Voltage regulation of Alternator by EMF method

The voltage regulation of alternator by EMF method involves the EMF quantities of all the armature parameters (armature resistance, Armature leakage reactance, armature reaction). The drop due to armature reaction is not considered, because it does not occur due to any of the physical element but due to interaction of armature flux with main flux.

Hence, in order to quantify the voltage drop due to armature reaction, armature winding is assumed to have a fictitious reactance called armature reaction reactance X_ar Ω/phase.

Now, the Sum of armature leakage reactance and armature reaction reactance is called synchronous reactance of an alternator X_S.

X_S = X_L + X_ar

In EMF method, the voltage drop due to armature resistance (R_a) and the drop due to synchronous reactance(X_S) is considered, both the drops are emf quantities.

The impedance of armature winding is expressed as Z_S = R_a + jX_S Ω/phase, which is nothing but the synchronous impedance of an alternator and since the drop due to the synchronous impedance is considered, this method is called synchronous impedance method.

This method is also called pessimistic method, because the voltage regulation obtained by this method is more than the actual value.

The EMF method requires the following data’s to determine the voltage regulation of alternator.

• Armature resistance/phase
• Open circuit characteristics (OCC)
• Short circuit characteristics (SCC)

Armature Resistance per phase

Armature Resistance per phase can be obtained by conducting stator resistance test on the alternator. It is done by connecting the dc voltage supply to the stator armature winding and the corresponding current is measured.

By doing so, the dc stator resistance is calculated and then by using the formula R_ac = 1.6 R_dc the ac stator resistance is determined.

Open Circuit Characteristics(OCC)

• Open circuit characteristics is obtained by conducting open circuit test in the Alternator. To do that, the connections are given as per the following circuit diagram.
• To perform this test, the stator windings are kept open.
• The Alternator was made to run at synchronous speed by adjusting the field rheostat of the dc motor.
• The field current of the alternator was varied in steps until the machine attains its maximum voltage. The corresponding readings were noted down.
• From the readings, a graph is drawn as below, where OCC represents the open circuit characteristics.

Short Circuit Characteristics(SCC)

• Short circuit characteristics is obtained by conducting short circuit test in the Alternator. To do that, the connections are given as per the above circuit diagram.
• The stator windings of alternator are Shorted and an ammeter is connected to measure the current flow.
• The Alternator was made to run at synchronous speed by adjusting the field rheostat of the dc motor.
• The field current of the Alternator was adjusted so that the armature current reaches its maximum rated value.
• Note the corresponding current readings and draw the graph. SCC in the graph below represents the short circuit characteristics.

Determination of Zs from the graph

The value of Zs to be determined for the “SAME VALUE OF FIELD EXCITATION”. Follow the simple procedure to draw the graph and obtain the voltage regulation.

1. Plot the OCC and SCC curve in a graph.
2. For the rated full load current(I_sc) of alternator[which is to be found from the rating of alternator], draw a line that cuts the SCC curve, from that draw a vertical line towards the x-axis and find the field current(I_f).
3. For that field current, extend the line so that it cuts the OCC curve and find the open circuit voltage V_oc volts (phase value).
4. Now, we know the open circuit voltage V_oc volts and short circuit current I_sc. From this, determine the value of Z_S using the formula,

5. From the known resistance value and determined Z_S found the value of Xs using the formula,

6. Now, using the following formulas, obtain the value for Eph, [obtained from phasor diagram]

For Lagging Power factor,

For Leading Power factor,

7. Finally the voltage regulation of alternator can be determined from the formula,