Troubleshooting a VFD

Read the drive before you reach for a meter

A VFD is the best-instrumented device in the panel — it is watching its own bus voltage, output current, and heatsink temperature hundreds of times a second, and when it trips it tells you which of those protections fired. The fault code is the diagnosis's first half. Your job is the second half: prove why that protection tripped, because the drive reports the symptom, not the cause. A ground-fault code does not tell you whether the motor or the cable failed. An overvoltage code does not tell you whether the fix is a decel time or a brake resistor.

So the workflow is always the same. Read the keypad's fault log and its monitor values first. Then verify with the meter at three places: the input, the DC bus, and the output. Each answers a different question, and together they separate a drive problem from a supply problem from a motor problem.

The input — is the supply healthy?

Read all three line-to-line voltages at the drive's input. Three balanced 480s means the supply is good and the fault is downstream. One pair reading full voltage while the two pairs that share a line read low or zero means you have lost that line — a blown input fuse, an open disconnect pole, or a loose lug. A drive fed on two phases sags its DC bus and trips on undervoltage, and if you keep resetting it, it cooks its own rectifier and bus capacitors.

The DC bus — the drive's centre of gravity

On a 480 V drive the bus sits around 650 V — roughly the peak of the line. Read it on the DC range across DC+ and DC−. High — up near 800 V — is overvoltage: the motor is regenerating into the bus on a fast stop faster than the drive can dump it. Lengthen the decel or add a braking resistor. Low points back at the input: a lost phase or a sagging supply. The bus voltage, more than any other single reading, tells you whether the drive's problem comes from the line side or the load side.

The output — motor and cable

Output faults are found dead, locked out and discharged, because the drive shuts its output off the moment it trips — there is nothing to read live. Ohm the three output pairs and megger each output lead to ground, exactly as you would test a motor across its own leads:

• Two pairs read O.L., one normal. An open — an output phase loss. A loose motor-box terminal, an open lead, or an open winding.
• One pair reads near zero. A phase-to-phase short — the drive's instantaneous overcurrent. Cable or motor.
• One lead low to ground on the megger. A ground fault. Split the motor from the cable by meggering each separately.

Lifting the motor leads and repeating the test is what splits the motor from the cable feeding it — the single most useful move on any output fault, and the one that decides whether you are pulling a motor or chasing a cable.

The one that is not electrical

Overtemperature is the trap. Every voltage and resistance reads perfect, and a tech who only trusts the meter is left with nothing. The answer is on the keypad: a heatsink temperature above its limit and a stopped fan. A drive throttles and then trips to protect its transistors from heat it cannot shed. The fix is a fan, a cleaned heatsink, or ventilation for a baking enclosure — not a single electrical part.

What normally goes wrong, in order

1. Overtemperature from a failed fan or a clogged, dusty heatsink
2. Overvoltage on fast stops where the load should have a braking resistor
3. Input phase loss from a blown fuse or a loose supply connection
4. Output faults from cable damage, wash-down moisture, or a loose motor terminal
5. Nuisance trips from parameters that do not match the motor or the load

Notice that only a few of these are the drive's own fault. Most drives that get replaced were doing exactly what they were designed to do — protecting themselves from a supply, a motor, or an environment that was the real problem. Reading the drive, then proving the cause, is what keeps a good drive from being thrown away.