Factors affecting alternator regulation

Voltage regulation of an alternator is the change in terminal voltage from no load to full load (speed and field current constant), expressed as a percentage of the full-load voltage:

$$\%\text{Reg} = \frac{E_0 - V}{V}\times100$$

where $E_0 = \sqrt{(V\cos\phi + IR_a)^2 + (V\sin\phi \pm IX_s)^2}$. The factors that affect it are:

1. Armature resistance drop ($IR_a$)

The resistance of the armature winding causes a drop in phase with the current. It is small in large machines but increases with temperature.

2. Leakage reactance drop ($IX_l$)

Part of the armature flux links only the armature conductors (slot, end-winding and tooth-tip leakage). It causes a reactive drop $IX_l$.

3. Armature reaction ($IX_a$)

The armature mmf changes the main flux: cross-magnetising at unity pf, demagnetising at lagging pf, magnetising at leading pf. It is represented by $X_a$; usually it is the biggest factor. $X_s = X_l + X_a$.

4. Power factor of the load

• Lagging pf: large positive regulation (voltage falls).
• Unity pf: small positive regulation.
• Leading pf: low or negative regulation (voltage rises).

5. Magnitude of load current

All drops are proportional to the current; regulation rises with load.

6. Saturation and field excitation

With saturation, the actual change in voltage is less than predicted by unsaturated $X_s$; this is why the EMF method gives pessimistic (higher) and the MMF method optimistic (lower) values, while ZPF (Potier) gives accurate values.

7. Speed and frequency

A change in prime-mover speed changes $E = 4.44fk_w\Phi T$ and $X_s = 2\pi fL$; constant speed is assumed.