THEORY OF OPERATION

Rectifier diodes mounted on the rotor of a brushless generator or motor

fail by either opening or shorting. If a rectifier shorts, very high current

flows through the associated exciter armature winding and causes

excessive heating and probable failure of the exciter.

If a rectifier opens, the voltage regulator will substantially increase the

excitation to maintain the operating level. This constant, high level of

excitation could lead to failure of the regulator.

The EDM-200 determines the status of the rotating rectifiers by sensing

the ripple content of the exciter field current. The waveform shown in

Figure 2A is the normal ripple measured with all diodes functional.

Figure 2B illustrates a waveform where one diode is open. Figure 2C

illustrates a waveform where one diode is shorted.

Automatic Gain Controlled Amplifier

As the output of the input amplifier varies from 1 to 4 volts, the associated

ripple content also varies. To prevent the monitoring circuits from

interpreting the 1 to 4 volt change as a diode failure, the automatic gain

controlled amplifier offsets the signal variations so that the monitoring

circuit monitors the ripple in a constant amplitude signal.