General stuff on phase converters

FWIW, following is an email message sent to a reader of Home Shop Machinist
that had questions re. my article in the Nov/Dec 2001 issue of HSM.
Hi Jim,
I received a US mail letter re. your question on figures 13 and 14 of my
article in the Nov/Dec 2001 edition of Home Shop Machinist. I am sorry if
the pictures generated any confusion. They were not intended to be actual
working circuits but were to serve as examples of voltmeter testing of
rotary phase converter circuits.
Please go back and read the text starting on p.32 "Idler Motor Rotary Phase
Converter, Type 1".
Fig. 12 shows a rotary phase converter consisting of a 3 phase motor with no
capacitance. As per the text, a converter like Fig. 12 would be OK to run a
three phase motor if the idler motor was several times larger than the load
motor. The box labeled "Start" is there to represent the starting method
employed. Starting could be accomplished by the static converter circuit
shown in Fig. 10 or the button controller method of Fig. 11, or it could be
as simple as a pull rope wrapped around the idler motor shaft. As in all
the start "boxes" shown, start capacitance should be on the order of 70 to
100 microfarads per horsepower of the idler motor. Generally speaking, the
more economical electrolytic capacitors are used for starting.
Figures 13 and 14 show respectively, "unbalanced" and "balanced" rotary
phase converters. As per the text, in the unbalanced type, Fig. 13, the
single run capacitor is chosen on the basis of approximately 30 microfarads
per horsepower of the idler motor. Adjustments of capacitance are made in
small increments above or less than 30 microfarads/HP to bring the third leg
voltage to the same as "line in" voltage plus 8 or 10 percent.
Figure 14 is a "balanced" or Fitch type of rotary phase converter.
Capacitance is started out at about 30 microfarads/HP with about 40% in one
leg and 60% in the other - balancing proceeds as stated in the text under
"Voltmeter Adjustment of The Balanced Voltage Rotary Phase Converter" on
p.35. Interestingly, when voltage balance is achieved the amounts of
capacitance will not be the same. This is an anomaly related to the
direction of rotation. Specific amounts of necessary capacitance will vary.
For example, my "balanced" 7.5 HP rotary has 93 microfarads in one leg and
167 in the other. For years when it ran "unbalanced" it had 212 microfarads
to make the unloaded 3rd leg voltage 10% higher than line in.
Thank you for your interest in my article. I hope this answers your
questions. Please contact me if I can be of any more help.
Bob Swinney
Reply to
Bob Swinney
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Hmm. Well that clears *that* up. I always wondered about that.
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Reply to
jim rozen
Yeah. It took me a while to tumble to that idea, myself.
Bob Swinney
Reply to
Bob Swinney

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