# Testing a motor and/or drive



## 5volts (Jan 11, 2008)

Ok, I know we all have ran in to situations where we get a "OVERLOAD" fault on a vfd. Is it the Motor? Is it the VFD?

The steps I take are pretty simple and I have seen many electricians get bit before because they either didn't take a megger reading, or didn't well...its pretty basic

1. If the motor is suspected of causing the fault first use an ohmmeter, I prefer a simpson meter. Take readings Phase to phase with the T leads disconnected from the VFD. T1-T2,T2-T3,T1-T3. Determine if these are balanced.

2. Use a megger to read from phase to ground. This checks for breakdown of insulation of motor windings and feeds. the rule of thumb is 1000ohms per volt. Then .5 or half a meg to ground would be acceptable, however some VFD's don't like low megs. You decide!

3. If the motor reads bad at the panel check it from the motor pecker head. If the motor reads good check the feeds to the motor.

4. If the motor and feeds pass the meter check, check the drive (VFD). There are several possible things and we need to eliminate them one by one. Check all three fuses, I have seen where one is blown and the indication is still active at the HIM. Next with the motor T leads disconnected (put the motor in hand) use a volt meter to read the output of the drive to determine if the voltage output is balanced.

5. If steps 1-4 test good, another possible test is to connect the motor to a known good drive in the panel if there is one available and try it from there.

6. Make sure that no one has accidentally hooked a brake up to the T leads in the pecker head when the motor has a VFD.

7. Make sure the brake (if it has one) is released during testing.

What you guys think? Any one else have ideas, or any more suggestions?


----------



## TheRick (Apr 13, 2008)

Yeah that is prety much the procedure I follow for testing a motor on a VFD;

Disconnect T-Leads at the VFD.
Read motor winding resistance to check for an open winding
Megger the T-Leads individually to ground
Disconnect at the peckerhead and repeat.
If all of these readings are good, check the VFD output, this can sometimes be difficult as many are load sensitive, and will not put out any voltage w/o a load attached so you may need a known good test motor for this purpose. READ THE MANUAL FOR THE VFD
A couple of other points to remember...you should never attach any type of test equipment without first knowing what you expect to see for a reading...what will a "good" reading be, and what will a "bad" reading be. And never assume a motor is bad (grounded) based on one megger reading at the source, always megger again in the peckerhead. I have seen a few "bad" motors changed out, only to find after the new motor is in place and wired up :blink: there is still a ground fault, because it was in the T-Leads to the motor all along!

_"Forget the adadge about learning from your mistakes...it is safer and much more entertaining to learn from the mistakes of others!" :jester:_


----------



## John Valdes (May 17, 2007)

Most standard VFD's will operate without a motor connected. (no feedback device) This is a good first step to make sure the control is not the problem. Sometimes adding a load is when the problem you are facing will surface. Another option is to operate the control and load from the keypad. This can indicate where the problem lies, or at least get you a head start.


----------

