Re: single to three phase conversion

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> 


 > The gadgets described are capacitor-start arrangments for the three
 > phase motor, generating enough torque to get it running after which it
 > runs on a single phase with accordingly reduced power.  On inspection
 > they contain a large ELECTROLYTIC starting capacitor, a resistor, and a
 > relay.  I assume the Lindsay book gives construction details.  Both my
 > lathe and my mill have three-phase motors which I run this way and they
 > are noticeably deficient in torque.  There is another technique which is
 > sometimes used, but not at all convenient.  A large three-phase motor is
 > started single phase that way and, once it's running three phase power
 > can be drawn from its three connections, only two of which are connected
 > to the single phase input line.  No magic really, the three-phase motor
 > is operating as a three-phase induction generator.  Cumbersome and not
 > very efficient, but can be used.
 >

I don't know that it's all that inefficient, compared to other ways of
making three phases from one (Motor generator, solid state inverter).. the
rotary converter DOES depend on the load being reasonably balanced.

The other thing is that the 3phase motor being used has to be sized
appropriately. I seem to recall a rule of thumb that the motor has to be
twice the size of the load (i.e. If you want to run a 2 HP three phase load,
you'll need a 5HP three phase motor for the converter).. probably has to do
with the current in the windings.

  They also make specialized devices for this application with somewhat
different rotor design and stator windings (I don't know that the wining for
the stator uses different gauges on the line and load windings, or, even, 4
windings). Certainly, the rotor design can be optimized from the usual
induction motor rotor designs (which are tailored to give the desired torque
vs speed curves, depending on the mechanical load characteristics.  A
compressor or a punch press is a very different load than a fan)