3phase air compressor

My starter box burned up and was wanting to put another one on do you need a special one or can you make any one work of the same voltage I have 240 and how do you wore the pressure switch to it also it is a 7 1/2 hrs thanks Howard

I'm not a 3 phase expert but,,, you need a 3 phase relay or you you have one phase on at all times with a 2 contact relay. Probably not good if there is a winding to ground or neutral. Definitely not good for safety. You could use a 220 or 110 air switch to control a 3 ph relay.

Japery it was suppose to say wire the pressure switch

To run a three phase compressor you'll need a contactor/motor starter with three poles in order to break all three power legs going to the motor. The starter will need to have a coil voltage equal to whatever you use as a control voltage in order to pull the contacts in.


You'll then need to run the control voltage, most likely 240, given that you can get that from two legs of the 3 phase, through a pressure switch in order to control the actual contactor. Basically you use a two pole pressure switch, and when the two contacts close they send 240 to the coil on the contactor. The contactor, in turn, pulls in and sends full 3 phase to the motor.

As far as running some kind of a timer, that would probably have to be set up on the control side of things. In other words the timer would only let the 240 control voltage through to the pressure switch for the amount of time set.

Ultimately given the questions your asking, it sounds like your no that familiar with three phase to begin with. That being the case your best bet would be to get an electrician to come in and do the wiring for you. Looking at it another way, the original stuff "burned up" and that had to have happened for a reason.

Too, if the original system had been wired properly, it probably wouldn't have burned up...beyond the occasional contactor coil going bad...if it had been wired using the correct overloads, etc. It's going to take an electrician to figure out the reason it burned up, and put in a new setup that will be safe.

how did it burn up? Take all your nameplate specs to the supply house with you and the counterman will help you choose the new starter.Also need to know control voltage.

For a 7 1/2 HP motor you need a size 1, 3 pole magnetic starter with a 230 volt coil.

If you go back with the same brand, you can use your existing overload relay, providing it wasn't damaged.

If you have to replace the overload relay, you'll need to size the overload heaters to the FLA rating on the motor name plate.

If you will take this information to the supply house, they can get you the parts you need. If in doubt about installing it, get some help, this stuff is expensive, do it wrong, things can go bad!

I believe the reason it burned up was because of age it is probably over 40 yrs old n your right I know just enough to be dangerous I think I now realize I need a starter box with a coil that will switch it on when the pressure switch activates it thanks

See this at work every so often. Possible that your contactor points are worn out. "Cooked" You most likely dropped a phase. They will flash over between the phases. They burn through the plastic in the contactor. The big dish machines at work do this a couple times a year. They have 35 and 45 amp contactors on them. One each for all three pumps and three banks of calrod heater elements. Have 100% moisture in the air almost all day long. Pretty fantastic fire works with 2hp pumps and 480 three phase power!!

Here is one that will do the job. If you need a 240 volt coil they are interchangeable.
Check Here

(quoted from post at 09:28:56 03/17/15) My starter box burned up and was wanting to put another one on do you need a special one or can you make any one work of the same voltage I have 240 and how do you wore the pressure switch to it also it is a 7 1/2 hrs thanks Howard

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Time to call in a pro electrician. Yes I know it costs but costs less than arcing and sparking. Why mess with something that is dangerous and important that doesn't forgive . And is outside your area of expertise

Age does not have much to do with it. Condition does but I have plastic machines that were manufactured in the early 1960's that the motor starters are fine. I have one machine that was made in the 1940's with an original motor starter. Sometimes they can run for a week without opening the contacts. An air compressor without a cutout can beat a motor starter up if it is used a lot. It really wouldn't have to take 40 years.

Like DickL said, age usually has nothing to do with the problem in circumstances like this. In the years I did industrial maintenance, I've seen electrical 'devices' that were 50 years old, plus, that were still in operation.

A lot of times what happens is that constant starting and stopping will burn contacts if they aren't rated properly. With burnt contacts, they will often pass current, but have enough resistance due to the burn, to create a lot of heat and cause a major malfunction.

In the end, it's no telling exactly what happened, so you really ought to have things checked out before spending a lot of money to get the power to the motor, only to have it burn up also because of a problem in the motor itself, and have to spend double.

Thank you so much it's really cool that you would go to the trouble to look it up for me I'm going to buy it that should work I had no idea you could buy a new one that cheap thanks again