The widespread use of motors in fields ranging from manufacturing to household applications means that downtime due to a malfunctioned motor can result in a great loss of productivity. A step-by-step approach is the best way you can analyze and subsequently troubleshoot a failed system.
As you read, you’ll learn the essentials to motor troubleshooting, but before you should do so, try to collect the following tools by your side:
- DC power source
- Series field DC power source
- Magnetic compass
- Digital Multimeter
Always start off with checking the motor thoroughly, spending a few minutes trying to find out what went wrong, e.g. burnt windings, broken rotor, etc. Remove the power connections to the motor & starter and record the nameplate information. Here are a few things you should check for:
- Rotate the shaft to see if it rotates freely
- Listen carefully for any unusual noises
- Smell for burned insulation
- Try to feel excess heat at the stator core
- Check for discoloration or hot spots
Once you’re done, re-tighten everything you’ve unhooked.
First, engage the starter with manual rotation and measure the resistance across it. If the resistance is more than 0.10 ohms, this indicates abnormality. Then, disengage the starter and carry out a ground test using the megaohmmeter. The general rule for AC devices is normal operation under 2 megaohms when tested across ground, while DC drives operate not less than 1 megaohm. A precaution during the test would be to disconnect any supplementary electronic equipment as it could get damaged through the high-voltage megohmmeter.
The resistance measured would represent the horsepower, e.g. 50-hp motor would show 0.05-ohms, and so on.
Carry out continuity test using a Multimeter across each fuse. If you find a blown fuse, remember that it would only indicate a faulty line, and replacing it won’t be a solution. Replacing a faulty fuse with a new one would only make it blow again, and if one with a higher-rating is installed, it would pose a greater risk to the motor’s components.
Measure line-to-line voltage and check to see if the change in voltage imbalances is within 5%.
Motor Junction Box
By now you would be aware of the symptoms and the approximate region where the fault exists. Open the motor lead junction box, and check for continuity, abnormal resistances, loose nuts and bolts, etc.
If during the testing you observe low ground readings, then test the stator winding phase resistance, which would give you an accurate idea of the location of the fault. You’ll need to break the connections and first test the motor in one direction and then the other.
In the direction of the motor, if you catch a phase-on-phase short, then the motor would require the winding to be replaced. On the other hand, in the direction of the supply, use the megohmmeter to conduct an insulation resistance test.
Once the problems have been identified and the motor repaired, re-install the motor and the motor control. Run it, and record voltage, current and other measurements to make sure they match the nameplate specifications.