Ohms?

Problems related to residential installations.

Ohms?

Postby alewicki81 » Sun May 22, 2011 9:35 pm

How many ohms should you expected across a coil on a contactor rated 220v, 40 amps? Double pole, double throw. I have two of the same in a pressure fryer. They both read open. One works one doesnt. Im thinking my meter is going or needs calibration.
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- Ohms?

Postby 4ftBanger » Fri May 27, 2011 9:24 am

The resistance of the coil on a contactor varies by type, size and manufacturer. I'd expect anywhere from 75 ohms to around 2500 ohms give or take. Definitely should not read open (Infinite or OL) or shorted (0 ohms) - make sure you have the multimeter set for ohms and not continuity - continuity is only looking for a closed circuit and, depending on the meter, the readings can be pretty inaccurate.

Typically I'd be testing for control voltage on the coil. If you have the right control voltage and the contactor doesn't pull in then it's bad.
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- Ohms?

Postby nomadpeo » Sun Jun 12, 2011 9:05 pm

agree with mr. banger - problem is possibly a signal missing to the coil. that signal could be interrupted from your controls feeding that signal to the relay.

your readings are indeed in question. if you read open on both coils and one works, use a different meter and follow 4ftbanger's advice. if possible, power up the relays and test for voltage present at the coils of each.

if you are not trained to do this safely, please get help. you are working with high voltage.
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- Ohms?

Postby okctech » Wed Jun 15, 2011 2:13 am

Ohm's Law * E I R or volts * amps x resistance so volts/amps * resistance
When I read someone refer to a contactor as a "double throw" and needs to know how many ohm's resistance to be looking for I've gotta second the op and recommend calling a professional.
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- Ohms?

Postby nomadpeo » Thu Jun 16, 2011 7:26 am

as 4ft pointed out, the resistance, though certainly not irrelevent, is not the focus of the testing - primarily because the ohm value design is an unknown. the more important information is the voltage rating of the coil and whether that signal is being delivered to the coil. if it is and the coil is not pulling in the contacts, replace it. if the coil is not being energized appropriately according to the sequence of operation for that equipment, the problem is somewhere else.
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