Applicable Systems
FC-30
FC-51
FC-101
FC-202
FC-302
Symptoms
The drive trips. The display shows Alarm 7, indicating DC link overvoltage. The DC bus voltage has exceeded the safe maximum limit.
Possible Causes
- Deceleration time too short, causing regenerative overvoltage
- No brake chopper or resistor installed
- Brake resistor open or disconnected
- Supply voltage too high
Troubleshooting Steps
- Step 1: Check the deceleration time parameter in the drive settings
Deceleration time is long enough for the regenerative energy to dissipate - Step 2: Verify that a brake chopper and resistor are installed if deceleration is fast
Brake chopper and resistor are present and correctly connected - Step 3: Measure the brake resistor resistance and verify continuity
Brake resistor has the correct resistance and is not open circuit - Step 4: Measure the input supply voltage at the drive terminals
Supply voltage is within the rated range
How to Reset
To reset: 1) Fix the overvoltage cause (increase deceleration time, install/fix brake resistor). 2) Press RESET. Caution: DC bus overvoltage can damage the drive capacitors and IGBTs. Always address the root cause.
Frequently Asked Questions
- Q: How do I size a brake resistor for a Danfoss drive?
Calculate the braking power based on the load inertia, speed, and deceleration time. Select a resistor with the correct resistance (from the drive manual) and sufficient power rating. Danfoss provides brake resistor selection tables in the design guide.
- Q: What happens if the brake resistor is too small?
The resistor may overheat and burn out during extended braking. The drive will then trip on DC overvoltage because the excess energy cannot be dissipated.
Source: Danfoss VLT FC-302 Design Guide, 2023
Category: PLC & VFD