Transcript The DC generator

The DC generator
Types of DC generators
DC Generator
Self Excited
Separately Excited
shunt
Shunt
series
Compound
compound
cumulative
cumulative
Differential
Differential
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.
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.
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
Self excited shunt generator
 At the armature terminal, the
voltage Va is given by the
equation:
 Va= Ea-IaRa
 And KCL at the armature
terminal node gives:
 Ia=IF+IL
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 IF( in
the linear portion of magnetizing
curve), a plot between Ea and IF
reflects the relation between Ea and Ф
while ω is held constant by the prime
mover.
The Magnetization curve or Open Circuit
characteristics of a shunt Generator
 The characteristics curve between
Ea and IF 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.
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 Eares.
 The residual voltage Eares 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:
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.
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 Rsh and adjustable
resistance Radj used to adjust the
field current ( and hence flux Ф)
i.e, RF = Rsh + Radj
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 Ia causes
an internal voltage drop across the
armature resistance Ra. The available
voltage across the load terminal ,
termed VT is therefore less then internal
generated voltage Ea. This is given by
the equation:
 Va= Ea-IaRa
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.
The rated armature voltage and current
 The armature terminal voltage Va 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.
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 = VT(N.L) - VT(F.L) x 100
VT(F.L)
Thank You