Zero Dc Bus Voltage Test; Dc Undervoltage Test - Danfoss VLT HVAC Drive FC 100 Service Manual

6 Test Procedures

6.3.5 Zero DC Bus Voltage Test

If no voltage is present at power card connector MK105 (A) and (B), check the condition of the
DC power supply fuse. The fuse is located on the power card mounting plate next to the power
card. It can be tested without disassembling the unit.
1.
.
6
If the fuse is open, it indicates a failure of the power supplies on the power card. The power card
and fuse require replacement. If the fuse checks good, there may be a problem with the soft
charge circuitry. Proceed to the static checks of the soft charge and rectifier circuits earlier in this
section.

6.3.6 DC Undervoltage Test

The initial charge of the DC bus is accomplished by the soft charge circuit. If the DC bus voltage
is below normal it would indicate that either the AC line voltage is out of tolerance or the soft
charge circuit is restricting the DC bus from charging. Conduct the input voltage test (6.3.2) to
ensure the AC line voltage is correct.
If excessive input power cycling has occurred, the PTC resistors on the soft charge card may be
restricting the bus from charging. If this is the case, expect to read a DC bus voltage in the area
of 50 VDC.
1.
2.
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Remove power to the adjustable frequency drive and make sure the DC bus is fully dis-
charged by measuring the voltage at the power card connector MK105 (A) with respect
to MK105 (B).
If the DC power supply fuse is open (blown), it is not possible to detect the presence
of bus voltage at these terminals. If uncertain, wait 20 minutes for D-frame sizes or
40 minutes for E-frame sizes to allow the DC bus to fully discharge. See label on
front of adjustable frequency drive door for specific discharge time.
2. With the ohmmeter set on a diode scale or Rx100, measure from power card connector
MK105 (A) to any bus bars coming from the DC inductor. Bus bars are visible at the lower
edge and beneath the power card mounting bracket. Depending on the bus bar readout,
look for a diode drop or a complete short. In either case, this indicates a fuse is in the
circuit providing a path for continuity. An open reading indicates an open fuse.
Check DC bus voltage by reading power card connector MK105 (A) with respect to MK105
(B). If verified, disconnect the power from the adjustable frequency drive and allow it to
cool for approximately 20 minutes.
Reapply power to the adjustable frequency drive after 20 minutes and recheck the DC
bus voltage. If voltage remains, a short circuit may exist within the intermediate circuit
preventing it from charging. Proceed to static checks (6.2) earlier in this section.
MG.90.L1.22 - VLT
High Power Service Manual for VLT
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FC Series
Adjustable Frequency Drives