Advanced Motor Control Circuit Troubleshooting

A dead rung is an easy call. You measure along it and find the open contact. The calls a senior technician gets are harder. The motor drops out twice a shift and never while anyone is watching. The meter says 40 volts on a wire that should be dead. An open contact traces back through four relays to a pressure switch on another floor. This covers those faults, and it assumes you can already read a ladder diagram and find the last good and first bad point on a rung.

What You Will Be Able to Do

• Trace a chain of permissives from an open contact back to the real cause.
• Tell real voltage from ghost voltage.
• Predict what a control circuit ground will do, and why the wiring arrangement matters.
• Trap an intermittent fault with a meter, a recorder or a programmable logic controller (PLC).
• Tell a PLC module fault from a field wiring fault.
• Work on safety circuits without defeating them.

Warning: Voltage readings in a live control panel are energized work for qualified people, with a rated meter and the personal protective equipment the task calls for. Panels can hold more than one source. A control circuit may stay live with the motor disconnect off, so read the print and test before you touch anything. Never jump out a safety device to run a machine.

Permissives: Reading the Chain

A permissive is a condition that must be true before a motor may run. Lube pressure, guard closed, upstream conveyor running, and timer done are all permissives. On a print, they appear as contacts in series with the coil. Many of those contacts are driven by other rungs.

An open contact often is not the fault. It is doing its job, and telling you that something else is wrong.

Worked Example: Following the Chain

A pump starter, M2, will not pull in. The rung reads:

 X1                                                       X2
  |                                                        |
  +--[STOP]--[START]--[CR1]--[TR1]--( M2 )--[OL2]----------+

With a reference lead on X2, and Start held:

1. After START: 120 volts.
2. After CR1: 0 volts. So CR1 is open.

Do not replace CR1 yet. Use the cross-reference number on the print to find the CR1 coil, on rung 14:

 X1                                     X2
  |                                      |
  +--[PS1]--[LS3]-----( CR1 )------------+

1. Measure at the CR1 coil. If it has 120 volts and CR1 is not pulled in, the relay has failed.
2. Here the coil has 0 volts. After PS1 you read 120 volts, and after LS3 you read 0 volts.
3. LS3 is open. It is a limit switch on a guard, and the guard is ajar.

The fix took a closed guard and no parts. A technician who stopped at step 2 would have bought a relay.

Build a Permissive Table

On a machine with many conditions, write one out before you start. It takes five minutes and saves an hour.

Anything open is a lead. Work on those first, and do not move on until you know why each one is open.

Ghost Voltage

A digital meter has a very high input impedance, so it draws almost no current. A wire that is open at one end, but runs for a long way beside live wires, picks up voltage through capacitive coupling. The meter shows it, often tens of volts on a 120-volt panel. There is no real power behind it.

Ghost voltage fools people in two ways. A wire that should be dead reads voltage, so you think a contact is closed. Or a wire reads a low voltage, so you think a contact is dirty.

To tell the difference, use a meter with a low-impedance (LoZ) setting. It loads the wire, and a ghost voltage collapses toward zero, while a real source stays up. A solenoid-type tester does a similar job.

Capacitive coupling can also do real work. On a long run, it can leak enough current to hold a small relay in after the contact has opened. A coil that will not drop out, with a long wire back to a field device, is a sign of this. Fixes include a load resistor across the coil, a shorter run or a lower-voltage control scheme, as an engineer specifies.

Grounds in the Control Circuit

Most control transformers have one side of the secondary grounded, and that side goes to the coils. NEC 430.74 says that where one side is grounded, an accidental ground outside the controller must not start the motor or bypass a shutdown device. The arrangement is why that works.

Keep every remote switch and contact on the hot side of the coil, and the coil on the grounded side. Overload contacts are the usual exception. They are often drawn between the coil and the grounded side, as in the rung above, because they stay inside the controller and are not remote devices.

The second row is why the rule exists, and why a contact in the grounded leg is a serious find. If someone has rewired an old panel that way, report it.

An ungrounded control circuit behaves differently. The first ground does nothing visible, so it can sit there for months. The second ground, in the right place, can bypass a contact and start the motor. If a control circuit acts strangely after a wet shift, lock out the control power and test each wire to ground, or use the ground detection if it has one.

Finding a Blown-Fuse Ground

If a control fuse blows when you try to start, suspect a ground on the hot side.

1. Lock out the control power.
2. Lift the wire to the field devices at the terminal strip.
3. Test each field wire to ground.
4. A near-zero reading is your grounded wire. Work outward from the strip, by halves.

Water in a limit switch, a pinched cable at a door hinge and a chafed wire at a conduit fitting are common causes.

Intermittent Faults

An intermittent fault needs a trap. Do not stand and watch.

Use the Process First

Ask what the motor was doing, what the temperature was and whether vibration changes at that step. Machines that trip only when warm, only on start or only at one position are telling you something.

Use Your Meter

Many meters have a min/max recording mode. Connect across a suspect contact or on the coil, and let it run. A brief drop shows up as a minimum reading.

Use a Recorder or First-Out Data

On a long chain, you can wire a spare input or a recorder to each contact in the chain, with plant approval. Many PLCs also have a first-out signal, which tells you which permissive opened first. The first to open is usually the cause, and the others followed. Without it, every alarm goes off at once and tells you nothing.

Physical Checks

With the circuit locked out, check for these:

• Loose terminal screws. Use a screwdriver, not a look.
• Ferrules that have pulled off wires.
• Wire worn through at a door hinge or cable track.
• Corroded relay sockets.
• A relay that fits loosely in its base.

Vibration-related faults often lie at a moving part, like a cable track or a machine door.

Control Power That Sags

A coil draws far more current in the instant it pulls in. That is its inrush, and it can be many times the current it needs once it is sealed.

Worked Example: The Chattering Bank

A panel has a 100 VA control transformer and four contactors. Each one uses 25 VA sealed and 200 VA inrush, according to its data sheet.

1. With three contactors on: 3 x 25 = 75 VA sealed.
2. The fourth pulls in: 75 + 200 = 275 VA.
3. The transformer is rated 100 VA, so its voltage falls during that moment.

Most coils need about 85 percent of rated voltage to pull in reliably. If the voltage sags too far, the new coil chatters and the others may drop out. A meter shows 120 volts until the moment of the start. The fix is a larger transformer sized for inrush from the maker's table, not a new contactor.

PLC or Field Fault

When a PLC runs the logic, split the problem at the module. Watch two things: the status light, and the voltage at the terminal.

Outputs

Triac outputs leak a small current when off. That can hold in a small relay or keep a pilot light glowing. The fix is a bleeder resistor or a different output type, as the manufacturer directs.

Inputs

If the input light is off and the device has changed state, measure at the terminal. If voltage is there and the light is off, suspect the module. If voltage is not there, the fault is in the field. Sourcing and sinking wiring must match the card, so a replaced sensor with the wrong output type gives a dead input.

Warning: Do not force an input or output to find a fault, unless your plant's procedure allows it and the machine is safe to move. A forced bit can start a machine with no warning, and it stays forced after you leave.

Safety Circuits

Safety relays and safety PLCs watch two channels at once. A dual-channel E-stop opens both channels together. If one opens and the other does not, the relay faults and will not reset. This discrepancy fault is doing its job.

• Check both channels for the fault. A stuck contact block on one side is common.
• Check that the reset is a monitored reset, and that it has been released.
• After any repair, test the function, using the machine's validation steps.

Do not jump a channel. Bypassing a safety device to find a fault can get someone killed, and the jumper may be left behind.

Repeat Failures

Coils produce a voltage spike when they drop out, and that can damage PLC outputs and nearby contacts. Suppressors across the coil are the usual protection.

Common Mistakes

Check Your Understanding

1. Three permissives show as not satisfied on a machine that will not start. How do you work through them?

Write them in a permissive table, and find out why each one is not satisfied. Each may be correctly reporting a real condition somewhere else, so do not move on until you know the reason.

2. A terminal reads 117 volts on your meter and falls to 55 volts on LoZ. What does that tell you?

There is a real source, but it cannot carry current. Look for a high-resistance connection between that terminal and the supply.

3. A control wire chafes against a grounded cabinet on the coil's hot side, in a grounded control circuit that is wired correctly. What happens?

When the contacts close, the control fuse blows, so the motor stops or never starts. It will not start by itself.

4. A PLC output light is on, and there is no voltage at the output terminal. Where is the fault?

The output itself or its fuse, not the program and not the field wire.

5. A motor drops out when a second, large contactor pulls in. The coil reads 120 volts at rest. What do you do?

Record the coil voltage during the start. A sag points to control power, so check the transformer rating against inrush and check the connections.

6. A machine needs a guard switch jumped to find a fault, according to an operator. What do you say?

No. Work with the machine stopped, test the switch and its wiring with the power off, and find the fault that way.

Key Takeaways

• An open contact is often a message, so follow it back to the real cause.
• Use LoZ to tell real voltage from ghost voltage.
• Keep remote contacts on the hot side and coils on the grounded side, so a ground blows a fuse and does not start a motor.
• Trap intermittent faults with min/max, recording and first-out data.
• Split PLC faults at the module, using the light and the terminal voltage.
• Never force an output or jump a safety device to find a fault.