Magnetization Characteristics of a D.C. Shunt Generator
Aim: To determine experimentally the magnetization or open circuit characteristic of a generator and
determine the critical field resistance and critical speed.
Procedure:
1. Connect the circuit as shown in the fig.
2. Set the potential divider to zero output keeping motor field rheostat in minimum resistance
position.
3. Switch on the supply and start the motor with the help of the starter.
4. Adjust the speed of the motor generator set to the rated speed of the generator by controlling the
motor field resistance the set speed is to be maintained constant throughout the experiment.
5. Note down the voltmeter reading at zero field current (lf).
6. Increase the field current lf uniformly in steps, by changing the potential divider tapping,
simultaneously note down the field current (lf) and the terminal voltage (E) across the generator
armature terminals.
7. Continue the experiment till saturation of the field is reached.
8. Continue the experiment for decreasing values of If in steps by decreasing the tapping of potential
divider.
THEORY:
Critical Field Resistance:
It is that value of the field resistance at which the D.C. shunt generator will fail to excite.
Critical Speed:
It is that speed for which the given shunt field resistance becomes the critical field resistance. Critical
field resistance is obtained by plotting the OCC as in fig.2 and determining the slope of the tangent to the
linear position of the curve from the origin. While drawing the tangent, the initial position of the O.C.C is
neglected.
Due to residual magnetism in the poles some EMF is generated even when If = 0. Hence the curve starts a
little way up. The slight curvature at the lower end is due to magnetic inertia. It is seen that in the first
part of the curve is practically straight. Hence the flux and the consequently the generated EMF is directly
proportional to the exciting current. However at the higher flux densities where it is small iron path
reluctance becomes appreciable and straight.Field windings are connected parallel to the armature and it
is called DC shunt generator. Due to residual magnetism some initial emf and hence some current will be
generated. This current while passing into the field coils will strengthen the magnetism of poles. This will
increase pole flux which will further increase the generated emf. Increased emf and flux proceeds till
equilibrium reached. This reinforcement of emf and flux proceeds till equilibrium reached at some point.
GRAPH between If and Eg* :
Precautions:
1. Perform the experiment at constant speed.
2. Readings are to be taken for uniformly increasing and then uniformly decreasing field current.
3. Check must be made for residual magnetism otherwise, the field terminals may be required to be
reversed.
Rresult:
Thus an OCC and LOAD characteristics of a separately excited generator was performed and the
respective graphs were drawn.