VFD Overvoltage Trip on a High-Inertia Induced-Draft Fan

Why the trip occurs

When the VFD reduces frequency, the fan keeps turning at a higher speed for a short time. The motor is driven above its synchronous speed and becomes an induction generator. A diode-front-end drive cannot send this energy back to the supply, so the energy charges the DC-bus capacitors and raises the bus voltage. The 5 s deceleration demands a large braking torque and power; the drive's losses and the fan load are not enough to absorb it, so the bus reaches the 800 V trip threshold.

Approximate energy method

[E=\frac{1}{2}J\omega^2,\qquad P_{brake}\approx\frac{E}{t}+P_{loss}]

At 50 Hz, (\omega=2\pi\times50/2\approx157\ \text{rad/s}). If the fan-plus-motor inertia is, for example, 5–15 kg·m², the stored energy is roughly 60–185 kJ. Over 5 s that is about 12–37 kW of braking power; over 30 s it falls to about 2–6 kW. This is an estimate only.

Data needed for a precise calculation

• The actual moment of inertia J of the fan and motor, or the manufacturer's coast-down curve.
• The fan load torque versus speed curve.
• The drive's DC-bus capacitance and the exact overvoltage threshold.
• The drive losses and the motor losses during deceleration.
• The required stopping time and the duty cycle of stops.

The DC-bus capacitor energy alone is small: for a typical capacitance, the rise from 585 V to 780 V stores only a fraction of the kinetic energy. The sustained regeneration is the main problem.

Extending deceleration to 30 s

Increasing the ramp from 5 s to 30 s reduces the braking power roughly in proportion, by about six times. It uses the fan and motor losses to absorb the energy and costs nothing in hardware. It is acceptable if the process can tolerate a 30 s run-down and if the drive can control the motor without excessive braking torque. Check that the deceleration torque does not exceed the motor and drive current limits, and enable the drive's overvoltage stall-avoidance function so that the ramp is automatically extended if the bus voltage rises. Monitor the DC-bus voltage during the first few stops; it should stay below the trip threshold with a margin.

Braking resistor

A braking chopper and resistor give a defined stopping time independent of the process. Size the resistor from the peak braking power and the chopper's minimum resistance, and size the resistor's thermal duty from the energy per stop and the number of stops per hour. Do not choose a resistor by kW rating alone; the ohmic value and duty cycle matter. A 5 s stop may require a large resistor and a chopper rated for the current, whereas a 30 s stop may need only a modest resistor or none at all.

Recommended solution

First try a 20–30 s deceleration with overvoltage stall avoidance enabled and record the DC-bus voltage. If the process requires a faster stop, fit a braking chopper and resistor selected from the drive manufacturer's data, and verify the chopper's duty. Do not disable the overvoltage trip. For either option, check that the motor can dissipate the braking losses and that the driven fan is not damaged by rapid deceleration.

A standard IE2 motor can run on a VFD, but the PWM output and the 30 m unshielded cable need attention:

1. Insulation withstand – the drive's fast switching produces voltage reflections and a high dv/dt at the motor terminals. Check the motor's insulation class and the manufacturer's maximum cable length and carrier-frequency limits. If the cable is long or the insulation is not suitable, fit an output reactor or a dv/dt filter.
2. Cable type and termination – replace the unshielded cable with a shielded or armoured VFD cable, or at least run the motor cable away from control cables and earth the armour/PE at both ends with 360° gland terminations. The 30 m length is moderate at 4 kHz, but the installation must be checked against the drive manual.
3. Earthing and common-mode current – provide a low-impedance earth between the drive, motor frame and panel. Check for shaft-voltage/bearing current problems, especially on larger frames; use a shaft earthing ring if measurements show damaging voltages.
4. Motor cooling at low speed – a standard TEFC motor's self-cooling fan becomes ineffective at low speed. If the fan runs for long periods below about 30 Hz at high torque, check the motor temperature or provide separate cooling.
5. Drive settings and protection – enter the correct motor nameplate data, set the overload protection to the motor FLC, check the carrier frequency and the maximum output frequency, and confirm that the drive is rated for the 30 kW motor at the site ambient.
6. Baseline tests – record insulation resistance, no-load current and vibration before and after the VFD installation, and check the motor terminal voltage waveform if a reflection problem is suspected.

The braking-resistor decision should not be made in isolation; the motor insulation, cable and earthing are part of the same VFD installation review.