A drive trips on DC-bus overvoltage every time the machine is stopped. The most likely cause?

A. The decel time is too short for a high-inertia load, so the motor regenerates into the bus

B. The supply voltage is too low

Which nameplate data should be entered into a drive before the first run?

B. Motor voltage, full-load current, base frequency, base speed (and power) — then an autotune if the drive offers one

D. Nothing — drives detect the motor

An instant overcurrent or ground-fault trip right at start, even with a long accel time. What do you check first?

B. The motor and cable insulation (with the leads lifted from the drive) and the output connections

Why is a contactor between a running drive and its motor a problem?

B. Opening or closing it while the drive is outputting can cause current spikes that trip or damage the drive, unless the drive is set up for it

Rappel pour compagnons Refresher 8 of 10 · about 9 min

Rusty on VFDs?

What you’ll learn

The three sections of a drive and the DC-bus voltage to expect.
The parameters you set on every drive, and why the nameplate FLA matters.
The common trips and what usually causes each one.
The two habits that protect you and the drive: bus discharge, and lifting leads before insulation tests.

Before this: How a VFD works

Every drive: fixed AC in, a DC bus in the middle, variable-frequency AC out. The bus capacitors hold their charge after the power is off.

Parameters you set on every drive

Parameter	From	Why it matters
Motor voltage, FLA, base Hz, base rpm, power	Motor nameplate	Motor overload protection and the motor model
Min / max frequency	The process and the machine maker	Keeps a pump from dead-heading or a fan from overspeeding
Accel / decel time	The load	Too short: overcurrent on start, overvoltage on stop
Control mode (V/Hz, sensorless vector…)	Load type	Torque at low speed; V/Hz for multiple motors on one drive
Start / stop / speed source	The control design	Keypad, terminals or network — the usual “it won’t start” answer
Stop mode	The process	Ramp to stop versus coast

The common trips

Trip	Usual causes	First check
Overvoltage (DC bus)	Decel too fast on a heavy load; high or spiking supply	Decel time; braking resistor; supply voltage
Overcurrent	Accel too fast; jammed load; output short	Accel time; load turns freely; insulation (leads lifted)
Ground fault	Motor or cable insulation failing; water in a box	Insulation test with leads lifted from the drive
Undervoltage	Supply sag or lost phase on the input	Three input voltages, input fuses
Motor overload	Real overload; wrong FLA entered	Motor current vs nameplate; the FLA parameter
Overtemperature (drive)	Blocked fan or filter, hot enclosure	Fans, filters, heat sink, room temperature

Worked check: what should the numbers be?

Drive on 480 V. DC bus at no load ≈ 480 × 1.414 = 679 V; loaded ≈ 480 × 1.35 = 648 V.
Motor: 460 V, 60 Hz, 1770 rpm (4-pole, 1800 synchronous, 30 rpm slip). V/Hz = 460 ÷ 60 = 7.67.
Commanded 40 Hz: output ≈ 7.67 × 40 = 307 V; shaft ≈ 120 × 40 ÷ 4 − 30 = 1200 − 30 = 1170 rpm.

Sur le chantier

Tips

Photograph the parameter list (or save it to the keypad or a laptop) before you change anything.
The fault history — with the time, current and bus voltage at the trip — is the best clue you will get.
Use a true-RMS meter with a low-pass (drive) setting on the output side.

Common mistakes

Meggering a motor with the drive still connected.
Touching the bus terminals because the display is dark.
Shortening decel to fix a slow stop and getting overvoltage trips instead.
Running a TEFC motor slowly for long periods without checking it can cool itself.

Check yourself

0 right of 0 answered 6 questions · pick one answer each

1.A drive trips on DC-bus overvoltage every time the machine is stopped. The most likely cause?
2.Roughly what DC-bus voltage do you expect on a 480 V drive with the motor stopped (light load)?
3.A 460 V, 60 Hz motor on a drive in V/Hz mode is running at 30 Hz. About what voltage is the drive putting out?
4.Which nameplate data should be entered into a drive before the first run?
5.An instant overcurrent or ground-fault trip right at start, even with a long accel time. What do you check first?
6.Why is a contactor between a running drive and its motor a problem?

Answers

A. The decel time is too short for a high-inertia load, so the motor regenerates into the bus — Slowing a heavy load quickly turns the motor into a generator, pumping energy back into the DC bus. Lengthen the decel, or add a braking resistor or regenerative unit if the process needs the fast stop.
C. About 650–680 V — The rectifier and bus capacitors charge toward the peak of the line voltage: 480 × 1.414 ≈ 679 V at no load, sagging toward about 1.35 × 480 ≈ 648 V under load. It stays dangerous after power is removed until the capacitors discharge — check with a meter after the wait the maker specifies.
B. 230 V — V/Hz = 460 ÷ 60 ≈ 7.67. At 30 Hz: 7.67 × 30 = 230 V. (Low-speed boost adds a little at the very bottom of the range.)
B. Motor voltage, full-load current, base frequency, base speed (and power) — then an autotune if the drive offers one — The drive’s motor overload protection, current limit and motor model are built from the nameplate. Wrong FLA means wrong overload protection.
B. The motor and cable insulation (with the leads lifted from the drive) and the output connections — A trip before the motor has had any chance to accelerate points to a short or ground fault on the output side. Lock out, lift the leads at the drive, insulation-test the cable and motor.
B. Opening or closing it while the drive is outputting can cause current spikes that trip or damage the drive, unless the drive is set up for it — Switching the output under load is hard on the drive’s transistors. If an output contactor is needed (bypass, isolation), it is interlocked so it only switches with the drive stopped.

Questions fréquentes

Why does the drive show a different current than my clamp meter?

The output of a drive is a chopped (PWM) waveform. An average-responding clamp can read it badly; a true-RMS meter with a low-pass filter setting made for drive outputs reads it properly. The input current is different again, because the rectifier draws current in pulses.

Can I run a motor above 60 Hz on a drive?

The drive can; whether the motor and the machine can is a question for their makers. Above base frequency the drive holds the voltage at rated, so torque capability falls, and the mechanical speed limit of the motor and the driven load matters.

How long do I wait after power-off before touching a drive?

As long as the drive label and manual say — often several minutes — and then prove the DC bus dead with a meter. The capacitors hold a lethal charge after the display goes dark.

Voir aussi

How a VFD works — the full lesson
VFD parameters and troubleshooting — the full lesson
Calculateur de fréquence et de vitesse d’un variateur (VFD)
Power quality and harmonics
Motor troubleshooting flowcharts

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