Checking Electric Motors

[Unregistered]

Hi,
Just wondering the proper way to check electric motors. I am under the impression that you check for ohms. I was just wondering the hook up (ex: power to neutral) on a 115v motor, and 208 motors (single and 3 phase) and what the general reading should be. Thank You for any help.

[Mr T]

Theres way too many factors involving a motor's specs to answer that question directly.

A motor typically has 2-3 coils in it. WHen the motor is not spinning, its just a coil of wire and will have a resistance. If you spin the motor you create voltage in the coil and it will change your "resistance" reading.

Best quick test is to hook up a AC voltmeter to the terminals and give it a spin by hand. A motor works as a generator when you spin the shaft. You should get a reading of anywhere from less then a volt up to a few volts. IT wont be alot.

If you want to get really indepth. A single phase motor has 2 windings in it, a 3 phase motor has 3 windings. You can pull apart the junction box and do the same for each winding. Be very carefull that you get the connections back together exactly as they were, otherwise you can change the motor direction, or the makup of the motor (single phase- series, compound ect). Depending on your motor makup, you may not get a voltage reading on a single coil..but it will measure a resistance....nothing to worry about.

If you have a bad starting cap the motor wont start under load, and MIGHT start with no load. If it doesnt start, CAREFULLY give it a spin (no load) by hand and it will start up.

A motor most likley will have a thermal coutout also. Ones that "auto reset" often do not when they trip. THere are also "one shot" thermo fuses, and they may not be directly servicable. ALso some motors have a overload breaker. If either of these is tripped you will have a open circuit.

IF it is dead or weak, check to see if it has brushes... IF so, they may need replaced.

Note: If your motor happens to be a series motor (there isn't much of a chance that you will run across one). It must have a load on it. If you run it without load, it will spin out of control. Just a disclaimer. :o

IF this "guide" doesnt help, please post more details (what the motor is powering, size/specs of motor, ect)