Megging a motor

[mull982]

Hello

When Megging a motor to determine if it is good or not what is an acceptable reading on the megger that will tell you that the motor is good and everything is terminated properly (eg.. no shorts to ground). What is a minimum reading that you want to see on your megger to verify that a motor is good? Below what reading should you check your motor feeders or determine that you have a bad motor?

Thanks

Mull982

 

[ptonsparky]

Really does depend on the motor you are testing. Larger ones have different values then smaller. Keeping track from day one and megging on a regular basis is the best policy.

We megged several for a customer last fall with one of the motors and feeder testing at over 1 meg. No bad, but all the rest had checked out at 1G. Investigation found an occasional ground that had developed some carbon tracking.

 

[Classified]

For a rule of thumb, a 10 meg-ohm reading would be acceptable in most any applications for a motor megger test.

Obviously a 20, 50 or higher meg-ohm reading is better. Depends on the motor being tested to some extent.

 

[mdshunk]

I condemn any motor that megs less than 20 megohm.

 

[brian john]

3 techs 3 answers.

We like to see 50 meg or better, depending on age, ect...below 50 meg start start budgeting for a replacement, below 10-20 open the check book, below 5 run it only in an emergency.

As noted above the best approach to meggering motors (or any distribution equipment) is to track it from day one. Have a baseline readings with temperature and humidity recorded at regular intervals. This allows for planning for replacements.

With larger motors there are other megger test you can do.

 

[zog]

4 techs - 4 answers. Lets make it 5. (And this one comes with a reference ( NETA MTS 2005 ))

IR 1 minute = kV+1 for most windings made before 1970, all field windings, and others not lited below.

IR 1min=100 Meg for most DC armature and AC windings built after 1970

IR 1min= 5 Meg for most machines and random wound stator coils and form wound coils rated below 1kV.

All values temp corrected to 40 degrees C.