external relay to trigger starter coil on AC motor

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Old 06-30-08, 10:51 PM
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external relay to trigger starter coil on AC motor

hi..... I need advise... as to if there is any kind of a device that will give me the results I desire..
I own a small 1hp rated air compressor.. Built not in the USA.
At times the contacts stick when starting up,and the starter coil will over heat,to the point of smoking.. very hot..
This is frustrating,mainly because I'm afraid to leave from sight of the compressor...
Ya never know when its going to fail to release power to the starter coil..
It's my understanding that there are contacts mounted inside the AC motor housing somewhere..That do the relay to the starter coil,the sticking event,scares me..
Just to say.... the contacts always apply power,no failure there.It's just that sometimes there is failure to disconnect,after the motor starts up...
Now with all that text explaining....
I need to know if there is an after market style of relay,that I can purchase,and will do the job of the relay thats sticking inside the motor...
My plains would be to simply run the two leads that go to the coil now,to the external relay,then from the relay to the coil...
The contacts inside the motor would still be in charge of apply current to the external relay,but the new external relay would in fact work as a disconnect between the motor and coil...Even if the contacts inside the motor fail to disconnect...
Hence in my non Electrical mind,I can see this working..
Is there any kind of device that will supply me with this idea of a sticking relay fix..???
Thanks for any help....... If it matters the compressor is 120v ... single coil starter...
 
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Old 06-30-08, 10:56 PM
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It's my understanding that there are contacts mounted inside the AC motor housing somewhere
the only contacts inside a motor I am aware of are;
1. thremal overload contacts if the motor has internal thermal protection

2. contacts on a cacitor start motor cutout switch.

neither of which would be able to be used how you suggest.

It may be you have a starter not rated for high enough current. What is the FLA on the motor and what size is the starter contactor?
 
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Old 06-30-08, 10:57 PM
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Before I can comment on it more can you tell me what manufacter it is ??


Most single phase motor have start and run winding and when the motor is in starting mode both start and run winding are engerized when the motor get up to near full running speed the start winding will go off line by cenfuige switch. then run winding will take care the rest of it.

But however if the compressor do have starting capaitor some case it willbe either shorted or open .,,

If shorted it will have hard starting but open winding all it will do just buzz until you turn it by hand.

Merci,Marc
 
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Old 06-30-08, 11:52 PM
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..dosen't sound promising.......

Thanks for the replies...
As far as the manufacturer,the motor is made in China...
The compressor is a direct motor drive connection type..
The cheaper versions.....
I have already bought a new compressor....
I was just trying to get the older smaller one working.....
It's more suited to the occasional tire airing event than the new bigger one.
If the winding in the motor is responsible for switching in and out of the starter coil,then this is probably a no fix problem.Sense replacing the motor,is virtually impossibly.
The coil in use now,is much higher rated than what came with the compressor originally.
I have swapped the old coil for a higher rated one,nearly double the output,in micro farads.
The comment about the wiring being shorted may be correct in assumption.
A year ago or so,the compressor,wouldn't start with a load on it...It would just buzz,till it tripped out,from the internal overload.
I figured it was a faulty release valve on the tank,causing head pressure.
That wasn't it.....So I opted to go with a higher rated coil...
It seemed to fix the not starting problem,but shortly afterwards,was when I first noticed the coil smoking,after a restart....
I have sprayed contact cleaner,in the motor,by the cans..
While running.......and at rest,hoping to help anything that was sticking...
So far the contact cleaner seems to have had no effect.....
I guess it's time to salvage parts,the gauges,and maybe save the tank,and send the rest off to the scrap heap.
I just had hoped that there was some kind of an external relay that I could add to the mix..
One that would open after a small duration,operation via thermal reaction maybe.....
It seems that would ease my coil not releasing problem..
Or a version that was opened via duration,a timer version..
Anyway.....I was just mentally contriving in the dark....
Thinking that there was something out there that would solve the problem of power not shutting off to the starter coil.......
I guess such a device hasn't been needed enough to cause it's invention yet....
I just need a devise that will relay 120v,for about two seconds then open....
If I'm not wrong....... I think there is such a thing as external starter coil relays.....In other words (laymen terms) a starter coil not triggered via the motor itself....
I was talking to a air compressor tech person.....about a week ago....(not about this issue).
He stated that his company had starting using external relay devices,for some of there electric compressor motors.
Maybe I'll call him back,and challenge his statement,with asking just how his external device works,and where they buy them....
 
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Old 07-01-08, 07:04 AM
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Yes, external relays are available.

I have one here somewhere but I can't seem to find it.
If it turns up I will post the make and model.

The one I have is solid state and has the ability to use a capacitor or not.
You need to remove the wires from the starting switch and redirect them to the relay.

If memory serves me correct though I think the one I have cost between $40.00 and $50.00.
Considering that the problem you are having could just as easily be the motor as the starting switch going further with this might be a waste of time...........Considering that your compressor could likely be had for under $100.00.
 
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Old 07-01-08, 07:36 AM
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http://www.allelectronics.com/?iorb=4764&0
and
http://www.hosfelt.com/
and places like them have good price/quality ratios, if they have a suitable part.
Digi-key, Mouser, Jameco, Grainger, HVAC supply houses after that.

If you come close to the necessary part, a few R's, L's and C's and a xformer may complete the interface.
 
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Old 07-01-08, 10:12 AM
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I just need a devise that will relay 120v,for about two seconds then open....
If I'm not wrong.......
that is easy. It's called a "one shot" relay. They have been around for a long time.


due to the situation, I think all you would be doing is cobbling a short term fix that wouldn't be worth the money.

dump the junk.
 
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Old 07-01-08, 10:50 AM
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I am pretty much agree with Ibpooks about the set up and many cheap portable aircompressor some don't last very long but few good one last pretty long time.

Sound like the start winding is pretty much burnted.

The other thing with some of the cheap compressor motor they just can't take undervoltage very well at all.
Like example long extendsion cord with aircompressor when the unit try to start up the cord will act like a " choke " due the voltage drop can get pretty serve to point where the motor can overheat pretty fast.

With small portable aircompressor if the motor go out you can use the air tank just take the motor off and used the tanks as portable air tank to fill up the tires etc.

Merci,Marc
 
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