Effect of Change in Excitation

Effect of Change in Excitation

In case of alternators a field rheostat may be used to change the excitation or its field current. If alternators are running in parallel, a change in the field current will not change the active power shared significantly but will change the operating power factor. With change in the excitation the armature current will change which will change the active power by a small amount. Let us consider the effect of change in excitation on alternator with and without load.

1. Alternator on No Load

Consider two alternators on no load and working in parallel. If their excitations are adjusted properly then the e.m.f.s E₁, and E₂ will be equal. Thus there will not be any current in local circuit.

Now say excitation of alternator 1 is increased then magnitude of E₁ will be more than that of E₂. This will cause resultant voltage  that will appear in the local circuit. This can be shown in the phasor diagram shown in the Fig. 3.14.1.

This resulting voltage will set up a synchronizing current I_SY in the local circuit and since the synchronizing impedances are mainly reactive, this current lags E_r by approximately 90°.

For alternator 1, I _SY lags behind E1 by 90°. This lagging current will produce demagnetizing effect and will try to reduce the generated e.m.f. Alternately for other alternator, I _SY leads E₂ by 90°. There will be leading current which will produce magnetizing effect and the field will be strengthened which will try to increase the generated e.m.f. Thus E₁ will be reduced whereas E₂ will be increased. Hence the circulating current will try to make the two generated e.m.f.s equal at no load whereas the power angle δ will remain at zero degrees.

From the Fig. 3.14.2.