A protection circuit for a wind
turbine generator that includes a PWM
Brake that works in conjunction with known
Brake Relays is disclosed. The
Brake Relay is used to short the generator output terminals at a first
threshold voltage. The PWM Brake includes one or more switching devices, coupled across the generator output. The PWM Brake is under the control of a PWM
Brake Control Circuit which actuates the PWM Brake at a second
threshold voltage that is relatively lower than the first
threshold voltage. In accordance with an important aspect of the invention, the PWM
Brake Control Circuit includes a novel speed sensing circuit for providing a
signal representative of the speed of the
turbine generator The novel speed sensing circuit eliminates the need to
mount a speed sensor on the pole top mounted
turbine generator. As such, the need for adding cabling from the pole top mounted wind turbine generator is eliminated. The novel speed sensing circuit provides a
signal representative of the rotational speed of the turbine generator based upon the
duty cycle of a
pulse width modulated (PWM)
signal that is derived from the drive signal developed by the PWM
Control Circuit. This signal is used to alternatively actuate and close the Brake
Relay to minimize actuation of the
centrifugal switch.