BASLER ELECTRIC EXCITER DIODE MONITOR EDM-200

BASLER ELECTRIC EXCITER DIODE MONITOR EDM-200

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.

CONTROLS AND INDICATORS

EDM-200 controls and indicators are illustrated in

Figure and described in Table 1.

Table 1. Control and Indicator Descriptions

Locator

Description

Threshold Control. During calibration and a

no-load condition, this potentiometer is

rotated clockwise until the Threshold segment

of the bar graph display (locator C) just lights.

This sets the threshold to the automatic gain

control amplifier output level.

Null Control. This potentiometer is used to

null out the voltage regulator ripple

component of the field current.

Bar Graph Display. This ten-segment LED

displays the ripple level of the exciter diode

current.

Gain Control. This potentiometer adjusts the

threshold for indicating an alarm and

activating the output relay.

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.

Publication

9177200990

Revision

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.