I have a 230 volt, single phase, 5 hp motor on an air compressor. Not long ago the capacitor starting switches failed, and I was unable to find new ones. I did however find a couple of used ones to get going again temporarly.
What I would like to do is replace the centrifical/mechanical switches with a timed relay, but I'm not sure how to size the relay. The motor draws 23 full load amps, but I'm just swiching the capacitor start circuit. What's minumum rating I should consider when I look for a relay?
Use a contactor - maybe 50 Amps? You can parallel up the contacts for more capacity if you use a smaller model. Relays aren't a good choice here.
One older method was to measure the current drawn by the motor and switch out the start winding when it dropped enough, however that is tricky to do. The timer should be OK - about 5 seconds so a 1 - 10 second adjustable timer should work.
It is a big motor indeed, but I never considered it would take that much to switch off the starting cap. If it's going to be that expensive, I'd probably be better off buying a new three phase motor. But thanks for the help Homer.
Well now, it does have a 5 hp contactor that works off a pressure switch now. On at 150 psi, off at 175 psi. That contactor makes connection of the two 120v legs that supply power. But it also has a spring loaded switch that switches out the starting capacitors at about 1400 rpm or so, it's a 17XX rpm motor. The 5 hp contactor and power leads to the motor are something different.
The relay I want to ad would switch on the starting capacitors immediately when the power is switched on by the existing 5 hp contactor, and break that connection about 1.5 seconds later. So although I know the main contactor has to be rated for switching a 5 hp single phase motor, that normally draws
23 amps ( nd has a lock rotor rating of much much more), I don't know how much amperage is being supplied through, and has to be broken, by the capacitor circuit. Now that I think about it, it's only about a 14 or 16 gage wire, so it would have to be a lot less aperage than the power legs though the 5 hp contactor. The contactor closes a 30 amp circuit wired with #10 copper.
For sure you don't want the contacts to weld together however it may be that the starting current through the cap start circuit is quite reasonable once the motor gets up to speed. So any half way reasonable contactor should do the job OK - inc. a pre owned one!