To determine the regulation of an Alternator at full load and  at various power factor by EMF and MMF method.


                                The voltage regulation of all alternator is defined as the" rise in voltage when full load is removed (field excitation and speed remaining the same) divided by the rated terminal voltage”.

                                %Regulation = (Eo –V) /  V * 100
                                Eo - No load induced emf or open circuit voltage per phase
                                V - Rated terminal voltage per phase.
                             The no load induced emf can be, found by,
                                1. Synchronous impedance or EMF method
                                2. Ampere turn method
                                3. Zero power or Potier method
                                4. ASA method

All these method require the following,
                                1. Armature or stator a. c resistance Rac,
                                2. Open circuit or no load characteristics
                                3. Short circuit characteristics
                                4. Zero power factor lagging characteristics for Potier method

.                                In this method, the Armature reaction is treated as another armature    leakage reactance. The synchronous impedance ZS , found is always more than its  value under normal voltage conditions and saturation. Hence, the value or regulation so obtained is always more, than that found from an actual test. That is why it is called pessimistic method.


1. Synchronous Impedance, Zs = (E1 / I1). (From Graph)
2. Synchronous Reactance, Xs  = (Zs2-Ra2)
3. Open Circuit Voltage (Eo)
  (i) For leading power factor
    Eo = (Vph Cos + IRac) 2 + (Vph Sin _ IaXs) 2
  (ii) For lagging power factor
          Eo = (Vph Cos + IRac) 2 + (Vph Sin+IaXs) 2
   (iii) For unity power factor:
          Eo = (Vph Cos + IRac) 2 + ( IaXs) 2
    4. Percentage Regulation = (Eo – Vph) / Vph*100

Stator resistance method:

Rdc=Measured value/2
Ra(ph)=Rac=1.6 Rdc


Open circuit test:

•    Connections are given as per the circuit diagram.
•    The motor field rheostat should be kept in minimum resistance position
•    The D.C Motor is started and its speed is adjusted to the synchronous speed of the alternator.
•    The field rheostat (Alternator) is varied and for various values of field current, the corresponding values of open circuit voltage are noted down.
•    The open circuit characteristic is drawn. (If Vs Eo)

Short circuit test:

•    The alternator is made to run at synchronous speed.
•    The Armature terminals of the alternator are shorted by an Ammeter.
•    The field rheostat (Alternator) is varied up to rated current (short circuit current) is circulating in short circuited stator winding corresponding meters reading is noted down.
•    The Short circuit characteristic is drawn. (If Vs Isc).


    The Regulation of an alternator at full load ant at various power factors by EMF and MMF method was determined and the characteristics curves are drawn.

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