Electric motor testing using milliohm meters

[Cgmaster1]

Hi am do work as a service technician on automation and controls and have run into issues testing motors to verify they are good. They are 3phase usually 220v sometimes 380-480v. We deal with different types of motors from standard, servo, seldomly stepper and multipole motors.

 I am looking for a good milliohm meter to test motors. I already have a megger but I may look into getting a better one as well. The one I have is a fieldpiece made for HVAC techs and only rated to 240v

I am looking for one that can run on AC power mainly but being able to use batteries would be nice. I hate leaving battery's in equipment due to problems I have had with them leaking previously.

The reason for this is this is I have a machine we just installed a spindle drive into that blew the IGBT W output. The original had a bad V output. The spindle spins freely and we megged it and everything looks good. I have had this happen before on another machine we ended up changing the motor even though what we tested was good it was blowing drives.

I spoke to a motor shop and they said we need to get a milli ohm meter. Unfortunately he didn't know much about it.

Lastly on testing what test should we do on motors and drives? Currently I check incoming voltage to the drive, Meg the motor and check it rotates freely. 

[CaptDon]

A 4-wire milliohm meter could help, however, It still may be impossible to determine that the motor has one or two shorted turns. The reason is that on a good multi-phase motor there is already a 3 to 5% difference in the readings obtained and one or two shorted turns may not be evident. You would almost have to test and record the readings of good 'same make and model' motors to compare with suspected bad motors. If the short is across several turns of the winding it would be more evident to a milliohm meter as well as more likely to blow up a VFD. There are plenty of DMM's on Ebay that are capable of 4-wire milliohm measurements as well as all standard measurements you may encounter. Some of the gray plastic Fluke 'lab grade' meters are nice choices. I made my own 4-wire test leads. There will be two red wires, one end of each will have a red banana plug and the opposite ends get soldered together onto an alligator clip. The same is done to create the 'black' test lead. If you don't understand 4-wire test leads the object is that the method eliminates test lead resistance (which may be higher than what you want to measure) and gives you a true idea of what the resistance is that your gator clips are attached to, a.k.a. the unit under test. Once you own this meter and test lead you should create a record of the milliohms of every new motor you install for future reference and remember this very important fact, the measured milliohms will be MUCH higher on a hot motor so you may need a 'hot' reference and a 'room temperature' reference. I sometimes also use a low a.c. voltage like 6 or 12 volts from a control transformer and a series ammeter to look at comparative current draw. Even at this low voltage a motor with shorted turns will draw more current than a good motor but once again, you will need some sort of reference. Cheers mate, Merry Christmas.