Hardwiring 230VAC Compressor Question

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On Sat, 6 Dec 2003 14:37:52 -0500, Checkmate wrote

Checkmate, can you elaborate on this boost transformer setup? Also, any idea what the equipment would cost (new or surplus) to gt it done? Thanks, -N.
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On Thu, 11 Dec 2003 08:46:14 GMT, -n. put forth the notion that...

Certainly. You take a 240 to 24 volt transformer, and wire the primary and secondary windings in series by connecting one end of the primary winding to one end of the secondary winding. Now, you have what's known as an autotransformer. Next, you connect your 208 volt input to the opposite end of the primary winding, and the common ends of the primary and secondary windings. Finally, you connect your output to the same end of the primary that you connected your input to, plus far end of the secondary winding, and you'll get 230 volts out. It's important that you connect the windings so that they're in phase. By that, I mean you don't want the windings in the primary to be connected to the windings in the secondary in such a way that one coil is wound clockwise, and the other is wound counterclockwise. Acme transformer has some excellent information on this, as well as the correct wiring diagrams for each of their transformers. Here's the link:
http://www.electricalpowertransformer.com/buck-boost.html
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Checkmate
Copyright 2003
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Rick wrote:

Most new areas are seeing three phase power distribution. Single phase service is provided by tapping off one phase and neutral. The result is 208V.
Ted
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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

S/A: Challenge, The Bullet Proof Boat, (Under Construction in the Southland)
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