Air Compressor Controller

[jean dore]

I am trying to draw a circuit for an air compressor controller and I am having difficulty figuring it out. Any help would be greatly appreciated.
This is a 460 3 phase Air compressor with a 115v pressure switch, 115v low oil switch and a dump valve on a timer.
I will first tell you the parts I have.
30A Breaker
Motor start with overloads
460/120 Transformer with fuse
Phase rotation monitor
Pressure switch
Low oil switch
Dump valve on a timer\
8 terminal relay
10 contact terminal board
On switch/ momentary
E-stop/ momentary
Phase rotation pilot light
On pilot light.
I understand the concept and that the PS,LOS and PR monitor all have to be closed in order for On push button to work. But adding them all together is giving me a headache... Please help

 

[Minder]

I assume you want to have automatic restart when the PS resets after opening?
You would normally have a series string consisting of a N.C. E-stop followed by a N.O. start P.B. that has a N.O. contact from the 3 ph contactor in parallel with the start, also the timer output contact across the PB.
the series string would continue with the contactor O/L's and Phase rotation contact and low oil switch the final output can either be the contactor coil itself or use the control relay and use a N.O. contact on it for the coil.
I presume the Press.S. will input to the timer for delay restart.
Telemecanique wiring diagrams
M.

 

[Sunnysky]

de morgan's law states inverting all inputs and output inverts function from/to AND/OR
If contact is normally closed, call that "0" , so all inputs in series will be ANDed when output is Closed="0". Active low means negative logic if contacts are looped to ground. If all contacts are connected to V+ or Vac to an ac relay coil with otherside gnd (V-) or AC neutral, then output is inverted,so we call this not AND or NAND.

One can also invert the inputs (open) and wire in parallel and use wired OR. But for safety on sensor connections, NC is preferred. (normally closed)

So see @Minder's diagram.. START IS PARALLEL and STOP is series.

 

[Minder]

It would help to also know the nature of the timer, I made a wrong assumption on the Pr.Sw either a N.O. contact on the contactor or use the control relay if not enough contacts, and use this to trigger a on delay timer which will pick up the dump valve after the delay, this way every time the motor starts the dump valve will be open for the time set.
The pressure switch normally has a N.C. a common and a N. O. so the N.O. section would be across the start P.B., and the N.C. in series with the start P.B. for automatic start
I could sketch it out if needed.
M.

 

[Bluejets]

Unusual to have a dump valve electrically controlled.
I'm assuming this is to dump head pressure at shut off to allow the compressor to re-start with zero head pressure.
Don't really see the need for phase rotation monitor unless the unit is portable.

 

[Minder]

Don't really see the need for phase rotation monitor unless the unit is portable.

Tecumseh state that their 3 ph compressors Must include phase rotation detection otherwise it can be harmful and lead to premature failure of the compressor.
It can happen if someone upstream or source of the supply happens to change the phase rotation.
M.

 

[jean dore]

Thanks for all the input. I think I having been using the wrong term. What I always call a dump valve is actually a condensate drain valve. That's what the timer is for. I'm sorry for the confusion. If it is not too much trouble minder I would greatly appreciate a sketch of this system. I am starting to grasp the concept in pieces but as a whole it is still very confusing to me. Also the phase rotation is because on the waterfront this is being fed by cam locks and people tend to not put them in order.

 

[Bluejets]

Tecumseh state that their 3 ph compressors Must include phase rotation detection otherwise it can be harmful and lead to premature failure of the compressor.
It can happen if someone upstream or source of the supply happens to change the phase rotation.
M.

Yes, I've seen the need with Tecumseh in RAC systems but not air compressors.
No problem though, just another contact.
Some phase rotation relays will even include undervoltage protection.

 

[Bluejets]

What I always call a dump valve is actually a condensate drain valve. That's what the timer is for. .

Fair enough.
I'd tend to call that a condensate drain valve but different countries.....different names...
So I take it then the pressure switch has a mechanical dump valve on the compressor head?
It's usually seen as a small tube running from the bottom of the pressure switch to a flap valve housing on the top of the tank where the main pressure line from the compressor enters the tank.

 

[jean dore]

True.

 

[Minder]

I thought some thing along these lines, you will modify it to suit, I assumed the dump valve was a pressure relief on the head, modify as needed.
M.

 

[jean dore]

Thank you very much.

 

[Bluejets]

Phase rotation contacts will be n/o (should be drawn as such) until correct rotation is achieved.
Control circuits are drawn, no power.
Circuit does not take into account (due to shortcoming of info on equipment used) the 110v transformer and associated controls.
i.e. it was mentioned the pressure switch and the low oil switch were 110v but no mention of coil voltages on contactors and timer etc.
Adjustment to drawings will have to be made to allow for this.
Is the dump valve required to operate at each run of the compressor?
If not, then a selector switch or 7 day timer or whatever would be needed here.
As Minder says, the drawing is "along these lines" so he has provided a guide to final circuit.

 

[Tha fios agaibh]

This is a 460 3 phase Air compressor with a 115v pressure switch, 115v low oil switch and a dump valve on a timer.
I will first tell you the parts I have.
30A Breaker.....

You should start with the motor specs. The 30A breaker may not be sized correctly to your application.
On thing I think compressors should have is a redundant pressure switch that would kill the main control circuit in case the normal pressure switch fails. (And they do fail)

 

[Bluejets]

On thing I think compressors should have is a redundant pressure switch that would kill the main control circuit in case the normal pressure switch fails. (And they do fail)

They do have a mechanical overpressure release valve.
When they release it is very difficult to ignore.
30A should be ok for around 7 to 10hp as they start no-load.
Breaker should be sized to both protect the cable used (size) and allow normal motor start without tripping.
A different curve breaker can also be used in some circumstances.

 

[hevans1944]

This brings back some fond memories...

They do have a mechanical overpressure release valve.
When they release it is very difficult to ignore.

 

[Tha fios agaibh]

They do have a mechanical overpressure release valve.
When they release it is very difficult to ignore.
30A should be ok for around 7 to 10hp as they start no-load.

Ah, good point on the mechanical pressure release. I forgot about those.
But for an extra $20, it wouldn't be a bad plan to have the electrical redundancy also.

Yes,the 30A cb would work for a 7.5hp (standard size) but it'd likely be undersized for a 10hp and giving nuisance trips at start up.

For motors it's generally better to use a fuse.
They tolerate the inrush currents better and are often faster than a breaker.

I guess my point was, that you should start by knowing exactly what your load is before designing the rest of your circuit.

 

[jean dore]

So there are still a few things I left I guess. Let me try and fill in. As for the coil. That is 115v. The motor though is sized correctly. The plate states 8.97A for the FLA. the NEC calls for 11A for that hp motor and voltage but the plate overrides that.

 

[jean dore]

I did do the load calculations for that motor. 250% of fla for the breaker since it is instantaneous. The wire to the motor will be 12AWG. There is also a manual pressure release on top of tank. I really would appreciate a wire diagram of this set up if anybody feels bored. Thanks for all the help everybody. Greatly appreciated.

 

[Bluejets]

Don't use AWG here in Aus but looked up equivilent and you are in the ball park.
I would have gone for 4 mm2 which is a tad bigger but the AWG doesn't change sizes in proportion to mm2.
Depends how far you will run the final feed.
At the stated FLA I'd say around 5 to 6hp for your motor.