(1)
No-load Characteristic gives
the variation of armature generated emf Eg with field current If for constant speed N. The circuit connection
diagram is shown below.
As armature generated
voltage (Eg) = PΦZN / 60A
Therefore, for constant
speed N
Eg ∝ Φ ∝ IF ∵ {
Φ
= NF IF / reluctance }
The No-load
characteristics of a separately excited dc generator is shown below.
It is seen that even when
field winding is not energised, the voltmeter indicates a small voltage, due to
presence of residual flux in the field poles as shown by OA in the
characteristics.
When IF is
increased from zero, the curve is found to be a straight line, because the
entire field mmf is almost spent in forcing the flux through the air gap and
mmf required by the iron is almost negligible as its reluctance remains almost
constant.
With larger IF
, saturation sets in and mmf required by the iron increases rapidly as its
reluctance increases rapidly.
(2)
Load Characteristic gives
variation of terminal voltage Vt with field current for constant armature
current and armature speed.
Circuit connection is
shown below.
To obtain load
characteristic of separately excited dc generator, run the armature at rated
speed and close the switch S.
Now, adjust IF
till Ia is equal to
rated armature current and take instrument readings.
Vary the load and field
current such that Ia and N
remain constant, but terminal voltage Vt changes and keep taking more
readings.
The characteristic is
shown below.
(3)
External Characteristic shows
variation of terminal voltage Vt with load current IL for
constant speed N and constant field current IF .
Make circuit connections
as shown below.
Firstly, generator is run
at rated speed and its field winding is excited to give rated terminal voltage
at no-load.
Now, close the switch S ,
vary load resistance and note Vt and IL . Keep repeating
it for various readings.
The external characteristic
is shown below.
The decrease in terminal
voltage with increase in load is due to
the voltage drops caused by armature reaction and armature resistance (
includes brush contact resistance ).
