Danfoss VLT series, Service Manual

Page: 75 / 158

5-9
VLT is a registered Danfoss trademark
5.1.5 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.
Depending on the size of the unit, either one, two, or three
IGBT modules are present. The drive also has 3 snubber
capacitors on each IGBT module.
CAUTION
!
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, interface
and power cards. Do not disconnect the cable to connector
MK105 on the power card since the path for continuity would
be broken.
5.1.5.1 Inverter Test Part I
1. Connect positive (+) meter lead to (+) positive
DC bus connector MK105 (A) on power card.
2. Connect negative (–) meter lead to terminals U,
V, and W in turn.
Each reading should show infinity. The meter will start at a low
value and slowly climb toward infinity due to capacitance within
the drive being charged by the meter.
5.1.5.2 Inverter Test Part II
1. Reverse the meter leads by connecting negative
(–) meter lead to positive (+) DC bus connector
MK105 (A) on power card.
2. Connect positive (+) meter lead to U, V, and W
in turn. 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 in accordance with the
disassembly instructions in Section 6 or 7. It is further
recommended for units with two IGBT modules that both
modules be replaced even if the second module tests correctly.
5.1.5.3 Inverter Test Part III
1. Connect positive (+) meter lead to the negative
(-) DC bus connector MK105 (B) on power card.
2. Connect negative (–) meter lead to terminals U,
V, and W in turn. Each reading should show a
diode drop.
5.1.5.4 Inverter Test Part IV
1. Reverse the meter leads by connecting negative
(–) meter lead to negative (-) DC bus connector
MK105 (B) on power card.
2. Connect positive (+) meter lead to U, V, and W
in turn.
Each reading should show infinity. The meter will start at a low
value and slowly climb toward infinity due to capacitance within
the drive being charged by the meter.
Incorrect Reading
An incorrect reading in any inverter test indicates a failed IGBT
module. Replace the IGBT module in accordance with the
disassembly instructions in Section 6 or 7. It is further
recommended for units with two IGBT modules that both
modules be replaced even if the second module tests correctly.
Indications of a failure in this circuit
IGBT failures may be caused by the drive being exposed to
repeated short circuits or ground faults, or by extended drive
operation outside of its normal operating parameters. Following
an IGBT failure, it is important to verify the gate drive signals
are present and of the correct waveform. See the dynamic
test section on checking IGBT gate drive signals.
5.1.5.5 Gate Resistor Test
Mounted to each IGBT module is an IGBT gate resistor board
containing, among other components, the gate resistors for
the IGBT transistors. Based on the nature of the failure, a
defective IGBT can produce good readings from the previous
tests. In nearly all cases, the failure of an IGBT will result in the
failure of the gate resistors.
Located on the gate drive card near each of the gate signal
leads is a 3 pin test connector. These are labeled MK 250,
350, 450, 550, 650, 750, and, if the drive is equipped with a
brake option, 850.
For the sake of clarity, refer to the 3 pins as one, two, and
three, reading left to right. Pins 1 and 2 of each connector are
in parallel with the gate drive signal sent to the IGBTs. Pin 1 is
the signal and Pin 2 is common.
1. With ohm meter, measure pins 1 and 2 of each
test connector. Reading should indicate 7.8K
ohms for units with single IGBT module, 3.9K
ohms for units with dual IGBT modules, and
2.6K ohms for units with three IGBT modules.
Incorrect Reading
An incorrect reading indicates that either the gate signal wires
are not connected from the gate drive card to the gate resistor
board or the gate resistors are defective. Connect gate signal
wires, or if the resistors are defective, the entire IGBT module
assembly requires replacement. Replace the IGBT module in
accordance with the disassembly procedures in Section 6 or 7.