The output current should be balanced from phase to phase and no phase should be more than
2 to 3% different from another. If the above tests are successful, the adjustable frequency drive
is operating normally.
3. If a greater imbalance exists than described above, disconnect the motor leads and
repeat the voltage balance test.
Since the current will follow the voltage, it is necessary to differentiate between a load problem
and an adjustable frequency drive problem. Should a voltage imbalance in the output be detected
with the motor disconnected, it is necessary to test the gate drive circuits for proper firing. Proceed
to the gate drive signals test (6.3.11).
If the voltage was balanced but the current imbalanced when the motor was connected, then the
load is suspect. There could be a faulty connection between the adjustable frequency drive and
motor or a defect in the motor itself. Look for bad connections at any junctions of the output wires
including connections made to contactors and overloads. Also, check for burned or open contacts
in such devices.
6.3.11 IGBT Gate Drive Signals Test
This procedure tests the gate drive signals at the output of the gate drive card just prior to them
being delivered to the IGBTs.
A simple test to check for the presence of the gate signals can be performed with a DVM, however,
to actually check the waveforms, an oscilloscope is required.
Disable the DC bus when performing this test with Test Cable p/n 176F8437. Failure
to do so could result in damage to the adjustable frequency drive if the probe is
inadvertently connected to the wrong pins. Additionally, AC line power bus bars are
in close proximity to these test points. Exercise caution when working close to high
voltage components.
Prior to beginning the tests, ensure that power is removed from the unit and that the DC Bus
capacitors have been discharged.
Check for the presence of DC bus voltage by measuring power card connector MK105 (A) with
respect to MK105 (B). The voltage should be zero (0) before proceeding.
1.
For D-frame size units, follow the procedure in Section 7 for soft charge card removal
and disengage the soft charge card far enough to disconnect the cable plugged into the
MK3.
2.
Disconnect the cable from the MK3 connector on the soft charge card and connect one
end of the test cable into the MK3.
3.
For D-frame size units, reinstall the soft charge card.
4.
Disconnect connectors MK100 and MK105 on the power card.
5.
Connect the free end of test cable into MK105.
6.
Connect the SCR gate shorting plug (included with test cable 176F8437) into the cable
that was removed from MK100.
6 Test Procedures
High Power Service Manual for VLT
®
FC Series
Adjustable Frequency Drives
6-30
MG.90.L1.22 - VLT
®
is a registered Danfoss trademark
6
Summary of Contents for VLT HVAC FC 100
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