Subject: Re: rotary phase converters

From: [email-masked] (Daryl Bender)
Date: Wed, 05 Apr 1995 17:30:26 -0500

In article <[email-masked]>, [email-masked]
(Paul Amaranth) wrote:

> In article <[email-masked]> [email-masked] (David Erickson) writes:
>
> snipped excellent info
> 
> BTW, the input phases are not 180 apart, your input power is single
> phase 220 since you're not referencing it to the neutral.  With 2 wires
> you get 1 phase, with 3, you can have 2 or 3.
>
> more snipped excellent info.
> 
He is partly right here in that a house typically receives two 120V inputs
that are 180¡ out of phase.When measured from +120 to -120  you get a
single phase 240. Essentially the same thing you said though.

-------------
                 )
                 )
                 )
                 )  ------------------------------------ +120 (or 120V with
zero phase angle)
                 ) (                 
                 ) (
                 )  ------------------------------------
                 )  ------------------------------------ tied to form
Neutral
                 ) (
                 ) (
-------------   ----------------------------------- -120 (or 120V with 180
phase angle)
         Tranformer
        on Hydro Pole

Daryl
-- 
Working, but not speaking for BNR.

---

From: [email-masked] (Antonio Palama')
Date: 07 Apr 1995 11:25:57 GMT

In article <[email-masked]> 
[email-masked] (David Erickson) writes:

   I have a few questions about rotary phase converters.  I just finished
   wiring a magnetic starter and some outlets for mine, and the electrical
   connections surprised me.  There were only three connections inside the
   box on the phase converter - is this typical?  I had expected five
   connections (2 in and 3 out).  Instead there are 2 in and 1 out.  I
   thought a phase converter was a motor generator, but all it does is
   generate the third phase.  

The phase converter you describe is, most probably, of the Ferraris-Arno
type; in August 1994 I mailed an article to this newsgroup with the references
to the theory of the method and a practical discussion of how to implement it
using an ordinary 3-phase induction motor. I do not have a copy of that posting
handy but, should someone be interested, I could repost it next week.

   So, how does this third phase relate to the 220V input?  There is no
   way that the three phases are balanced... (how could they be, since the 
   input phases are already 180 degrees apart).  How many volts will there
   be between the two input phases and the generated phase?  How will this
   vary with load (i.e., is it a bad idea to have the phase converter
   running without any load, or with a very small load in relation to its
   size)?

   I'm going to run some experiments to find out, but I'd welcome hearing
   from any EE's who could explain some of the theory here. 

   Another thing that surprised me: I opened the box on top of the phase
   converter that holds the electrical components that generate the
   third phase.  All that was inside the box were two large capacitor
   banks.  Actually, I don't know if those banks were used in generating
   the third phase or in starting the phase converter.  Any information
   that could shed some light on this would be appreciated. 

A simplified theory of the method is the following:
One can write the single phase AC voltage as the sum of two 3-phase systems
of the same voltage and rotating in opposite directions. An asyncronous
induction motor running near syncronous speed (e.g. 2950 RPM for a 2-pole
50 Hz motor) has a very high impedence for the 3-phase system rotating in
the same direction of the motor and a very low impedence for the other one.
So one of the two 3-phase systems is short circuited by the motor while the
other one is left undisturbed.
The role of the capacitor is both that of starting the phase converter and that
of improving the balancing of the output 3-phase system. In theory one should
use a specific capacitance for every load on the converter but, in practice,
a reasonable compromise can be worked out.
This method is inehrently risky for the load motor since the 3-phase system
produced by the converter is unbalanced and this can overheat the load
motor. Nevertheless I had no problem using this type of converter in my 
workshop.

Good luck.

Antonio.

---

From: [email-masked] (Antonio Palama')
Date: 07 Apr 1995 11:25:57 GMT

Good luck.

Antonio.

---

From: [email-masked] (Alan Frisbie)
Date: 11 Apr 95 14:13:46 PDT

In article <[email-masked]>, 
[email-masked] (David Erickson) writes:

> I have a few questions about rotary phase converters.  

Call GWM Corporation (see below) and ask for a copy of
their booklet, "Understanding and Selecting a Phase Converter".
While it *is* a sales tool and will try to steer you towards
their higher-end models, it does do a decent job of explaining
how phase converters work.    You will also get a selection
of sales literature, but that is educational as well.

        GWM Corporation (Gerhard Werner Motorwerke)
        P.O. Box 705
        Peculiar, MO 64078

        816-380-7247 (Voice)
        816-887-3889 (FAX)
        800-437-4273 (Voice)

When I retire, I hope I can find a place to move to with
as good a name as "Peculiar".   :-)

--  Alan E. Frisbie               [email-masked]
--  Flying Disk Systems, Inc.
--  4759 Round Top Drive          (213) 256-2575 (voice)
--  Los Angeles, CA 90065         (213) 258-3585 (FAX)