4 the dc generator

DC Motors

1. The DC generator
Characteristics & equivalent
circuits

2. Types of DC generators
DC Generator
Self Excited Separately Excited
Shunt
Compound
shunt
series
compound
cumulative
Differential
cumulative
Differential

3. According to the way of field
excitation
Separately Excited DC generator
The field winding is excited from dedicated DC
supply
Self Excited DC generator
The field winding is excited from the armature. No
need of separate DC source.

4. According to the connection of Field
winding with respect to Armature
winding
Shunt Generator
When field winding is connected in parallel with armature
winding. The field winding is termed as Shunt winding.
Series Generator
Field winding is connected in series with the armature.
Compound Generator
Both series and shunt winding are used to get combined
characteristics of the above two types of generators.

5. The Shunt DC Generator
Separately Excited-
Shunt field winding is
excited from a separate
DC source.
Self Excited – Shunt
Field winding is excited
from the armature voltage

6. Self excited shunt generator
At the armature
terminal, the voltage Va
is given by the
equation:
V
a
= E
a
-I
a
R
a

And KCL at the
armature terminal node
gives:
I
a
=I
F
+I
L

7. The No Load characteristics of
a DC generator
The armature is run by the
prime mover. There is no load
connected to armature
terminals while the field current
is increased gradually.
Since Ea=kФω and the flux Ф
is proportional to the field
current I
F
( in the linear portion
of magnetizing curve), a plot
between Ea and I
F
reflects the
relation between Ea and Ф
while ω is held constant by the
prime mover.

8. The Magnetization curve or Open Circuit
characteristics of a shunt Generator
The characteristics curve
between Ea and I
F
under the
condition of open circuited
armature is called the open
circuit characteristics (OCC)
or the magnetizing
characteristics of a DC
generator.
DC generators are usually
operated near the
saturation( or knee) point of
the OCC.

9. The armature residual voltage
When the field winding is not excited, the
armature terminal will still show some voltage
as the flux does not collapse fully in
ferromagmetic Field Poles. This flux is called
the residual flux.
This voltage is due to the residual flux in the
field poles, hence termed the residual voltage
Ea
res.

10. The residual voltage Ea
res
provides current in
the field winding and the flux grows.
The cumulative action of field strength and
voltage build up depends on several
conditions. These are:

11. Conditions for voltage build up
The presence of residual flux. If there is no residual
voltage, the generator must first be excited from an
external DC source. The process is termed Flashing
of field.
The field winding must be connected properly across
the armature in such a way so as to strengthen the
flux in the poles.
Resistance of field circuit must be less than critical
resistance.

12. Critical resistance
The maximum resistance of
field circuit above which no
voltage build up is possible
is called the critical
resistance..
The field circuit includes the
shunt field resistance R
sh
and
adjustable resistance R
adj

used to adjust the field
current ( and hence flux Ф)
i.e, R
F
= R
sh
+ R
adj

13. The load characteristics
The field winding is fully excited
up to the saturation level which
results in a No-load Armature
terminal Voltage Va
NL
(i.e., Ea)
The armature is then switched
to load with the result a current
flows in the armature circuit.
This current I
a
causes an
internal voltage drop across the
armature resistance R
a
. The
available voltage across the
load terminal , termed V
T
is
therefore less then internal
generated voltage Ea. This is
given by the equation:
V
a
= E
a
-I
a
R
a

14. Effect of armature reaction
In a self excited shunt
generator, if there is no
compensating winding to
make up the loss of flux
with increasing load, the
armature terminal voltage
drops sharply beyond the
Breakdown point.

15. The rated armature voltage
and current
The armature terminal voltage V
a
at full load
condition is the armature Rated voltage.
Similarly the full load armature current is
called the rated armature current. This is
usually determined by the permissible
temperature rise in the winding. This in turn
depends on the thickness of armature coils
and insulation class of the winding.

16. The voltage regulation
It is given by the percent change in the
armature terminal voltage from no load to full
load condition, with respect to the rated
armature voltage.
V.R = V
T
(N.L) - V
T
(F.L) x 100
VT(F.L)

17. Example problem
The generator armature voltage of a
separately excited DC generator is 151 Volts
at a speed of 151 Amperes when the field
current is 2.8 A.
a) Find the armature voltage for a field current of
2.4 A at 1450 rpm.
b) find the generated voltage for a field current of
2.1 A at a speed of 1600 A.
Answer: 129.43 Volts, 124.97 V,

18. Example problem
A 50kW,250V, DC shunt generator delivers
rated load at rated voltage for a generated
armature voltage of 255.1 V. Assume that the
armature resistance is 0.025 ohms, find the
shunt field resistance and the generated
armature voltage when the generator is
delivering half rated load at the rated terminal
voltage.
Answer: 62.5 ohms,