Interactive trainer

Troubleshooting a three-phase motor

A 480 V motor that will not run, or runs hot and trips. Six faults, four instruments — meter, megger, clamp — and the order a good tech works them in: isolate and test the motor, then energise and read the supply.

By Francisco Padilla Interactive Technician level

Before you start — job brief

This simulator behaves like a live 480 V motor circuit, because that is what you would be standing in front of. Resistance and insulation tests are done dead and locked out; voltage and current are the only readings you take live, and only when the reading itself is the diagnosis. On real equipment 480 V will kill you and an arc flash will do it faster.

Only a qualified person works on or inside an energised motor circuit
Lock out and prove dead before every resistance or insulation test
Never apply an ohmmeter or a megger to a live circuit
Arc-rated PPE and the correct meter category (CAT III/IV) for live work
Prove your meter on a known source before and after you trust a zero

The order you work it in

A three-phase motor gives you four questions and four instruments to answer them, and the sequence matters. Test the motor first with it isolated, then energise and test the supply. Two checks — winding resistance and insulation — are done dead, locked out, with a meter and a megger. The other two — supply voltage and running current — can only be taken live. Work them in that order and most faults name themselves before you ever close the disconnect.

The single idea underneath all of it is balance. A healthy three-phase motor on a healthy supply is symmetrical: three equal winding resistances, three equal insulation readings, three equal voltages, three equal currents. Every fault breaks one of those symmetries, and the instrument that shows the break tells you where the fault lives.

1 · Winding resistance — dead, across the leads

Lock it out, separate the motor leads, and read across them: T1–T2, T2–T3, T1–T3. On a small motor that is a couple of ohms, and the number matters far less than whether the three agree. Within about 5% of each other is healthy.

Two pairs read O.L., one reads normal. An open winding. The motor is finished — confirm the leg, rule out a broken lead in the box, then replace or rewind.
One pair reads low and the set is unbalanced. Shorted turns. It will run, draw uneven current, and cook itself. Replace it.

An ohmmeter cannot see insulation, so a ground fault reads perfectly normal here. That is what the megger is for.

2 · Insulation — the megger, each lead to ground

Still locked out, put 500 or 1000 V from the insulation tester between each motor lead and the frame. A healthy motor reads hundreds of megohms into the gigohms. The IEEE 43 floor is roughly one megohm per kV of rating plus one — about 1.5 MΩ for a 480 V motor — but anything down in the low single-digit megohms is a motor on its way out. One lead reading a fraction of a megohm while the others read a thousand is a winding broken down to the frame.

Field note

Never megger a live circuit, and never chase a suspected ground fault with an ordinary ohmmeter — the fault may only show at test voltage. A motor that got wet often reads low and recovers with a bake-out; a motor that broke down to ground under load does not. When you can, take a one-minute and a ten-minute reading: a polarization index above 2 means the insulation is dry and sound.

3 · Supply voltage — now it is live

Close the disconnect and read all three line-to-line voltages at the motor. Balance again: three equal 480s is what you want.

One pair reads full voltage, the other two read low. Single-phasing — a line is lost upstream. The two low readings are a phantom, back-fed through the windings; do not trust them. The motor itself ohms out fine, which is the tell that the fault is in the supply, not the motor.
All three are present but unequal. Voltage unbalance. NEMA says derate above 1% and never run above 5%, because a small voltage unbalance drives a current unbalance six to ten times larger. It is a distribution problem — a loose connection, uneven single-phase loading, or a weak utility leg.

4 · Running current — the clamp

With it running, clamp each line in turn. Near full-load amps and balanced is healthy. The pattern of the unbalance points straight at the cause:

One leg at zero, two running high. A phase is missing — single-phasing from the supply, or an open winding inside. The resistance check you already did says which.
One leg high, the others normal. A shorted winding pulling extra current on its phase.
All three high and balanced. The motor is drawing locked-rotor current and not turning. That is mechanical.

When the motor is innocent

The hardest call is the one where every electrical test passes. Balanced windings, good insulation, a balanced 480 V supply — and the thing still trips the instant it starts, drawing six times full load on all three legs, balanced. Nothing electrical is wrong. The rotor is not turning. Lock it out and turn the shaft by hand: a seized bearing, a jammed load, or a bound coupling will all do this, and no amount of electrical work fixes any of them. Uncouple the motor and spin it alone to split the motor from what it drives.

Safety, without the lecture

Winding resistance and insulation are dead tests — locked out, leads isolated. Voltage and current are live tests, taken only because the reading itself is the diagnosis, and only by someone qualified to open an energised 480 V enclosure in the right PPE. Prove your meter on a known source before and after you trust a zero, and never put an ohmmeter or a megger on a live circuit.

What normally goes wrong, in order

Single-phasing from a blown fuse or a burnt starter pole — and it takes the next motor with it
Voltage unbalance from loose connections or uneven loading
Bearings — the most common mechanical failure, and the one a vibration check catches early
Insulation breakdown from moisture, heat, or age
Shorted or open windings — usually the end stage of one of the problems above

Notice how many are symptoms of something upstream. A motor that single-phased and burned a winding did not fail on its own — the blown fuse did it, and a replacement motor on the same supply will fail the same way. Finding the cause, not just the dead motor, is the whole job.