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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 7497
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Airwell gc48rcfx contactor will not engage. When switched manually

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Airwell gc48rcfx contactor will not engage. When switched manually everything works cannot see 24v on contactor. No room thermostat temp sensor wired into ceiling distribution checked with ohm meter and are open circuit id this the promplem?
Welcome to Just answer!.

If you can locate wiring diagrams on the service panel of the outside unit and the brand and model number of the ceiling distrubution device you are referring to, I might be able to help you.

You can post those photo's to photo sharing site and then post a link to them here.

The problem may very well be in that open circuit you mention, but without a much better system description I will be unable guess which circuit is involved.

I hold questions open after positive ratings to allow for unlimited follow up. Please be patient though. The research & typing take time

I will be off to bed in an hour, if I miss you I will be back in the morning west coast of the USA time, we can go from there.

Customer: replied 3 years ago.
I have. Diagrams will upload to some site and get back, however i now think it might be the three phase mix protector as this has the switch to supply contacator. Strange thing is if i jiggle the main isolator switch sometimes the contactor engages?
Hello again, your wire jiggling test is very important... what ever is on the end of it is most likely the problem.

Regarding the 3 phase mix protector... it operates a shut down switch when the 3 phase fails or goes out of balance... you can jumper that *switch* but send a photo of it and its diagram so I can make sure you jumping the right switch... if you accidentally jumper high voltage line power to the 24 volt circuit you will cause a lot of hard to fix damage.

I will see you in the morning, 9-10 hours from now.


Customer: replied 3 years ago.
Hi Phil

Links above to most of the info, i can see what your saying about the jumper between A an C alsonrealised its an 415v ac coil in the contactor not 24 v. Confused because all web searches kept refering to the contactor activation is 24v.

Ok couldnt wait its freezing so connected A and C and all working fine checked unit switches off when set temp reached all good. From the pictures i reckon its the JQC - 3F T73 24v relay . Is it ok to run with this phase protector out of the loop? Or do ineed to replace? Or is there something wrong with the phase?



Hello again...

That is good work on your end.

Phase protection for 3 phase systems is not a common feature, as a rough guess 99% of 3 phase systems run without any phase protection and rely on heat sensitive motor or compressor overload devices for protection.

The 3 phase motors will generally not start if a 3 phase system loses one of its legs and 'single phases'.

Some 3 phase motors will run if one leg of the 3 phase drops out once the motor has already started... generally low torque fan or blower motors, but not 3 phase compressor motors.



You can check the amperage on each phase... if there are no single phase motors wired to just 2 legs of the power, the amperage will be in the same range... for an example 31 amps. 29 amps and 33 amps on lines 1, 2 and 3... all on the same motor.

That happens when the phases are slightly out of balance, that is common up to 10% or so... (I will elaborate and attach a few links)


'out of balance' means that the phases are not exactly 120 degrees apart as they should be.

That happens when other users on the utility power grid are running single phase motors off of two of the legs of power, line 1 and line 2 for example....


My experience is that most electrical engineers I meet in industrial plants think 'back EMF' only applies to electronic circuits and not to motor circuits and definitely not back into the utility company power grid.... that is not true however.

Back EMF = Electro Motive Force fed BACK into the incoming power line...

That 'Back EMF' is generated by a running electric motors windings, 180 degrees out of phase with the incoming power on each leg

Further explanation: A running motor to a slight extent also acts as a generator, and sends that current back into the power lines.. and that power, being 180 degrees out of phase with the incoming power, impedes that incoming power, and causes it to shift out of phase slightly.

The electric motor is not designed to run efficiently with power that is not exactly 120 degrees (thats phase separation).

So that the motor slows down a bit... when the motor slows down... it offers less impedance to incoming power, so draws *more amperage on that leg... and that is why you see different amperage readings on each leg of a 3 phase motor.

These are called 'power factor' issues in the industry... if you read an electric bill you will see that the power company measures this 'phase shift' on average and adjusts your power usage charge accordingly since a motor running on less than perfect power draws more amperage... and runs hotter.


(that overheat is what trips the overloads on those motors, that is the most common motor protection.)

Let me know if that makes any sense on your end or not.

Both of these articles are accurate.

Summary: I don't know the details of how your particular system is arranged or what power issues there might be, to be safe I would replace the defective device.

Let me know if this makes any sense on your end or not... we can go from there with no time limits if you choose to rate my answers to date positively.


Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 7497
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
Phil and 3 other HVAC Specialists are ready to help you
Customer: replied 3 years ago.
Thanks Phil, i get your explanation, i come from a more electronics background and specalise in pressure sensors working for that great american company GE. I get the back EMF thing we use this principal in a low pressure sensor measuring back emf on a conductor placed on a diapragm like prox probe the opposing emf is converted to pressure.

I had two repair companies come out and they didn't have a clue. All they wanted to do was re fill, the second one replaced the main isolator switch because of the jiggle thing. They never heard of a phase protector. The ducted aircon i have came with the house and its made in china an australian company gone bust but i have found aplace to get parts. However the whole protector unit is 150 bucks but i can get the relay for 60c so will trynand replace that first (do ypu think thats enough).

Many thanks for your advice great learning experience i have given you the highest rating

Cheers Phil

Best Regs

Hello again! Thanks for the compliments and excellent rating. I don't know how that switch is built or what signal it gets to cause it to open... it might be quite a trick ro replace.

Let me know how it goes if you wish, I will hold the question open so you can do that.


Customer: replied 3 years ago.
Hi Phil, please keep it open and i will let you know how it goes


I will keep the question open, I will be interested in the switch details


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