Subject: Rotary phase converters

From: [email-masked] (Antonio Palama')
Date: 12 Apr 1995 06:56:46 GMT

This is a reposting of an article which I mailed to this newsgroup
in august 1994.
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There have recently been some questions in this newsgroup about 
the conversion of single phase to 3-phase power and specifically 
about the use of an asyncronous induction motor as a phase 
converter. I have been using this conversion technique for a 
long time in my workshop where my lathe, shaper and drill press 
have retained their original 3-phase motors and are powered by a 
single conversion system.

This conversion technique is known in the electrical literature 
as "Ferraris-Arno Conversion". The best theoretical analysis I 
know of this subject is in:

BROWN J.E., and RUSSELL L.R.: "Symmetrical-component analysis 
applied to phase convertors of the Ferraris-Arno type" 
Proceedings I.E.E. Paper no. 2704 U Oct. 1958 (105 A, p. 538).
Reading requires some electrical engineering background.

For the practical point of view, here is a description of what I 
did in my workshop where 220 V 50 Hz power is available.

1) I found an old induction motor at the local junk yard whose 
rated power was close to the power of the machine's induction 
motor (work motor). This motor was used as the converter (pilot 
motor). 

2) I connected both the work and the pilot motor windings so that 
the rated 3-phase voltage (hot wire to hot wire) was 220 V.

3) I wired the motors in the following way:
Let R,S,T be the leads of the pilot motor and X,Y,Z the work 
motor's ones. 
I connected R,S to the power mains through a single phase (2 wires) 
switch S1. 
I connected a tentative capacitance (about 50 microF) across S 
and T. This capacitance was made up by connecting capacitors in 
parallel; the capacitors were old, single-phase motor, NON 
ELECTROLYTIC startup capacitors; their voltage rating was above 
400 V AC.
I connected R,S,T to X,Y,Z through a 3-phase (3 wires) switch S2, 
as such I used the normal machine switch.

4) I tested the start-up of the pilot motor:
With S2 OPEN I closed S1 and verified that the pilot motor 
started and reached synchronous speed in a short time. When it 
didn't start or the run-up time was too long I increased the 
capacitance.

5) I tested the start-up of the work motor:
With S1 closed and the pilot motor running, I closed S2 and 
observed the start-up of the work motor. I exchanged any 2 of 
X,Y,Z when the work motor started in the wrong direction. In all 
my experiences the work motor started without problems and with a 
good starting torque.

6) I measured the unbalance of the work motor currents:
I used a clamp AC ampere meter to measure the currents in leads 
X,Y,Z. Unbalance could also be measured through the voltages 
X-Y, Y-Z, Z-X but this was a measure much less sensitive than the 
above.

7) I fine tuned the capacitance:
I experimented with different capacitors connected in parallel 
to the start-up ones with the work motor running at average load 
and with the aim of reducing the unbalance to a minimum i.e. making 
the currents in leads X,Y,Z as equal to each other as possible. This 
usually required a substantial increase of the capacitance above what 
was required for the pilot motor's start-up. Typical best balanced 
currents were in the ratio 4:4:5. In my workshop I had to compromise a 
little in this fine tuning since a single pilot motor had to serve 3 
different work motors.

8) When I operate my machines I close S1 first and, once the 
pilot motor has reached synchronous speed, I close S2. One WRONG 
procedure that could seriously overload both motors and which I 
am very careful to avoid is to close S2 first and S1 second.

9) I never asked the work motor to deliver full rated power 
for a long time. The unavoidable unbalance in the work motor 
currents implies overheating and hence reduction of the available 
maximum power. Being afraid of burning my motors, I use to 
monitor their temperature during operation.

This summarizes my experience, the technique worked for me but 
it is not risk-less. If you decide to give it a try use your own 
judgment and do it at your own risk.

Ciao Antonio.