Electrical Troubleshooting
of an Electrical Motor Control Circuit

NEVER work on live circuits unless the proper safety equipment and procedures are in place.

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Troubleshooting an electrical motor control circuit

Shut off the power to a circuit before performing any work on it. You must secure all sources of harmful energy before a system may be considered safe to work on. In industry, securing a circuit, device, or system in this condition is commonly known as placing it in a Zero Energy State

It is best to always start with preparation. Make sure you're equipped with documentations and diagrams of the system you're working on and have a thorough understanding of how it was designed to operate. Also make sure you have the proper tools including the PoziDriv for replacing IEC electrical parts

Troubleshooting Relays and Contactors

Next, review the description of the problem and observe the system as you find it.

Note the site gauge. If it reveals that the tank is full and that the mixture appears to be correct, but that the status of previous batch was "timed out", meaning it didn't reach its destination, you'll need to test.

First, make sure all the lights on the control panel are off, and the emergency stop button is pressed.

Proceed to operate the system and look for signs of abnormal behavior.

Before starting the cycle, release the emergency stop to drain the tank. Wait for it to drain fully and for the ready light to switch on.

When the system is restarted, the cycle and intake one lights switch on and the tank begins to fill with the first fluid

At the midway point, intake one stops, and intake two starts.

Once tank is full, the second intake stops.
The agitator light should switch on, and the agitator should start, but it doesn't.

Inside the power box, the agitator's overload is tripped.
Resetting the overload causes the agitator contactor to pick up, but the overload trips again after a few seconds.

Some observations you can make:

The cycle operates normally until the point when the agitator should operate, at which time its overload trips.

You know the control portion of the circuit works as expected. And in the power circuit, you can exclude the two phases which supply the control circuit. Since the overload trips, you know there is current in at least one phase of the agitator circuit. Therefore, the problem area must be confined to the agitator power circuit and third supply phase.

Next, consider possible causes which could be any one of these problems:

  • Agitator mechanism jammed
  • Seized motor
  • Open motor winding
  • Detective overload heater
  • Open contactor contact
  • Open wiring in one phase
  • Blown fuse
Troubleshooting Relays and Contactors

Now, you must consider if there's an obvious probable cause. Several are more likely, such as open fuse, or problems with motor, but if one does not stand out, you will have to test. Because the overload trips, you know that there is excessive current in one or more phases.

For your first test, use an ammeter to measure the current in each phase.

If you read locked rotor current in two phases, but no current in the center phase, you will know that there is an open in the center phase.

Now that you've established a new problem area, the fuse is now the best place to test, since that can be an obvious probable cause.

To safely test voltage in a power circuit

Troubleshooting Relays and Contactors

you must open the breaker each time you place the leads.

If the readings all show full voltage indicating that the fuses are okay, you will now know the fault is an open in the center phase somewhere between the bottom of the fuse and the motor start point.

Since there is no obvious probable cause, you will need to run tests to reduce it.

To find an open in a three phase motor circuit, an ohm meter is the best choice of tools.

Lock out the circuit.

Next, sectionalize the circuit where it leaves the power box, by disconnecting a wire and measuring from there to the fuse with the contactor closed.

If you read infinite resistance, that means the open is within the control box.

Testing at the top of the contactor with it closed indicates that the contact is okay.

The only possible causes left are one wire and its connections.

If you begin to disconnect wire and find a loose terminal, which fits the symptoms, try tightening the terminal to repair the circuit.

To verify that the repair was effective, test a full path through the motor.

If you get a low resistance reading, that indicated that the path is complete.

Test operate system to ensure that there are no other problems.

When restarting system, observe that the tank fills with the first fluid, then switches to the second fluid.

Once the tank is full, listen for the agitator the start. If it does, you have repaired the malfunction.

The cycle should now continue mixing and heating fluid. When the proper temp is reached, the agitator and heater should stop, and the mixture should then be pumped to its destination. The batch status should now show passed.

You were successfully able to troubleshot and repair the electrical motor control circuit!

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