Starting shunt generator on no load and voltage build-up

Why a shunt generator is not started on load

A shunt generator builds up from residual magnetism, and its field current is taken from its own terminal voltage ($I_f = V/R_f$).

• If a load (low resistance) is connected at starting, the small residual EMF drives most of its current through the load, and the armature drop $I_aR_a$ becomes large compared with the tiny EMF.
• The terminal voltage applied to the field therefore stays very low, so the field current, and hence the flux, cannot grow.
• In terms of the characteristic: with the load connected, the field gets only part of the armature current, and the effective resistance line becomes $E = I_f\left[R_f + R_a\left(1 + \dfrac{R_f}{R_L}\right)\right]$. For a low load resistance $R_L$ (below the critical load resistance) this line is steeper than the critical-resistance line, so it cuts the OCC only near the residual voltage and the generator fails to build up.

Therefore the load switch is kept open until the generator reaches rated voltage, and only then is the load connected.

Voltage build-up process

   +---------+------o/ o----+
   |         |    switch    |
 ( G )     [Rsh]   (open)  load
   |         |              |
   +---------+--------------+

1. Residual flux: the field poles keep a small residual magnetism. Driving the armature at rated speed induces a small EMF $E_r$ (2-5% of rated).
2. Field current starts: $E_r$ drives a small current $I_{f1} = E_r/R_f$ through the shunt field.
3. Flux increases: if the field winding is connected so that its MMF aids residual flux, flux rises and the EMF rises to $E_1$ (read from the OCC at $I_{f1}$).
4. Cumulative rise: $E_1$ drives a larger $I_{f2}$, giving a higher $E_2$, and so on, step by step.
5. Stable point: the process stops where the OCC meets the field resistance line $V = I_fR_f$. Iron saturation bends the OCC, so the intersection is definite.

  E ^              Rf line
    |             /
    |            /  _____-- OCC
    |           /.-'
    |          /'  <- final voltage
    |       .'/
    |     .' /
    |   .'  /
    | .'   /
 Er +-----/----------------> If

Conditions for build-up: residual magnetism present, field MMF aiding residual flux, field resistance below critical resistance, speed above critical speed, and no (or light) load at starting.