[awerby]

Since people here on the Zone know so much about so many things, I thought I'd toss out a problem I just ran into. I recently got a new (to me) 7.5 hp air compressor, and the motor says it wants to run on 208 volts 3 phase. I've got 3-phase power, but its delivered at up to 240v. Is that going to kill my motor? If so, will it do so quickly and spectacularly, or slowly, like over a period of years? Will it draw more amps or less? Is this going to affect any other parts of the system, like the pressure switch? If it will run at all, I'd just as soon use this motor, and replace it when it eventually dies. But if it's going to burst into flames or something, I'll have to rethink that...

[Al_The_Man]

Motors like transformers are fairly rugged items, but it is a tad over the recommended limit which is normally around 228.
You could try it and monitor the running current, both off and on load and compare to the FLA on the plate, if it is shown.
It will not affect any other part of the system, also you should have overloads and fusing in place to prevent a complete burn out.
Max.

[awerby]

Yes, I'll put an amp clamp on it when I start it up, and see how much current it draws, which should be greatest at startup. I understand about fuses, and plan to use the slow type rather than the instantaneous-blow kind. But when you say "overloads" do you mean the "heaters" in the motor starter, or something else?

[Al_The_Man]

But when you say "overloads" do you mean the "heaters" in the motor starter, or something else?

Overloads now are generally electronic, the old style used 'heater' elements.
They are generally mounted integral or stand-alone on the bottom of the contactor.
Al.

[jbaz]

I wouldn't worry about it on a compressor so much unless it was an expensive one, IF YOU REALLY HAVE 240v and not higher... have you measured the legs yourself to see if they are 240v and you don't have a hotter supply? If you're getting in to the 248 range then it might be pushing it more than I would. But at 240v, especially if it drops a little under load, you're only about 5% outside the usual threshold. If it is an expensive compressor or you want to do it right buy a set of buck boost transformers. For a motor that size you can buy a used set on ebay for around $100 if you hunt long enough or a new set of Federals for about $200.

[awerby]

Thanks for all the help, guys! I'm now sufficiently emboldened to try running this thing; it sounds like undervoltage is a much bigger problem for motors than overvoltage. I had a safety switch with fuses, and I got my new starter yesterday, it has the old-fashioned kind of heaters I'm used to. I'm pretty sure my voltage stays somewhere between 220 and 240, but I'll check it when I get the circuit run, and monitor the amperage when I start it up.