Diagnosing your problem starts with using your senses
Got a motor down? No matter how urgent the problem — and motor failure can cause some very urgent problems — always develop a plan of attack before you start repair work. Gather up any background material you may have on hand and then use your senses to inspect the site. Then conduct resistance, fuse, and motor tests to help pinpoint the cause of the failure.
Initial inspections. Spend a few minutes with the operator and get as much history as you can on the failed motor. Find out if anyone else recently attempted repairs or modifications. If so, what did they do and when? Sometimes an inexperienced person can unwittingly create additional problems while trying to help. Knowing what has already been done can shorten troubleshooting time.
Armed with background information, you're ready to start your inspection process. Begin by removing power to the motor and starter — following approved and accepted lock-out/tag-out procedures — and disengaging the motor. Then after recording the motor nameplate information, turn the shaft to determine if it rotates freely. Your basic senses can also help you determine a great deal about what's wrong. Listen carefully for unusual noises (such as scraping), smell for burned insulation, and feel for excess heat. Now look at the motor starter for loose connections and hot spots indicated by discoloration. Check all fasteners, including the mounting hardware, and re-tighten. This completes your initial pre-measurement inspections. Now it's time to begin some testing.
Resistance of line and load circuits to ground. Manually engage the starter and measure the resistance through its contacts. You should read 0.09 ohms or less. Disengage the starter and inspect the contacts closely. You'll need to use your megohmmeter to ground test line and load circuits at the starter. This will effectively identify the resistance to ground of the starter, line circuits to the disconnect, and load lines to the motor and starter windings.
Generally, AC devices can safely operate at not less than two megohms to ground, and DC devices can safely operate at not less than one megohm to ground. But be careful: before ground testing, make sure you disconnect any electronic controls because they can be destroyed by misapplied high-voltage test equipment.
The resistance you see will depend on the horsepower of the motor. For example, a 50-hp motor should ideally show 0.05-ohm resistance. Measurements between phases should be roughly equivalent. Given the variety of motors in use, you may not have access to the specific engineering data for each one. However, the exact measurement value is less important than the balance between phases. While you can't expect identical readings, they should be close.
For single-phase and DC applications, the resistance measurement is suitable for identifying open circuits and for historical use.
Fuse checks. Check each fuse for continuity (one fuse per phase). Remember that a blown fuse is a symptom, not a cause. Don't assume that replacing a fuse will fix the motor. By doing so, you'll run the risk of causing even more damage and downtime.
Then check the fuse holders for spring tension. Repeated fuse replacement can “expand” the fuse holder (usually a clip) and compromise the spring tension necessary for good contact. Pinhead-sized arc damage anywhere on the clip, fuse, or surrounding areas is evidence that the fuse holder has lost tension. Inspect breakers and busses for overheating and arc damage. Inspect the cord caps (connectors) for correct circuit installation and tightness.
Line to line checks. Now you're ready to check voltage value and balance on the line (supply) side of the fuseholder. You're testing line to line, so operating voltage doesn't have a path to ground. If you find an unreasonable voltage imbalance between any two phases, that's a problem. A 5% voltage unbalance is normal and reasonable.
But don't assume the supply is healthy if you get good readings — even the weakest electrical connection will pass a voltage reading without applied load.
Motor junction box. By this point you've made preliminary determinations of where the problem exists. Now it's time to open the motor lead junction box and check the connections inside. Even if you found nothing wrong during the preliminary testing, you should still check the motor connections — many motor failures result from poorly installed wire nuts or insufficiently insulated connections grounding inside the junction box or shorting together.
If during your testing you recorded low ground readings or open readings at the load side of the starter, your next step is to test stator winding phase resistance and resistance to ground. This will help you determine whether the discrepancy is in the motor or in the line circuit. To conduct the tests, break the motor connections and test first in one direction (the motor) and then in the other (the supply).
For the motor, test the stator winding resistance phase-to-phase and phase-to-ground. If you find a phase-on-phase short, the motor needs evaluation for a rewind or for replacement. If you find a phase-to-ground short, a motor shop may be able to do an in-place repair for a large motor. However, you might end up needing to rewind or replace it.
For the supply, test first with the motor disconnect open and locked out. Use your megohmmeter to test the insulation resistance to ground. If the wiring fails that test, look for problems in the connections or in failed conductor insulation. Note: A stator phase resistance test will identify a bad motor but isn't an absolute verification of a good motor. Final load voltage and current tests verify a good motor. You never know when you'll find a motor that tests well but has a thumb-sized hole blown from the coils.
Final tests and procedures. Once you've corrected all identified problems and the motor is installed and aligned, you're almost done. Energize the motor and test the controls and overall system operation. While the motor is running, record the operating voltage and current, check the balance, and verify that your measurements are within the nameplate specifications. Then apply the sense tests again:
Listen for unusual noises.
Smell for smoke or hot insulation.
Feel the motor for excessive heat or vibration.
Look for possible obstructions.
Now engage the load. Apply the four sense tests again. Once you're sure you've addressed all of the problems with this motor, it's time to go home.
Toland is the foreman at Buzzell Electric Works in San Francisco. This article was prepared for and supported by Fluke Corp.
Sidebar: Tools You Need
Minimum 500V output with high impact rating.
Must measure AC and DC and feature multiple jaw sizes.
- General DC power source
An example is a 35A bridge rectifier configured with a 120V power cord and test leads.
- Series field DC power source
A 9V battery will suffice.
- Magnetic compass
Used to quickly identify field coil polarities.
- Digital multimeter
Minimum 600V, CAT III true RMS rating, with low resistance (0.01 ohms or lower) featuring a cycle function and capacitance test.