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