Hi,

I've stumbled across an old 5 hp compressor that I'm going to put back together. One hurdle that I’m going to have to cross at some point is the wiring of the magnetic switch. I’ve posted a picture of it here.

I have a 30A 2-pole breaker in the panel and will pipe over to the compressor at some point. I’ve got the following: the source (220 VAC) a 2-pole pressure switch, the mag-switch, and the motor. All that I know for sure is that the coil, when energized, allows current to pass from top to bottom or vice versa. The switch also has a ‘common’ and a ‘normally open’ tab on the bottom left, and a dial that swings between 110 and 90 (hard to see in the photo, on the bottom left). I don’t know what those are about, I haven’t worked with motor controls before.

If anyone can point me to a typical wiring diagram for this setup I’d be eternally grateful. There’s absolutely no rush on this, it’s sort of a summertime project.

Thanks in Advance,

-Steve G

 

Steve,

Looking at the starter picture, I noticed that it is for a 3 Phase circuit. Is the compressor motor 3 Phase? Also, do you have 3 phase power, or only single phase residential power? I can help you with this, but need a little more info.

Nashcat

 

My understanding is that this switch (and the motor) could be used for either. The name plate on the motor says: V 208, 230 and gives all other specs in dual format. When it was last in use it was run on single phase 220, and that's how I would use it. I don't have 3 phase available.

If there are alternate methods of powering the motor, I'm open to suggestions.

-Steve G

 

Steve,

If the motor name plate says 208/230V, then everything should be fine. I've got a couple of more questions, then I can sketch you a wiring diagram.

First question. You say that you have a two pole pressure switch. Are both poles switched, or do you have a common, normally open, and normally closed terminals. A picture of the pressure switch would help.

Second question. Can you identify the voltage that is marked on the coil in the starter. It doesn't matter what voltage is marked anywhere else on the starter. The coil should have its own markings.

I'm assuming that you would run your power through a separate disconnect switch, located near the compressor, and use the pressure switch to cycle the motor to control/maintain pressure.

Nashcat

 

I’ve posted pix of the coil & pressure switch here.

Thanks for taking the time to do this.

-Steve G

 

Oops, make that here and here.

-Steve G

 

Steve,

After seeing the picture of the pressure switch, you may not have to use the magnetic starter. The contacts in the pressure switch appear to be heavy enough to cycle the compressor. Looking at the pressure switch, if you lable the contacts 1,2,3, and 4, in order, you would hook your 230V service to contacts 1 and 3, and hook your motor leads to contacts 2 and 4. I would use a simple manual disconnect, located near the compressor, to turn it on when needed.

Let us know if this helps, or if you need more info.

Nashcat

 

First of all, thanks again for taking the time to help me with this. I would have no problem wiring the motor through a disconnect and pressure switch, but it leads me to another question: what is the magnetic starter all about? It came with the compressor and was in use before the pump failed (bad connecting rod bearing). I wonder why they would have used it in the first place and what purpose a magnetic switch serves. I'm happy to exclude it if I don't need it (just one more thing to go wrong & it doesn't look cheap).

Regards,

-Steve G

 

The compressor-control contacts "make & break" the coil-contol circuit which preserves the life of the pressure-control contacts because the current thru the contacts is 1 amp or less.

With the pressure-control contacts in the motor-circuit,the current, on starting, is 40-60 amps thru the pressure-control contacts The starter-contacts are specificly designed for hi motor-starting currents.

Also , a motor "across-the-Line" starter is equipped with over-load devices that will open the control-circuit if the motor-current is excessive. This protects the motor-windings from the destructive effects of over-heating due to a current-value that exceeds the rated current-value of the motor.

 

The first picture appears to be a starting capacitor.
The magnetic contactor may have been a way to turn of the compressor at the end of the day.

 

The magnetic starter would also allow you to have remote start stop switches for the compressor. These start/stop switches could be wired with much smaller wire, such as #16, to prevent having long runs of larger wire at the higher amperage required for the motor.

Nashcat

 

Thanks to all, especially Nashcat, for clearing that one up. I'll pipe to a disconnect, then through the pressure switch.

-Steve G