Air Compresser motor 230vac 1 phase hooked to 208vac ?


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Old 12-09-04, 11:05 PM
skip159
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Question Air Compresser motor 230vac 1 phase hooked to 208vac ?

Can I hook up a 230vac 1 phase motor to a 208vac panel box ? Its a 7.5 hp motor rated at 40 amps 230v 1 phase. The power coming in is 208vac 3 phase. It would be a 75 ft run and it has #8 wire in 3/4 pipe already there. The breaker box has a 50 amp breaker in it for the motor it use to run, using a 125% rating for 40 amp load on the label of the motor number #8 wire is sized right but if you have 208VAC instead of 230VAC that the motor calls for would raise the amps for the motor to 45 amps with the lower voltage at the panel.
Would a 230v motor run on 208 volts ?
 
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Old 12-10-04, 12:48 AM
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I don't know too much about motor theory, but this doesn't seem workable to me. - Surely when you run a 230 single phase motor, it's operating on two 180 degree opposed alternating currents. In a three phase system you have 120 degree phase separation , so logically if you take only a two output feed off it you will get 1/3 less total voltage,( ie. in this case only140 or so volts )
Any motor theorists around??
 
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Old 12-10-04, 05:48 AM
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Skip,

Essentially all '230V' single phase motors will operate at 208V. Many will work fine, but some will burn out; the motor will almost certainly run hotter, the question is 'will the motor run too much hotter'?. There is no hard and fast rule; check the nameplate, and if you can get them, the manufacturer's instructions. If the motor nameplate does not say something like 208-230/460, then I would recommend against using it in this fashion.

If you use this as a 208V motor, you will need to size conductors for 208V. That makes your #8s too small.

What sort of overload protection does your motor have?

nomind,
Your 3 phase math is wrong. It's called 120/208V because you have 208V line to line. Where phase angles come in is that you have 120V line to neutral, but because of phase angles you get (120 + 120) * sqrt(3) = 208V, rather than the 240V that you would expect in single phase service.

In addition, a load connected to any _two_ conductors of a three phase service is a _single_ phase load. Phase angle difference only matters when you have more than one circuit. Think about it this way: current flowing in an AC circuit follows a sine wave. You have no way to say 'phase angle' because you have no other sine wave to reference against. But when you have _two_ sine waves from two separate circuits, you can identify a phase angle difference between them. With three wires you have three circuits, and phase angle matters, but with two wires you only have a single circuit, so phase angle doesn't matter.

-Jon
 
 

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