Electric motor starter

[benchracer1]

I am in the process of converting my 7.5 hp two stage compressor to single phase. From what I've read anything over about 5 hp should be protected by a magnetic starter. My question is can a magnetic starter be used in conjunction with any electric motor or are there motors specific to magnetic starters. I was told by the salesman that I could go from power source to pressure switch then motor. While this is all well and good I also have heard that absent a starter that can overwhelm the contacts on the pressure switch. Any electrical guys feel free to educate me....steve

 

[American Locomotive]

Any motor can run on a magnetic starter. You just need to make sure you get a magnetic starter rated for 7.5 HP single phase. Your pressure switch will need to control the magnetic starter.

Pressure switches are commonly available up to 5 HP, but they tend to live short lives.

Additionally, if the motor does NOT have built in thermal protection (it will say so if it does!), you should use some kind of overload protection to protect the motor. Most magnetic starters have the overload protection built in. Most pressure switches do not.

 

[matt_i]

I also have heard that absent a starter that can overwhelm the contacts on the pressure switch.

That's the main driver if you ask me. The pressure switch is amp-limited whereas a magnetic device (relay, contactor, motor starter) can be literally any size matched to the motor.

At the point you get the "magnetic device" the pressure switch is just a signaling device, it basically switches milliamps on and off after that. Its just energizing the electromagnetic winding that pulls in the iron core of the relay.

 

[Spareparts]

matt i is correct, the pressure switch will last longer without that amp draw of the motor

 

[benchracer1]

Thanx guys. After reading here and looking at some schematics I was able to figure out how a mag starter works. Going to go ahead and order a baldor or leeson 7.5 motor and a 7.5 hp mag starter...steve

 

[benchracer1]

Got my 7.5 hp motor installed and the magnetic starter. Works beautifully. All I need now is a pressure cutoff switch . Can anybody recommend a simple pressure switch that doesn't have the on off or unloader feature and works with a magnetic starter? ...Steve

 

[redmondjp]

Got my 7.5 hp motor installed and the magnetic starter. Works beautifully. All I need now is a pressure cutoff switch . Can anybody recommend a simple pressure switch that doesn't have the on off or unloader feature and works with a magnetic starter? ...Steve

Go on Amazon and search for "square D pressure switch" and then find one with the right pressure range. It doesn't matter if it has the unloader feature - you can just leave that unconnected and it won't hurt a thing.

I just did a quick search, and this switch p/n should work perfectly for you - it has a 30psi differential which is adjustable and no on/off switch: 9013FHG19J55X

https://www.amazon.com/dp/B0078S7Z78/?tag=atomicindus08-20

If you want to see all of the Square D options for their pressure switches, go to Chapter 22 of the digest (their catalog) which the link below goes to - you can either download as a pdf or look at it online. Then take the p/n for the exact pressure switch that you want and paste it into Amazon to see if they have it. Note that you will probably want to pick one of the more common ones (may have switch & unloader which you may not need) for the lowest price. Also note that since you are controlling your motor starter with it, you won't care what the electrical current or HP ratings are, so long as it is rated for the AC voltage:

https://www.schneider-electric.us/en/download/document/0100CT1501_SEC-22/

Happy hunting!

 

[benchracer1]

I was kinda curious if I had to worry about horsepower. I figured it wouldn't matter since I wasn't running the 220 through the switch to the motor. I'll take a look at square d. I'll report back...steve

 

[NC Rick]

It’s good if you can buy them as a matched set. The starters have selectable “heaters” to provide the correct level of thermal protection for the motor. The values can be determined by calculations or tables. That’s not something I have had to do in nearly 40 years so I can’t do a better job of describing how to do that.

 

[toplessHO]

dont forget to add SF in with calcs