Danfoss VLT Series Service Manual page 26

Section Three
TESTING GATE DRIVE
FIRING CIRCUITS
3 - 8
CAUTION: The gate firing signals are referenced to the negative DC
Bus and are therefore at Bus potential. Extreme care must be taken
to prevent personal injury or damage to equipment. Oscilloscopes,
when used, should be equipped with isolation devices.
The individual gate drive firing pulses originate on the Interface/ILD Card. These
signals are then distributed to the individual IGBT's. An oscilloscope is the
instrument of choice when observing waveforms; however, when a scope is not
available, a simple test can be made with a DC voltmeter. When using a voltmeter,
compare the gate pulse voltage readings between phases. A missing gate pulse
or an incorrect gate pulse have a different average voltage when compared with
the other pulse outputs. At very low frequencies (below 10Hz) the voltmeter
reading will tend to bounce around as the pulses rise and fall. Above 10Hz the
reading will stabilize. When using an oscilloscope, the test points remain the same,
as shown. These tests must be made with the motor disconnected. The internal
impedance of a meter or scope can induce problems to the IGBTs.
1. With power off, remove and re-install the Control Card as shown to allow
easy access to the Interface/ILD Card.*
2. Measure the resistance at each of the six test points. Each test point should
read approximately 2.2kΩ.
3. Apply power and run the unit up to 20Hz. Measure each of the six IGBT gate
pulse signals.
4. If gate pulses are missing or the readings are inconsistent, remove power,
remove the three IGBT gate wire harnesses from the Interface/ILD Card
and measure the gate pulse signals directly at the Interface/ILD Card
Connectors.
There may be a slight DC shift in voltage readings between the positive and negative
half IGBT gate signals.