VFD trips on DC bus overvoltage during fan deceleration
A 90 kW, 415 V, four-pole centrifugal fan is driven by a 110 kW variable-frequency drive. The fan runs at 1480 rpm and the total rotating inertia is 25 kg·m². The drive has a DC bus of about 585 V nominal and an overvoltage trip at 800 V. A brake chopper is fitted but no brake resistor is connected. On a controlled stop with 8 s deceleration the drive trips on DC bus overvoltage after about 3 s; on coast-to-stop it does not trip. The process needs the fan to stop in 8 s to avoid reverse airflow and duct damage. The existing supply is 415 V, +6% at light load. The drive is located 40 m from the motor and uses unshielded cable. The panel is in a 45°C ambient. The maintenance team proposes increasing the deceleration time to 30 s and disabling the overvoltage trip.
1. Explain why the DC bus rises during deceleration and why coasting avoids the trip. Estimate the kinetic energy and the average regenerative power for an 8 s stop from 1480 rpm.
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During a controlled deceleration the VFD reduces output frequency, so the motor synchronous speed falls below the actual rotor speed. The motor then acts as an induction generator and the fan inertia drives energy back into the drive. With no brake resistor, that energy charges the DC-link capacitors and the bus voltage rises until the overvoltage trip operates. On coast-to-stop the drive removes torque and lets the fan slow naturally against its own losses; little energy is returned, so the bus does not rise.
Kinetic energy:
- ω = 2π × 1480/60 ≈ 155 rad/s
- E = 0.5 × J × ω² = 0.5 × 25 × 155² ≈ 300 kJ
Average power over 8 s = 300/8 = 37.5 kW. With constant deceleration torque, the peak power at the start of the ramp is higher: T = Jα = 25 × (155/8) ≈ 484 N·m, and P = Tω ≈ 484 × 155 ≈ 75 kW. The resistor and chopper must be checked for this peak, not only the average.
2. Estimate a brake resistor value and power rating for the chopper, stating assumptions. Would increasing deceleration to 30 s remove the need for a resistor?
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Assume the chopper switches at about 750 V. For the average braking power of 37.5 kW, R ≈ V²/P = 750²/37,500 ≈ 15 Ω. For the estimated 75 kW peak, R ≈ 750²/75,000 ≈ 7.5 Ω. A practical starting point is therefore a resistor around 8-10 Ω, rated for the drive's chopper duty: peak power of at least 75 kW for a few seconds and an average rating that depends on stops per hour. If the fan stops once per minute, the 8 s ramp is about 13% duty, so a 10 kW continuously rated resistor with a high short-time overload may be adequate, but the manufacturer's minimum resistance and thermal model must be followed.
Increasing deceleration to 30 s reduces average braking power to about 10 kW and peak to about 25 kW, but it does not create a place to dissipate the energy. The bus will still rise unless the drive has enough internal loss or an overvoltage stall routine that reduces torque. More importantly, the process requires an 8 s stop. The proposal also includes disabling the overvoltage trip, which risks capacitor rupture and should be rejected. Fit a properly rated resistor or use a regenerative drive.
3. Recommend a safe solution and two settings or checks before commissioning.
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Install a brake resistor rated for the calculated peak and duty, connect it to the chopper, and set the chopper threshold below the 800 V trip - typically around 720-750 V - so the resistor takes over before the trip level. Keep the 8 s deceleration. Enable overvoltage stall prevention only as a backup, not as the primary control. Verify the resistor thermostat and overtemperature interlock.
Before commissioning, check the drive manufacturer's minimum allowable resistance and the chopper continuous current; too low a resistance can destroy the chopper. Also check the 45°C ambient derating and the motor cable arrangement: 40 m of unshielded cable can cause reflected-wave overvoltage at the motor terminals, so use shielded cable or a dv/dt filter if the motor insulation is not rated for the drive. Do not disable the DC-bus overvoltage trip. Record DC-bus voltage and resistor temperature during several stops before handing over.