The induced EMF in a DC generator is given by the equation E g = PZ N/ 60A
volts. Since P,Z and A are constants the above equation can be rewritten as E g = K N.
If the speed of the generator is also maintained constant then Eg =K1 , but the flux is
directly proportional to the field current, hence E g = K2 If. From the above equation it
is clear that the induced EMF is directly proportional to the field current when speed
is maintained constant. The plot between the induced EMF and the field current is
known as open circuit characteristics of the DC generator. The typical shape of this
characteristic is shown in figure.
The induced EMF when the field current is zero is known as residual voltage.
This EMF is due to the presence of a small amount of flux retained in the field poles
of the generator called residual flux. Once the O.C.C is obtained the parameters such
as critical field resistance, critical speed and the maximum voltage to which the
machine can build up can be determined. If required the O.C.C at a different speeds
can also be obtained. Critical speed is minimum speed below which the generator
shunt fails to excite.
PRECAUTIONS: (Not to be included in the Record)
1. Remove the fuse carriers before wiring and start wiring as per the circuit
2. Keep the motor field rheostat at minimum resistance position and generator field
rheostat at maximum resistance position.
3. The SPST switch is kept open at the time of starting the experiment.
4. Fuse calculations. As the test is a no-load test the required fuse ratings are 20%
of motor rated current.
5. Replace the fuse carriers with appropriate fuse wires after the circuit connections
are checked by the staff-in-charge.