Voltage build-up; series generator at no load

Voltage build-up in a DC shunt generator

The shunt field is across the armature, so the generator excites itself from residual magnetism.

1. When the armature rotates, the residual flux induces a small emf $E_r$ (1–3 V).
2. This emf drives a small current $I_f = E_r/(R_f + R_a)$ through the shunt field.
3. If the field connections are such that this current strengthens the residual flux, the flux rises, which raises the emf, which raises $I_f$ further.
4. The process continues until the field-resistance line ($V = I_fR_f$) meets the open-circuit characteristic. The voltage at this point is the no-load voltage.

 E, V
  |           ____---- OCC
  |        .-'    /  V = If.Rf
  |     .-'     /
  |   .'      /
  | Er.-----/
  +--------------------- If

Conditions: residual magnetism present, correct field polarity, field resistance below the critical value, speed above the critical speed.

Why a series generator is not started at no load

In a series generator the field winding is in series with the armature and the load, so the field current equals the load current.

• At no load the circuit is open, so $I_a = I_f = 0$.
• With no field current there is only residual flux, so the generated emf is just the small residual emf (a few volts). The machine cannot build up its voltage.
• Voltage appears only after a load is connected, and it then rises with the load current (until saturation).

So the series generator must be started with a load (a closed circuit) connected to get its normal voltage.