Volt Drop across a contactor 1

[easpe]

What is the acceptable volt drop across a electrical contactor before replacing it. Never seen a standard spec for replacement.

 

[dpc]

Probably depends on how it's being measured. I'd be nervous if I saw much voltage drop at all if we're talking about a contactor used for power circuits. Compute the power being lost and being converted to heat.

 

[itsmoked]

Probably a better check would be temperature easpe. Since that is where everything in a discussion like this is going anyway. AC, DC, frequency, duty cycle, ambient, and the specific current all affect where the contact temperature and hence life of the contacts and the contactor itself are going. To worry about just the contact resistance is myoptic to the whole picture. Using a simple cheap IR heat gun would let you or any service guy determine if the "whole system" is being pushed or not, quickly.

As an example a contact that has a 'relatively' hih resistance may last forever if itonly has a small current running through it. The same contact might fail in a week if instead a large ocurrent is running through it.

Good'ol I*squared*R.

 

[wfowfo]

All that I have seen have been rated as to acceptable micro-ohmmeter readings. I would imagine (as Itsmoked stated above) that this resistance is determined according to the maximum rated current of the contactor so as not to exceed a thermal breakdown limit.

 

[dpc]

Contact resistance for power equipment such as circuit breakers is normally measured with a relatively high current micro-ohmmeter designed especially for this type of testing. 100A current levels are pretty common. These are heavy devices - my arm still hurts from carrying one through a few airports.

 

[itsmoked]

Is the arm pain from security twisting it to "get the truth" about the suspicious package?

 

[cenelect]

Probably too late to help, but ZERO! Anything over a few millivolts would indicate that the contacts are 'shot', or you've got a 'grounding problem' or poor wiring in the control box.

 

[Hewlett]

I remember we did actually use mV measurements as criteria at one workshop. We had many hundreds of contactor controlled rentalunits for controlling fixed heat loads (for annealing of steel pipelines). Units had to be checked between each rental. Many of the units was going to remote areas where equipment returns was "very undesirable" so the boss virtually "required us to predict breakdown". We found that checking mV over power input/power output with full testload was actually a quick and very good indicator of contact condition (keep in mind, we used always same testload.).
We decided on a "replace contactor" limit after looking on measured values and checking contacts for 30-40 units.
I remember this as a much more reliable indicator than pure resistance measurements or visual check, and we discovered a few bad crimpings and terminations during these tests also. Looking back I think this method was very useful. I do not remember exact value we used...think it was somewhere in the 50-200 mV range with full load, but the best is to do some testing on your own with some representative equipment, and use measurements as reference values.
When mV increases with same load, it is indeed an indicator of worsening contact sets. How good indicator it is will probably be disputed by some...thermal imaging is nowdays very common and very good method for checking continously connected equipment, but with an immense amount of rental equipment to check....and quickly....I still think this old method was pretty good...

 

[jraef]

My opinion follws the others regarding thermal imaging / scanning. Even the cheapest thermal scanner is going to give you a better idea of what is really happening in the contacts than a mV or mOhm reading is. mV and mOhm readings are what we used when there was no better alternative.

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