Voltage regulation of Alternator by MMF method

Voltage regulation of alternator by MMF method requires mmf (which is a product of field current(I) and number of turns(N) of field winding) for two separate purposes.

1. It must have an mmf which is necessary to induce the rated terminal voltage on open circuit condition.
2. It must have an mmf to circulate the full load current equal and opposite to that of armature reaction mmf.

The field mmf (I_f1) required to induce the rated terminal voltage is obtained by conducting open circuit test.

This method replaces the effect of armature leakage reactance by an equivalent armature reaction mmf. I_f2 is the field current required to circulate the full load armature current by balancing or overcoming the armature reaction, which is obtained by conducting the short circuit test.

This method is also called an optimistic method because the voltage regulation obtained by this method is less than the actual value.

The MMF method requires the following data’s to determine the regulation

• 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 Voltage regulation for Leading Power factor

• Plot the OCC and SCC curve in a graph.
• Find the value of induced emf with the resistance drop using the formula , if resistance is given,

• For the rated voltage(E_oph), draw a line that cuts the OCC curve, from that, draw a vertical line and find the field current I_f1 and mark it as A.
• [If the resistance is not given, neglect it and hence for the rated voltage V_ph, find the corresponding field current I_f1]
• Find the field current I_f2, that is responsible to circulate the full load short circuit current (I_sc) by balancing the armature reaction. This is obtained by drawing a line for full load short circuit current.
• Now, With A as centre and I_f2 as radius, draw a semi-circle.
• For Leading power factor, current leads the voltage, so considering AB as voltage phasor, draw the current phasor such that it leads the voltage by Φ inside the semi-circle(the angle between this current phasor and x-axis is 90-Φ in anti-clockwise direction) and mark the point on the semi-circle as C.
• Join OC, which is the resultant Field current (I_f) that is responsible for generating rated voltage with the drops.
• Now, in order to find the corresponding voltage, with O as centre and OC as radius, draw an arc that cuts the x-axis at point D.
• From this point D, draw a vertical line which cuts the OCC curve, from that draw a horizontal line to y-axis and find the no load voltage (E_ph).

Finally the voltage regulation o alternator by mmf method can be determined from the formula,

Determination of Voltage regulation for Lagging Power factor

To draw the graph for lagging power factor, the same steps are repeated as above to determine the voltage regulation, except for the following change.

For Lagging power factor, current lags the voltage, so considering AB as voltage phasor, you have to draw the current phasor such that it lags the voltage by Φ inside the semi-circle(the angle b/w this current phasor and x-axis is 90-Φ in clockwise direction) and mark the point on the semi-circle as C.

Determination of Voltage regulation for Unity Power factor

For Unity power factor, repeat the same procedure but for a small change. Here current is in phase with voltage, so considering AB as voltage phasor, draw the current phasor such that it is in phase with the voltage inside the semi-circle (on the voltage phasor itself) and mark the point on the semi-circle as C and continue with the same steps to determine the voltage regulation.