Testing AC motors

[westmc]

Hi,

How can you test a three phase AC motor for single phasing?

From what i understand you can check the switchgear stabbings for heat damage and also check the 3 cores for 415V between the phases.

What else can you check with just a megger or fluke?

Would checking the balance of the windings or meggering the phases to earth prove the loss of a phase?

 

[dpc]

With the motor running, you should see a pretty good current imbalance if you've lost a phase. Voltage tests can be misleading because the motor will tend to provide the missing voltage if the load isn't too great.

Meggering probably isn't going to help much.

 

[Skogsgurra]

Agree. You cannot do serious work on drives without a clamp-on ammeter. Get one thet can be connected to your fluke and you will be a better electrician.

Gunnar Englund

http://www.gke.org

 

[westmc]

Unfortunatly the motor will not run, are there any other tell tale signs or tests that can be done to check if a motor has been single phasing?

 

[itsmoked]

If the motor is now dead... One phase will probably be burnt out. You might be able to use your nose.. But I would think meggering would show a problem (why not dpc?) furthermore you could see a resistance difference between phases.

Have you ohm'd out the motor? What did you see?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[westmc]

yeah ohm'd out the motor from the substation and all was balanced at 0.27ohms respectivly

iv had a look and a smell and couldn't smell anything, the motor connections seem okay, and i have checked the switchgear apart from the contactor and everything seems fine. I think i will check the contactor tomorrow anyway

 

[dpc]

If you are concerned about the motor windings, a megger test is a good idea.

The reference to "single-phasing" lead me to think the OP was concerned about the loss of one phase of the power supply to the motor. Megger test of the motor in that situation isn't going to help much.

I'm still confused - will the motor run or not? If the motor will run, a clamp-on ammeter should be used to check the motor current.

 

[itsmoked]

dpc; What part of [green]"Unfortunatly the motor will not run"[/green] do you need help with?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[54494060]

westmc,
You need to go back to the beginning. Look at the source voltage (make a voltage check). If the motor will not operate and you have the correct voltage (all three phases)search for the electrical and "mechanical" integrity of your starter and control circuit. Control transformer, fuses, start stop station(s) etc. If you have no voltage at the line side of the supply breaker and you have voltage to the bus, you could (!! Caution !!) pull the 'STARTER BUCKET and look (inspect) the STABS to see if they are lining up with the bus, or the SUPPLY CONDUCTORS to the line side of your breaker may have failed. Ensure all electrical connections are sound (Tight).
One other thing, if you are not sure about the troubleshooting events mentioned, be sure to ask someone that may be more qualified.

12fish
As Iron Sharpens Iron, so does a man sharpen the countenance of his friends

 

[dgallagher]

You may want to consider a tool such as the AllTestPro 31 for motor testing. It detects any type of motor electrical fault: shorted windings, winding contamination, insulation breakdown, poor connections, ground faults, phase unbalances, and all types of rotor faults.

http://www.reliabilitydirect.com/motormonitor/pdf/AT31.pdf

Dave G.

http://www.ReliabilityDirect.com

 

[machmech]

Hello westmc

Be very careful that is a lot of energy! You should have power off then manually activate starters or relays to check for continuity through out circuit if possible. You mentioned using a Fluke which is most likely at least cat 2 rated this is a good safety measure.

Good luck, Chuck

 

[dpc]

itsmoked,

Thanks for unsolicited sarcasm. I probably need help with a lot of things. But the fact that a motor won't run and a megger test shows a shorted or open winding, does not prove that "single-phasing" was the cause of the failure. Motors won't run and fail for all kinds of reasons.

I'd still like to understand how a megger test of a motor is going to prove that one of the supply phases to the motor was lost.

 

[bigbillnky]

westmc

I guess we are all just trying to figure out exactly what you are asking help in trying to figure out.

Are you attempting to root cause a motor that is known to have failed due to single-phasing, or are you trying to determine why this motor will not run, thinking it could be due to a single phase condition?

The best way to verify that a motor actually failed due to a single phase condition is to open the motor and examine the stator windings. If the motor was delta connected, only one of the three windings will be charred. If the motor was wye connected, two of the three windings will be charred.

As stated by skogsgurra previously, an amp clamp is an absolute must when performing motor diagnostics.

dpc,
I have seen many motors that single-phased due to a supply conductor or t-lead grounding out and NOT tripping the overcurrent protection out.

Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[bigheadted]

hi
you say the motor tests to be balanced between phases and doesnt smell. a megger test would show if the insulation had broken down, but this would have probably blown fuses.
isolate the suply
reconnect the motor, have someone stab in the contactor manually and if the resistance readings are similar to that that you measured across the motor windings then i would say that the motor starting circuit is likely to be ok. if not try measuring across the contact of the contactor as a first port of call for high resistance(should be low). if this all apears to be in order
and the supply is definately ok, having the u1 and u2,v1v2etc connections mixxed up so that the current is flowing the wrong way through the windings can look like single phasing, has the motor ever run?

 

[westmc]

Thanks for all your replies