MMF method:Voltage Regulation of Alternator By MMF method

Voltage Regulation of Synchronous Generator  By MMF method:-

This method is also known as amp - turns method. In this method the all the emfs produced by rotor and stator are replaced by their equivalent MMFs (fluxes), and hence called mmf method. 

In this method also it is assumed that the magnetic circuit is unsaturated. In this method both the reactance drops are replaced by their equivalent mmfs. Figure shows the complete phasor diagram for the mmf method. Similar to emf method OC and SC characteristics are used for the determination of regulation by mmf method. Using the details it is possible determine the regulation at different power factors.

From the phasor diagram it can be seen that the mmf required to produce the emf E1= ( V + IRa) is FR1.In large machines resistance drop may neglected.
The mmf required to over come the reactance drops is (A+Ax) as shown in phasor diagram. The mmf (A+Ax) can be found from SC characteristic as under SC condition both reactance drops will be present.

Following procedure can be used for determination of regulation by mmf method.
(i) By conducting OC and SC test plot OCC and SCC as shown in figure 2.
(ii) From the OCC find the field current If1 required to produce the voltage, E1= ( V + IRa).
(iii) From SCC find the magnitude of field current If2=( A+Ax) to produce the required armature current. A+Ax can also found from ZPF characteristics.
(iv) Draw If2 at angle (90+Ø ) from If1, where Ø  is the phase angle of current w. r. t voltage. If
current is leading, take the angle of If2 as (90- Ø ) as shown in figure 36.
(v) Determine the resultant field current, If and mark its magnitude on the field current axis.
(vi) From OCC. find the voltage corresponding to If, which will be E0 and hence find the regulation.

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Note : In most of the cases as number of turns on the field winding is not known, the m.m.f. is calculate and expressed i terms of the field current itself.

Because of the assumption of unsaturated magnetic circuit the regulation computed by this method will be less than the actual and hence this method of regulation is called optimistic method.

Another Derivation For MMF Method:-

Theory is same as above

The two components of total field m.m.f. which are FO and FAR are indicated in O.C.C. (open circuit characteristics) and S.C.C. (short circuit characteristics) as shown in the Figure.

Zero lagging p.f. : As long as power factor is zero lagging, the armature reaction is completely demagnetising. Hence the resultant FR is the algebraic sum of the two components FO and FAR. Field m.m.f. is not only required to produce rated terminal voltage but also required to overcome completely demagnetising armature reaction effect.

       This is shown in the Fig

       OA = FO

       AB = FAR   demagnetising

       OB = FR = FO + FAR

       Total field m.m.f. is greater than FO.

Zero leading p.f. : When the power factor is zero leading then the armature reaction is totally magnetizing and helps main flux to induce rated terminal voltage. Hence net field m.m.f. required is less than that required to induce rated voltage normally, as part of its function is done by magnetising armature reaction component. The net field m.m.f. is the algebraic difference between the two components FO and FAR. 

This is shown in the Fig.

OA = FO

       AB = FAR magnetising

       OB = FO - FAR = FR

       Total m.m.f. is less than FO.

Unity p.f. : Under unity power factor condition, the armature reaction is cross magnetizing and its effect is to distort the main flux. Thus and F are at right angles to each other and hence resultant m.m.f. is the vector sum of FO and FAR.

OA = FO 

       AB = FAR cross magnetising

OB=FR=FO=FAR.

Generalized Formula:-

(FR)²= (FO)² + ( FAR)²-2(FO)( FAR)Cos(FO^FAR)

FO^FAR =90-Φ if Φ leading

          =90+Φ if Φ is lagging

Regulation :-

%R=Eph-Vph/Vph *100