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#170527 - 11/05/07 09:09 AM correct way to wire a thermal overload switch  
MMK  Offline
Member
Joined: Nov 2007
Posts: 20
Ireland
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.


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#170537 - 11/05/07 01:44 PM Re: correct way to wire a thermal overload switch [Re: MMK]  
mikesh  Offline
Member
Joined: Jun 2006
Posts: 613
Victoria, BC, Canada
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.


#170540 - 11/05/07 02:21 PM Re: correct way to wire a thermal overload switch [Re: mikesh]  
MMK  Offline
Member
Joined: Nov 2007
Posts: 20
Ireland
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.


#170550 - 11/05/07 07:21 PM Re: correct way to wire a thermal overload switch [Re: MMK]  
classicsat  Offline
Member
Joined: Nov 2002
Posts: 456
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.


#170551 - 11/05/07 07:39 PM Re: correct way to wire a thermal overload switch [Re: classicsat]  
MMK  Offline
Member
Joined: Nov 2007
Posts: 20
Ireland
Classicsat,
Could you please explain your suggestion a little more. What type (manufacturer) of motor are you using?
Michael.


#170624 - 11/07/07 12:27 PM Re: correct way to wire a thermal overload switch [Re: MMK]  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
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?


#170630 - 11/07/07 03:23 PM Re: correct way to wire a thermal overload switch [Re: pauluk]  
MMK  Offline
Member
Joined: Nov 2007
Posts: 20
Ireland
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.


#170634 - 11/07/07 05:35 PM Re: correct way to wire a thermal overload switch [Re: MMK]  
Alan Belson  Offline
Member
Joined: Mar 2005
Posts: 1,803
Mayenne N. France
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!



Wood work but can't!

#170694 - 11/08/07 10:28 PM Re: correct way to wire a thermal overload switch [Re: Alan Belson]  
classicsat  Offline
Member
Joined: Nov 2002
Posts: 456
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.


#174326 - 02/01/08 09:47 PM Re: correct way to wire a thermal overload switch [Re: MMK]  
iansettle  Offline
Member
Joined: Jul 2007
Posts: 19
WIRRAL, UK
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.

Last edited by iansettle; 02/01/08 09:49 PM.

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