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correct way to wire a thermal overload switch

Posted By: MMK

correct way to wire a thermal overload switch - 11/05/07 01:09 PM

What is the correct way to wire the inbuilt thermal overload switch in an AC motor? I want to protect the motor in event of stall, but it should be safe, i.e. not start up automatically when motor cools down - in case some operator might get injured.
Michael.
Posted By: mikesh

Re: correct way to wire a thermal overload switch - 11/05/07 05:44 PM

I think if you use a magnetic starter you need to wire for low voltage protection like a standard 2 button stop start. Place the overload in series with the stop button.
The start button is N.O. with holding contacts. Most other connections I can think of would let the motor restart on cooldown if there is no manual reset for the over load.
Posted By: MMK

Re: correct way to wire a thermal overload switch - 11/05/07 06:21 PM

Mikesh,
Our current solution is to use a manual resettable current trip - external device.
Now though we are trying to use a dual voltage motor (see my other post) so that we can standardise machines for US(110) and European (230vac) markets. Problem is the current will differ by a factor of 2 and so the current trips would have to be different for both. Hence looking at using the inbuilt thermal switch in motor.
Posted By: classicsat

Re: correct way to wire a thermal overload switch - 11/05/07 11:21 PM

The ones on the motors we use have two circuits, one through the heater, one not (or it could be dual heater/contact). When the motor is wired in 120V mode, the start and one run winding runs through the heater, the other run winding through the non heater contact. In 240V mode, the "other" run winding is in series after the first run/start winding.
Posted By: MMK

Re: correct way to wire a thermal overload switch - 11/05/07 11:39 PM

Classicsat,
Could you please explain your suggestion a little more. What type (manufacturer) of motor are you using?
Michael.
Posted By: pauluk

Re: correct way to wire a thermal overload switch - 11/07/07 04:27 PM

One way or another, I think you're going to have to extend the thermal cutout contacts to the no-volt release holding circuit. Are you going to try to use contactor coils which are split wound to allow for parallel and series connection for the different voltages as well?
Posted By: MMK

Re: correct way to wire a thermal overload switch - 11/07/07 07:23 PM

It is a single phase AC motor, 100W approx, so we are only planning to use a simple relay for on/off.
Can you get split wound contactor coils? This might be the solution for another application where we would like to use the same contactor for larger (5-11kW) pump motors, and which will operate on American 3ph (460V) and European 3 ph 380V.
Having to fit differing coils and/or different contactors becomes a logistical nightmare.
Posted By: Alan Belson

Re: correct way to wire a thermal overload switch - 11/07/07 09:35 PM

Can I just say with my ex-production engineer and machine designer hat on: Build for the market. Customers can source spares easier, and you will find in the end that off the shelf parts will be less cost overall than complex circuits and 'special' components added to try and make 'one machine fits all'.

KISS = KEEP IT SIMPLE SIR!

Posted By: classicsat

Re: correct way to wire a thermal overload switch - 11/09/07 02:28 AM

They are Doerr (and other manufacturers) 1Ø Split Phase 60hz 1/2 to 2HP dual voltage capacitor start motors. We use them all over the place here, and I just rebuilt one a couple month's ago.

FWIW, I have, in a box someplace, some 3 pole contactors that are dual-winding; they can be set up for 120v or 240v use.
Posted By: iansettle

Re: correct way to wire a thermal overload switch - 02/02/08 01:47 AM

Originally Posted by MMK
What is the correct way to wire the inbuilt thermal overload switch in an AC motor? I want to protect the motor in event of stall, but it should be safe, i.e. not start up automatically when motor cools down - in case some operator might get injured.
Michael.


Michael
If I am thinking right you have a thermal protective device which is within the windings of your motor.

To stop the motor from starting automatically when motor has cooled you need to locate the terminations in the motor terminal box for the termal protctive device. When you have found them you put them in series with the stop circuit of the motor control circuit.
Posted By: Trumpy

Re: correct way to wire a thermal overload switch - 02/02/08 06:21 AM

You have to ensure that any controls on any machinery don't start without prior knowledge.
Having a lockable isolator next to the pump or whatever, to make sure that the thing won't start on you is a good idea.
Even so, there should always be a warning sign on the gear itself:
This equipment is controlled by an automatic switch, it may start without warning, please isolate all supply to this motor before working on it.
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