Variable Slip

From Wind wiki

Variable slip represents a compromise between fixed- and variable-speed operation. The variable-slip generator is essentially an induction generator with a variable resistor in series with the rotor circuit, controlled by a high-frequency semiconductor switch. Below the rated wind speed, it acts just like a conventional fixed-speed induction generator. Above rated, however, control of the resistance effectively allows the air-gap torque to be controlled and slip speed to vary, so the behavior is then similar to a variable-speed system. A speed range of about ten percent is typical.

This is cheaper than a variable-speed system, and gives some of the advantages, in particular the control of torque in the drive train and the smoothing of aerodynamic torque variations above the rated wind speed. It does not offer increased aerodynamic efficiency below rated (although it does not suffer from frequency converter losses), and it does not allow any control of the power factor. Electrical flicker will, however, be reduced above the rated wind speed.

Slip rings can be avoided by mounting the variable resistors and control circuitry on the generator rotor. An advantage of mounting these externally via slip rings is that it is then easier to dissipate the extra heat which is generated above the rated wind speed, and which may otherwise be a limiting factor at large sizes.