wiring compressor with HOA switch, magnetic contactor and no-volts release

[metalmagpie]

I am putting together an air compressor. Its motor is 7.5hp 3 phase 240 volts. I have a size 1 magnetic contactor which has several auxiliary contacts both NO and NC. I also have an HOA switch and a pressure switch for the tank.

I have seen diagrams of a normal 3 phase HOA (hand-off-auto) switch controlling a magnetic contactor which switches a motor. But the circuits I've seen lack the feature I need which is if the power goes off and back on again I want the machine to not come on again. I've heard this called "no-volts release" which is a good succinct phrase so that's what I'm calling it.

I do have a background in electrical circuits but am looking for help on a schematic to put my machine together.

Thanks,

metalmagpie

 

[rons]

The circuit is common in some RPC designs.

Control Wiring - 3 Wire Control - Start Stop Circuit — TW Controls - Helping You Become a Better Technician

The most common use of 3 wire control is a Start/Stop control.

twcontrols.com

 

[SomeoneSomewhere]

I'm pretty sure you will need two contactors (or a contactor and a relay) to deliver both no-volt release, and automatic pressure-based control.

A small signal relay held closed by the presence of mains power, with the standard 'stop' and 'start' buttons.
Power from that, through the pressure switch (and possibly an A/O/M switch, though that's usually not a great idea on compressors unless you want to test your safeties) feeds your main contactor.

 

[TimWilborne]

Thanks for the plug @rons . Here it is with the no power up option. That might make a good video.

 

[SomeoneSomewhere]

Note you can use the same NO contact as both the sealing contact and for the big contactor. That means you can use an SPST relay, rather than DPST.

 

[metalmagpie]

Thank you, Tim. I'm sorry to be so dense, but in your drawing it would appear that the leg through the S/S switch, once activated, would power the motor regardless of the state of the HOA switch or pressure switch. The motor connection is implicit in your drawing, maybe I don't understand where the motor should be.

metalmagpie

 

[metalmagpie]

Thank you, S/S. It looks to me in the second photo above that if the start button were pressed and the state of the HOA and pressure switch were right, that there would then be the same voltage on both sides of the big contactor and no current would flow. Still confused. Thanks for helping!

metalmagpie

 

[SomeoneSomewhere]

This is a 'ladder diagram'. Hidden by his hand is the power source on the left, so typically the left side is your supply voltage (e.g. +24V DC, or 24/120/240V AC hot), and the right side is your common (0V DC, or neutral, or maybe L2). When a connection is made all the way through from left to right, the relevant coil/load (the big circles) is energised.

This is just a controls diagram, it doesn't show the high power connections. The implication is that when the 'big contactor' pulls in, that switches your compressor on.

 

[TimWilborne]

@SomeoneSomewhere is correct, it is only the control portion of the wiring diagram and my hand is covering the power supply. The wires coming out of the right side of the small relay and big contactor would go to the DC- or N. The wires coming out of the left side of each go through the pilot devices and to the +DC or L1.

 

[rons]

It took a bit of time looking up a diagram of the circuit. Wasn't like that 15-20 years ago.
All the google links were connected with selling something.

 

[TimWilborne]

It is probably still a good practice on a lot of stand alone systems today where someone could easily be injured due to a sudden start. We should lock out equipment, but you would be surprised at the crazy ways machines react to losing power.

 

[TimWilborne]

@metalmagpie and @rons . Does this video show the problem and solution well? It was a good exercise.

 

[rons]

This should cover what the op wants.

 

[TimWilborne]

Ok I must have misread. I thought they needed a pressure switch in the circuit also.

 

[SomeoneSomewhere]

Yeah, OP definitely needs a pressure switch here per their original post.

 

[rons]

I use this digital pressure switch on my tank.

https://www.motion.com/products/sku/05176563?utm_source=google&utm_medium=cpc&utm_campaign=performance_max&utm_content=&gclid=EAIaIQobChMI8sDViZqfgQMV90t_AB3yBg-bEAQYAiABEgJ_1_D_BwE

 

[SomeoneSomewhere]

OK, that's a trickier one because it requires a 12-24VDC supply and can only switch a 24V 80mA load. So you're going to need another small signal (interposing) relay and a 24V power supply.

Unless you have a great need for those specifically, I would get a more conventional pressure switch and a gauge. A standard air compressor pressure switch might also come with an unloader valve, which will make starting much easier.

 

[metalmagpie]

@metalmagpie and @rons . Does this video show the problem and solution well? It was a good exercise.

It took me a long time to realize that your small relay had one coil and two switches i.e. was DPST. It might have been more clear if you'd drawn them both using the same symbol. Anyway, once that particular light turned on then all became clear.

metalmagpie

 

[TimWilborne]

It took me a long time to realize that your small relay had one coil and two switches i.e. was DPST. It might have been more clear if you'd drawn them both using the same symbol. Anyway, once that particular light turned on then all became clear.

metalmagpie

Glad it helped and thanks for the feedback. It can be challenging to relay...no pun intended...which components in my videos equate to which components out in the field sometimes. I have a trainer that has a basic motor starter set up that might have made that clearer but I was already set up to do videos on that PLC trainer.