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Motor Phases
On 21 Oct 2003 23:20:39 -0400, (DoN. Nichols) wrote:
2. Rotary converter - full power, could be cheap if you use an old 3
phase motor to make your own, or more expensive if you buy one new.
Could be noisy. Often shared by multiple machines.
There are techniques for starting them other than the one
mentioned under "static converter", including:
a) "self-starting": where the tuning capacitors are a
compromise value which gets the motor spinning without
any switching at all.
b) "Pony motor": A smaller motor is connected with belt
to the converter motor, to get it started spinning, and
then the belt is thrown by raising the pony motor to
slack the belt.
No need to throw the belt, just switch the power off to the pony and let
it freewheel.
c) A pull rope, used to start the motor spinning, just
before switching on the power.
Note that a rotary converter is better made self starting, either
from the inclusion of the parts of a static converter, or from
self-starting capacitor balance. One reason for this is that if
you walk out of the shop leaving something running (even just
the rotary converter), and power drops for long enough for the
converter to spin down, when it returns, the motor will sit
there humming and drawing excess current. If the circuit
breakers are properly sized, it will pop the breaker.
Otherwise, it might fry the motor without a self-starting mode.
Always use a contactor to supply power to the rotary. The contactor
is activated with a momentary push button, then it is held in by auxillary
contacts on the contactor. If the power fails, the contactor releases,
and won't re-engage until you push the start button again. This way,
if the power fails, the converter will be *cold* until you manually restart
it after the power returns. You'll also want a normally closed push
button in series with the contactor coil. This is your stop button.
Gary
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