The zero power factor or Potier method is based on the separation of armature leakage reactance drop and the armature reaction effects. The experimental data required are ( i ) no load curve ( ii ) full load Zero power factor curve (not short circuit characteristics) which is also called wattles load characteristic. It is the curve of terminal volts against excitation when armature is delivering full load current at zero powerfactor. The reduction in voltage due to armature reaction is found from above and (iii) voltage drop due to armature leakage reactance XL ( also called as Potier reactance) is found from both . By combining these two, E0 can be calculated. It should he noted that if we vectorically add to V the drop due to resistance and leakage reactance XL. We get E. If to E is further added the drop due to armature reaction (assuming lagging power factor) then we get Eo.
This method is a modification of MMF and potier methods. The field current corresponding to the rated voltage V is found out by referring to the air gap line. So that the field current required to circulate the load current under short circuit conditions is added at an angle of ( 90 + ) or ( 90 - ) to get the resultant field current. This resultant field current is increased by a small amount which takes into account its saturation. For this total field current value, corresponding no load voltage ( Eo) is noted & the regulation of alternator for the particular power factor can be calculated & This method is reliable for both salient and non-salient pole machines.
FULL LOAD ZERO POWER FACTOR TEST:
The connections are given as shown in the circuit. The DC motor is started and the speed is adjusted to rated speed of the alternator. The terminal voltage is brought to its rated voltage by adjusting field rheostat of alternator. Then the load side TPST switch is closed. The inductive load is gradually increased up to rated stator current of alternator and wattmeter should be reads zero. Corresponding meters are noted down.
i) To find the field current required overcoming the saturation effects
1) O.C.C curve and the air gap line are drawn.
2)Rated voltage (V) is plotted at the power factor angle of with the X - axis. A scale of voltage scale of OCC may be used.
3) IRa drop line is drawn from V (which is parallel to X – axis)
4) IXI, drop line is to IRa. (Taken from Potier triangle)
5) Eg line is drawn (join IXI, to the origin).
6) Project V & Eg by arcs to the vertical axsis.
7) These points are projected horizontally to cut the OCC curve.
8) Measure the difference between the air gap line & O.C.C corresponding to projected Eg which gives IFs, to overcome the saturation effects.
ii) To find the field current corresponding to Eg
1) Measure Ifn ( upto air gap line ) corresponding to rated voltage.
2) Add voctorizally or using the formula (shown below) the Ifsc, (required to
Circulate load current under short circuit condition) at the required power factor to Ifn which is designated as Ifg.
i) Ifg = (Ifn 2 + If sc2 + 2 Ifn Ifsc Cos ( 180 - (90 +))
ii) Circulation of voltage regulation
1) Ifo = Ifs + lfg
2) Obtain the value or E0 for Ifo from OCC
3) Percentage regulation is calculated.