Alternator on Load
Alternator on Load:
Fig. (1) shows Y-connected alternator supplying inductive load (lagging p.f.). When the load on the alternator is increased (i.e., armature current Ia is increased), the field excitation and speed being kept constant, the terminal voltage V (phase value) of the alternator decreases. This is due to
(i) Voltage drop IaRa where Ra is the armature resistance per phase.
(ii) Voltage drop IaXL where XL is the armature leakage reactance per phase.
(iii) Voltage drop because of armature reaction.
(i) Armature Resistance (Ra)
Since the armature or stator winding has some resistance, there will be an IaRa drop when current (Ia) flows through it. The armature resistance per phase is generally small so that IaRa drop is negligible for all practical purposes.
(ii) Armature Leakage Reactance (XL)
When current flows through the armature winding, flux is set up and a part of it does not cross the air-gap and links the coil sides as shown in Fig. (2). This leakage flux alternates with current and gives the winding self-inductance. This is called armature leakage reactance. Therefore, there will be IaXL drop which is also effective in reducing the terminal voltage.
(iii) Armature reaction
The load is generally inductive and the effect of armature reaction is to reduce the generated voltage. Since armature reaction results in a voltage effect in a circuit caused by the change in flux produced by current in the same circuit, its effect is of the nature of an inductive reactance. Therefore, armature reaction effect is accounted for by assuming the presence of a fictitious reactance XAR in the armature winding. The quantity XAR is called reactance of armature reaction. The value of XAR is such that IaXAR represents the voltage drop due to armature reaction.
Fig. (3) shows the equivalent circuit of the loaded alternator for one phase. All the quantities are per phase. Here
E0 = No-load e.m.f.
E = Load induced e.m.f. It is the induced e.m.f. after allowing for armature reaction. It is equal to phasor difference of E0 and IaXAR.
V = Terminal voltage. It is less than E by voltage drops in XL and Ra.
Therefore, E = V Ia (Ra j XL )
and E0 = E Ia ( jXAR )