Viva Questions on DC- Motor
1. What
Is The Principle Of Operation Of A DC Motor?
Ans: 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 DC motor and a DC generator.
The same DC machine can be run as a generator or motor.
2. What
Is Back EMF Or Counter EMF?
Ans: When the armature of a DC motor rotates under the influence of the driving torque, the armature
conductors move through the magnetic field and hence EMF is induced in them
as in a generator. The induced EMF acts in opposite direction to the applied
voltage V (Lenz’s law) and in known as back or counter EMF. It is always
less than the applied voltage V, although this difference is small when the
motor is running under normal conditions.
3. What
Is Speed Regulation?
Ans: 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. Why
Series Motor Cannot Be Started On No-load?
Ans: Series motor cannot be
started without load because of high starting torque. Series motor are used in
Trains, Crane etc.
5. Which
Type Of Motor Is Used In Trains, What Is The Rating Of Supply Used?
Ans: 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.
6.
Explain Different Methods Of Speed Control Of DC Shunt Motors?
Speed
control of a DC shunt motor can be done using:
- Flux control method
- Armature control method
- Voltage control method
7.
Explain Different Methods Of Speed Control Of DC Series Motors?
Speed
control of a DC series motor can be done using:
- flux control method
- Armature-resistance control control method
8. Why Is
The Starting Current High In A Dc Motor (or) Why Do We Use A Starter?
Ans: For DC motors, Voltage
equation is
V=Eb-IaRa
Where V = Terminal voltage,
Eb = Back emf in Motor
Ia = Armature current
Ra = Armature 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. How
Can You Reverse The Direction Of Rotation Of A DC Motor?
Ans: 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.
10. Why
The Field Of A DC Shunt Motor Should Not Be Open?
Ans: The shunt motor will
achieve dangerously high speed and may destroy itself.
11. What
Will Happen If The Direction Of Current At The Terminals Of A DC Motor Is
Reversed?
Ans: The direction of rotation
of motor remains the same as current in armature and field.
12. What
Happens When A DC Motor Is Connected Across An AC Supply?
- The motor will run but it would not carry same load as it would on DC supply
- More sparking at the brushes
- Eddy currents will be high and will cause overheating and may eventually burn on AC supply
13.
Mention The Methods For Starting An Induction Motor?
- DOL – direct online starter
- Star delta starter
- Auto transformer starter
- Resistance starter
- Series reactor starter
14. What Are The Types Of Starters Used For
Starting Of DC Motors?
Ans: There are two principal
types of DC 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. What
Are The Advantages Of Star-delta Starter With Induction Motor?
Ans: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.
16. What
Are The Different Tests Conducted On DC Machines?
- Brake test
- Swinburne’s test or No-load test
- Regeneration test or Hopkinson’s test
- Retardation or Running down test
17. Why
Is Swinburne’s Test Conducted?
Ans: Swinburne’s test is
conducted at No-load condition to predetermine the efficiency at different loads
and to measure the losses separately.
For details theory about dc motor click link Electric Motor
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