VVVF Drive Overvoltage on an Empty Car Traveling Up
A 12-story gearless traction elevator has a 2,500 lb capacity, a 3,500 lb car, a 4,750 lb counterweight, and a rated speed of 350 fpm. The motor is 40 hp, 480 V, three-phase, and the new VVVF drive has a nominal 650 V DC bus. During a test run with an empty car traveling up, the drive trips on DC bus overvoltage at about 320 fpm. The dynamic braking resistor is marked 15 ohm, 1,500 W, and is switched by a contactor controlled by the drive. The resistor measures open circuit. Line voltage is 492 V. The drive decel ramp is set at 2.5 ft/s2. A mechanic proposes to slow the ramp and install a 50 ohm, 5,000 W resistor to stop the trips.
1. Why does an empty car traveling up create a DC bus overvoltage condition, and how much overhauling power is involved at rated speed?
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The counterweight is heavier than the empty car by 1,250 lb (4,750 - 3,500). An empty car moving up is therefore an overhauling load: gravity is pulling the counterweight down and driving the motor through the drive. The motor acts as a generator, and the drive pumps energy back into the DC bus.
At 350 fpm, speed is 350/60 = 5.83 ft/s. Power = force x velocity = 1,250 lb x 5.83 ft/s = 7,292 ft-lb/s. Divide by 550 ft-lb/s/hp: about 13.3 hp, or about 9.9 kW. At the 320 fpm trip point it is about 12.1 hp, or about 9.0 kW. That energy must be absorbed by the braking resistor, stored, or dissipated; if the resistor circuit is open, the DC bus rises until the drive trips. The open resistor is the primary fault, not the decel ramp.
2. Evaluate the proposed 50 ohm, 5,000 W resistor. Is it an acceptable substitute?
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Probably not. At a 650 V DC bus, a 15 ohm resistor would draw about 43 A and dissipate about 28 kW instantaneously; a 50 ohm resistor would draw only about 13 A and dissipate about 8.5 kW. The higher resistance absorbs less power, so it may not keep the bus below the overvoltage threshold during the same decel. The 5,000 W rating is also below the calculated 8.5 kW instantaneous dissipation, although braking is intermittent, so the duty cycle and drive manufacturer's short-time energy rating must be checked.
Replace the resistor with the drive manufacturer's specified resistance and energy rating for this motor and duty. Do not use a higher resistance to 'fix' an overvoltage trip without checking the drive's minimum resistance and braking unit requirements. A resistor that is too high can leave the bus unregulated and cause repeated trips or drive damage.
3. Outline the diagnostic and repair steps before returning the car to service.
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First lock out the drive, verify DC bus is discharged, and inspect the braking resistor, its wiring, contactor, and terminals. The open resistor explains the trip. Replace it with the correct rated assembly, then verify the contactor pulls in during decel and that the drive's braking parameters match the resistor.
Next, check line voltage and tap settings; 492 V is within a normal range but confirm the drive is set for the actual voltage. Review the decel ramp, but do not use a slower ramp to mask a failed braking circuit. Run the car empty up and down, then with rated load, and record DC bus voltage during decel. Verify the brake sets after the drive stops and that no regen energy is being dumped through the motor. Finally, check for intermittent resistor connections caused by vibration and confirm the drive has no stored overvoltage faults before releasing the car.