Repaso para oficiales 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
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.
En la obra
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
-
1.A drive trips on DC-bus overvoltage every time the machine is stopped. The most likely cause?
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.
-
2.Roughly what DC-bus voltage do you expect on a 480 V drive with the motor stopped (light load)?
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.
-
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?
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.)
-
4.Which nameplate data should be entered into a drive before the first run?
The drive’s motor overload protection, current limit and motor model are built from the nameplate. Wrong FLA means wrong overload protection.
-
5.An instant overcurrent or ground-fault trip right at start, even with a long accel time. What do you check first?
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.
-
6.Why is a contactor between a running drive and its motor a problem?
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.
Preguntas frecuentes
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.
Ver también
- How a VFD works — the full lesson
- VFD parameters and troubleshooting — the full lesson
- Calculadora de frecuencia y velocidad de un VFD
- Power quality and harmonics
- Motor troubleshooting flowcharts
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