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Re: single to three phase conversion

Original poster: "john cooper" <tesla-at-tesla-coil-dot-com> 

Here's a link to the scanned Lindsay booklet pages covering rotary phase 



---------- Original Message ----------------------------------
From: "Tesla list" <tesla-at-pupman-dot-com>
Date:  Sat, 03 Apr 2004 10:12:09 -0700

 >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)