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J.Ramsey
04-12-2012, 05:38 PM
Last week while out in the shop working the air compressor was running and started to make weird noise and about that time i saw sparks,flame then major smoke coming from the on off switch and realized the compressor was running on one leg of the 240 ,ran across the shop to kill the main breaker it was too late and toasted the motor as well.

The compressor 60 gal. upright 2 stage with a 6.5hp. motor,the switch was SquareD 60 amp with 10 hp rating pictured below.
The copper or brass contact on the right side of the switch melted and its remains were found in the bottom of the switch box, all wires were tight.

As of now( with the new motor) I am turning it on and off at the breaker box as its on its own circuit.
Anyway I'm open to suggestions for a better switch, maybe the old style 2 line knife and fork type? http://img687.imageshack.us/img687/7214/aircompressorswitch.jpg

Ian B
04-12-2012, 06:00 PM
This was a 3 phase motor? If one or more phase drops out, the other phases get overloaded. It's easy enough to wire 3 relays up with their contacts in series, one to each phase, such that if any phase drops out, the holding coil on the contactor de-energises and all 3 phases are broken. That'll stop future burnouts.

Ian

Boucher
04-12-2012, 06:19 PM
I will suggest a few things for your consideration. Coming from the incoming breaker supplying the power I like to put an A/C type disconnect near the compressor. The cost is ~ $15. Next a magnetic contactor is used to actually perform the current connection. This contactor is turned on/off by the pressure switch. The pressure switch is lightly loaded since it is only carrying the coil current for the contactor. I up size the contactor to get better current carrying capacity. Breaking an inductive load is always going to arc and erode the contacts so the larger the better.

Black_Moons
04-12-2012, 06:36 PM
Don't use a circuit breaker to switch loads, they are not designed for it and will wear out fast. Particularly not an inductive load near the breakers ratings.

Catshooter
04-12-2012, 07:59 PM
You don't say the voltage of your motor, but looking at the breaker it must be 240 volt single phase.

That's a 60 amp breaker. Does it say "SWD" on the handle? That stands for Switch Duty and it is rated for use as a switch. But not for that motor.

Your 6.5 hp motor draws roughly 110 to 125 amps to start. If you use that breaker to start it all the time then that would explain it's failure. If that motor is running 480 volt you're lucky that the breaker didn't explode as it's only rated for 250 volts.

There are cheaper/easier ways to start it. If you interrupt the control circuit with a switch you can use a much smaller simple switch to do so.

You still want a way to cut off the power completly in case of need, such as that 60 amp breaker.

If it was me, I'd replace that breaker with one just like it and then install a switch in the control circuit to start/stop the compressor. But then as you can probably tell I'm an electrician.


Cat

Carld
04-12-2012, 08:21 PM
That burned breaker looks like a two pole breaker and the amp rating is to high for protection. Also, as stated you shouldn't use a breaker for a disconnect switch.

You should have a breaker correctly rated for the motor in your service panel and that should run to a disconnect switch and from there to the air compressor motor.

I suspect the breaker failed from being used as a disconnect switch. Turning a breaker on and off while it is under a load will make it fail rather quick.

lakeside53
04-12-2012, 08:42 PM
If it's single phase it can't "run on one leg of the 240". That would imply one side of the motor is connected to neutral, which it isn't.

That breaker style 60 amp switch (it's not a breaker) may have had a bad contact which led to undervoltage and a resultant motor overload, or, the motor took out the switch. That switch and enclosure sells for about $7 at my local Home Depot...

The NEC requires overload protection for your motor. When you replace the motor, buy a real motor starter with overload protection. Cheap on Ebay. And as other have said -don't use your panel breaker as an on-off switch..

J.Ramsey
04-12-2012, 09:05 PM
"This was a 3 phase motor?",

2 line knife and fork type?

"Breaking an inductive load is always going to arc and erode the contacts so the larger the better."

Which is why I asked about the knife and fork switch, at least I can inspect it by opening the encloser.

"Don't use a circuit breaker to switch loads,"

I bought the compressor new in 1985, and it was turned on and off that way with a breaker never had a problem, I built my new shop in 2003 and thought I'd put in a proper switch as pictured.

"If it was me, I'd replace that breaker with one just like it and then install a switch in the control circuit to start/stop the compressor. But then as you can probably tell I'm an electrician."

You no read good, Its a switch ....not a breaker.

"That burned breaker looks like a two pole breaker and the amp rating is to high for protection. Also, as stated you shouldn't use a breaker for a disconnect switch.

You should have a breaker correctly rated for the motor in your service panel and that should run to a disconnect switch and from there to the air compressor motor."


I suspect the breaker failed from being used as a disconnect switch. Turning a breaker on and off while it is under a load will make it fail rather quick"

You also READ NO GOOD, ITS A SWITCH RATED FOR 60 AMPS AND 10 HP. ITS NOT A BREAKER FOR CHRIST SAKES,the compressor is as stated on its own circuit which has a 30 amp breaker.

J Tiers
04-12-2012, 10:38 PM
Stuff happens.... switches fail.

I'd get another switch setup, and go. Probably not a knife switch, while they can be HP rated for switch duty, they are basically disconnects. A "manual motor starter" switch will do what you need just like that one used to.

It isn't a bad idea to instead get a contactor-type motor starter, which will drop out and stay out if power drops and then comes on again.

Bob Ford
04-12-2012, 10:59 PM
If I understand right.
1. You have 3 phase 240volt
2. You were only disconnecting 2 phases leaving one hot to motor at all times.

If this is right then you have 3 phase delta service one leg called the stinger leg has roughly 180 volts to ground.
By not disconnecting one phase as the motor deteriorated the one phase was free to work on the windings 24 hours a day.
When the motor was running one part of the winding was already hot, like overload.
You should check the wiring between disconnect and motor looking for melted insulation.

The correct wiring is.
1. Breaker to protect wiring to disconnect.
2. Disconnect to allow working on motor with no hot wires.
3. Motor starter to protect motor and to stop start motor.
4. A stop start switch which operates the motor starter.

Bob

Catshooter
04-13-2012, 08:12 PM
Well your picture is now a red X, but it sure looked like a breaker to me.

Was just trying to help you. Sorry. I won't make that mistake again.


Cat