VOLTAGE REGULATION OF THREE PHASE ALTERNATOR The voltage regulation of an alternator is defined as the rise in voltage when full load is removed divided by the rated terminal voltage. % Regulation = (E 0 – V rated ) / V rated × 100 %

METHODS EMF Method MMF Method ZPF Method ASA Method

ZPF METHOD (OPEN CIRCUIT AND SHORT CIRCUIT TESTS)

PROCEDURE OPEN CIRCUIT TEST Initially TPST Switch is kept open. Supply is given by closing the DPST Switch in the Motor side. Using the DC three point starter, start the motor. The field rheostat of motor is adjusted to run the motor at rated speed and note down the corresponding alternator field current. Increase the field current in steps by varying the resistance of the alternator field and note down the corresponding open circuit voltage.

PROCEDURE SHORT CIRCUIT TEST Supply is given by closing the DPST Switch in the motor side. Using the DC three point starter, start the motor. The field rheostat of motor is adjusted to run the motor at rated speed. The TPST Switch is closed. The alternator field rheostat is adjusted for the rated short circuit armature current and note down the corresponding field current.

ZPF TEST

PROCEDURE ZPF TEST Supply is given by closing the DPST Switch in the motor side. Using the DC three point starter, start the motor. The field rheostat of motor is adjusted to run the motor at rated speed. The TPST Switch is closed. The alternator field rheostat is adjusted for the rated short circuit armature current. Vary the inductive load such that the voltmeter reads the rated terminal voltage and note down the corresponding field current.

PROCEDURE TO DRAW POTIER TRIANGLE Plot open circuit characteristics on graph. Plot the excitation corresponding to zero terminal voltage i.e short circuit full load zero power factor armature current. This point is shown as A in the model graph which is on the x-axis. Another point is the rated voltage when alternator is delivering full load current at zero power factor lagging. This point is P as shown in the model graph. Draw the tangent to O.C.C through origin, which is line OB as shown dotted in the model graph. This is called air line. Draw the horizontal line PQ parallel and equal to OA.

PROCEDURE TO DRAW POTIER TRIANGLE From point Q draw the line parallel to the air line that intersects O.C.C at point R. Join RQ and join PR. The triangle PQR is called Potier triangle. From point R, drop a perpendicular on PQ to meet at point S. The zero power factor full load saturation curve is now be constructed by moving a triangle PQR so that R remains always on O.C.C and line PQ always remains horizontal. The dotted triangle is shown in the model graph. It must be noted that the Potier triangle once obtained is constant for a given armature current and hence can be transferred as it is.

PROCEDURE TO DRAW POTIER TRIANGLE Through point A, draw line parallel to PR meeting O.C.C at point B. From B, draw perpendicular on OA to meet it at point C. Triangles OAB and PQR are similar triangles. The perpendicular RS gives the voltage drop due to the armature leakage reactance IX L. The length PS gives field current necessary to overcome demagnetizing effect of armature reaction at full load. The length SQ represents field current required to induce an e.m.f or balancing leakage reactance drop RS.

POTIER TRIANGLE

CALCULATION OF REGULATION Open circuit voltage per phase, E 0 (ph) E 0 (ph) = [(V ph cos Φ+I ph R a ) 2 + (V ph sin Φ ±I ph X Lph ) 2 ] 1/2 in Volts + for lagging, - for leading where, V ph = Rated terminal voltage per phase I ph = Rated short circuit armature current per phase X Lph = Armature leakage reactance per phase I f = [I f1 2 + I f2 2 – 2 I f1 I f2 cos (90 ±Φ)] 1/2 in Amps + for lagging, - for leading where, I f = Field current in Amps I f1 = Field current corresponding to Open circuit voltage per phase E 0 (ph) in Amps I f2 = length of PS % Regulation = (E 0 – V ph ) / V ph × 100 %

% REGULATION Vs POWER FACTOR