MCQs on Electrical Machines

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01․ The direction of rotation of a D.C. series motor can be changed by
interchanging supply terminals.
none of the options.
both (A) and (D).
interchanging field terminals.

By changing the field terminal of D.C motor the polarity of the field electromagnet is reversed and consequently the direction of flux produced changes thus resulting in reversal of direction of rotation of dc motor.

02․ DC Motor
DC Shunt Motor
remains constant.

In shunt motor, Ish = V / Rsh. If V is constant Ish will also be a constant. Hence the flux is constant.

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

In dc series motor,the flux φ is produced by the armature current flowing in the field winding and so φ  is directly proportional to Ia. Now since φ  is inversely proportional to speed. ∴ Ia is also inversely proportional to speed .

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

In compound motor having 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 motor.

05․ A dc motor with armature resistance of 0.5 ohms 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 E is zero. The initial armature current Ias is given by Ias = ( V - 0 ) / Ra i.e. V / Ra. Armature resistance is 0.5 ohm and voltage is 230 V therefore Ias = V / Ra = 230 / 0.5 = 460 A.

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

Thinner the lamination, greater is the resistance offered to the induced emf smaller the current and hence lesser the I2R loss.

07․ DC Motor
DC Shunt motor.
DC compound motor.
Separately excited motor.
DC series motor.

Separately excited motor always obtains accurate speed and most suitable for applications requiring speed variation from very low value to high value, like its required in a paper mill.

08․ In electric locomotive which of the following motor are used ?
DC series motor.
Separately compound motor.
Synchronous motor.
Shunt motor.

In DC series motor starting torque is very high up to 500 % resulting maximum momentary operation of 400 % thus it is employed in locomotive.

09․ On what factors the speed of dc motor depends?
Applied voltage.
Field flux.
Armature Electric current Ia.
All of these.

Speed N proportional to Eb / φ N proportional to V - IaRa / φ.

10․ The speed of a motor falls from 1100 r.p.m at no-load to 1050 r.p.m at rated load. The speed regulation of motor is
2.56 %.
2.57 %.

(No load sped - Rated speed) / Full load speed No load speed = 1100 rpm Full load speed = 1000 rpm ∴ % of Regulation = (1100 - 1050) / 1050 = 4.76%.