2. E.M.F. EQUATION OF A D.C. GENERATOR
• We shall now derive an expression for the e.m.f. generated in a d.c. generator.
• Let
• f = flux/pole in Wb
• Z = total number of armature conductors
• P = number of poles
• A = number of parallel paths = 2 ... for wave winding
• = P ... for lap winding
• N = speed of armature in r.p.m.
• Eg = e.m.f. of the generator = e.m.f./parallel path
3. Cont. ..
Then, e.m.f. of generator,
• Eg = e.m.f. per parallel path
• Where,
• A = 2 for wave winding
• A = P for lap winding
60
g
P ZN
E
A
4. Armature Resistance (Ra)
• The resistance offered by the armature circuit is known as armature
resistance
(Ra) and includes:
i. resistance of armature winding
ii. resistance of brushes
• The armature resistance depends upon the construction of machine. Except
for small machines, its value is generally less than 1Ω.
5. Types Of Generators
• Generators are usually classified according to the way in
which their fields are excited.
• Generators may be divided into
a. Separately – excited generators
b. Self – excited generators
6. Cont. …
a. Separately – Excited
Are those whose field magnets are energized from an
independent external source of d.c current.
7. Cont. …
b. Self – excited generators
Are those whose field magnets are energized by the
current produced by the generators themselves. Due to
residual magnetism, there is always present some flux in
the poles. When the armature is rotated, some e.m.f. and
hence some induced current is produced which is partly
of fully passed through the field coils thereby
strengthening the residual pole flux.
8. Cont. …
There are three types of self-excited generators named
according to the manner in which their coils (or windings)
are connected to the armature.
i. Shunt Wound
The field windings are connected across or in parallel
with the armature conductors and have the full voltage
of the generator applied across them.
9.
10. Cont. ..
ii. Series Wound
In this case, the field windings are joined in series with
the armature conductors. As they carry full load current,
they consist of relatively few turns of thick wire or strips.
Such generators are rarely used except for special
purpose i.e. as boosters
11.
12. Cont. ..
iii. Compound wound
Its is a combination of few series and a few shunt
windings and can be either short-shunt of long-shunt as
seen in the figure.
In a compound generator, the shunt field is stronger
than the series field..
15. Brush Contact Drop
• Its is the voltage over the brush contact resistance when
current passes from commutator segment to brushes and
finally to the external load.
• Its value depends on the amount of current and the value
of contact resistance.
16. Cont. ..
• This drop is usually small and includes brushes of both
polarities..
• However, in practice, the brush contact drop is assumed to
have following constant values for all loads.
a) 0.5 V for metal-graphic brushes
b)2.0 V for carbon brushes
17. Examples
1. A shunt generator delivers 450 A at 230 V and the resistance
of the shunt field and armature are 50 Ω and 0.03 Ω
respectively. Calculate the generated e.m.f.
(Ans: Eg = 243.638V)
18. Examples
2. A long-shunt compound generator delivers load current of 50
A at 500 V and has armature, series field and shunt field
resistance of 0.05 , 0.03 , and 250 respectively. Calculate the
generated voltage and the armature current. Allow 1 V per
brush for contact drop.
(Ans: Eg = 506.16V)
