ECN Forum Forums General Topics of the Trade General Discussion Area motor starter

Ever get those jobs where you think you "know" something can be done, but due to brain cramp, can't figure it out??

With a push button motor starter (1 stop, 1 start. Not a manual button starter but one controlled with coil), connected to a compressor with pressure switch. When the pressure switch opens the circuit and breaks the coil contact, it takes the operator to push the start button again once the pressure drops to get the motor going again.

Argh! the holding contact, I believe there is a way around this..... and can't quite remember how to do this. Wire the pressure switch into the aux contacts?

I think a compressor needs an off-auto switch.

You need an off-auto switch, or a locking run push button

I don't think I would leave it that way. I would fill the holes with a press-to-test indicator and a momentary-off-on rotary selector switch. The pressure switch should have a pressure and deadband or differential adjustment. The Off position breaks path to N.C. opens on P increase, switch terminals. The momentary, "Hand" position, parallels P-switch, to check operation without bleeding the tank. The N.O. aux switch would be used to drive your indicator. Do you have any E-stops in the mix? You wouldn't need any holding contacts unless you were going to use 2 separate pressure switches in series. In that case, the holding contact is in parallel with the low pressure switch. It helps to think of one as "closes on low pressure", and the other as, "opens on high pressure"
Joe

Seems like someone is trying to defeat a safety device ... to me. If there's a high-pressure cut off switch, it's there for a reason. I'm not about to give advice on defeating it.

If you want the compressor to automatically re-start when the pressure has dropped ... well, that's a normal control problem, and would be solved by adding a pressure switch - not by bypassing an existing one.

I'm real close to locking this thread. Did I understand the request correctly?

I didn't read this as an over pressure switch, just the one that establishes proper tank pressure.

BINGO!!!

BINGO!

Thanks for the clarification.

Forget the holding contact

You need to bypass the holding contact entirely, and run the coil directly from the pressure differential switch.

A MAINTAINED contact switch can be inserted anywhere in the control circuit to turn it off.