# MCQs on Electrical Machines

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01․ Which of following winding is connected in series with armature winding of DC machine?
Auxiliary winding.
Compensating winding.
Both Compensating and Commutating pole winding.
Commutating pole winding.

Commutating pole or inter-pole windings are connected in series with armature winding. So that they carry full armature current and they induce an e.m.f. in the coil (under commutation) which helps the reversal of current. This e.m.f. is proportional to the armature current and ensures automatic neutralization of reactance voltage which is also due to armature current. Compensating pole windings are also connected in series with the armature. They work outside the commutating zone.

02․ Direction of rotation of DC motor is reversed by
Reversing supply connection.
Interchanging armature and field connection.
Reversing armature connection or field connection.

The direction of rotation of a motor can be reversed by reversing the current through either the armature winding or the field windings. If the current through both is reversed, the motor will continue to rotate in same direction as before.

03․ If the field connection of a DC Shunt Motor is changed then
it will run in same direction by slowly.
motor will not run.
it will run in opposite direction.
it will run in same direction.

If the field winding connection of DC shunt motor is changed then the motor will run in opposite direction.

04․ With the increase in speed of a DC motor
back emf increases but line current falls.
back emf falls and line current increases.
both back emf as well as line current increase.
both back emf as well as line current fall.

In case of DC motor, the speed is proportional to the back emf (Eb ∝ N). So with the increase in speed, the back emf also increases. Therefore, armature current is also decreased, . In case of series motor, armature current is equal to the line or load current.

05․ If the back emf in DC motor vanishes suddenly the motor will
burn.
run at very high speed.
run at very low speed.
start haunting.

If the back emf in DC motor vanishes suddenly then current flow through the armature becomes large, as Eb is zero. So, armature current increases to a very high value suddenly and hence, large amount of heat is produced which will burn the armature winding.

06․ The no load and full load speed of DC motor are 1400 rpm and 1200 rpm respectively. What is the percentage speed regulation of the motor ?
14.28%.
15.30%.
16.67%.
18.25%.

07․ An electric motor with constant output power will have a torque speed characteristics in the form of a
Straight line parallel to the speed axis.
Straight line through the origin.
Rectangular hyperbola

In case of motor for the constant output power, Eb×Ia = T×ω. As T×ω = Constant, so the torque speed characteristic is in the form of rectangular hyperbola.

08․ The output power of any electrical motor is taken from the
Field.
Coupling mounted on the shaft.
Armature.
Motor frame.

The output power of any electrical motor is always taken from the coupling mounted on the shaft because the motor converts electrical power into mechanical power.

09․ The output power indicated on the name plate of any motor is always the
Power drawn in KW.
Power drawn in KVA.
Output power at the shaft.
Gross power.

In case of electric motor (AC or DC), the mechanical power is available at the shaft in horse power is written on the name plate of the motor.

10․ The speed of a DC motor is
directly proportional to flux and inversely proportional to back emf.
directly proportional to both back emf and flux.
inversely proportional to both back emf and flux.
directly proportional to back emf and inversely proportional flux.

From the equation Eb = φZNP/60A, the speed of DC motor is given by N α Eb/φ (if the other terms remains constant). Where N = Speed in rpm, Eb = back emf and φ = flux per pole. From the above relation, we get that the speed of DC motor is directly proportional to its back emf and is inversely proportional to flux.

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