Check an Electric Motor
When a motor fails, it is often difficult to see why it failed just by looking at it. A motor placed in storage may or may not work, regardless of its physical appearance. A quick check-out can be done with a simple ohm meter, but there's a lot more information to collect and weigh before actually putting it to use. At no time during the motor check out is power required. If it is connected - disconnect it before attempting the steps below.
Contents
Steps
Checking the Outside of the Motor
- Check the outside of the motor. If the motor has any of the following issues on the outside, they may be problems that can shorten the life of the motor because of previous overloading, wrong application, or both. Look for:
- Broken mounting holes or feet
- Darkened paint in the middle of the motor (indicating excessive heat)
- Evidence of dirt and other foreign matter having been pulled into the motor windings through openings in the housing
- Check the nameplate on the motor. The nameplate is a metal or other durable tag or label that is riveted or otherwise affixed to the outside of motor housing called the '"stator" or "frame". Important information about the motor is on the label; without it, it will be difficult to determine its suitability to a task. Typical information found on most motors include (but not limited to):
- Manufacturer's Name — the name of the company the made the motor
- Model and Serial Number — information that identifies your particular motor
- RPM — the number of revolutions the rotor makes in one minute
- Horsepower — how much work it can perform
- Wiring diagram — how to connect for different voltages, speeds and direction of rotation
- Voltage — voltage and phase requirements
- Current — amperage requirements
- Frame Style — physical dimensions and mounting pattern
- Type — describes if frame is open, drip proof, total enclosed fan cooled, etc.
Checking the Bearings
- Begin to check the bearings of the motor. Many electric motor failures are caused by bearing failures. The bearings allow the shaft or rotor assembly to turn freely and smoothly in the frame. Bearings are located at both ends of the motor which are sometimes called "bell housings" or "end bells".
- There are several types of bearings used. Two popular types are brass sleeve bearings and steel ball bearings. Many have fittings for lubrication while others are permanently lubricated or "maintenance free".
- Perform a check of the bearings. To perform a cursory check of the bearings, place the motor on a solid surface and place one hand on the top of the motor, spin the shaft/rotor with the other hand. Closely watch, feel, and listen for any indication of rubbing, scraping, or unevenness of the spinning rotor. The rotor should spin quietly, freely and evenly.
- Next, push and pull the shaft in and out of the frame. A small amount of movement in and out (most household fractional horsepower types should be less than 1/8" or so) is permitted, but the closer to "none" the better. A motor that has bearing-related issues when run will be loud, overheat the bearings, and potentially fail catastrophically.
Checking the Windings
- Check the windings for short circuiting to the frame. Most household appliance motors with a shorted winding will not run and will probably open the fuse or trip the circuit breaker instantly (600 volt systems are "ungrounded," so a 600 volt motor with a shorted winding may run and not trip a fuse or circuit breaker).
- Use an ohmmeter to check resistance value. With an ohmmeter set to the Resistance or Ohms test setting, place test probes into the appropriate jacks, usually the "Common" and "Ohms" jacks. (Check the meter's operation manual if necessary) Choose the highest scale (R X 1000 or similar) and zero the meter by touching both probes against each other. Adjust the needle to 0 if possible. Locate a ground screw (often a green, hex head type) or any metal part of the frame (scrape away paint if needed to make good contact with metal) and press a test probe to this spot and the other test probe to each of the motor leads, one at a time. Ideally, the meter should barely move off the highest resistance indication. Make sure your hands are not touching the metal probe tips, as doing so will cause the reading to be inaccurate.
- It may move a fair amount, but the meter should always indicate a resistance value in the millions of ohms (or "megohms"). Occasionally, values as low as several hundred thousand ohms (500,000 or so), *may* be acceptable, but a higher number is more desirable.
- Many digital meters do not offer the ability to zero, so skip the "zeroing" information above if yours is a digital meter.
- Check that the windings are not open or blown. Many simple "across the line" single-phase and 3-phase motors (used in household appliances and industry respectively) can be checked simply by changing the range of the ohm meter to the lowest offered (R X 1), zeroing the meter again, and measuring the resistance between the leads of the motor. In this case, consult the wiring diagram of the motor to be sure that the meter is measuring across each winding.
- Expect to see a very low value of resistance in ohms. Low, single digit resistance values are expected. Make sure your hands are not touching the metal probe tips, as doing so will cause the reading to be inaccurate. Values greater than this indicate a potential problem and values significantly greater than this indicate the winding has failed opened. A motor with high resistance will not run - or not run with speed control (as is the case when a 3-phase motor winding opens while running).
Troubleshooting Other Potential Problems
- Check the start or run capacitor used for starting or running some motors, if equipped. Most capacitors are protected from damage by a metal cover on the exterior of the motor. The cover must be removed to access the capacitor for inspection and testing. A visual inspection might indicate oil leaking from the container, bulges in the container, or any holes in the container, burnt smell or smoke residue — all potential problems.
- Electrically checking a capacitor can be done with the ohm meter. Placing the test probes on the capacitor terminals, the resistance should start low, and gradually increase as the small voltage supplied by the meter's battery gradually charges the capacitor. If it stays shorted or does not rise, there is probably an issue with the capacitor and may need to be replaced. The capacitor will have to be allowed 10 or more minutes to discharge before attempting this test again.
- Check the rear bell housing of the motor. Some motors have centrifugal switches used to switch the start / run capacitor (or other windings) "in" and "out" of the circuit at a specific RPM. Check the switch contacts are not welded closed or are contaminated with dirt and grease that could prevent a good connection. Use a screwdriver to see if the switch mechanism and any spring can be operated freely.
- Check the fan. A "TEFC" type motor is a "Totally Enclosed, Fan Cooled" type. The fan blades are behind metal guard on the back of the motor. Make sure it is securely fastened to the frame and is not clogged with dirt and other debris. The openings in the rear metal guard need to have full and free air movement; otherwise, the motor will overheat and eventually fail.
- Choose the right motor for the conditions in which it will be run. Check that drip-proof motors are exposed to directed water spray or moisture, and that open motors aren't exposed to any water or moisture at all.
- Drip-proof motors can be installed in damp or wet locations, so long as they are installed in such a way that water (and other liquids) can not enter due to gravity and must not be subjected to a stream of water (or other liquids) directed at or in it.
- Open motors are, as the name implies, completely open. The ends of the motor have rather large openings and the windings in the stator windings are plainly visible. These motors should not have these openings blocked or restricted and should not be installed in wet, dirty or dusty areas.
- TEFC motors on the other hand, can be used in all the previously mentioned areas but must not be submerged unless designed specifically for the purpose.
Tips
- A NEMA Quick Reference List can be consulted for all dimensional motor data.
- It is not that uncommon for a motor's windings to be both "open" and "shorted" at the same time. At first look, this may seem to be an oxymoron, but it really isn't. An example could be an "open" circuit caused by an electrical failure caused by a foreign object that falls into or is drawn magnetically into the motor or excessive voltage that literally causes a wire in the winding to "blow up" or melt. This results in a broken path - or "open circuit". Either if end of the wire at the open point - or if some melted copper wire should encounter the motor frame or other grounded part of the motor - a "short circuit" results. It doesn't happen often - but it does happen.
