MCQs on Electrical Machines

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01․ A 500 V, 55 kVA single phase alternator has an effective resistance of 0.5 Ω. An excitation current of 10 A produces 200 A armature current on short circuit and an emf of 450 V on per circuit. The synchronous reactance will be
2.241 Ω.
2.781 Ω.
25.564 Ω.
2.74 Ω.

The synchronous impedance (ZS) value is the ratio of the short circuit voltage and the short circuit current. Therefore ZS = 450/200 = 2.25 Ohms. Now, the synchronous reactance (XS) = \

02․ Methods of synchronizing of alternator may be done by
all dark lamp method.
all bright lamp method.
one dark two bright.
all of these.

The method of connecting an incoming alternator safely to the live bus-bars is called synchronizing. The equality of voltage between the incoming alternator and the bus-bars can be easily checked by a voltmeter. The phase sequence of the alternator and the bus-bars can be checked by a phase sequence indicator. Differences in frequency and phase of the voltages of the incoming alternator and bus-bars can be checked by one of the following two methods: (i) By Three Lamp (one dark, two bright) method, (ii) By synchroscope.

03․ In a stepper motor the detente torque means
maximum of the static torque with the phase winding excited.
minimum of the static torque with the phase winding unexcited.
minimum of the static torque with the phase winding excited.
maximum of the static torque with the phase winding unexcited.

In a stepper motor the torque means-minimum of the static torque with the phase winding unexcited.

04․ What are the starters used for starting of cage motors?
Star-delta starter.
Auto transformer starter.
DOL starter.
All of these.

In the starting of the squirrel cage induction motors, we use two different starting methods. The one is that the stator is supplied by the controlled power supply voltage and the next one is that the stator is directly connected with the supply voltage. The star-delta starter and autotransformer starter are used to provide controlled supply voltage and the DOL starter is used to provide the direct supply voltage to the stator winding of the induction motor.

05․ The direction of rotation of a 3 - Φ induction motor is clockwise when it is supplied with 3 - Φ sinusoidal voltage having phase sequence A - B - C. For counterclockwise rotation of motor, the phase sequence of the supply should be
B - C - A.
C - A - B.
A - C - B.
B - C - A or C - A - B.

:To reverse direction of rotation phase sequence of the supply has to be reversed. Then only it creates the reverse rotating magnetic field and thereby the rotor of the induction motor tries to rotate in the counterclockwise direction.

06․ In a single phase induction motor driving a fan load, the reason for having a high resistance rotor is to achieve
low starting torque.
quick acceleration.
high efficiency.
reduced size.

We know that the lagging load means that is inductive load. Usually in inductive loads, the current gets demagnetized. Also, the winding of the synchronous generator is an inductive part. So, due to the current is getting demagnetized, the armature reaction is also demagnetizing.

07․ A synchronous generator is feeding a zero power factor (lagging) load at rated current. The armature reaction is
cross – magnetizing.

The four-point starter eliminates the drawback of the three-point starter that if we adjust the field resistance in a three point starter to adjust the certain level of field current, the main coil gets de-energized. So, the motor is suddenly getting turned OFF. But, in the four point starter, the main voltage coil is not connected with the field resistance. Instead of it is directly connected to the armature circuit. It does not allow the motor to stop even if we adjust the field current with very low value of a DC shunt motor. Hence, it is suitable for a dc shunt motor with field weakening control.

08․ Regenerative braking is possible in
DC shunt motor with armature resistance control.
DC shunt motor with field weakening control.
DC series motor.
DC compound motor.

In a regenerative braking, the applied field current should be greater than the armature current (or) the armature is supposed to disconnect from the power supply but the field winding should be connected with power supply. In case of DC series motor, It is not possible to make field current more than the armature current and also we cannot disconnect the armature from the power supply since it is serially connected with the series field winding. Therefore regenerative braking with DC series motor is not possible.

09․ Regenerative braking is not found in which motor?
DC series.
DC shunt.
DC compound.
Induction motor.

Basically, the rotor of a stepper motor is made up of permanent magnet. Hence, there is no windings, no commutator and no brushes.

10․ Centrifugal switch is absent in which of the single phase induction motor?
Shaded pole motor.
PSC (permanent split capacitor) motor.
Split phase motor.
None of the above.

The permanent split capacitor motor is similar to a capacitor start motors. But, the auxiliary winding and the capacitor are continuously energized because it is connected permanently. Hence, the centrifugal switch is absent in this case.