DC Motor Interview Questions And Answers
1. Question 1. What Is The Principle Of Operation Of A D.c Motor?
Answer :
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.
2. 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.
3. 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.
4. Question 4. Why Series Motor Cannot Be Started On No-load?
Answer :
Series motor cannot be started without load because of high starting torque. Series
motor are used in Trains, Crane etc.
5. Question 5. Which Type Of Motor Is Used In Trains, What Is The Rating Of
Supply Used?
Answer :
Dc series is in the used in trains to get high starting torque while starting of the
trains and the operating voltage is 1500v dc.
Electronic Circuits Interview Questions
6. Question 6. Explain Different Methods Of Speed Control Of D.c Shunt
Motors?
Answer :
Speed control of a d.c shunt motor can be done using:
1. Flux control method ()
2. Armature control method
3. Voltage control method
7. Question 7. Explain Different Methods Of Speed Control Of D.c Series
Motors?
Answer :
Speed control of a d.c series motor can be done using:
1. flux control method
2. Armature-resistance control control method
Electrical Engineering Interview Questions
8. Question 8. Why Is The Starting Current High In A Dc Motor (or) Why Do
We Use A Starter?
Answer :
For DC motors, Voltage equation is
V=Eb-IaRa
Where V = Terminal voltage,
Eb = Back emf in Motor
Ia = Armature current
Ra = Aramture resistance.
At starting, Eb is zero. Therefore, V=IaRa, Ia = V/Ra, where Ra is very less like
0.01ohm.i.e, Ia will become enormously increased. The excessive current will
damage commutator and brushes and eventually blow out the fuses.
9. Question 9. How Can You Reverse The Direction Of Rotation Of A D.c
Motor?
Answer :
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.
Induction Motors Interview Questions
10. Question 10. Why The Field Of A D.c Shunt Motor Should Not Be Open?
Answer :
The shunt motor will achieve dangerously high speed and may destroy itself.
11. Question 11. What Will Happen If The Direction Of Current At The
Terminals Of A D.c Motor Is Reversed?
Answer :
The direction of rotation of motor remains the same as current in armature and
field.
Electrical Power System Interview Questions
12. Question 12. What Happens When A D.c Motor Is Connected Across An A.c
Supply?
Answer :
1. The motor will run but it would not carry same load as it would on d.c
supply.
2. More sparking at the brushes
3. Eddy currents will be high and will cause overheating and may eventually
burn on a.c supply
Electrical and Electronics Engineering Interview Questions
13. Question 13. Mention The Methods For Starting An Induction Motor?
Answer :
1. DOL – direct online starter
2. Star delta starter
3. Auto transformer starter
4. Resistance starter
5. Series reactor starter
14. Question 14. What Are The Types Of Starters Used For Starting Of D.c
Motors?
Answer :
There are two principal types of d.c. motor starters viz., three-point starter and
four-point starter. As we shall see, the two types of starters differ only in the
manner in which the no-volt release coil is connected.
15. Question 15. What Are The Advantages Of Star-delta Starter With
Induction Motor?
Answer :
The main advantage of using the star delta starter is reduction of current during
the starting of the motor. Starting current is reduced to 3-4 times of current of
Direct online starting. As the starting current is reduced, the voltage drops during
the starting of motor in systems are reduced.
Transformer Interview Questions
16. Question 16. What Are The Different Tests Conducted On D.c Machines?
Answer :
1. Brake test
2. Swinburne’s test or No-load test
3. Regeneration test or Hopkinson’s test
4. Retardation or Running down test
17. Question 17. Why Is Swinburne’s Test Conducted?
Answer :
Swinburne’s test is conducted at No-load condition to predetermine the efficiency
at different loads and to measure the losses separately.
