Danfoss VLT Series Service Manual page 24

Section Three
STATIC TEST PROCEDURES
TESTING THE HEATSINK
THERMAL SWITCH
DYNAMIC TEST
PROCEDURES
TESTING FOR OUTPUT
PHASE VOLTAGE
IMBALANCE
3 - 6
There are thermal switches included on all units covered by this manual. The
smaller drives monitor the ambient temperature within the unit. The thermal switch
is mounted on the ILD Card. All units with Interface Cards have a stand alone
thermal switch mounted on the heatsink.
VLT 3006-3022, 230V
VLT 3011-3052, 380V/460V
The heatsink temperature is sensed by a thermal switch. The thermal switch
harness is connected to the top of the Interface Card at connector MK401. The
switch will open when the heatsink temperature exceeds 100°C/212°F and will
close when the heatsink temperature falls below 50°C/122°F. By unplugging the
connector from the Interface Card, the thermal switch continuity can be checked.
When testing phase imbalances, it is practical to measure both voltage and current.
A balanced voltage reading, but unbalanced current, indicates the motor is drawing
uneven current. This could be caused by a fault in the motor windings or in the
wiring connections between the drive and motor. When both voltage and current
are unbalanced, it indicates a switching problem or a faulty connection within the
unit itself. This can be caused by improper gate drive signals as a result of a faulty
interface board. A faulty IGBT or loose wire connection between the IGBT and
the output terminals may also be the cause.
NOTE: When monitoring output voltage an analog voltmeter should be used.
Digital meters are sensitive to the switching frequency and usually read erroneously.
1. Remove the motor leads from the output terminals of the unit.
2. Conduct the Inverter Test Procedure in Section Three.
3. If the Inverter Test Procedure proves good, power the unit back up. Initiate a
Run command with a speed reference greater than 40Hz.
4. Read the phase-to-phase output voltage. The actual value of the readings
is of less importance than the phase-to-phase balance. This balance should
be within 8 volts per phase.
5. If a greater-than-8-volt imbalance exists, measure the gate drive firing signals.
6. If the phase-to-phase output voltage is balanced, recheck motor and
connections for faults. Consult the factory for additional assistance.
VLT 3508-3532, 230V
VLT 3516-3562, 380V/460V