Troubleshooting Motor Control Circuits

When a motor stops, production stops, and everyone stares at you. Good troubleshooters do not swap parts until something works. They use a print and a meter to divide the circuit, and each test removes half of what might be wrong. This guide shows that method on real motor control faults, from a starter that does nothing to a motor that keeps tripping.

What You Will Be Able to Do

• Work through a motor failure in a logical order.
• Tell a power problem from a control problem.
• Trace a control circuit and interpret voltage across contacts.
• Test motor windings and insulation safely.
• Match common symptoms to their likely causes.

Warning: Voltage testing in a live starter is energized work. It is for qualified people only, with a rated meter and the protective equipment the task calls for. Everyone else watches from outside the boundary. Do all you can with the power off, and lock out before you put your hands on conductors or parts.

Start Before You Open the Door

Ask the operator three questions: what happened, what changed, and what else was running. Then look. A tripped overload, a burned smell, a jammed machine or a stop button still pushed in may tell you the answer in a minute.

Check the simple things first:

1. Is the disconnect on, and does it have power?
2. Has the overload relay tripped?
3. Are any E-stops, safety gates or interlocks open?
4. Is the machine in the right mode, such as Hand or Auto?

Many calls end here. If an overload has tripped, do not just reset it. Find out why first.

Power Circuit or Control Circuit?

A motor starter has two circuits:

• The power circuit carries motor current through the disconnect, fuses, contacts and overload to the motor.
• The control circuit carries the small current that operates the coil.

Divide the problem by watching the contactor when someone presses Start.

That one observation cuts the job in half.

Tracing a Control Circuit

Get the schematic before you touch anything. Trace the logic on paper, and write down what you expect to read at each point. Here is a typical circuit:

 X1 (120 V)                                                X2
  |                                                         |
  +--[ E-STOP ]--[ STOP ]--+--[ START ]--+--( M )--[ OL ]---+
                           |             |
                           +--[ M aux ]--+

Measuring to a Reference

Put one meter lead on X2, the return side, and keep it there. Move the other lead along the rung, one point at a time.

1. Measure at X1. You should read 120 volts. If not, check the control fuse and transformer.
2. Measure after the E-stop, and after STOP. Both should read 120 volts.
3. Have someone press Start, and measure after START. You should read 120 volts.
4. Measure at the coil.

The fault is between the last point with voltage and the first point without it.

Measuring Across a Contact

You can also put the leads across a single device.

A closed contact has almost no voltage across it, because there is no resistance to drop voltage. An open contact in a series circuit shows full voltage, because the meter reads through the rest of the circuit back to the source.

Worked Example: A Dead Conveyor

A conveyor will not start, and the contactor stays still. With X2 as your reference:

1. X1 reads 120 volts. Control power is fine.
2. After the E-stop: 120 volts.
3. After STOP: 0 volts.

The fault is in the STOP button or the wire to it. With power off and locked, a continuity check across the button shows it is open while released. A failed contact block is the cause. You found it in three readings.

Worked Example: Voltage at the Coil

The same symptom, but now you read 120 volts at the supply terminal of the coil while Start is held. Move your lead to the other side of the coil. If that side also reads 120 volts to X2, the path back to X2 is open, which usually means an open OL contact. If it reads 0 volts, the coil has full voltage across it and still will not pull in. Turn the power off, lock out and check the coil resistance. An open reading means a burned-out coil, and a very low reading suggests shorted turns.

When a Contact Depends on Another Coil

If an interlock contact is open, the problem may not be that contact. Find its coil on the drawing, using the cross-reference numbers, and read the rung that drives it. Keep following back until you reach the rung that is really broken. Sequence interlocks and PLC outputs often work this way.

Testing the Power Circuit

If the contactor pulls in and the motor does not run, check all three phases at each stage: line side of the disconnect, load side of the fuses, load side of the contactor, and load side of the overload.

• All three phases are present and balanced at the motor leads, but the motor does not turn: look at the motor or the load.
• One phase is missing: the motor is single-phasing, and the fault is at the point where the voltage disappears. Look for a blown fuse, a burned contact or a loose lug.
• A big difference between phases at one device: a contact or connection there is failing.

A clamp meter on each phase while the motor runs shows imbalance. Currents should be close to each other. If the high reading follows the supply phase when you roll the leads one position, the cause is on the supply side. If it stays with the same motor lead, the problem is in the motor. Do this only as a planned test, with the circuit locked out while you move the leads.

Testing the Motor

Lock out first, and disconnect the motor leads from the starter or drive.

Winding Resistance

Measure each pair of leads: T1 to T2, T2 to T3 and T1 to T3. Readings are low, often under a few ohms, and they should match within a few percent. An open reading or a clear mismatch means a damaged winding. Use a meter with a good low-ohms range, because a small difference can be hard to see.

Insulation Resistance

A megohmmeter, or megger, applies a high DC voltage, typically 500 or 1,000 volts on a 480-volt motor, and measures the leakage to the frame.

1. Lock out, prove dead and disconnect the motor at the starter or drive.
2. Connect one lead to the motor frame, on bare metal.
3. Connect the other to the motor leads.
4. Run the test for 60 seconds, and note the reading.
5. Discharge the winding to ground afterward.

A long-used rule of thumb sets the minimum at 1 megohm plus 1 megohm for each 1,000 volts of rating, or about 1.5 megohms for a 480-volt motor. Newer IEEE guidance sets higher minimums, such as 5 megohms for most low-voltage motors. Healthy motors usually read in the hundreds of megohms or more.

Record the readings over time. A falling trend shows trouble before a failure happens. Moisture also lowers the reading, so a motor that has sat idle in a damp spot may recover after it is dried.

Warning: A megger will destroy electronics. Disconnect drives, soft starters, electronic overloads and PLC outputs before you test.

Symptom Guide

Motor will not stop. If pressing Stop does nothing, shut down at the disconnect. Then find out whether the contacts are welded. Welded contacts usually mean a past short, or a contactor that is too small.

After the Repair

1. Make sure tools are out and covers are on.
2. Remove your lock, following the procedure.
3. Start the motor and check current on all phases, plus the direction of rotation.
4. Listen and look for anything unusual.
5. Write down what failed and why, and mark any changes on the drawing.

Fixing the part is not the end. Fixing the reason it failed is what keeps you from going back.

Common Mistakes

Check Your Understanding

1. A contactor pulls in, but the motor only hums. What do you check next?

Check all three phases at the motor side of the overload. One missing phase is the common cause. If all three are there, check for a jammed load.

2. You read 120 volts across a closed-looking STOP button while the circuit should be running. What does that mean?

The contact is actually open, since full voltage appears across an open contact.

3. A motor runs only while Start is held down. Where is the fault?

The seal-in path. Check the M auxiliary contact and its wiring.

4. A 480-volt motor reads 0.4 megohm to ground. What do you do?

That is too low. Do not run it. Check for moisture, dry it if that applies, retest, and plan for repair or replacement.

5. An overload has tripped twice this shift. An operator asks you to set it higher. What is your answer?

No. Find out why it trips. Measure current, check the load and the voltage, and compare the setting to the nameplate.

Key Takeaways

• Look and ask first. Then check the disconnect, the overload and the safety devices.
• Watch the contactor. That one observation separates control faults from power faults.
• The fault is between the last good point and the first bad one.
• A closed contact reads about 0 volts across it. An open one reads full voltage.
• Isolate the motor from drives and electronics before an insulation test.
• Always find out why something failed before you put the machine back in service.