Indications of a failure in this circuit
IGBT failures may be caused by the adjustable frequency drive being exposed to repeated short
circuits or ground faults, or by extended adjustable frequency 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.
Gate resistor test
Mounted to each IGBT module is an IGBT gate resistor board containing, among other compo-
nents, 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 (see
Figure 6-17). These are labeled MK 250, 350, 450, 550, 650, 750, and, if the adjustable frequency
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 ohmmeter, measure pins 1 and 2 of each test connector. Reading should indicate
7.8K ohms for D1/D3 frames, 3.9K ohms for D2/D4 frames, and 2.6K ohms for E1/E2
frames.
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 the gate signal
wires, or if the resistors are defective, the entire IGBT module assembly requires replacement.
Replace the IGBT module according to the disassembly procedures in Section 7 or 8.
6.2.6 Brake IGBT Test
This test can only be carried out on units equipped with a dynamic brake option. If a brake resistor
is connected to terminals 81 and 82, disconnect it before proceeding. Use an ohmmeter set on
diode check or Rx100 scale.
Brake IGBT test part I
1.
Connect the positive (+) meter lead to the brake resistor terminal R+ (82).
2.
Connect the negative (-) meter lead to the brake resistor terminal R- (81).
The reading should indicate infinity. The meter may start out at a value and climb toward infinity
as capacitance is charged within the adjustable frequency drive.
Brake IGBT test part II
1.
Connect the positive (+) meter lead to the brake resistor terminal R- (81).
2.
Connect the negative (-) meter lead to the brake resistor terminal R+ (82).
The reading should indicate a diode drop.
High Power Service Manual for VLT
®
FC Series
Adjustable Frequency Drives
6 Test Procedures
MG.90.L1.22 - VLT
®
is a registered Danfoss trademark
6-13
6
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