Resistance / Voltage Drop Across a Contactor

[zimmerDN]

What is an acceptable level of voltage drop across the line and load side of a motor contactor? Is there an IEEE standard or manufacturer recommendation on this?

 

[oldfieldguy]

Many manufacturers have specifications for contact resistance in their acceptance/maintenance testing information.

In the absence of this data, you can derive some useful information from checking like equipment, i.e., you have three contactors, two of which have contact resistance in the 60-70 microohm range and one with readings in the 2-300 range. You'd be wise to suspect a problem with that errant unit assuming it is indeed the same rating for current and voltage. The same goes for each of the contacts in the contactor. If one diverges from the other by more than 50%, then it is suspect.

Lastly, on legacy (read 'so old nobody's seen a manual in decades') equipment, look at the I^2*R loss across the contactor. I always operated on the supposition that if an air-breaker contactor had more than 25 watts loss at rated current, there was going to be an issue due to the equipment's ability to deal with heat in the contact assembly.

Some air-break contactors have a bad characteristic of contacts 'glazing' over with an oxidized layer that renders high readings at the low voltages that test equipment uses. Proper cleaning will often correct this problem.

old field guy

 

[wroggent]

I don't think an acceptable voltage drop would be specified since, for example, a 600V contactor could be applied at several different (lower) voltages; unless this is given in percent or per unit. I think NETA recommends (per ATS-2007) that all phases would have a resistance that does not deviate more than 50% from one another. In addition to that, NETA has a table that recommends actions based on the results of a thermographic survey. Bottom page of first result:

https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=neta table 100.18

Manufacturer may have a recommendation as well. I am doubtful that a general resistance value exists that you could test all products against since the allowable loading and corresponding voltage drop for a particular contactor is going to be dependent on how well that specific contactor dissipates heat.

 

[DRWeig]

One UL test requirement for a motor rated contactor is 50 mV at rated current and highest rated voltage. As wroggent said, the three contacts should be consistent.

Note that a brand-new contactor won't meet this on first bang. It's gotta clean itself under load.

Best to you,

Goober Dave

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[mikekilroy]

This question begs a response of "WHY?"

Is it because you are concerned over a 'bad' contact? Or?

Maybe better would be to consider what the RESULT of that bad contact would offer you in clues.....

Assuming you can even find a milliohm spec at YOUR particular voltage and current, why not instead just buy a $25 IR thermal gun and point it at the contact?

I believe this may be the bottom line; if the contact is not 'good enough,' the much higher temp than ambient will tell you there is a 'bad' contact issue.....

http://www.KilroyWasHere<dot>com

 

[jraef]

In NEMA ICS1-2000 there is no spec on voltage drop, only temperature rise over ambient. 65C

"Will work for (the memory of) salami"