Basler Electric EDM-200 Exciter Diode Monitor

INTRODUCTION

The EDM-200 Exciter Diode Monitor connects to the exciter field circuit

and monitors the output of the brushless exciter power semiconductors.

The EDM-200 can detect the failure of a power rectifier in the brushless

exciter which fails in either the open or shorted mode.

A set of Form C contacts provides alarm annunciation. A bar graph display

enables easy onsite calibration and monitoring of EDM-200 operation.

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.

FUNCTIONAL DESCRIPTION

EDM-200 functional description is illustrated in Figure 3 and described

in the following paragraphs.

Input Amplifier

Resistor R is connected in series with the negative lead to the exciter

field. The small voltage drop developed across R is amplified by the

input amplifier to approximately 1 volt when the generator is unloaded.

During various load conditions, the amplifier output will increase from 1

volt to about 4 volts.

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.

Variable Bandpass Ripple Amplifier

When all of the rotating diodes are operating normally, the field current

ripple is of small amplitude and high frequency. When a diode is faulty,

the ripple is of large amplitude and low frequency (one-third or one-sixth

of normal). Thus, the ripple amplifier has a high gain at low frequencies

and a low gain at high frequencies.

The amplifier provides adjustment of the high frequency gain to provide

compatibility with a wide variety of brushless generators.

Precision Rectifier

This rectifier converts the ac output of the ripple amplifier to a dc signal.

Inverse Timer

The dc output of the precision rectifier passes through an inverse timing

network to provide prompt tripping for a shorted diode but allow longer

delays for an open diode.

Relay Driver

The relay driver consists of a transistor that controls the relay (K1) coil

voltage.

Bar Graph Display

This ten-segment indicator graphically displays the level of ripple and is

used during EDM-200 calibration and for monitoring EDM-200 operation.

AVR Ripple Amplifier

Since the AVR output consists of pulses, a strong ripple component

consisting of these pulses is present in the field current and makes the

detection of an open diode difficult. To eliminate the AVR ripple, the

AVR voltage at terminal F+ is passed through a filter, a null adjustment,

and the AVR ripple amplifier. The amplifier output is then used by the

input amplifier to cancel out the AVR ripple component in the field current.

Power Supply

EDM-200 circuitry operates from a regulated +14 Vdc source that is provided

by the transformer-rectifier type power supply. The relay output will be

inhibited and the bar graph display may indicate incorrect information until

the input voltage is at least 50% of nominal.

MOUNTING

The EDM-200 should be mounted in a position that