# MCQs on Electrical Machines

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01․ The mechanical power is developed by a DC motor is maximum when
back emf is equal to twice of applied voltage.
back emf is equal to half of applied voltage.
back emf is equal to applied voltage.
none of above.

For motor, Eb = Vt − IaRa Mechanical power developed by the motor is given by Pm = EbIa ⇒ VtIa − Ia2Ra. Mechanical power developed by the motor will be maximum if dPm/dIa is zero. i.e. Vt - 2IaRa = 0 ⇒ IaRa=Vt/2 ∴ Back emf Eb = Vt − Vt/2 = Vt/2.

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

In case of DC Motor the direction of rotation of conductors can be determined by Fleming's left hand rule. When thumb, forefinger and middle finger of the left hand are placed right angles to each other, if forefinger indicates the direction of magnetic field and the middle finger indicates the direction of current, the thumb will give the direction of rotation of conductors.

03․ If the excitation is increased, the power factor is
unity power factor
lagging power factor
none of the above

Reactive power From the above equation, the reactive power generated or delivered significantly depends on excitation. When the excitation is increased (over excited), then E cosθ > V, which means generator delivers or supplies reactive power to the load and operates at lagging power factor.

04․ If the excitation is reduced, the power factor is
unity power factor
lagging power factor
none of the above

Reactive power From the above equation, the reactive power generated or delivered significantly depends on excitation. If the excitation is reduced (under excitation), then E cosθ < V, which means the generator absorbs or draws reactive power from the bus bar and operates at leading power factor.

05․ If the temperature of armature resistance is increased, the speed of a series motor would
not change.
decrease.
increase.
None of above.

In a series motor, when the resistance increases due to rise in temperature, the IR drop increases, which reduces the effective voltage applied to the armature, hence speed decreases.

06․ The slot edges in a DC machine are made of
Cast iron.
Fibre.
Silicon steel.
Mild steel.

Slot edges of DC machine are made of cast iron.

07․ For a DC Motor for constant load torque, what will happen to the armature current if armature resistance increases?
Remain unchanged
Increases.
Decreases.
None of above.

For a constant load torque, changing the armature resistance will not change the armature current.

08․ Which of the following statements are correct ?
The leakage reactance of the induction motor should be small.
A small value of leakage reactance will increase the maximum power output of motor.
Both 1 and 2.
None of the above.

As leakage reactance creates additional voltage drop hence it should be small. Maximum power Therefore, small value of leakage reactance will increase the maximum power output.

09․ Field winding of a DC series motor is usually provided with thick wire
as it carries large load current.
to provide large flux.
in order to reduce eddy current to provide large flux.
to reduce the insulation of materials.

In DC series motor, the cross sectional area of wire used for field coils has to be fairly large because the field coils carry the load or armature current. As the cross-sectional area is large, the resistance is minimum and series drop is also minimum.

10․ For same HP rating and full load speed which of following motor has poor starting torque?
Series.
Shunt.
Cummulative compound.
Differently compounded.

In differential compound motor, the series field opposes shunt field. It has poor starting torque as the resultant flux is minimized by the opposition. The flux further decreases with increase in load. The decrease in flux cause the starting torque to be less than the other motor.

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