Subject: Rotary phase converters

From: [email-masked] (Bill W. Martin)
Date: Wed, 19 Jan 1994 21:37:31 GMT

Hello,
	If anyone has info on using a large 3-phase motor as a phase 
converter, ie 1-phase power to 3-phase power, I have a couple of questions.
First, how much power is consumed at idle load condition, relative to the 
size of the motor being run?
Second, can a dual-voltage 3-phase motor be configured as a kind of "
rotating transformer" to step up 220V to 440V ?
I recall seeing some discussion on the general topic of 1->3 phase 
conversion with a 3-phase motor, but no mention of either of my questions.
	Thanks in advance for any info you may have!

Bill Martin

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From: [email-masked] (James Kirkpatrick)
Date: 20 Jan 94 19:15:21 MST

[email-masked] (Bill W. Martin) writes:

|>	If anyone has info on using a large 3-phase motor as a phase 
|>converter, ie 1-phase power to 3-phase power, I have a couple of questions.
|>First, how much power is consumed at idle load condition, relative to the 
|>size of the motor being run?

I don't have a direct answer to this.  However, what I have read suggests
that the converter be about twice the size of the load, i.e. use a 2-HP
motor as a converter to run a 1-HP 3-phase motor.

|>Second, can a dual-voltage 3-phase motor be configured as a kind of "
|>rotating transformer" to step up 220V to 440V ?

Here I am only guessing at "no".  At least with the few 1-phase motors
I've run across, you have to jumper some wires to configure for a
particular voltage, not just use some and let others dangle like you
might with a simple transformer.  Also, with a "normal" converter, one
of the output phases is simply the input power, so you'd have to figure
out how to double that.  However, it might still be possible to select
leads in a non-standard way and run power in to just 1/2 of one winding,
but you'd probably have to derate efficiency even more.

I've been collecting HSM and PIM articles on this (there is an update
in the latest HSM which I have not read yet), and ordered (but not yet
received) a book from Lindsay publishing.  I'll look for any reference
to voltage conversion when I can and report.

Jim

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From: [email-masked] (Jay Stryker)
Date: Fri, 21 Jan 1994 18:07:58

>>size of the motor being run?
***************************************
I have seen ordinary three phase motors used in this way...you can wrap a
rope around the shaft, give a strong pull, and then switch on 1 phase power.
Once running, it will draw a few amps, but as long as there is no load on
the shaft, it "idles".  When you place three phase motors on the output
(recall that you have to use a three phase contactor) the current will rise.
How much...hmmm...maybe twice or more?  Anyone know?   I have a commercial
single to three phase converter....I have measured the three phase outputs
for balance but never measured the input current change.   I run mine from
a 70 Amp circuit into the workshop; when I start the converter the house
light level drops about the same as an appliance starting.  When I start
the 7 hp turret lathe motor it dims the house lights for about one second
as it comes up to speed.  
***************************************
>>rotating transformer" to step up 220V to 440V ?
***************************************
This sounds feasible if the windings are electrically separate (eg. the
220 is not a mid-tap on the 440).  Heating, arc-over and insulation limits
may be involved here.  If you are using a "bigger" three phase, say 7 horse,
motor, for this, and the driven motors are 2 or 3 horse, the heating may not
be a problem.  Three phase motors are rugged beasties, but you want balanced
power in all three feeds to prevent overheating/meltdown.
Cautions: high voltages, high currents, high shaft torques, possible open
or short circuits, possible insulation fires, possible contactor problems,
etc.....this can be an expensive laboratory experiment....!!!!   Check with
your local expert(tm)! 
****************************************

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From: [email-masked] (John De Armond)
Date: Sun, 23 Jan 94 08:21:32 GMT

>>>size of the motor being run?

Yes, this works well. and if you connect a capacitor in series with 
another winding hooked in parallel with the first, the motor will start
itself and will draw much less idling current.  For 3P motors in the
5 hp range, I'd start out with about 20 uF at the appropriate voltage.
If you're interested in best efficiency, tune the capacitor by varying
it to get the least idling current.

>for balance but never measured the input current change.   

Amazing how power factor works, eh :-)  

>may be involved here.  

This works fine as long as the FLA limitation of the converter is
respected.  For dual voltage motors, simply connect the windings in 
series for the higher voltage and then apply the incoming power to
between one end and the junction of the series windings.  Take the output
power from the ends of each phase.

Another little known fact is that an induction motor driven by another
prime mover (engine, etc) is a fine generator.  It generates lots of
leading VARs so it counteracts the low powerfactor of the motors and the
"converter."  Simply connect the 3P converter motor to the prime mover,
bring the rotor speed up to nameplate (1800, 3600, etc) and apply
excitation as before.  Then crank the throttle open on the prime mover
and apply the 3P load.  Within the current rating of the converter, the
rotor will remain locked roughly in sync with the line and no govenor is
needed.  Watch the phase current to keep it within the FLA rating of the
converter.  Particularly watch the phase connected to the line; "excess"
power is fed back out the power line.

john

-- 
John De Armond, WD4OQC, Marietta, GA    [email-masked] 
"Would you fire on US Citizens while in the process of confiscating their guns?"
    A question on a survey given to members of the U.S. counterterrorist 
    unit Navy SEAL Team Six by their commanders last September.

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From: [email-masked] (Mark Williams)
Date: Mon, 24 Jan 1994 23:43:44 GMT

Yes, 3 phase motors can serve as rotary phase converters, wiht reduced
power from the target motor.  

You basically run the 3 phase motor off 220, on two of the phases, and
the spinning motor will generate a third phase.  This is a hack, since
two of the phases are the 220 volt line (180 instead of 120 degrees out
of phase) and the third phase will be of lower voltage (160 v) and between the
other phases.  

You also need a means to start the 3 phase motor/generator,
either mechanical or electrical.  Either another motor and cluch or a
motor starting capacitor to the third leg will spin it up, then should
be disconnected.  

Woodworkers sometimes use this scheme to run bargain 3-phase saws, jointers,
etc.  See writeups in "Fine Woodworking on Making and Modifying Machines",
available from Taunton Press.

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From: [email-masked]
Date: 24 Jan 94 21:24:25 PST

> conversion with a 3-phase motor, but no mention of either of my questions.

There is a good article in the current (Jan/Feb) issue of 
The Home Show Machinist on page 41.   Also, there was an
earlier article in the Jan/Feb 1991 issue.

Alas, I did not see either of your questions answered in
the current article, but there is a lot of other good 
information.

--  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)

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From: [email-masked] (Ron Ginger)
Date: 25 JAN 94 13:41:26

In article <[email-masked]>, [email-masked]  (Jay Stryker) writes...
>>>size of the motor being run?

I measured the input current to my 2hp phase convertor, at idle and its
ony about 1 amp. All its drawing is the windage and bearing loss, since its
doing no work.

My convertor control is about as simple as possible- a two pole switch, a
single pole momentary push button, and a motor starting capacitor- total
cost under $20. I flip the switch with my finger, while holding
the push button with my thumb, as soon as I hear the motor up to speed I let
go of the button. Much siimpler than relays.

ron

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From: [email-masked] (Adam Karpowicz)
Date: Wed, 26 Jan 1994 01:04:59 GMT

>go of the button. Much siimpler than relays.

I have a 2hp 3-phase motor too - can you tell me what starting capacitor
you use for your motor (in uF)? The motor I have is 220VAC. 

Is it possible to actually use the converter motor for driving tools?
How efficient it would be?

Adam    [email-masked]

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From: [email-masked] (Eric Perozziello)
Date: Wed, 26 Jan 94 19:43:00 GMT

>you use for your motor (in uF)? The motor I have is 220VAC. 

While on the subject, I've been looking for an Air Compressor
(leaning towards the Quincy), and some of the models say

"No Starter Required"  and another model says:

"Magnetic Starter mounted and wired"  and another says:

"capacitor run and start"

Can someone explain the difference and which is better?

-- 
--
Governments that don't trust honest people with weapons, deserve no trust.

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From: [email-masked] (Scot Heath)
Date: Thu, 27 Jan 1994 23:03:54 GMT

Mark Williams ([email-masked]) wrote:
: Yes, 3 phase motors can serve as rotary phase converters, wiht reduced
: power from the target motor.  

: You basically run the 3 phase motor off 220, on two of the phases, and
: the spinning motor will generate a third phase.  This is a hack, since
: two of the phases are the 220 volt line (180 instead of 120 degrees out
: of phase)

This is not correct.  If you were to look at the voltage vrs. time plot of
the 240v available in a home or the 240V available between 2 phases on a
240v 3 phase circuit, you would see the same thing, a 339v peak, 60Hz sine
wave.  The two leads of the motor connected to the 230v line see this also.
The voltage from either of the applied leads to the third lead on the motor
will be, under ideal conditions, 240v and 120 degrees out of phase with the
applied voltage.  This is due to the geometry of the motor, which may be
modeled as a transformer with a rotating secondary.

I have seen this mis-information in many posts regarding 3-phase
converters.  I think it is a result of trying to incorporate the center tap
"neutral" line of household single phase systems into the problem.  This
tap has no bearing on the applied 240V.

-Scot

--
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Everything I write is my opinion only.  Nobody else would want it.

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Scot E. Heath                          "Nothing beats turning clay to dust."
Fort Collins, CO
[email-masked]