checking contact resistance on motor starters

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steveng

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when using a megger to check a motor starter contact resistance, is it as simple as measuring the contact closure? when energized, with no load?

i understand you have to clean the pitted contacts, are there any other checks you can make?

i have some older ge motor starters, 40 hp fan motors, and when i start checking these i am seeking advice or any helpful tips.;)

thanks,
Steve
 
steveng said:
when using a megger to check a motor starter contact resistance, is it as simple as measuring the contact closure? when energized, with no load?

i understand you have to clean the pitted contacts, are there any other checks you can make?

i have some older ge motor starters, 40 hp fan motors, and when i start checking these i am seeking advice or any helpful tips.;)

thanks,
Steve
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I don't know what any standards body says, but we put a voltmeter across a loaded closed contactor and used an amprobe; basic R=E/I was assumed.
 
No so sure I'd pick a megger for that test, unless you were checking for phase to phase or phase to chassis faults. You will get better unenergized results by using a DLRO or do an FOP test while energized.
 
marc, can you help a guy out with the definitions of those abbreviations?;)

thanks , georgeb,
 
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DLRO, Digital Low Resistance Ohmeter. For breaker and starter contacts you should have at least a set with a 10A output.

FOR, Fall Of Potential (Voltage drop) test, done while energized.
 
steveng said:
when using a megger to check a motor starter contact resistance, is it as simple as measuring the contact closure? when energized, with no load?

i understand you have to clean the pitted contacts, are there any other checks you can make?

i have some older ge motor starters, 40 hp fan motors, and when i start checking these i am seeking advice or any helpful tips.;)

thanks,
Steve
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There is a lot more to check, see NETA MTS standard for guideance. Also refer to the equipments maintenance manual for specific checks like contact pressure, alignment, gap, wipe, lubrication guides and approved lubricants, spring pressures, etc....There is a lot to do on these.

What do you plan on using to clean the "pitted contacts"?
 
While I have burnished contacts in extreme circumstances, I always tell customer new contacts are cheap insurance (if inspection determines they are worn), a lot cheaper than a new motor.
 
Fluke provides a couple of meters with a Low-ohms feature for this reason/applications. I have the 1520, and the 1507. The 289 has it as well. I have the 87V which has a conductance mode, the 189, and 289 do as well. It measures in Siemens.
 
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76nemo said:
Fluke provides a couple of meters with a Low-ohms feature for this reason/applications. I have the 1520, and the 1507. The 289 has it as well. I have the 87V which has a conductance mode, the 189, and 289 do as well. It measures in Siemens.
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You cant use an ohmeter to measeure microhms accuratly, you need a bridge.
 
thanks zog

thanks zog

There is a lot more to check, see NETA MTS standard for guideance. Also refer to the equipments maintenance manual for specific checks like contact pressure, alignment, gap, wipe, lubrication guides and approved lubricants, spring pressures, etc....There is a lot to do on these.

What do you plan on using to clean the "pitted contacts"?


I see you are neta certified in electrical testing, i wanted to say thanks for the advice you gave.

we have neta company that has been doing some work for us, i plan on getting some training , we have some 30 yr old ge starters, air handler starters, pump starters, that need the maint now!

do you know some helpful links?
 
steveng said:
There is a lot more to check, see NETA MTS standard for guideance. Also refer to the equipments maintenance manual for specific checks like contact pressure, alignment, gap, wipe, lubrication guides and approved lubricants, spring pressures, etc....There is a lot to do on these.

What do you plan on using to clean the "pitted contacts"?


I see you are neta certified in electrical testing, i wanted to say thanks for the advice you gave.

we have neta company that has been doing some work for us, i plan on getting some training , we have some 30 yr old ge starters, air handler starters, pump starters, that need the maint now!

do you know some helpful links?
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Well i am pretty sure I know who you are using based on your location and if I am right you ar in good hands.

www.netaworld.org is the NETA website, you have to buy the standards but you can view archived articles from the NETA world magazine for free, therea are some great articles archioved there about breaker and starter testing.

Then there is a book by Paul Gill called something like "Power system Mintenance and Testing" great book to get you started, available on Amazon
 
thanks again zog

thanks again zog

i plan on purchasing some new meters, can you give me some specific models i should consider for checking motor starters and motor resistance values? i have mostly 40 hp and under on motors, mostly air handler loads and chill water pumps , 480 vac. 3ph 40 hp and under, i will replace several of the starters but would like to thoroughly check them first to determine which ones need to be replaced first, since i have over 20 starters.;)


zog said:
Well i am pretty sure I know who you are using based on your location and if I am right you ar in good hands.

www.netaworld.org is the NETA website, you have to buy the standards but you can view archived articles from the NETA world magazine for free, therea are some great articles archioved there about breaker and starter testing.

Then there is a book by Paul Gill called something like "Power system Mintenance and Testing" great book to get you started, available on Amazon
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Not to stir up a controversy, but I was always taught to NEVER clean or file contacts with anything other than a spray-on contact cleaner. Any abrasion will drastically reduce the remaining electrical life and add the potential for welding as you get down into the substrate, not the silver alloy. As contacts become "pitted", the opposite side has peaks that match the valleys so to speak. This actually maximizes the contact surface area and when you file or sand them down, you are back to the original state (as they were when new), minus about 10,000 electrical operations minimum (maybe more if you are too aggressive about it). The carbonization you see is relatively meaningless, remember that carbon is a conductor. The real potential damage is a film that can build up over time as plastic parts in the control equipment begin to deteriorate. that's why you occasionally need the contact cleaner spray.
 
Jraef said:
Not to stir up a controversy, but I was always taught to NEVER clean or file contacts with anything other than a spray-on contact cleaner. QUOTE]

Who taught you that, check the plash point of contact cleaner!

Scoth Brite is OK to use on contacts, dont file them, dont use emery cloth or sandpaper, dot us eanything but scotchbrite. Just be sure not to be too aggressive, pitted contacts should be replaced if <70% surface area or if copper is exposed.

Carbon is a conductor, but not a very good one, all carbon should be removed. The brown stuff you see is silver oxide which is a great conductor (Unlike copper oxide) and can be ignored, thats the readon for the silver surface area on the contacts.
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steveng said:
i plan on purchasing some new meters, can you give me some specific models i should consider for checking motor starters and motor resistance values? i have mostly 40 hp and under on motors, mostly air handler loads and chill water pumps , 480 vac. 3ph 40 hp and under, i will replace several of the starters but would like to thoroughly check them first to determine which ones need to be replaced first, since i have over 20 starters.;)
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Any of the microhmeters listed here for contact resistance

http://www.aemc.com/

this is my favorite megometer for inductive loads

http://www.megger.com/us/products/ProductDetails.php?ID=729&Description=
 
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