Tuesday, April 3, 2012


To determine experimentally the magnetization or open circuit characteristic of agenerator and determine the critical field resistance and critical speed
Prepare a List of the apparatus required based on the name plate ratings and Circuitdiagram in the following format.
4.Rheostat250 ohm/5A
1. Connect the circuit as shown in the fig2. Set the potential divider to zero output keeping motor field rheostat in minimumresistance position3. Switch on the supply and start the motor with the help of the starter. Adjust the speedof 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 Notedown the voltmeter reading at zero field current (lf). Increase the field current lf uniformly in steps, by changing the potential divider tapping, simultaneously note downthe field current (lf) and the terminal voltage (E) across the generator armature terminals.4. Continue the experiment till saturation of the field is reached.5. Continue the experiment for decreasing values of If in steps by decreasing the tappingof potential divider.

Critical Field Resistance:
It is that value of the field resistance at which the D.C. shuntgenerator will fail to excite.
Critical Speed:
It is that speed for which the given shunt field resistance becomes thecritical field resistance. Critical field resistance is obtained by plotting the OCC as infig.2 and determining the slope of the tangent to the linear position of the curve from theorigin. 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 curvestarts a little way up. The slight curvature at the lower end is due to magnetic inertia. It is seen that in thefirst part of the curve is practically straight. Hence the flux and the consequently the generated EMF isdirectly proportional to the exciting current. However at the higher flux densities where it is small iron pathreluctance becomes appreciable and straight.Field windings are connected parallel to the armature and it is called dc shunt generator. Due toresidual magnetism some initial emf and hence some current will be generated. This current while passinginto 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 reinforcementof emf and flux proceeds till equilibrium reached at some point.
GRAPH between If and Eg* :

1. Perform the experiment at constant speed.
2. Readings are to be taken for uniformly increasing and then uniformly decreasing fieldcurrent.
3. Check must be made for residual magnetism otherwise; the field terminals may berequired to be reversed.

Thus an OCC and LOAD characteristics of a separately excited generator was performed and therespective graphs were drawn

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