MCQs on Electrical Machines

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01․ Where the shunt generators are widely used?
Where the load is very high.
Where a constant voltage is required over a narrow load range.
Where often load fluctuation is occurred.
Where voltage variation is needed over a wide load range.

Basically, the DC shunt generator provides the constant output voltage because the connected load across the shunt generator does not affect the internal generated EMF. That means the flux production in the field winding is not affected by the connected load across the generator. So, it generates the constant output voltage over a narrow load range and it can be widely used in those kinds of required places.

02․ What is the function of inter pole in DC generator?
To serve the economical aspects.
To neutralize the distortion of main field flux due to armature reaction.
To get a superior commutation.
Both option B and C.

We use the compensating winding in DC machines whichis placed on the pole facesof the machine and it is connected in series with the armature circuit in such a way that the direction of current in compensating winding is opposite to that in the armature conductor. It may be noted that the compensating winding cannot nullify the quadrature (MNA) axis armature flux completely. Additional small poles called inter polesare provided in between the main poles in large machines to get rid of the commutation problem arising out of armature reaction.So, it is used to neutralize the armature reaction effect and to get the good commutation also.

03․ What type of connection is made in between the inter-pole and the field winding?
None of the options.

The inter-poles are used in DC machine to create reversing voltage to neutralise reactance voltage due to armature reaction. Hence the EMF generated by inter poles should be proportional to armature current as the armature reaction is proportional to the armature current. The polarity of inter-poles is same as the main pole ahead in the case of DC generator show that voltage induced by the inter-poles opposes the reactance voltage thereby lead to a spark less commutation. This is the reason; the inter-pole coils are always connected in series with the armature winding.

04․ In DC generator, lap winding is used in
high current and low voltage applications.
high voltage and low current applications.
where constant speed is required.
where greater load is connected.

In a DC generator, the lap windings means the winding has the same number of parallel paths as the number of magnetic poles The machine may not be able to generate higher voltage but due to the existence of parallel paths the machine can deliver high current. That is why the lap winding design is used in higher current and lower voltage applications.

05․ In DC generator, which component plays the vital role to produce the direct current?

In DC generator, the internal voltage generation in an armature circuit is the sinusoidal waveform. But, the commutator acts as a rotary mechanical converter that converts the sinusoidal voltage to pulsating DC voltage. That is how the commutator plays the vital role to produce the direct current in DC generators.

06․ If an AC servomotor has one of its winding excited by AC, then voltage measured at the other winding with rotor running will be
proportional to rotor speed.
proportional to square of rotor speed.
constant independent of speed.

In an AC servomotor, the angle is measured by the movement of the rotor. This movement is measured by generating the voltage in the external winding which is connected to the same shaft of the servomotor's rotor. Hence, the voltage is generated by the speed of the shaft. Thus, the measured voltage is directly proportional to the shaft of the rotor.

07․ An inverted V curve of synchronous motor shows variation of
power factor and DC excitation at constant load.
supply voltage and field current at constant excitation.
power factor and supply voltage during hunting.
supply voltage and excitation current at constant load.

Under construction.

08․ A small air gap in poly phase induction motor helps to
reduce chance of crawling.
increase starting torque.
reduce chance of cogging.
reduce magnetizing current.

The presence of air-gap between the stator and rotor of an induction motor increases the reluctance of the magnetic circuit. Consequently, an induction motor draws a large magnetizing current (Im) to produce the required flux in the air-gap. That is why the small air-gap is preferred for the induction motor which helps to reduce the magnetizing current.

09․ Which of the following motors are suitable for computer printer drive ?
Reluctance motor.
Hysteresis motor.
Shaded pole motor.
Stepper motor.

In computer printer drive, we don't want the continuous rotation. For a particular instruction only it has to the role the papers in printing. Hence, the stepper motor is suitable for the computer printer drive.

10․ The direction of rotation of a DC motor can be determined by
Lenz's law.
Ampere's law.
Fleming's right hand rule.
Fleming's left hand rule.

Whenever a current carrying conductor is placed inside a magnetic field, it experiences the mechanical force acting on it. Depending upon this principle a DC motor rotates. The direction of the mechanical force experienced by the current carrying conductor inside a magnetic field is determined by Fleming's left-hand rule. Hence naturally, the direction of rotation of a DC motor is also determined by Fleming's left and role.