ASSIGNMENT:

3 TDC

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Ans. A magnet is permanent while electromagnet is shows magnetic properties until the
current flows through it.

The strength of electromagnet can be changed while that of magnet can’t be changed.

Q2. Name the Six component of DC motor?

Ans. The basic parts of a DC motor are axle, rotor, stator, commutator, field magnet and
brushes.

Q3. Why are armature laminations notched on outer periphery?

Ans. Copper losses are reduced when copper thickness is increased. Increasing copper thickness for
given space constraints comes by reducing number of turns in armature, but leads to undesirably
high amount of current flow for given voltage. High current flow would require larger core to store
the flux, otherwise it'll behave like a short circuit.

Electronic Control circuit regulates current and supports such armature construction. It can also
eliminate need for commutation and therefore, it affects armature construction in many ways.

Q4. What material are brushes made up of?

Ans. Brushes are made up of carbon.

Q5. What components of DC motor reverse the magnetic property of the

Ans. It depends on the motor construction.

For a “standard” brushed DC motor, it is easiest to do on a separately excited motor, where the
field winding has separate terminals to the (main) armature winding. This is typically done on
a motor designed for speed control. To reverse the motor, you need to change the polarity of
the supply voltage to either the field winding or the armature winding, but not both.
Generally it is better to reverse the field voltage because the field current is less than the
armature current, so your reversing switchgear is more lightweight. If you cannot separate the
field terminals and armature terminals, then reversing cannot be done.
 For a brushless DC motor, reversing the supply polarity won’t work and will possibly
damage the motor. The electronic commutation needs to be reversible, which has to be
part of the motor design.

Q6. Define Torque?

Ans. Torque is turning force that tends to cause rotation around axis.

Q7. What are the two primary advantages of DC motor?

Ans. The primary advantages of DC motor are that it can develop constant torque over a
wide speed application.

The speed can be controlled from zero to speeds above rated.

Q8. What is dynamo?

Ans. Dynamo is basically a DC generator i.e. an electrical machine which coverts mechanical
power into direct current electric power.

Q9. Can a generator produce electricity?

Ans. A generator doesn’t actually create electrical energy; instead it uses the mechanical
energy supplied to it to force the movement of electric charges present in the wire of its
windings through an external electric current.

Q10. What is the function of commutator?

Ans. The function of commutator is to keep the torque on a DC motor from reversing every
time the coil moves through the plane and to the magnetic field by reversing the current at
that point.

Q11. How can convert DC generator into AC generator?

Ans. It can be done by replacing the commutator with slip rings.

Q12. Faraday law of a DC generator?

Ans. Whenever conductor cuts magnetic flux, dynamically induced emf is produced in it
according to Faraday’s law of electromagnetic induction.

Q13. Principle of DC generator?

Ans. DC generator is based upon the principle that whenever a conductor is placed in
varying magnetic field an emf is induced in it.

Q14. Give the application of DC generator?

Series generators are ideal for use as boosters for adding voltage to the transmission line
and to compensate the line drop.

Q15. Difference between DC generator and DC motor?

Ans. A DC generator converts mechanical energy into unidirectional emf, while a DC motor
converts DC current to rotator work or torque.

Q16. Define hysteresis loss in transformers?

Ans. Hysteresis loss is due to the reversal of magnetization of transformer core whenever it
is subjected to alternating nature of magnetising force.

Q17. Draw Equivalent circuit of single phase transformer.

Ans.

Q18.What is meant by saturation in transformers and its effects on exciting current?

Ans. it is the condition where a transformer core is fully magnetised and producing
maximum magnetic flux. It will result in increased temperature and unwanted harmonic
distortion to output.

Q19. Distinguish between two winding transformer and auto transformer?

Ans. Auto transformer has only one winding while transformer has two.

Auto transformer is smaller in size while other transformer is larger in size.

Q20. Write different types of winding connections in 3 phase transformer?

Ans. Star to Star

Delta to Delta

Star to Delta
Delta to Star

Q21. Compare transformer and motor?

Ans. A transformer works on AC only but a motor can work on DC as well as AC.

A transformer is used to change the voltage i.e. step up or down while a motor is used to
give torque.

Q22.what is meant by regenerative braking?

Ans. It is a form of braking in which the kinetic energy of the motor is returned to the power
supply system.

Q23. Define Eddy current loss?

Ans. Eddy current loss occur due to loops of electric current induced within conductors by
changing magnetic field in the conductor.

Q24. Is copper loss depends on load?

Ans. Yes the copper loss depends upon the density of load. More is the load more will be
copper losses.