APU Speed Control

The speed control for each auxiliary power unit provides positive speed control, starting and stopping, and overspeed protection by regulating the flow of hydrogen peroxide to the decomposition chamber.

The key component of each speed control is a controller which contains the necessary circuits for sensing unit operation through the frequency output of a tachometer generator. During normal operation, the frequency of the power generated by the tachometer generator, being proportional to the turbine speed, matches a preset frequency of the controller. Any change in turbine speed due to a change in load, or from any other disturbance, causes a proportional change in frequency of the tachometer generator. This frequency change is sensed by the controller, which in turn adjusts the opening of the flow control valve to bring turbine speed back to the normal operating level.

During normal operation, the speed of the unit is automatically controlled to maintain 51,200 rpm by the speed-sensing element of the controller. Should an overspeed condition occur (56,000 rpm or greater), the overspeed sensing element of the controller automatically acts to energize the solenoid-operated APU shutoff valve to the closed position, thereby shutting off the flow of the propellant. The unit then decelerates and stops. It cannot be restarted until the APU switch is first cycled to the OFF position.

In this section:

APU Speed Control Switches and Lights