About Motor troubleshooting and the use of Meggers…

This particular activity is potentially very dangerous because power needs to be applied and locked out frequently, so please follow proper lockout-tagout procedures!
Meggers are a must in the area of industrial electrical troubleshooting because they can help quickly identify some sources of malfunctions while eliminating other sources as the cause of the malfunction.
As an example, let’s say that a motor refuses to turn. The problem could be coming from let’s say 3 different directions.

1. it is not receiving the necessary signal or voltage needed.

2. The motor itself may have become defective

3. The load is too great for the motor to physically overcome.

We will cover the other 2 possibilities but focus on number 2, which is the scenario that requires the use of our megger.

Scenario number 1:The motor is not being told to run.

Typical VFD, (Variable Frequency Drive)

Something is going to tell this motor when it should run, it could be a hand operated on-off switch, a pilot device like a level probe or temperature sensor, or possibly a Programmable logic Controller executing code to run a process or machine.
Once it is determined that Control is the problem, then that is the direction the troubleshooting must take.
While on this step it would probably be advisable to make sure the VFD (if used) or motor starter is not defective. For the VFD look at any error codes on the display and for a starter, with ALL power off,  (use a meter across the top of the starter to be sure) manually activate and read( top to bottom) across the contacts with an ohmmeter, they should read near zero ohms. Also physically inspect the contacts, if practical.

Scenario number 2
The motor itself has become defective.

Three-phase induction motor

This is where our megger comes in.
The megger is similar to an ohmmeter except that the voltage it puts out is in the hundreds or thousands of volts range, rather than say 5 to 12 volts of the ohmmeter.
The reason for this is that sometimes the windings within the motor will not short out until a certain voltage threshold is reached, therefore an ohmmeter may be misleading.
When using a megger it is EXTREMELY important to completely disconnect the motor leads from everything else so that you do not blow up any other equipment with those super high voltages.
The megger usually has output voltage options which the user needs to be aware of. Depending on what he or she is testing, it’s going to make a difference. For example, THHN or similar wiring may be guaranteed to 600v. This means that beyond 600v the insulation of the wire may break down, therefore if you are testing wiring with your megger it may show a short if you turn up the output voltage of the megger beyond 600v.

This would be a good place for me to stipulate that I am referring to 3 phase squirrel cage type AC motors.

Interestingly enough the old sniff test should be first thing tried, It should not smell like burnt metal.
When testing motors it is also important to know what you SHOULD be seeing with a good motor. If you test LEAD to LEAD you are going to see very low ohms, this is normal but the exact reading is critical. For a huge motor(50hp or so) it may be 1 ohm or even slightly less. Maybe a few ohms for smaller motors.
However, any lead to earth ground should be in the megohm range even at maximum megger voltage settings. (unlike wiring). If I saw less than 6 megohms I would be very suspicious. At that point you are getting into the area of trying a known good motor if you could find nothing else wrong.
This is not an exact science and those are general guidelines.
I have seen on many occasions motors that seemed ok but the problem was actually corrected by a new motor. Luckily though a bad motor is usually pretty obvious.
If the motor seems OK with the insulation test then go ahead and investigate scenario 3

Scenario number 3:
The physical load is too great for the motor.

This situation could manifest itself with blown fuses or tripped circuit breakers, so it is recommended to use a megger before proceeding here for the sake of not needlessly blowing expensive fuses.

(Please bear in mind that once the motor is physically disconnected, certain servo type systems may need re-calibration.)
If the motor checks out with the megger, then physically disconnect the motor from the load and check the bearings of the motor by turning the shaft by hand. Make sure you have followed proper lockout-tagout procedures first.
Then try moving the load, checking for excessive resistance.

Finally, physically secure the motor so that it doesn’t jump when started. Electrically reconnect the motor then remove the lockout and start the motor to see if it is willing to run normally. ( physically disconnected from the load).
Make sure your motor remains locked out unless you intend to apply run voltages. then re-lock.
By this time you hopefully have found the problem.
Some detailed information on the subjects of using a megger and all things about insulation may also be found here:

http://www.techni-tool.com/site/ARTICLE_LIBRARY/Megger%2520-%2520The%2520Complete%2520Guide%2520to%2520Electrical%2520Insulation%2520Testing.pdf

Happy Hunting and Godspeed!

Wayne Dover,

Admin@electricalequipmenttesters.com

2960total visits,6visits today

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