The synchronous impedance method or EMF method is one of the methods of finding voltage regulation of alternator or synchronous generator. In order to calculate regulation by this method, it requires the following data,- The per-phase armature resistance Ra,
- The open-circuit test data shows the relationship of the characteristics between open-circuit voltage and field current, and
- The short-circuit characteristic is the graph of short-circuit current against field current.
DC Resistance Test :
The per-phase armature resistance can be calculated by performing a dc resistance test. To measure the dc resistance armature-voltmeter method is used. The calculated dc resistance value is low when compared to ac resistance. So that the obtained dc resistance is multiplied by a factor of 1.3 to obtain ac resistance. The below shows the circuit diagram for the dc resistance test.
Open-Circuit Test :
The circuit diagram for the open-circuit test is shown below. Let the alternator is running at synchronous speed driven by the prime mover with no-load connected across the armature. The voltmeters connected across the armature open-circuited lines measure open-circuit voltage.
- At first, adjust the prime mover speed to the synchronous speed of the alternator.
- The rheostat in the field circuit ( rotor ) is set to its maximum value.
- Now increase the field current by varying the field rheostat so that flux produced by field winding increases.
- The increased flux increases induced emf in the alternator thereby increasing the terminal voltage of the alternator.
- The various values of open-circuit line voltage Voc at different field current If values are noted
- The below curve shows the open-circuit test characteristics drawn between field current and open-circuit voltage.
The observation table for the open-circuit test is shown below.
S.No. |
Field current Ia |
Open-circuit line Voltage Voc |
---|
Readings oberved from the test |
Short-Circuit Test :
A short-circuit test is performed by running the alternator at rated speed. The below shows the circuit diagram with short-circuited armature terminals. The ammeters connected in the armature circuit measures short-circuit current.
- Before short-circuiting the armature terminals, the field current is reduced to a minimum value by increasing the resistance of the field rheostat.
- Now, the field current is gradually increased till the armature winding reads full load current.
- The full load current is obtained through armature ammeter reading.
- The various short-circuit current Isc values at a various field current Ia values are noted.
- The relation between both values gives short-circuit characteristics of an alternator as shown below.
The observation table for the short-circuit test is shown below.
S.No. |
Field current Ia |
Short-circuit armature current Isc |
---|
Readings oberved from the test |