Air Compressor wiring hookup?
I have a buddys air compressor to hook up, and have a question regarding the wiring to to pressure switch. This is a 5hp , 575v 3-phase job. He picked this up used, and when I got over to his place, I found all the wiring removed from the unit. all it had was the wires hanging out of the motor. Kinda made me step back and have another look.
The pressure switch is a Furnas 69HA U1 , now this is the part that puzzles me? The Pressure switch has 4-terminals on it. The two outside ones are maked (line, L1 & L2) and the two middle ones are marked (Motor)
The inside of the cover says 2-pole rating, for 1-phase , 3-phase , and DC. The listing for 3-Phase says 240-600v -5hp. I,m not used to any of this compressor hook up, so shouldn,t this have 6-terminals L1-L2-L3 for line in, and T1- T2-T3 to the Motor? This just seems weird to me that it only has provision for what appears to be single phase hook-up.
Maybe compressors have there own way of wiring these pressue switches, I don,t know. After installing the disconnect, and running the wiring over to the compressor, I didnt go any farther with the job, because of this.
I did temporary hook the motor up direct, too see if it was ok,it started and ran, so that works.
If any one knows how to hook up this pressure switch , or let me know if there is a site that shows how they are normally wired, would be most helpfull.
As it says, it is a 2 pole switch which will physically switch a 3 phase line. Disconnect any 2 phases and the 3rd phase has nothing to conduct to, except ground, which shouldn't be an issue in this case.
That said, I am not aware of what the codes might be in Canada, but I doubt that it would be to code. In any event, this isn't air compressor specific, it really isn't a good idea to switch only 2 of the 3 hot lines as it raises the possibility of having a live line after the switch to stick with a finger/screwdriver even when the switch is off, which shouldn't be live anyhow cause we all remember to shut the breaker off first??
Use the pressure switch to operate a 3-phase motor contactor.
Three Phase Compressor Wiring
That pressure switch is intended to control the pilot circuit of a three phase magnetic contactor. The contactor will have four sets of contacts. Three of these are sets are used to connect the three motor leads to the three incoming power leads. The contactor normally has heaters that are specific to the motor and voltage used. The wires from the coil are hooked to the pressure switch and the fourth set of contacts in a manner that stops operation if the heaters trip. I can never remember the details of the fourth contact hook up and will have to dig into my old manuals if you need that.
The OP stated the switch was rated for 5 HP.
Originally Posted by Boucher
That seems way more than what is needed for pilot duty?
Two pole switching on a DEDICATED three phase motor load was common 40 years ago, not now. The biggest problem is motor protection as the pressure switch has no overloads. Today, I would use the pressure switch to control a motor starter (contactor with overloads). As the pressure switch is rater to 600 volts I would use a starter with a 575 volt coil and simply go from L1 to the pressure switch then from the pressure switch to the starter coil then coil to overloads then back to L3. Obviously not all on the same terminal of the respective devices .