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# DC Motor Interview Questions and Answers

## Question 1.  What is the Principle 25 of ∅ ½ √2 operation of a DC Motor?

The operation of a D.C Motor based on the principle that when a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. Basically, there is no constructional difference between a d.c. motor and a d.c generator. The same d.c. machine can be run as a generator or motor.

#### Question 2.  What is back E.M.F or Counter E.M.F?

Answer : 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 V (Lenz’s law) and in known as back or counter e.m.f Eb. It is always less than the applied voltage V, although this difference is small when the motor is running under normal conditions.

#### Question 3. What is speed regulation?

Answer : 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.

#### Question 4. Why Series Motor Cannot Be Started On No-load?

Series motor cannot be started without load because of high starting torque. Series motor are used in Trains, Crane etc.

Question 5. What are the Different Methods Of Speed Control Of DC Shunt Motors?

Speed control of a d.c shunt motor can be done using:

• Flux control method
• Armature control method
• Voltage control method

## Question 6. Explain Different Methods Of Speed Control Of D.c Series Motors?

Speed control of a d.c series motor can be done using:

• Flux control method
• Armature-resistance control control method

#### Question 9. How Can You Reverse The Direction Of Rotation Of A D.c Motor?

We can reverse the direction of rotation of a D.C Motor by either reversing the field current or armature current. If both the currents are reversed the motor will run in original direction.

The problem is 22 x 510

√2

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