Danfoss FC 302 trips A7 on ramp-down, what to check?
My Danfoss FC 302 drive keeps tripping with Alarm 7 (DC Overvoltage) whenever I try to ramp down the motor, especially with heavier loads. It seems to happen only during deceleration. What should I look into first?
Alarm 7 (DC Overvoltage) on a Danfoss VLT FC 302 indicates the DC-link voltage exceeded its limit, causing the drive to trip. This typically occurs when the motor regenerates energy back to the drive faster than it can be absorbed or dissipated, often during rapid deceleration of high-inertia loads or with overhauling loads. A supply sag can also trigger this alarm.
Likely causes and checks
1Ramp-down time is too fast for the load
- Points to it
- The motor is returning energy to the drive too quickly during deceleration, causing the DC-link voltage to rise above its limit. This is common with high-inertia loads.
- Check
- Lengthen the 3-42 Ramp 1 Ramp Down Time parameter or consider changing the ramp type to one that better manages regenerative operation. You might see Warning 5 DC Link Voltage High before the trip.
2Brake resistor is missing or inactive
- Points to it
- If the process requires fast stops of a heavy load, a brake resistor is needed to dissipate the regenerated energy. If it's not present or the brake function is disabled, the DC-link voltage will rise.
- Check
- Verify that a brake resistor is connected to the drive and check the 2-10 Brake Function parameter to ensure it is active. Warnings/Alarms 25–28 can also indicate a problem with the resistor or chopper.
3Overvoltage control is not enabled
- Points to it
- Without a brake resistor, the drive's internal overvoltage control might be disabled, preventing it from managing the regenerated energy effectively.
- Check
- Consider enabling the 2-17 Over-voltage Control parameter if no resistor is fitted. Note that Danfoss advises against enabling OVC in hoisting applications.
4Supply sag occurred
- Points to it
- A temporary drop in the main supply voltage can sometimes lead to an overvoltage condition in the DC link, especially if it coincides with other factors.
- Check
- Observe if the trip coincides with any supply disturbances rather than solely during a stop. If so, consider enabling kinetic back-up (14-10 Mains Failure) to help the drive ride through brief power sags.
Next check: First, confirm if the trip consistently happens during deceleration. If so, start by adjusting the 3-42 Ramp 1 Ramp Down Time to a longer duration. If the issue persists, check the status of any connected brake resistor and the 2-10 Brake Function parameter.
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