Alternator Tests

 

ALTERNATOR TESTS

To find the regulation of an alternator, armature resistance, open-circuit characteristics and short-circuit short characteristics are required. These data can be obtained by conducting the following tests on an alternator.

1.       Armature resistance test

2.       Open-circuit (O.C) Test or No-Load test

3.       Short-circuit (S.C)

Armature resistance test is conducted to determine the armature resistance and the O.C and S.C Tests are conducted to determine the synchronous impedance.  

Armature Resistance Test

This test is conducted to determine the armature resistance Ra. The armature resistance per phase can be determined by voltmeter-ammeter method or by using Wheatstone bridge. The connection diagram for measurement of armature resistance by voltmeter-Ammeter method is shown in fig below. The D.C source is connected between two phases connected in series. The voltmeter reading divided by ammeter reading gives the value of resistance of two phase connected in series. So resistance of each phase Ra is equal to half the resistance measured. The rated current can be applied by varying the rheostat. This test is conducted when the alternator is at rest, but under working conditions the armature resistance Ra is increased by about 60% due to skin effect. Hence, generally a value of 1.6 times the D.C value is taken.

Open-circuit Test (O.C Test) or No-Load Test

This test is conducted to determine open-circuit voltage per phase Eo from open-circuit characteristics (O.C.C). The connection diagram for obtaining open-circuit characteristics is show in fig below.

The field winding is connected to a D.C Source in series with a field rheostat and an ammeter. The alternator is run on no-load at the rated speed. The field current If is gradually increased in steps from zero by adjusting the field rheostat until the voltmeter readings are noted. The voltmeter indicates the no-load line voltage. The open-circuit voltage per phase Eo can be obtained by dividing the voltmeter reading by √3 (in star connection). A curve can be obtained between no-load voltage per phase Eo and field current If as show in below fig.

Short-circuit Test (S.C. Test)  

This test is conducted to determine the short-circuit current per phase Isc from short-circuit characteristics. The connection diagram for obtaining shot-circuit characteristics show in below.

All the three phases are shorted and one ammeter is connected in any one of the three phases. The alternator is run at rated speed. The field current If is gradually increased in steps from zero by adjusting the field rheostat until the short-circuit armature current Isc is about twice the full-load current. The reading of the ammeter A1 and A2 are noted, the ammeter A2 Reads the short-circuit current per phase (in star connection). A show in fig  below as know as short-circuit characteristics. From O.C and S.C tests, the synchronous impedance Zs can be determined as follows:

Synchronous impedance per phase, Zs = (open-circuit voltage / phase ‘Eo’ )/(short-circuit current/phase ‘Ise’)