Voltage build-up in isolated induction generator

An isolated (self-excited) induction generator is an induction machine driven above synchronous speed with a 3-phase capacitor bank across its stator terminals and no connection to the grid. The capacitors supply the reactive (magnetizing) power, and the terminal voltage builds up by itself from residual magnetism.

 Prime mover   +---------+
 (N > Ns) =====| Ind.    |===+====[S]===> Load
               | machine |   |
               +---------+  === C per phase

Requirements for build-up

1. Residual magnetism in the rotor core.
2. Rotor driven at a suitable speed (near or above $N_s$).
3. Sufficient capacitance, so that the capacitor line is less steep than the air-gap (linear) part of the magnetization curve.
4. Load disconnected (or light) during build-up.

Build-up process (step by step)

1. The prime mover drives the rotor. Residual flux in the rotor induces a small emf $V_r$ in the stator winding.
2. This small voltage drives a current $I_c = V_r\,\omega C$ through the capacitors. This current leads the voltage by 90°, which means it acts as a lagging magnetizing current for the machine.
3. The magnetizing current strengthens the air-gap flux, so the induced emf rises.
4. The higher emf drives a larger capacitor current, which raises the flux and emf further.
5. This cumulative action continues until the voltage reaches the point where the magnetization curve of the machine crosses the capacitor line $V = I_c X_c$. Beyond this point, saturation makes the machine need more current than the capacitors can provide, so the voltage stabilizes.

 V |                 capacitor line
   |                / V = Ic.Xc
   |        __.----*-- magnetization
   |     .-'     / |    curve
   |   .'      /   |
   |  /      /     |  * = operating
   | /     /       |      point
   |/    /         |
 Vr+  /            |
   |/______________|_____ I (magnetizing)

Factors affecting the voltage

• Capacitance: larger C gives a flatter capacitor line and higher voltage. Below a critical capacitance (line steeper than the air-gap line) the voltage does not build up.
• Speed: higher speed raises the magnetization curve (emf ∝ speed) and frequency, so voltage rises.
• Load: a lagging load takes part of the capacitor VAr, and resistive load demagnetizes the machine; voltage and frequency fall. Too heavy a load collapses the voltage, which gives inherent short-circuit protection.

Loss of excitation

If the residual magnetism is lost, it can be restored by briefly connecting a battery (DC) across a stator phase or running the machine as a motor from a supply for a moment.

Self-excited induction generators are widely used in micro-hydro and small wind plants in remote areas because they are cheap, rugged and need little maintenance; an electronic load controller keeps voltage and frequency steady.