Alternator on Load

ALTERNATOR ON LOAD

As the load on an alternator is varied, its terminal voltage is also found to vary as in DC generators. This variation in terminal voltage V is due to the following three reasons:

1. Voltage drop due to armature resistance (R_a)

2. Voltage drop due to armature leakage reactance (X_L)

3. Voltage drop due to armature reaction.

1. Armature Resistance (R_a)

The armature resistance / phase R_a causes a voltage drop/phase of IR_a which is in phase with the armature current I. However, this voltage drop is practically negligible.

2. Armature leakage reactance (X_L)

When current flows through the armature conductors, fluxes are setup which do not cross the air‒gap, but take different paths. Such fluxes are known as leakage fluxes.

The leakage flux is practically independent phase of saturation, but is dependent on I and its phase angle with terminal voltage V. This leakage flux sets up an emf of self‒inductance which is known as reactance emf and which is ahead of I by 90°. Hence, armature winding is assumed to posses leakage reactance X_L, such that voltage drop to this equals IX_L.

Therefore, E = V + I[R_a + jX_L]

This is shown in Fig. 4.6.

3. Armature Reaction

The voltage drop due to armature reaction is accounted for by assuming a fictitious reactance X_a in the armature winding. The phasor sum of X_L and X_a gives “synchronous reactance" (X_S). Hence X_S = X_L + X_a

As in DC generators, armature reaction is the effect of armature flux on the main field flux. In case of alternators, the powerfactor of the load has a considerable effect on the armature reaction.

(i) Unity p.f load

(ii) Lagging p.f load

(iii) Leading p.f load.

We will discuss about three cases of power factor.

(i) Unity p.f load

Fig. 4.7 shows the phasor diagram of an alternator for unity p.f. load. Here terminal voltage V is taken as the reference phasor. The current phasor I_a is in phase with terminal voltage V. The voltage drop I_aR_a is in phase with I_a while the voltage drop I_aX_s leads I_a by 90°. The vector sum of two voltage drops gives I_aZ_s. The vector sum of terminal voltage V and I_aZ_s gives E.

(ii) Lagging p.f load

Fig. 4.8 shows the phasor diagram of an alternator for lagging p.f load. Here terminal voltage V is taken as the reference phasor.

The current I_a is lagging behind the voltage by ϕ. The I_aR_a drops is inphase with I_a while the drop I_aX_s leads I_a by 90°, The vector sum of I_aR_a and I_aX_s gives I_aZ_s. Then the vector sum of terminal voltage V and I_aZ_S gives E.

(iii) Leading p.f load

Fig. 4.8 (b) shows the phasor diagram of an alternator for leading p.f load. Here again V is taken as the reference phasor. The current I_a leads V by ϕ. The I_aR_a drop is inphase with I_a while the drop I_aX_s leads I_a by 90°. The vector sum of I_aR_a and I_aX_s gives I_aZ_s Then the vector sum of terminal voltage V and I_aZ_s gives E.