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Motor control · lesson 12 of 12

Troubleshooting a control circuit

Every lab before this one worked. These do not. Each has one hidden fault — an open contact, a burnt-out coil, a welded contact — and the complaint you would get from the operator. Find it the way you would on a real panel, then tap the part you think is bad.

The method

  1. Listen to the complaint and operate it yourself. "Nothing happens" and "it runs while I hold Start" point at completely different parts. Watch the lights and the motor: did the coil pull in or not?
  2. Pick the rung. Which output is not doing what it should? That rung is where the fault is.
  3. Make the rung try to work. Hold the button that should energize it. You cannot find an open contact on a rung that is not being asked to carry anything.
  4. Walk the red lead from the supply end. Black lead on X2. Tap each dot along the rung, left to right. You read control voltage up to the fault and 0 V past it. The part between the last live dot and the first dead one is open.
  5. If there is voltage right up to the coil and it still will not pull in, the coil (or its return) is the problem.
  6. If the controls did their job and the motor did not, the problem is on the power side.

On a real panel this is live-voltage testing: the right meter, the right PPE, and only where your training and the job allow it. Here you can tap all you like.

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Reading the clues without a meter

The pilot lights and the sound of the starter tell you a lot before you open a door. A coil that clunks in and drops out as soon as you let go is a seal-in problem. A Stopped light that comes on while the motor keeps turning means the coil dropped and the power contacts did not. A starter that pulls in with no motor movement moves the search to the power side — fuses, the overload heaters, the motor leads. The fastest troubleshooters are the ones who can say which rung it is from the complaint alone.

Questions people ask

How do you troubleshoot a motor control circuit?

Operate it and watch what it does (and does not do), which narrows it to a rung. Then, with the black lead on the return side of the control supply, move the red lead along that rung from the supply end. The first point that reads 0 V is just past the part that is open.

What does it mean if there is voltage at the coil but it does not pull in?

Every contact in front of the coil is doing its job. Either the coil itself is open, or its connection back to the return side is. Check the return, then the coil.

The motor runs after Stop, but the coil has dropped. Why?

The main (power) contacts of the starter are welded. Stop worked — the coil and its auxiliary contacts dropped — but the power contacts did not open. Only the disconnect will stop it; replace the contacts.

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