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3 pase motor test solved 1

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rockytee

Electrical
Feb 17, 2007
6
I know it has been a while since my last post. I asked a question sometime ago about a problem one of my customers were having with a 3 hp 3 phase propane pump motor. Got alot of great ideas (thanks to you guys) we finally narrowed it down to a bad set of contactors (AFTER WE HAD REPLACED JUST ABOUT EVERYTHING ELSE!!!!) After overhauling motor, installing new fuses the only thing left were the contactor switch. After replacing it, all has been well. I suppose the contacts were pitted enough to allow current to pass through but would generate enough heat under load to kick out the heaters. Oh well when all else fails replace parts till it works again. LOL. I spent alot of time on this project that I did not bill the customer for for the simple reason that I felt like I was getting nowhere and he was spending a fortune repairing motors(which had been underwater and had ALOT of corrosion inside the connections box)replacing parts and fuses and still was not getting his customer satisfied. OH WELL it was a learning expierience for me. Just for future refrence is there a test for contactor switches.
Thanks again to everybody's input.
Rockytee
 
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Absolutely. With the motor running you can measure directly across the contacts. There will be a voltage. This is caused by the oxidation and wear experienced by the contacts which results in increased resistance.

As the resistance increases with time the operating temperature of the contacts increases. This increases the oxidation rate. => vicious circle. The result can be everything you've seen and more.

On any system I visit or put in service I automatically measure across the contacts to make sure I won't be back soon.

If you are troubleshooting an issue you should make repeated measures after the various starts as sometimes the contacts work well and next time one is not making good contact. A single check may not catch this case.

Contactors have data sheet contact resistance. You can measure the voltage across the contacts when the motor/load is running then measure the current with a clamp on.

R = V/A

Would give you the actual resistance which you can compare to the data sheet or make your own 'health' estimate.

V x A is the power being released in the contact. If you have a feel for heat dissipation you can estimate if what you're seeing is likely okay or needs more investigation.

In lieu of this direct measurement you can use a temp gun and directly measure, (non-contact), the contact temperatures if you can see them. One contact that seems substantially warmer than the others is probably on its way out.

Keith Cress
kcress -
 
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