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# DC Motor Interview Questions

Read Out “Electrical DC Motors” topic to better understand all those questions

1) What is the principle operation of DC Motors?

The principle operation of a DC motors is same as that of a DC generator that whenever a current carrying conductor came across an influence of a magnetic field, a mechanical force induces within it. Usually, there is no constructional difference between DC generator and DC motor. The same DC motor can be run as DC generator.

2) What is back e.m.f in a DC motor?

When the armature of a d.c. motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and hence e.m.f. is induced in them as in a generator. The induced e.m.f. acts in opposite direction to the applied voltage as per according to len’z law and is known as back e.m.f. It is always in less than the applied voltage.

3) Why is shaft torque less than armature torque in a DC motor?

Shaft torque is less than armature torque in a DC motor because some percentage of torque developed in the armature is used up in overcoming mechanical losses, i.e. iron and friction losses.

4) Why series motor cannot be started on no-load?

That is because series motor has a high starting torque value.

5) In a generator, the main flux is distorted in the direction of rotation due to armature reaction. Why is it distorted in the opposite direction of rotation in a DC motor?

In a generator, the armature current flows in the direction of induced e.m.f whereas in a motor, the armature current flows against the induced e.m.f. It should therefore, be expected for the same direction of rotation and field polarity, the armature flux of the motor will be in opposite direction to that of the generator. Hence, instead of the main flux being distorted in the direction of rotation, as in the generator, it is distorted in the opposite direction of rotation.

6) What is speed regulation?

Speed regulation is the change in speed when the load on the motor is reduced from rated value to zero. It is expressed as a percentage of rated load speed.

7) How can we control the speed of a DC shunt motor?

The speed of a DC shunt motor can be controlled by:

• flux controlled method
• armature controlled method
• voltage controlled method

8) How can we control the speed of a DC series motor?

The speed of a DC series motor can be controlled by:

• flux controlled method
• armature resistance control controlled method

9) Why is DC motor current so high?

Foe DC motor, voltage equation is given by:

V = Eb – IaRa

At starting, the back e.m.f voltage is equal to zero that makes it

V = IaRa

So, the current can be given as:

Ia = V/ Ra

The value of resistance is too low, of the order of 0.01 Ω, that makes current too high. Eventually, that excessive current can damage commutator and brushes.

10) What is the chief advantage of a DC series motor?