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
A
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.
B
Null Control. This potentiometer is used to
null out the voltage regulator ripple
component of the field current.
C
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
EDM-200
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