Danfoss VLT 3032 Service Manual page 28

TESTING AT THE
GATE CARD OUTPUT
IGBT GATE DRIVER SIGNAL
AT GATE CARD OUTPUT
10V/DIV 100uSEC/DIV
CAUTION: Allow sufficient time for the DC Bus to fully discharge
before beginning testing. The presence of bus voltage can be
tested by setting your voltmeter for 500VDC and reading the
terminals labeled 88 (–) and 89 (+).
Never power up the unit with any of the IGBT gate leads
disconnected from the Gate Drive Card. A disconnected gate lead
will cause the IGBT to switch on and result in its failure.
Testing the gate drive firing pulses at the output of the gate card insures
the signal has successfully made it to that point removing suspicion
from the interface board and the gate card. Testing in this area also
requires a greater deal of caution since the working space is tighter
and high voltages are in close proximity.
Unlike similar models of the VLT, the control card/interface board carrier
does not tilt down. It must be completely removed in order to gain access
to the Gate Drive Card. As this is the case, testing at the gate driver
requires that the test leads are connected and then the unit is
reassembled to conduct the tests. Extreme care must be taken to insure
test leads can not come in contact with other points of the card when
reinstalling the control card/interface board carrier.
If any of the readings as observed below appear
incorrect, replace the gate card. Prior to doing so,
insure the signals have also been tested at the
interface board to verify they are correct. If
replacement of the gate card does not correct the
problem, the IGBT(s) in that phase may be the cause.
Use an oscilloscope or an analog or digital DC
voltmeter set for 10VDC. Connect the negative (–)
meter lead or scope ground to pin 2 of connector MK2
of the gate card. Connect the positive (+) meter lead
or scope probe to pin 1 of connector MK2. Remember,
with the voltmeter the running speed of the VLT must
be above 10Hz. A voltmeter will read an average
voltage of approximately 2.2VDC and the scope will
exhibit a waveform similar to that shown on the left.
Repeat the test on the remaining connectors MK3,
MK5, MK6, MK8 and MK9.
29