A drive runs a heavy flywheel fine but trips on overvoltage every time it is told to stop. The most likely cause?

B. The decel time is too short, so the motor regenerates energy back into the DC bus faster than the drive can get rid of it

A. The supply voltage is too low

C. The motor has an insulation fault

A fan drive trips on overcurrent partway through every start, never while running. What do you look at first?

B. The accel time and whether the load is jammed or unusually heavy

You need to insulation-test (megger) a motor fed from a drive. What must you do first?

B. Lock out, verify the bus is discharged, and disconnect the motor leads from the drive output so the drive is not in the test

A. Nothing — meg through the drive output terminals

Why do you enter the motor’s nameplate full-load current into the drive?

B. So the drive’s electronic overload protects that motor at the right current, and so vector control has the right motor model

An outdoor pump drive starts tripping on ground fault after a storm. What is the sensible first suspicion?

B. Moisture in the motor, its junction box or the cable, letting current leak to ground

A drive trips on undervoltage only when a large motor elsewhere in the plant starts across the line. The drive is probably…

B. seeing a supply sag deep enough to drop its DC bus below the trip level

A drive has tripped. What is the most useful first step before pressing reset?

B. Read and write down the fault code, the fault history and the conditions at the time (speed, current, bus voltage)

Fourth year Lesson 2 of 12 · about 16 min

VFD parameters and troubleshooting

What you’ll learn

The handful of parameters you set on every drive, and where each value comes from.
How accel and decel ramps work, and why a fast stop trips on overvoltage.
What the four common trips — overvoltage, overcurrent, ground fault, undervoltage — usually mean.
A safe order for troubleshooting a drive, including how to insulation-test the motor without killing the drive.

Before this: How a VFD works

A drive only knows what you tell it. Most drive problems are either a parameter that does not match the motor or the job, or the drive correctly tripping to tell you about a problem somewhere else — in the supply, the cable, the motor or the load. The fault code is the drive’s best guess at which. Learn what each common one means and you fix most drives without changing a part.

The parameters you set on every drive

Parameter	Where the value comes from	Why it matters
Motor voltage, frequency, rpm	Motor nameplate	Sets the V/Hz pattern and the slip estimate
Motor full-load current	Motor nameplate	The drive’s electronic overload protects the motor from this
Min / max frequency	The process and the machine maker	Keeps pumps above their minimum and machines below their safe speed
Accel / decel time	The load: how heavy, how fast it must respond	Too short trips; too long makes the process sluggish
Start / speed source	The control design	Keypad, terminals, 4–20 mA or network — the drive must listen to the right one
Stop mode	The load	Ramp to stop or coast; a fan usually coasts, a conveyor ramps

Then run the drive’s auto-tune if it has one and the motor is safe to turn. Write your settings down, or save them to the keypad — the next person to replace that drive will thank you.

Ramps, and where the energy goes

Accel pulls energy from the supply. Decel faster than the load would coast and the motor becomes a generator, pushing energy back into the DC bus — the cause of most overvoltage trips.

Accel and decel times are nearly always set as the time for the full range, 0 to base frequency. A 10 s accel on a 60 Hz drive is a ramp of 6 Hz per second, whatever speed you are actually going to.

Worked: reading the ramp

Accel is set to 12 s for 0–60 Hz: the ramp is 60 ÷ 12 = 5 Hz/s.
The speed reference is 45 Hz. Time to get there: 45 ÷ 5 = 9 s.
Decel is set to 3 s. Stopping from 45 Hz takes 45 ÷ (60 ÷ 3) = 45 ÷ 20 = 2.25 s.
If that 2.25 s stop trips on overvoltage, the load cannot be slowed that fast without braking: lengthen decel (try 8–10 s), or fit the braking resistor the drive maker specifies.

The four trips you will see most

Trip	What the drive saw	Usual causes
Overvoltage	DC bus too high	Decel too short; overhauling load (a load that drives the motor); high or surging supply
Overcurrent	Output current too high, too fast	Accel too short; jammed or heavy load; short in the cable or motor; wrong motor data
Ground fault	Current leaking to ground on the output	Damaged cable insulation; water in the motor or terminal box; failed winding
Undervoltage	DC bus too low	Supply sag (a big motor starting nearby); lost phase; loose line connection; blown input fuse

Two more you will meet: drive overtemperature (blocked filters, a dead heatsink fan, a hot room) and motor overload (the load really is too heavy for too long — or the FLA parameter is wrong). The trip tells you where to look, not what to replace.

A safe order to work in

Read before you reset. Write down the fault code, the fault history and what the machine was doing.
Check the supply. Line voltage on all three phases at the drive input, and the DC bus against 1.35 × line.
Check the load. Does it turn freely? Has anything changed in the process?
Check the parameters against the motor nameplate and the job.
Test the motor and cable — isolated. Lock out, wait, verify the bus is dead, lift the motor leads at the drive, then insulation-test the cable and motor. Never meg through the drive.
Only then suspect the drive.

On the job

Tips

Photograph or save the parameter list the day a drive is commissioned.
A trip that only happens at one point in the cycle (starting, stopping, one speed) is almost always a setting or the load, not the drive.
Measure motor current with a clamp on the drive output and compare it to the nameplate FLA.
If the bus reads low with one input phase missing, the drive may still run lightly loaded and trip under load — check all three input fuses.

Common mistakes

Resetting three times and calling it fixed.
“Fixing” an overcurrent trip by raising the current limit instead of finding the jam.
Meggering the motor with the leads still on the drive output.
Setting FLA from the drive rating instead of the motor nameplate.
Working on the output terminals because the display went dark — the bus can still be charged.

Check yourself

0 right of 0 answered 8 questions · pick one answer each

1.A drive runs a heavy flywheel fine but trips on overvoltage every time it is told to stop. The most likely cause?
2.A fan drive trips on overcurrent partway through every start, never while running. What do you look at first?
3.You need to insulation-test (megger) a motor fed from a drive. What must you do first?
4.Why do you enter the motor’s nameplate full-load current into the drive?
5.An outdoor pump drive starts tripping on ground fault after a storm. What is the sensible first suspicion?
6.A drive is set to accelerate from 0 to 60 Hz in 12 s. How long does it take to reach 45 Hz?
7.A drive trips on undervoltage only when a large motor elsewhere in the plant starts across the line. The drive is probably…
8.A drive has tripped. What is the most useful first step before pressing reset?

Answers

B. The decel time is too short, so the motor regenerates energy back into the DC bus faster than the drive can get rid of it — Slowing a big inertia fast turns the motor into a generator, pumping energy into the bus and raising its voltage. Lengthen the decel, or add dynamic braking (a braking resistor) if the stop really must be quick.
B. The accel time and whether the load is jammed or unusually heavy — Overcurrent during acceleration means the drive is asking for more torque than it can deliver in that time — too short an accel ramp, a dragging or jammed load, or wrong motor data. Check the load turns freely, then lengthen the ramp.
B. Lock out, verify the bus is discharged, and disconnect the motor leads from the drive output so the drive is not in the test — A megger’s test voltage on the drive output would reach the inverter transistors and can destroy them. Isolate, verify dead, lift the motor leads at the drive, then test the cable and motor on their own.
B. So the drive’s electronic overload protects that motor at the right current, and so vector control has the right motor model — The drive models the motor’s heating from the current and the nameplate figure. With the wrong FLA the electronic overload trips too early or — worse — too late. Sensorless-vector and auto-tune also need the nameplate data.
B. Moisture in the motor, its junction box or the cable, letting current leak to ground — A ground-fault trip means current is leaving the motor circuit through the grounding path. Water in a motor terminal box or damaged cable is the classic cause. Isolate the drive and insulation-test the cable and motor separately.
B. 9 s — The ramp is linear at 60 ÷ 12 = 5 Hz per second. To reach 45 Hz: 45 ÷ 5 = 9 s. Accel and decel times are almost always set as the time for the full range, not the time to reach your set speed.
B. seeing a supply sag deep enough to drop its DC bus below the trip level — The big motor’s starting current sags the plant voltage for a second or two; the bus follows it down. The fix is on the supply side (the other motor’s starting method, the feeder) or in the drive’s ride-through settings — not a new drive.
B. Read and write down the fault code, the fault history and the conditions at the time (speed, current, bus voltage) — Resetting throws away the evidence. Most drives keep a fault log with the bus voltage, current and frequency at the moment of the trip — which usually points straight at the cause.

Questions people ask

What does auto-tune actually do?

It makes the drive measure the motor it is connected to — winding resistance, inductance and sometimes magnetising current — so its motor model matches reality. Sensorless-vector control depends on that model. A static auto-tune does it with the shaft still; a rotating one spins the motor, so the load must be safe to turn or uncoupled. Follow the drive manual.

Do I still need a separate overload relay with a drive?

Most drives include an electronic motor overload that, set correctly from the nameplate, protects a single motor. Where one drive feeds several motors, each motor usually needs its own overload. Whether the drive’s protection is acceptable for your installation is a code-book and drive-listing question.

What is a braking resistor for?

When a motor is slowed faster than friction would stop it, it regenerates energy into the DC bus. With nowhere to go, the bus voltage rises until the drive trips on overvoltage. A braking transistor and resistor switch that energy into heat, so the drive can stop a heavy load quickly without tripping.

How a VFD works — the previous lesson
VFD frequency & speed calculator
Rusty on VFDs? — the one-page refresher
Reading a motor nameplate — where the parameters come from
Meter safety and CAT ratings
Lockout / tagout walk-through

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