Just about any industrial facility, regardless of what business segment it represents, has an abundance of motors in use. When you’ve got motors, you’ve got spares. Shelf after shelf of spares, and motors are coming and going all the time. The shipping and receiving folks see a pallet with a motor strapped to it, they call maintenance/engineering and a forklift driver comes and hauls it to the designated shelf where it waits for its opportunity to get into the game. That’s the typical reality of receiving a new motor, but it shouldn’t be. The minute a new or rewound motor hits your dock, there’s an opportunity to determine if that motor should stay or go, based on its condition when you receive it.
Call it motor acceptance, Quality Assurance testing, it makes no difference. As every business, motor shops included, try to do more with less, things can fall through the cracks. Motor rewinding is an intricate process that requires an incredible level of attention to detail. Things can and do go wrong in the operation that can cause a motor to be in less than optimum condition fresh out of the process. Add to that the variable of material condition, like the porosity of the rotor or microscopic defects in stator laminations and the chance of receiving a motor that is 100% electrically and mechanically sound decreases. By following some basic testing steps however, you can separate the bad apples from the rest of the barrel and make sure your spares are ready to rock when you need them. Use any or all of the following test methods on new motors, preferably before they’re accepted from the vendor.
Polarization Index; we’ve talked about this one before. PI testing can be used to trend the condition of a motor, or to validate condition based on a standard. Insulation Capacitance can detect winding contamination, and is an excellent trending tool for motors in operation as well. Testing winding resistance for balance is very useful, because it tests the electrical conductivity through solder joints, end turn connections, essentially every connection internal to the motor. Imbalances can be the result of high resistance connections, and like many of these other testing methods is also useful for motors in service. Testing impedance of the windings can reveal turn-to-turn shorts, because they will cause an imbalance between phases. The Rotor Influence Check (RIC) is also an excellent test for a newly received motor. When the motor is received is probably the easiest time to perform a RIC test, since there’s nothing coupled to the shaft, so its position can be changed more easily. The RIC is essentially looking at impedance in different rotor positions, and can help find air gap eccentricity problems that resulted from the motor assembly process.
If your facility already has a comprehensive motor testing program, you may already be checking newly received motors. If you’re not, shhhhhh don’t tell anyone, just start doing it. If you don’t have a motor program, acceptance/QA testing is a great way to start one. Either way, check those motors when they hit the dock, and have increased confidence in your spares.