To obtain the no load characteristics of a DC shunt generator experiment and to determine the critical field resistance

AIM: To obtain the no load characteristics of a DC shunt generator and to determine the critical field resistance.

Name Plate Details:

Apparatus Required:

Circuit Diagram:

Theory:

Magnetization curve is relation between the magnetizing forces and the flux density B. this is also expressed as a relation between the field current and the induced e.m.f , in a D.C machine. Varying the field current and noting corresponding values of induced e.m.f can determine this. For a self-excited machine the theoretical shape of the magnetization Curve is as shown in the figure. The induced e.m.f corresponding to residual magnetism exists when the field current is zero. Hence the curve starts, a little above the origin on y-axis. The field resistance line Rsh is a straight-line passing through the origin. If field resistance is increased so much that the resistance line does not cut the OCC at all then obviously the machine will fail to exite .If the resistance line just lies along the slope, then machine will just excite. The value of the resistance represented by the tangent to the curve is known as critical field resistance Rc for a given speed.

Critical Field Resistance

It is the resistance of the field winding of the generator below which generator fail to build up the voltage.

First OCC is plotted from the the readings then tangent is drawn to its initial position .The slope of this curve gives the critical field resistance.

From the graph the critical field resistance Rc = AB/BC.

Procedure:

1. Connect the circuit as per the circuit diagram shown in fig.

2. Keep the motor field rheostat Rsh at minimum position and generator field rheostat at maximum position.

3. Check that the belt on the pulley is free so that there is no load on the pulley.

4. Switch on the DPST swatch. Start the motor slowly by using starter.

5. Adjust the current so that the motor runs at its rated speed.

6. Now vary the generator field rheostat to increase the field current and take the no load voltage and field current readings.

7. Take the no load voltage values until field gets saturated.

8. Finally set the field rheostats to initial positions then switch off the supply.

9. Draw the graph between generated voltage and field current. Find the critical field resistance from the tangent line.

Tabular Column:

Residual Voltage = Speed=

Model Graph:

Draw the graph between generated voltage at no load and field current. By taking Generated voltage Eg in volts on Y axis and field current If in amps on X-axis.

Precautions:

• The rheostat is connected such that minimum resistance is included in field circuit of motor.
• The rheostat is connected such that maximum resistance is included in field circuit of generator.
• Starter handle is moved slowly.

Result:

Viva Questions:

1. What is meant by critical field resistance?
2. Residual magnetism is necessary for self excited generators or not.
3. Why this test is conducted at constant speed?