Principle of operation of DC generator.ppt

Dc generators are used in generation of DC induced emf.

1. Describe an elementary generator
•ABCD – Rectangular loop of wire, Two metallic strips or brushes M
and S
•F- Ring , G- Shaft, xx’ – axis between the centre of the two magnets.
MAGENTIC LINES OF FORCE

2. What occurs during the second half of the revolution?

3. What may be said of the electromotive force during the second
half of the revolution?
•During the revolution of the loop
1. From 0
o
to 90
o
, the electromotive force
increases from 0 to maximum
2. From 90
o
to 180
o
, the electromotive force
decreases from maximum to zero
3. From 180
o
to 270
o
, current reverses and the
electromotive force increases from zero to
maximum.
4. From 270
o
to 360
o
, the electromotive force
decreases from maximum to zero

4. Reason for rise or fall of EMF with increasing or
decreasing rapidity

5. •The sine curve:
•Thus an alternating EMF is induced in the armature of
either an alternator or dynamo.
•i.e.
1. begins with zero emf
2. rises to a maximum
3. decreases again to zero
4. increases to a maximum in the opposite direction and
5. decreases to zero.
A wave – like curve is used to represent these several
changes, in which the horizontal distances represent
time, and the vertical distances represent the varying
value of the EMF.

6. Describe the construction and application of sine curve.

7. Position of armature at each quarter of the revolution

8. How the dynamo produces direct current?
•Difference between alternator and dynamo:
Alternator – delivers alternating current to the
external circuit.
Dynamo – delivers direct current to the external
circuit
•Similarity:
•Alternating emf are induced in the armature, but the
kind of current delivered to the external circuit
depends on the manner in which the armature
currents are collected.

9. •In an alternator:
•Each end of the loop is connected with an insulated
collector ring carried by the shaft, the current being
collected by means of brushes which bear against the
ring.
Important point:
•“each end of the loop is always in connection with the
same brush”
•A form of switch called the commutatorcommutator is placed
between the armature and the external circuitarmature and the external circuit and so
arranged that it will reverse the connections with the
external circuit at the instant of each reversal of current
in the armature.

10. How are the inductors connected to the commutator?
•The ends of each conducting loop or coil must be
connected with the commutator segments in a
certain order to correspond with the type of winding.

11. •Explain in detail how direct current is obtained
in a dynamo?

12. Elementary dynamo armatures
Single turn loopCoil of two turns in series
• Maximum EMF,
• Current generated is half
that generated with turns
in parallel
Coil of two turns in parallel
• Induced EMF is half that of
turns in series
• Current is maximum

13. COMMUTATION OF THE CURRENT
Figs. 174 Figs. 175 Figs. 176 Figs. 177 Figs. 178

14. •Is the direct current indicated by the sine curve in figs. 174
to 178 continuous?
•No;
•it is properly described as a pulsating current, or one,
constant in direction, but periodically varying in intensity
so as to progress in a series of throbbings or pulsations
instead of with uniform strength.
•What is generally understood by the word 'continuous‘ as
applied to the current obtained from a dynamo?
•It is usually accepted as meaning a steady or non pulsating
direct current; one that has a uniform pressure and
constant direction of flow as opposed to an alternating
current.

15. •Is a continuous current ever obtained with a
dynamo?

16. •The so called continuous current which it is said to
produce is in reality a pulsating current, but with
pulsations so minute.
•How is the so called continuous current produced by a
dynamo?
•Ans. In order to obtain a large number of small large number of small
pulsations per revolutionpulsations per revolution of the armature of the armature instead of
two large pulsations as with the single loop armature,
the latter must be replaced by one having a great
number of loops properly connected to commutator
segments and so arranged that the successive loops
begin the cycle progressively.

17. Gramme( 1871) - Ring Armature with one coil,
and characteristic sine curve below.
 With one coil as shown, there are two pulsations
of the current per revolution of the armature.

18. •Fig 183 - Gramme ring armature with two coils placed
180° apart.
•This arrangement gives double the pressure of the one
coil armature,

19. Fig 185 - Gramme ring armature with four coils.

20. Fig – 186 - Gramme ring armature with six coils

21. Resultant curve of figure 183,
185,186