International Islamic University,Islamabad

Electrical Machines LAB

EXPERIMENT # 2:

Demonstrate the effect of field excitation on the generation of voltage by an Alternator

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EXPERIMENT # 2:

Demonstrate the effect of field excitation on the generation of voltage by an Alternator

Theory

The effect of field excitation on the generation of voltage can be studied by considering the open circuit of an alternator. The open circuit test is carried out with the terminals of the machine disconnected from any external circuit. The basic test procedure is:

1. Open circuit the generator terminals2. Drive the machine at synchronous speed using an external mechanical system3. Slowly increase the field current and measure the open circuit terminal voltage

As there is no armature current, the measured terminal voltage is the induced voltage:

If the machine is wye connected, the measure voltage will be a line-line voltage and the per-phase induced voltage can be found from

Remember from the armature winding notes that the induced voltage vs. field current plot will have a similar shape to the flux vs. field current plot.

In the plot above, the air gap line (dotted blue) gives the relationship between mmf and flux density in the air gap. (Induced voltage is proportional to flux density). At higher field current levels, the iron in the machine saturates and the percentage of field mmf applied to the air gap is reduced. As a result, the induced voltage falls below the air gap line. As synchronous machines operate at effectively constant speed, the open circuit voltage test provides the relationship

between field current and induced voltage for all load conditions. The induced open circuit voltage is sometimes known as the "excitation voltage", or even simply as the "excitation".

Equipment and Components:

Three phase Alternator Separately excited DC Motor Variable DC power supply AC Ammeters AC Voltmeters DC Ammeters Variable Load Resistors Connecting wires

Connection Diagram

Procedure

1. Connect the apparatus shown in the figure.2. Mechanically couple the DC shunt motor with three phase Alternator, the DC shunt motor will

behave as prime mover.

3. Connect the voltmeters and ammeters as shown in figure, which will measure the excitation current and no-load output voltage of the three phase alternator.

4. Adjust the speed of DC motor to synchronous speed with the help of Variable DC supply.5. Now slowly increase the variable resistor of field, to increase the excitation current.6. Increasing excitation current note the voltmeter readings in the observation table given below.7. Keep the speed of prime mover constant during this whole process. 8. Plot the observations on a graph paper between no-load voltage and excitation current.

Observations

Synchronous Speed=___________

Open Circuit Test Measurements

No.Field Current I f (A)

Terminal Voltage per phase EOC (V)

1

2

3

4

5

6

7

8

9

10

11

12

13

Report

Draw the graph between Open circuit voltage EOC and field current I f ?