To determine the magnetization characteristic (O.C.C) of the given DC shunt Generator and to find out the critical field resistance and critical speed.

Aim

To determine the magnetization characteristic (O.C.C) of the given DC shunt Generator and to find out the critical field resistance and critical speed.

Apparatus Required:

Name plate details:-

Motor Generator

Voltage - 220V Voltage - 220V

Current -19A Current -13.6A

Speed - 1500 rpm Speed - 1500 rpm

Power - 5HP Power - 3KW

Excitation type–shunt Excitation type - shunt

Circuit diagram:

Circuit diagram for OCC:

Circuit diagram for finding field resistance:

Procedure:-

1. Make the connections as per the circuit diagram.
2. Initially keep the motor field rheostat in minimum resistance position and Generator field rheostat maximum resistance position.
3. Give the supply by closing DPST switch and start the motor with the help of 3- point starter.
4. Adjust the motor field rheostat till the rated speed is obtained.
5. Give the supply to the generator circuit.
6. Vary the generator field rheostat in steps to get field current and measure open circuit voltage at each step.
7. Repeat step no. 6 till we get 120% of rated voltage.
8. Decrease the field current in steps and note down the open circuit voltages
9. Switch off the supply by opening the DPST switch.

Tabular column:

Calculations:

Model graph:

From the graphs,

The critical field resistance = slope of the line

R _c= ΔEo / ΔIf ohms

Result:The magnetization characteristic (O.C.C) of the given DC shunt generator are determined and hence field resistance and critical field resistance are calculated.

Viva voce

1. What is the principle involved in the operation of generator?
2. Define critical field resistance?
3. Define critical speed?
4. What are the types of generators?
5. What are the causes of failures of excitation?