ASA Modification of M.M.F. Method

ASA Modification of M.M.F. Method

We have seen that neither of the two methods, M.M.F. method and E.M.F. method is capable of giving the reliable values of the voltage regulation. The error in the results of these methods is mainly due to the two reasons,

1. In these methods, the magnetic circuit is assumed to be unsaturated. This assumption is unrealistic as in practice. It is not possible to have completely unsaturated magnetic circuit.

2. In salient pole alternators, it is not correct to combine field ampere turns and armature ampere turns. This is because the field winding is always concentrated on a pole core while the armature winding is always distributed. Similarly the field and armature M.M.F.S act on magnetic circuits having different reluctances in case of salient pole machine hence phasor combination of field and armature M.M.F. is not fully justified.

Inspite of these short comings, due to the simplicity of constructions the ASA modified form of M.M.F. method is very commonly used for the calculation of voltage regulation.

Consider the phasor diagram according to the M.M.F. method as shown in the Fig. 2.16.1 for cos lagging p.f. load.

The FR is resultant excitation of F_O and F_AR where F_O is excitation required to produce rated terminal voltage on open circuit while F_AR is M.M.F. required for balancing armature reaction effect.

Thus  OB = FR = Resultant M.M.F.

The angle between F_AR and perpendicular to F_O ϕ is , where cos ϕ is power factor of the load.

But OB = F_R = resultant is based on the assumption of unsaturated magnetic circuit which is not true in practice. Actually M.M.F. equal to BB' is additionally required to take into account the effect of partially saturated magnetic field. Thus the total excitation required is OB' rather than OB. 

Let us see method of determining the additional excitation needed to take into account effect of partially saturated magnetic circuit.

Construct the no load saturation characteristics i.e. O.C.C. and zero power factor characteristics. Draw the potier triangle as discussed earlier and determine the leakage reactance X_L for the alternator. The excitation necessary to balance armature reaction can also be obtained from the Potier triangle. The armature resistance is known.

Now 

Construct ASA diagram, and draw phasor diagram related to the above equation.

The ASA diagram has x-axis as field current and y-axis as the open circuit voltage. Draw O.C.C. on the ASA diagram. Then asstuning x-axis as current phasor, draw V_ph at angle ϕ , above the horizontal. The V_ph is the rated terminal voltage. Add I_aR_a in phase with I_a i.e. horizontal and I_aX_L perpendicular to I_aR_a to V_ph. This gives the voltage E_lph.

Now with O as a centre and radius E_lph draw an arc which will intersect y-axis at E₁. From E₁  draw horizontal line intersecting both air gap line and O.C.C. These points of intersection are say B and B'. The distance between the points BB' corresponding to the field current scale gives the additional excitation required to take into account effect of partially saturated field. Adding this to FR we get the total excitation as FR. From this FR, the open circuit voltage Eph can be determined from O.C.C. using which the regulation can be determined. The ASA diagram is shown in the Fig. 2.16.2.

The results obtained by ASA method are reliable for both salient as well as nonsalient pole machines.

Review Question

1. Explain the ASA method of determine the regulation of an alternator.