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Old 05-01-2012, 08:07 PM   #1
Join Date: Dec 2008
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3 phase motor help

Hi all. I have installed many 5 horse power 3 phase motors in the past for large swamp cooler applications but encountered one this afternoon that I'm confused with.
When I removed the cover plate on a brand new one today that I need to wire, it has two small extra red wires and on the wiring diagram, it says "hook red wires to thermostat."
Does anyone know what they mean by this? Am I supposed to wire something in for overheat protection?
This is a simple replacement for a cooler that lost a motor on a warehouse that I service and none of the old motors have these extra two wires. Thanks

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Old 05-01-2012, 09:00 PM   #2
Join Date: Sep 2010
Location: South Florida
Posts: 38

3 phase motor help

Those 2 small wires are for the built in thermal switch. You should be able to read continuity through those 2 wires with a meter. If the motor overheats the thermal contacts would open and then, if wired into the start/stop circuit, would drop out the motor starter to keep the motor from burning out.

Sometimes though it could be too late but it's just another form of motor protection besides fuses and overload trip devices. You can just tape them up and not use them. (be sure the fuses and overloads are sized properly)

Generally these are N.C. switches (normally closed) but where I work we once had a conveyor system with several motors and each motor had a N.O. switch (normally open) when a motor got hot the switch would close and light a corresponding lamp on a control panel to indicate which motor was hot.
(problem was, if it cooled off before you looked at the panel the light would go off!)

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Old 05-01-2012, 09:35 PM   #3
Join Date: Apr 2010
Location: central wisconsin
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3 phase motor help

there is also the slight chance that these are motor heater leads but 99% likely these are for thermal overloads.
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Old 05-01-2012, 09:37 PM   #4
Join Date: May 2010
Location: winnipeg manitoba canada
Posts: 779

3 phase motor help

they are for inherant motor protection....
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Old 05-01-2012, 10:24 PM   #5
Join Date: Dec 2008
Posts: 423

3 phase motor help

Thanks guys. I usually see starters on these large motors but this building in particular has none on these coolers.
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