Exhaust Fan Motor Electrical Question

[Tom Bain]

I'm setting up a finishing room in my new shop and recently purchased a spiffy new 3-phase exhaust fan. The connection diagram on the motor plate indicates the need for thermostats (see pic below), however, I don't want the fan to be temperature controlled. I just want to be able to turn it on and off manually. There are two wires labeled P1 and P2 for the thermostat control. I'm wondering if I can just put a simple switch in between those control wires to turn the fan on and off? Or is it more complicated than that? Any advice from the electrical experts out there is certainly appreciated.

Thanks,
Tom

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[Tom Bain]

Ok, after a little more research I am realizing the P1 and P2 leads are actually for motor overload/overheating protection, not for on/off temperature control. So, the new questions are:

1) Do I really need to connect up the aux control circuit? That is not something I'm familiar with.

2) For turning the fan on and off, I assume I can use a 3-pole switch like this:
https://www.amazon.com/Leviton-MS303-DSW-Controller-Disconnect-Non-Grounding/dp/B07K5BC4L4/ref=sr_1_1?dchild=1&keywords=3+phase+leviton+switch&qid=1586470630&sr=8-1

[Paul F Franklin]

Well....you can do that, assuming your spiffy new motor draws less than 30 amps/phase (pretty likely!). You just won't have any overload/overtemp protection on the motor. How big a deal is that? Well, if your motor catches on fire and burns down the shop, the insurance company will be happy to see that particular shortcut. Sure, that's worst case and not very likely, but.... More likely? A bearing fails or cooling air gets blocked, the motor overheats and you burn out a winding or three.

[Tom Bain]

Paul — It’s only a 1 HP motor so not pulling a ton of amps, but point taken. Could I not just use a motor starter that has built-in overload protection and then not worry about the separate aux control circuit, since that function would be provided by the starter?

[Paul F Franklin]

Tom, I was envisioning a larger motor....a manual starter would be a good solution, and personally, I wouldn't lose sleep over your original plan on a small motor.

[Malcolm McLeod]

Paul — It’s only a 1 HP motor so not pulling a ton of amps, but point taken. Could I not just use a motor starter that has built-in overload protection and then not worry about the separate aux control circuit, since that function would be provided by the starter?

Review the motor nameplate (pic) carefully. It tells you about all you need to know.

If you use a typical (magnetic) motor starter, the control circuit/leg/voltage passes thru all the 'control' components: from the source, thru the operators, thru the thermostats, thru the overloads, finally thru the coil and a return to the source. The overloads don't kill the power to the motor; they kill the power to the starter coil and so the starter main contacts kill the power to the motor.

You CAN wire around the t'stats, but strongly advise you follow conventions in the overall control circuit - - and at that point the t'stats are just a matter of 2 more wire nuts.

[Tom Bain]

Malcolm — So, if I did want to connect the t’stats those would be wired into the magnetic starter utilizing the N.C. terminal, such that if the t’stats sense a high-temp condition they would would open the contact just like hitting the stop button? Do I have that anywhere close to correct? I’ve connected/replaced magnetic starters before, but never seen these extra t’stat wires.

[Malcolm McLeod]

Malcolm — So, if I did want to connect the t’stats those would be wired into the magnetic starter utilizing the N.C. terminal, such that if the t’stats sense a high-temp condition they would would open the contact just like hitting the stop button? Do I have that anywhere close to correct? I’ve connected/replaced magnetic starters before, but never seen these extra t’stat wires.

Exactly correct.

[Rob Luter]

Paul — It’s only a 1 HP motor so not pulling a ton of amps, but point taken. Could I not just use a motor starter that has built-in overload protection and then not worry about the separate aux control circuit, since that function would be provided by the starter?

Yes. The schematic illustrates separate contacts and thermal overloads. A definite purpose motor starter will have integral overload protection. Get a NEMA starter with integral overloads if you anticipate lots of on/off cycles per day.

[Bill Dufour]

Make sure the switch gear is located outside the explosion safe area. You do not want sparks from switches with fumes in the booth. They do make explosion safe switch housings but I would mount the contractor outside the danger area.
Bill D.

[Tom Bender]

So once you get all that worked out, consider adding a timer to the control circuit. That lets you leave the fan running for a while after you leave the shop to clear the air of dust or paint fumes. Also handy when you are in the shop and make the air bad, just crank up half an hour and forget it.

[Tom Bain]

Well, I got a magnetic starter and wired it up, hit the start button, and nada. I know the fan works and I know that I’m getting power on the line side, so need to do some troubleshooting as to why it’s not delivering power to the load side.

[Rob Luter]

Well, I got a magnetic starter and wired it up, hit the start button, and nada. I know the fan works and I know that I’m getting power on the line side, so need to do some troubleshooting as to why it’s not delivering power to the load side.

Does it have integral solid state overloads or does it require overload heaters? If you need heaters they are selected and installed separately based on anticipated amperage. Think of them as fuses. No fuses = no current. No offense, but you should have an electrician doing this.

[Frank Pratt]

Does it have integral solid state overloads or does it require overload heaters? If you need heaters they are selected and installed separately based on anticipated amperage. Think of them as fuses. No fuses = no current. No offense, but you should have an electrician doing this.

Agreed. At the very least get a pro to show you how to do it & then have a look when you're done. It's cheap insurance against motor damage & fire/shock hazard.

[Tom Bain]

It’s working! After some motor control schematics research, I was able to successfully troubleshoot the magnetic starter issue. Turns out the manufacturer forgot to install one of the jumpers (connecting L2 to the other side of the NC stop/reset contact). I put that jumper in (highlighted in the pic) and she started right up. I guess the guy assembling the starters that day had one too many caipirinhas!

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