MCQs on Electrical Machines

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01․ If the flux of DC motor approaches zero, its speed will
remain unchanged.
approach zero.
aprroach infinity.
between zero and infinity.

In DC motor, the relation of speed and flux is given by N ∝ (1/φ) from the equation Eb = φZNP/60A, when other parameters remain constant. Now if flux approaches zero then the speed will approach to infinity.

02․ The current drawn by a 220 V DC series motor of armature resistance 0.5 Ω and back emf of 200 V is
40 A.
44 A.
400 A.
440 A.

Where, Vt = terminal voltage Eb = back emf. Ia = armature current or line current. Ra = Armature resistance. .

03․ If the load current and the flux of a DC motor are held constant and voltage applied across its armature is increased by 5%, the speed of the motor will
reduce by 5 %.
increase by 5 %.
depend on other factors.
remain unchanged.

If the load current is constant then the armature current is also constant. Therefore back emf of DC motor is proportioned to the terminal voltage. So, 5 % increase of terminal voltage means that increase in 5 % of back emf. The speed of DC motor is directly proportional to back emf and inversely proportional to flux per pole. Since the flux is also constant, the speed only depends on the back emf and the speed is directly proportional to the back emf. Hence speed is also increased by 5 % with the increase 5 % of terminal voltage.

04․ The speed regulation of a DC motor is given as
NnL − NfL.
(NnL − NfL)/NfL.
(NfL − NnL)/NnL.

The speed regulation of a DC motor is defined as the change in speed when the load on the motor is reduced from full load to zero and is expressed of rated full load speed. .

05․ The direction of rotation of a DC compound motor can be reversed by interchanging
shunt field connections.
series field connections.
armature or field connections.
armature connections.

The direction of rotation of a DC compound motor may be conveniently reversed by reversing the connection of both series and shunt field winding or may be reversing the armature connection but not both at the same time.

06․ The armature shaft must be able to withstand
bending moment due to weight of the armature.
any unbalanced magnetic pull on the armature core.
twisting strains due to transmission of torque.
all of the above.

The armature shaft must be able to withstand
  • Any unbalanced magnetic pull on the armature core.
  • Bending moment due to weight of the armature and commutator.
  • Twisting strains due to transmission of torque.

07․ What will happen if the the supply terminals are reversed in case of a DC shunt motor?
The motor will run in reverse direction.
The motor will run at its normal speed in the same direction as before.
The motor will stop.
The motor will run much faster in the same direction.

The direction of eletromagnetic torque developed by armature depends upon the direction of flux or magnetic field and the direction of flow of current in armature conductors. If either of the two is reversed the direction of torque developed and direction of rotation will be reversed. When both (the direction of field as well as that of armature current) are reversed the direction of rotation will not change. So in case of DC shunt motor, if the supply terminals are interchanged, there is no change in direction of rotation.

08․ In a DC compound motor the field regulator is provided to
control the field flux.
demagnetize the field partially.
limit the armature current.
control the armature flux.

In a DC compound motor, speed control involves the variation of field flux by means of the field regulator. A field regulator is a high inductive resistance connected in parallel with the field to control the field current and hence the field flux.

09․ The DC compound motors are generally
Differentical compound.
Cummulative compound.
Level Compound.
None of these.

The DC cumulatively compound motor, starting torque is high up to 400 % depending the degree of compounding and speed regulation is varying depending upon of compounding. So it is used where high starting torque, fairly constant speed are possible. But in the differential arrangement the field flux become weak and speed-torque characteristics become rising. The speed increases when the load increases and the starting torque is also poor. So it is not practically used and only used for laboratory or research work.

10․ In case of a compensated DC series motor, the compensating winding is provided
As a separate unit.
In parallel with the armature winding.
In parallel with the field winding.
In series with the armature winding.

In DC series motor, compensating winding is provided in series with the armature windings to neutralize the cross magnetizing effect of armature reaction outside the effect of interpolar winding.