Hardwiring 230VAC Compressor Question

On Sat, 06 Dec 2003 20:51:50 GMT, Ted Edwards put forth the notion that...


One phase and neutral gives you 120 volts.
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Checkmate
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Checkmate wrote:

Your right. Sorry. Neutral to any phase is 120. As a result, from one phase to another is 208 volts.
Ted
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On Sat, 6 Dec 2003 10:24:57 -0500, Rick wrote

Rick, That is not news I want to hear. This goes for running 230VAC single phase on 208VAC single phase service? I had always thought there was some lattitude in voltage ratings of motors. Can anyone elaborate on why this would cause overheatng/shorting? Trying to learn something along the way. -N.
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There is some latitude. A nominal 230 motor will typically run fine on voltages between 220 and 240. But 208 is too low unless the motor is specially designed to handle it. Horsepower is voltage times current divided by 746. For a given horsepower demand, a lower supply voltage means the motor must draw more current to provide rated power.
Heat build up in the motor is a function of current squared times winding resistance. So the more current the motor has to draw to meet the load power demand, the hotter the motor will run. Notice that heat is a function of the square of current, so it doesn't take a whole lot of excess current to get the motor very hot.
Motors are designed for a specific maximum temperature. Exceeding that temperature will greatly shorten the life of the motor, ie you'll let out all the magic smoke if you persist in running a motor at its rated output power with lower than rated supply voltage.
Note that you could probably get away with running a 230 volt motor at lower than design voltage if you don't draw anything approaching its rated power output from it. In other words, if the motor is oversize for the load power demand, it'll tolerate running on a lower voltage. That's how motors rated to run on 208 to 240 are designed, ie they're bigger (larger windings and or more cooling air) than their nameplate hp would otherwise require them to be. That costs money, so equipment manufacturers typically don't use such motors unless you pay extra for them.
Gary
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Somebody wrote:

with
Absolutely true.
If you want to operate a 230V motor on a 208V supply, you must add a buck-boost transformer to the system.
By definition, a motor must be able to operate at +/-10% of nameplate voltage.
For a 230V motor, that translates to 207V minimum which sounds like it might work on a 208V supply; however, the utility supply voltage is allowed to operate at +/-10% which translates into 187V minimum.
Therein lies the rub. You can't get there from here.
Add a buck-boost x'fmr.
They are small and relatively low cost.
HTH
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Lew

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