rpm, φ = 0.4 mWb, Z = 1000, P = 4 and A = 4

09․ Which of the following DC Motor is used in paper machines?
DC series motor.
Separately excited DC motor.
DC Shunt motor.
DC compound motor.

Separately excited DC motor obtains very accurate speed and also suitable for applications requiring speed variation from very low to high value, like its required in a paper machines, diesel-electric propulsion of ships, in steel rolling mills etc.

10․ In electric locomotives which of the following motor are used ?
DC series motor.
DC compound motor.
Synchronous motor.
DC Shunt motor.

In DC series motor starting torque is very high up to 500 %, resulting maximum momentary operation at the time of starting and it has also poor speed regulation or good speed control from no-load to full load. Thus it is employed in locomotives.