Voltage build-up in induction generator

Voltage build-up in a self-excited induction generator is a regenerative process that starts from the residual magnetism of the rotor iron and is supported by a capacitor bank across the stator terminals.

Process

1. The rotor is driven above synchronous speed by a prime mover. The small residual flux cuts the stator conductors and induces a small emf $E_r$ (a few volts) at the terminals.
2. This emf drives a leading current $I_c = \frac{E_r}{X_c}$ through the capacitors.
3. The capacitor current flows in the stator winding as a magnetising current and increases the air-gap flux in the same direction as the residual flux.
4. The increased flux raises the terminal emf, which raises the capacitor current, which increases the flux further. The voltage thus builds up in steps.
5. The build-up stops at the point where the capacitor line ($V = I_c X_c$) intersects the magnetisation (saturation) curve. Beyond this point, saturation of the iron stops the rise of emf. This is the stable no-load voltage.

 V
 |           /  capacitor line
 |       _.-*   <- stable point
 |    _-' /
 |  .'  /
 | / ->/
 |/ /  Er
 +------------------> Im

Conditions for build-up

• Residual magnetism must exist in the rotor iron (if lost, flash the field with a battery).
• Capacitance must be greater than the critical value, so the capacitor line is flatter than the linear part of the magnetisation curve.
• Speed must be high enough, because the emf is proportional to speed.
• Load must not be too heavy, otherwise the voltage collapses.

With a higher capacitance the line is flatter and the final voltage is higher. A larger load needs a larger capacitance to hold the voltage.