Wiring question on Square D motor controller

[Tipton]

This motor controller is wired to my old upright air compressor. It's been running fine for years but curiosity got the better of me and I'm simply trying to piece together how it all works. I got this compressor years ago from my father in law. I have always questioned the basic light switch that he had wired into the system. It's running off of 240V single phase.

Looking at my pic it seems like the red wire is getting 120v from being connected at the same lug as one of the hot wires from the fuse box. That red wire goes up to the light switch. When the switch is turned on it sends power down the black wire to the pressure switch on the tank. Not sure what the white wires job is after that but the circuit basically turns the compressor on or off. In the photo the motor is currently not wired in because I'm replacing it with a new one.

Any ideas? To me it seems like the switch is handling a standard single-phase 120V leg so it's safe to keep it?

 

[TurnipTruck]

I found this schematic on the Journal here years ago and don’t know whom to attribute.

It appears your “light switch” is in series with the pressure switch and probably has 120v (to ground!) through it. I can’t tell if you have a jumper on the coil terminals or not; you may have to unscrew the two brass screws to remove the front cover to be sure, but I don’t see a neutral wire and you said it’s been working. You aren’t running a motor larger than 3hp are you?

 

[mm08822]

The motor starter is supplied with 240vac by 2 hot wires and an equipment ground wire is also run for safety purposes only.

The toggle switch is wired in series with the pressure switch. The toggle switch has to be on (closed) so when the pressure contacts close on low pressure, the motor starter coil is energized with 240 volts. When the tank pressure meets/exceeds the high pressure setting, the pressure opens the control circuit removing 240vac from the coil. Contactor opens, stopping motor. Similarly, the toggle switch could also have 240v across it too (when pressure switch contacts closed) and should be rated for 240-277v.

The white wires you see are not neutrals but actually hot wires in the control circuit. They should not have been white.

 

[wyliesdiesels]

the switch should be rated for 240v (277vac) as it can see 240v thru both sides of the coil.

 

[mm08822]

I found this schematic on the Journal here years ago and don’t know whom to attribute.

It appears your “light switch” is in series with the pressure switch and probably has 120v (to ground!) through it. I can’t tell if you have a jumper on the coil terminals or not; you may have to unscrew the two brass screws to remove the front cover to be sure, but I don’t see a neutral wire and you said it’s been working. You aren’t running a motor larger than 3hp are you?

That control circuit is 240vac. Yes each hot leg to ground (not neutral) is 120v b/c our 120/240vac systems are split phase.
When the pressure switch is open, there is 240v across its terminals (not 120). Similarly, the switch could also have 240v across it too and should be rated for 240-277v.

The diagram has 240v only supplied as there is no neutral wire shown.

 

[Tipton]

I took a look at that light switch. It says "120VAC 15 amp only". Damn thing has been working like that for years and years.

Yes there is a jumper on the two middle coil terminals. Looking at the little diagram it's wired for 240V.

 

[wyliesdiesels]

whats the HP rating on the motor?

looks like NM in FMC? what a poor choice....

whats the gauge on the NM wire?

 

[Tipton]

whats the HP rating on the motor?

looks like NM in FMC? what a poor choice....

whats the gauge on the NM wire?

The horse power rating of the motor is 3 HP 18 amps.

On the NM in FMC are you talking about the wiring in the flex conduit going to the tank pressure switch? If so I agree a poor choice. It looks to be a piece of 12 gauge Romex. It is a really short run though.

 

[Norcal]

I'll pile on about NM cable, AKA "Romex®" being used, ,where subject to vibration stranded wire should be used, I have only seen stranded NM cable once. The SQ D type S starter is a good one, beats the ones w/ a **** definite purpose contactor, or cheap ChiCom IEC starters.

 

[wyliesdiesels]

The horse power rating of the motor is 3 HP 18 amps.

On the NM in FMC are you talking about the wiring in the flex conduit going to the tank pressure switch? If so I agree a poor choice. It looks to be a piece of 12 gauge Romex. It is a really short run though.

as said above, should be stranded THWN wire....

 

[Tipton]

as said above, should be stranded THWN wire....

Yep, this is why I am going through this thing since it's down. I will be ditching that piece of Romex. Also installing a different switch and maybe a indicator light.

Crazy how that 120VAC switch worked just fine all these years.

 

[Cruzan80]

where subject to vibration stranded wire should be used,

Not questioning the rule, but curious about the why? Is it because of the vibrations causing stress fractures in solid, or more vibrations interacting negatively with the attachment pressure for solid wire?

Or is it just NM/b not as good, compared to THHN (even if both were solid)?

This is an "expanding my knowledge" kind of question, not in any way "I disagree with you".