air compressor

[Levi81]

7 1/2 hp implies three phase since that is an unusual size for single phase motors but common for three phase motors. That also means phase rotation needs to be correct or you will destroy the compressor. I've seen a nice 10 hp compressor destroyed because someone hooked it up with the wrong rotation and ran it with the resulting zero oil pressure until it siezed.

7.5 hp is very common in single phase, if not the most common go size. We have at least 12 on the farm that I can think of.
8 silo unloaded, 2 air compressors, 1 vacuum pump. I’m sure there are others I missed

 

[kencombs]

I see eBay has some from a compressor company (Belair) for $240 shipped. good price and that company has a good rep.

 

[Destroked 450]

I was thinking of using a three phase compressor and will 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 i use as a control voltage in order to pull the contacts in .
I am right on this ?

Do you have 3 phase, if so what line voltage do you have?
Most common is 240 or 480,

I was thinking of using a three phase compressor and will 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 i use as a control voltage in order to pull the contacts in .
I am right on this ?

Do you have 3 phase power, if so is it 240 or 480 volt
A 7 1/2 hp 240 volt 3 phase motor will draw up to 22 amps, you’ll need a size 2 contactor with a 27-30 amp overload
A 7 1/2 hp 480 volt 3 phase motor will draw up to 11 amps, you’ll need a size 1 contactor will a 14-15 amp overload
I like to run a second line with 120 volt single phase to the pressure switch to control a 120 volt coil in the contactor

 

[Geo-TH,In]

A starter box is rated by the HP of the motor and if it is 1 or 3 phase.
Or as drsportster said amperage used.
In other words a starter box for a 5 HP motor will not work on a 10 HP motor and a starter box for a 10 HP motor will not protect a 5 HP motor.

Mule Meat
Think of a starter box for a compressor as a starter relay for your cars starter.
The pressure switch now takes the place of the key.
So high amps goes to the starter box and directly to the motor by passing the pressure switch.
The pressure switch only tells the starter box when to cut on and off.
This saves the contacts in the pressure switch as they can not handle high amps.
Reason if you buy a big compressor at a box store you

John,
Well said. Totally agree with what you said.
I'll add that the relay coil voltage can be a different voltage.
Take a central AC that uses 220 vac and the relay coil is 24 vac.
All he would have to do is add a transformer if he wanted to use a different coil voltage.

 

[farmbj]

Do you have 3 phase, if so what line voltage do you have?
Most common is 240 or 480,


Do you have 3 phase power, if so is it 240 or 480 volt
A 7 1/2 hp 240 volt 3 phase motor will draw up to 22 amps, you’ll need a size 2 contactor with a 27-30 amp overload
A 7 1/2 hp 480 volt 3 phase motor will draw up to 11 amps, you’ll need a size 1 contactor will a 14-15 amp overload
I like to run a second line with 120 volt single phase to the pressure switch to control a 120 volt coil in the contactor

480 volt 3 phase motor .
All figured out

Thanks to everyone

 

[MF294-4]

I have a 7 1/2 hp 1ph motor on my compressor. Didn’t need one that big but got it for $100.