twin motor phase converter

I'm building a phase converter for my son... My third one.
We're using two motors. A 5hp start unit and a 20 hp run unit for a
total of 25 hp. Using a 3 phase load center panel to distribute all the wiring. Each of these motors will have a circuit breaker going to a motor starter contactor and then the motor. As an aside, the load center will make it easy for him to add machines as needed.
I'm doing the control work for him after he gets the power wiring and panels for the above installed. So, I read up on the needed start cap size here: http://www.metalwebnews.com/howto/ph-conv/ph-conv.html Looks like 200 MFD will bring it right up. Note: all these plans show a potential start relay to hold in these caps for a few seconds and then drop out - spendy little device.
Now I want to balance voltages in the running unit using run capacitors. An expert named Fitch gave me the attached report on balancing. it shows I'll need around 300 to 400 MFD run caps between each single phase leg and the "wild" leg
Then a huge simplification occurred to me. Why even use start caps? Put the 300 or so run caps between L1 and wild in with the first five hose motor. it will start right up.
OK, wait five seconds and start motor two, the large one. Put the rest of the caps behind the motor start contactor on this unit.
No start caps, no potential relay. Anybody see why this won't work?
Special note, I also asked on a place that will host Fitch's .pdf. Get it here: http://www.weaponsguild.com/forum/index.php?topicC541.msg604204 ;topicseen#new
Karl
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I did the same thing. It worked well. Of course, I no longer need the phase converter, since I moved my business to my warehouse.
i

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On 9/16/2013 7:15 AM, Karl Townsend wrote:

<snip>
What is the advantage to rotary phase converters vs VFD's? I never didn't have 3-phase so I never had to bone-up on this stuff. I just figured that if I ever wanted equipment at home, I would get the appropriate VFD and be done with it. Obviously, I'm missing something.
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IF you have the motors already, it's cheap. IF you can live with the imbalance of phases that's likely to occur (usually fine when running motors, and _can_ be tuned out) it works nicely. They usually occupy a good deal of space (although I've converted a machine's own 3-phase motor to run on single-phase with caps and a start relay, in the past)
VFD's tend to be noisy electrically, and more expensive than "scrap- built" rotary converters. They're also quieter acoustically.
Rotary converters and rotary generators used to be the only way to accomplish the task, so they stick around, and might even suit better depending on budget and space.
Lloyd
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"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote in message >> Obviously, I'm missing something.
No shit, Sherlocks.
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On Monday, September 16, 2013 4:11:43 PM UTC-4, Tom Gardner wrote:

Rotary converters are much cheaper in the larger sizes. The VFD is smaller , close to the same price in small sizes, and let you vary the motor speed.
Dan
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On Mon, 16 Sep 2013 15:49:02 -0700 (PDT), " snipped-for-privacy@krl.org"

#1 advantage is you can run multiple devices off one (1) rotory converter.
#1 DISadvantage of VFDs is you can only run one (1) device off a vfd.
Gunner
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VFDs cannot operate any control systems.
i
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Not even close to true, Ig.
VFDs, with proper low-pass filtering work fine on 3ph controls.
Lloyd
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On 9/16/2013 4:11 PM, Tom Gardner wrote:

no plug-reversing motors with VFD's for one
MikeB
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That's a matter of planning, rather than a specific advantage or disadvantage. The application matters.
VFD's decelerate then re-accelerate when switch-reversed.
RPCs do it more rapidly, but NOT instantly, which might be wanted in some applications. They also severely stress everything while doing it.
Lloyd
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On 2013-09-17, Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:

And if you plug reverse a big enough motor with a big enough inertia, it may happen that the motor will keep the rotation, but the phase converter idler willr everse.
i
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Yup! <G> But I don't know (off-hand) of a VFD that will reverse its phase arbitrarily, load or no.
Lloyd
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wrote:

On machines with multiple motors such as a horizontal mill...you need multiple VFDs to operate.
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http://www.weaponsguild.com/forum/index.php?topicC541.msg604204 ;topic seen#new

Karl, if you are going to run several machines off of the same converter buss then probably best to first balance your idler under no-load condition and then add correction capacitors at each machine on an as-needed basis.
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