6.2.5 Inverter Section Tests
The inverter section is primarily made up of the IGBTs used for switching the DC bus voltage to
create the output to the motor. IGBTs are grouped into modules comprised of six IGBTs. De-
pending on the size of the unit, either one, two, or three IGBT modules are present. The adjustable
frequency drive also has 3 snubber capacitors on each IGBT module.
Disconnect motor leads when testing inverter section. With leads connected, a short
circuit in one phase will read in all phases, making isolation difficult.
Before starting tests, ensure that meter is set to diode scale. If removed previously, reinstall the
soft charge card and power cards. Do not disconnect the cable to connector MK105 on the power
card since the path for continuity would be broken.
Inverter test part
1.
Connect the positive (+) meter lead to the (+) positive DC bus connector MK105 (A) on
the power card.
2.
Connect the negative (–) meter lead to terminals U, V, and W in sequence.
Each reading should show infinity. The meter will start at a low value and slowly climb toward
infinity due to capacitance within the adjustable frequency drive being charged by the meter.
Inverter test part II
1.
Reverse the meter leads by connecting the negative (–) meter lead to the positive (+)
DC bus connector MK105 (A) on the power card.
2.
Connect the positive (+) meter lead to U, V, and W in sequence. Each reading should
show a diode drop.
Incorrect reading
An incorrect reading in any inverter test indicates a failed IGBT module. Replace the IGBT module
according to the disassembly instructions in Section 7 or 8. It is further recommended for units
with two IGBT modules that both modules be replaced even if the second module tests correctly.
Inverter test part III
1.
Connect the positive (+) meter lead to the negative (-) DC bus connector MK105 (B) on
the power card.
2.
Connect the negative (–) meter lead to terminals U, V, and W in sequence. Each reading
should show a diode drop.
Inverter test part IV
1.
Reverse the meter leads by connecting the negative (–) meter lead to the negative (-)
DC bus connector MK105 (B) on the power card.
2.
Connect the positive (+) meter lead to U, V, and W in sequence.
Each reading should show infinity. The meter will start at a low value and slowly climb toward
infinity due to capacitance within the adjustable frequency drive being charged by the meter.
Incorrect reading
An incorrect reading in any inverter test indicates a failed IGBT module. Replace the IGBT module
according to the disassembly instructions in Section 7 or 8. It is further recommended for units
with two IGBT modules that both modules be replaced even if the second module tests correctly.
6 Test Procedures
High Power Service Manual for VLT
®
FC Series
Adjustable Frequency Drives
6-12
MG.90.L1.22 - VLT
®
is a registered Danfoss trademark
6
Summary of Contents for VLT HVAC FC 100
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