MOFFÂ Main Circuit Undervoltage / Contactor Open fault in Toshiba drive
MOFF Main Circuit Undervoltage / Contactor Open fault in Toshiba drive
Description This is a specific internal hardware fault found in larger Toshiba drives. The drive has an internal large DC contactor that bridges the AC input to the DC bus (bypassing the soft-charge resistors). MOFF means the control board detected that this main contactor opened while the motor was running, or the DC bus voltage dropped so fast that the contactor disengaged. Causes
1. Input Voltage Dip: A severe momentary sag in facility power caused the coil of the internal contactor to drop out.
2. Contactor Failure: The internal contactor is mechanically worn or sticky.
3. Loose Connectors: The ribbon cable or wire harness connecting the control board to the power board is loose.
4. Auxiliary Contact Failure: The small feedback switch on the contactor that tells the CPU "I am closed" is dirty or broken. Solution Check the incoming power stability. If the facility has large loads switching on (like arc welders), it may be causing voltage dips that drop the contactor.
If power is stable, this is likely an internal drive service issue. Power down the drive and wait 10 minutes (discharge time). Remove the front cover. Reseat all ribbon cables and white plug connectors between the main circuit board and the power modules. Inspect the internal contactor (if visible). If it looks charred or smells of ozone, the drive power stage needs rebuilding.
In some cases, this can be caused by a failing 24V control power supply on the drive's main board, which lacks the energy to hold the heavy contactor coil closed. This requires board repair.
Description This is a specific internal hardware fault found in larger Toshiba drives. The drive has an internal large DC contactor that bridges the AC input to the DC bus (bypassing the soft-charge resistors). MOFF means the control board detected that this main contactor opened while the motor was running, or the DC bus voltage dropped so fast that the contactor disengaged. Causes
1. Input Voltage Dip: A severe momentary sag in facility power caused the coil of the internal contactor to drop out.
2. Contactor Failure: The internal contactor is mechanically worn or sticky.
3. Loose Connectors: The ribbon cable or wire harness connecting the control board to the power board is loose.
4. Auxiliary Contact Failure: The small feedback switch on the contactor that tells the CPU "I am closed" is dirty or broken. Solution Check the incoming power stability. If the facility has large loads switching on (like arc welders), it may be causing voltage dips that drop the contactor.
If power is stable, this is likely an internal drive service issue. Power down the drive and wait 10 minutes (discharge time). Remove the front cover. Reseat all ribbon cables and white plug connectors between the main circuit board and the power modules. Inspect the internal contactor (if visible). If it looks charred or smells of ozone, the drive power stage needs rebuilding.
In some cases, this can be caused by a failing 24V control power supply on the drive's main board, which lacks the energy to hold the heavy contactor coil closed. This requires board repair.
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