Potier method of predetermining the voltage regulation of an alternator.

     ZERO POWER FACTOR METHOD (OR) POTIER METHOD :

  •     This method is based on the separation of armature leakage reactance drop and the armature reaction effect.
  •     This is more accurate than the emf and mmf methods. The experimental data required is
                             No load curve (or) O.C.C
                              S.C.C
                             Armature resistance
  •     Full load zero power factor curve (or) wattles load characteristics.

  •     The ZPF lagging characteristics is a reaction between terminal voltage and excitation when armature is delivering F.L. current at zero power factor.
  •     The reduction in voltage due to armature reaction is found from above and (ii) voltage drop due to armature leakage reactance XL   (also called potier reactance) is found from both. By combining these two, EO can be calculated.
  •     It should be noted that if we vectorially add to V the drop due to resistance and leakage reactance XL, we get E.
  •     If E is further added the drop due to armature reaction (assuming lagging power factor), then we get Eo.
  •     The zero power factor lagging curve can be obtained If a similar machine is available which may driven at no-load as a synchronous motor at practically zero power factor (or) By loading the alternator with pure reactors.
  •     By connecting the alternator to a 3Ф line with ammeter and wattmeters connected for measuring current and power and by so adjusting the field current that we get full-load armature current with zero wattmeter reading.
  •     Point B was obtained in this manner when wattmeter was zero.
  •     Point A is obtained from a short circuit test with full load armature current. Hence OA represents field current which is equal and opposite to the demagnetizing armature reaction and for balancing leakage drop at full load.
  •     Knowing these two points, full load zero power factor curve can be drawn as under.
  •     From B, BH is drawn equal to and parallel to OA.
  •     From H, HD is drawn parallel to initial straight part of N-L curve i.e., parallel to OC which is tangential to N-L curve.
  •     Hence, we get point D on no-load curve, which corresponds to point B on full-load zero power factor curve.
  •     The triangle BHD is known as potier triangle.
  •     This triangle is constant for a given armature current and hence can be transferred to give use other points like M,L etc.
  •     Draw DE perpendicular to BH.
  •     The length DE represents the drop in voltage due to armature leakage reactance XL i.e.,
  •     BE gives the field current necessary to overcome demagnetizing effect of armature reaction at full load and EH for balancing the armature drop DE.
  •     Let V be the terminal voltage on full load, then if we add to it vertically the voltage drop due to armature leakage reactance alone ( neglecting Ra ), then we get voltage E= DF ( and not Eo).
  •     Field excitation corresponding to E is given by OF.
  •     Add NA vectorially to OF (as in Rothert’s A.T method) we get excitation for Eo whose value can be read from N-L curve.
  •     FG (=NA) is drawn at an angle of (90°+Ф) for lagging.
  •     The voltage corresponding to this excitation is JK = Eo

                     % regulation=Eo-V
                                            V
  •     Assuming a lagging power factor with angle Ф, vector for I is drawn at an angle of Ф to V.
  •     IaRa is drawn parallel to current vector and IXL is drawn perpendicular to it.
  •     OD represents voltage E.
  •     The excitation corresponding to it i.e., OF is drawn at 90° ahead of it  FG (=Na=BE) representing field current equivalent of full load armature reaction is drawn parallel to current vector OI.
  •     The closing side OG gives field excitation for Eo.
  •     Vector for Eo is 90° lagging behind OG.
  •     DC represents voltage drop due to armature reaction.

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