Measurement of induced E.M.F. and magnetizing current under open-circuit condition in D.C. generators.

Experiment No.: 04

Aim of the experiment: –

Measurement of induced E.M.F. and magnetizing current under open-circuit condition in D.C. generators.

Objective of the Experiment:

To be written by student.

Apparatus Required: –

Sl. No.	Name	Specification	Quantity
01	Motor-Generator(M-G) set	Motor: Three-phase slip-ring IM, 440V 1400rpm, 50Hz, DC shunt Generator: Arm- 180V, Field- 180V	1 nos.
02	Tachometer	Digital	1 nos.
03	Voltmeter	(0-150)V, PMMC	1 nos.
04	Ammeter	(0-5)A, PMMC	1 nos.
05	Rheostat	(0-80)ohm	1 nos.
06	Connecting wires	PVC Insulated copper	As per required

Circuit Diagram: –

Theory: –

The open circuit characteristic or the magnetization characteristic of the DC generator gives the relationship between flux per pole and exciting current or the field m.m.f. per pole. If the machine is run at constant speed on no load, the induced e.m.f. is proportional to the flux per pole (and hence the field current); if e.m.f. is plotted against the exciting current the graph is nearly called the open-circuit characteristic. In a DC generator, the induced e.m.f. is given as E_g=(ZNɸP)/(60A) volt. Hence if speed N is constant, Eg=kΦ. It is bvious that when the field current, If is increased from its initial value, the flux and hence the generated e.m.f. Eg increases directly as If while the poles are unsaturated. But as the flux density increases, the poles become saturated, so a greater increase in If is required to produce a given increase in Eg than on the lower part of the curve , as shown. Also due to residual magnetism in the poles some e.m.f. is generated even when If is zero. Hence curve starts a little way up as shown. If the machine is on open circuit, the shunt circuit of the DC generator can be regarded as in series with the armature. A straight line is drawn through origin.

Procedures: –

1. Connect the circuit as shown in the diagram.
2. With the generator G field circuit OFF start the motor M with the help of the starter. Note the voltmeter reading and the speed of the machine. The machine may be run at rated speed by adjusting the field regulator of the motor. The voltmeter will indicate the voltage due to residual magnetism in the field of the generator (at If=0).
3. Adjust the output voltage potential divider at zero and then put the supply ON.
4. Increase the field current in regular steps, nothing at least ten sets of readings of voltage and field current, to about 125%of the rated voltage of the generator.(Note: while increasing the field current don’t decrease it to obtain a reading of particular value).
5. Put the motor and generator supply OFF (P.S.: the speed of the machine should remain constant throughout the experiment.
6. Draw a smooth curve as shown.

Observation Table: –

Sl. No.	Field Current (If) in Amps.	Induced Voltage (Va) in volts.	Speed N (RPM)
1
2
3
4
5
6
7

Precaution: –

1. Connection terminals should be tight, done as per circuit diagram.
2. Take observations reading properly.

Conclusion: –

To be written by student.