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building a rotary phase convertor


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  #1  
Old 10-17-2019, 06:50:32 PM
Tracy T Tracy T is offline
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Question building a rotary phase convertor

I would like to keep this thread simple! Its a bastard topic to start of with, it can be done, it cant be done etc. I for one know it can be done because i have seen it done and owned two different converters one came from the factory that way on a two post auto lift. and for the life of me i cant understand why, the friggen business it went into new had 3 phase! why the need? the other one i had was a rotary converter and was home built. what i would like to address is the sizing of the idler motor and the sizing of capicators for the desired load. so lets keep it stupid simple, ok?
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Old 10-17-2019, 07:01:44 PM
dalmatiangirl61 dalmatiangirl61 is offline
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Default Re: building a rotary phase convertor

Get a 3 phase motor, twice the size of largest motor you want to run, connect L1 and L2 to motor with leads to load motor, add lead from 3rd leg of motor to load motor, wrap a rope around shaft of idler motor, pull hard and throw switch to send 220 1ph to idler.

There are links to several designs over on practical machinist, Fitch Williams seems to be a popular one, there are others. 2:1 idler/driven is standard, if your load is really hard starting you might consider 3:1.
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Old 10-17-2019, 07:12:30 PM
Tracy T Tracy T is offline
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Default Re: building a rotary phase convertor

I should add some details of the design i am shooting for and have owned. this setup uses both start and run capps. start caps is on a timer to start the idler motor, maybe 3~5 seconds max. this eleminates the need to start your idler by spinning it with a rope or another motor or whatever other means. when the timer drops out the start caps the run caps come on instantly with the power feed of the third set of windings from the idler motor. clear as mud right? In theory the idler is runing on split phase single phase which acts like two phases and in doing so it is in fact generating part of a artifical 3rd phase making up the difference with the run caps. this is my understanding of how his 15 year ago setup was working and still is by the way. if i am not mistaken you do get a wild leg doing it this way. I do have a small setup currently if you guys need pictures that i built, but it wont carry any 15 HP load this i know! the setup i had years ago had a crap load of caps in it! and was reversable too.
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Old 10-17-2019, 07:16:46 PM
Tracy T Tracy T is offline
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Default Re: building a rotary phase convertor

dal caps are cheaper, in this case i would need a 30 hp and thats gonna set you back severel hundred franklings!
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Old 10-17-2019, 08:09:15 PM
dalmatiangirl61 dalmatiangirl61 is offline
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Default Re: building a rotary phase convertor

If you were closer to Tx (and I was there too), I could hook you up with a 30hp motor, but shipping it to you would kill the deal factor. By your description, it sounds like a Fitch Williams design, google that name.

Here https://www.practicalmachinist.com/FitchWConverter.pdf
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Old 10-17-2019, 09:10:44 PM
pegasuspinto pegasuspinto is offline
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Default Re: building a rotary phase convertor

You can use a VFD, many of them are rated for single phase input and 3 phase output, you get reversing, variable speed, soft start, etc.
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Old 10-17-2019, 10:59:43 PM
J.B. Castagnos J.B. Castagnos is offline
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Default Re: building a rotary phase convertor

I've made several for myself and others. I use two contactors from old a/c units. One side of one cantactor brings the starting cap to the third leg of the three phase motor, the other terminal pulls in the second contactor supplying line voltage to the other two legs of the three phase motor. This contactor is wired to feed itself and stay in until the power to the coil is interrupted with a kill switch. Starting is accomplished with a momentary switch to the first contactor. A quick tap is all that's needed to start the idler, almost instantaneous. Once it's running you can measure voltage on the three legs, capacitors from one leg to the third will increase voltage, caps to both legs doesn't seem to help.
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Old 10-18-2019, 12:08:20 AM
Jim Kennedy Jim Kennedy is offline
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Default Re: building a rotary phase convertor

Hi Tracy, I am curious as to what load you will be running? Only one motor or several motors.If you were only running one motor and it was in the difficult to start category, like an air compressor.
I would recommend the higher voltage autotransformer method. Very easy to connect a 240v to 120v single phase transformer to get 360v, add a run cap and a start cap and a voltage sensing relay.
What I have said here is for a 4 pole motor and ideally you need a reasonably constant load but you can run at full nameplate amps and have very good starting capabilities. With a Rotary, single phase is used to start the 3 phase idler as the motor rises in speed the Back EMF is increasing in the idler, so that is the 3rd phase. If you size the idler 5 times the load you don't even need run capacitors.

Jim
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Old 10-18-2019, 08:38:22 AM
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Default Re: building a rotary phase convertor

jim the motor i am trying to run never starts under a load and going by the numbers is a shade over 15HP. the name plate says 220 V 44 Amp. from what i have looked up a 15 HP in the same voltage was i think 42 amp. this being a older setup I imagine the 44 amps would be @ full load, which it probably will never see or rarley see. Its basically a 3 ph motor running a dc generator directly "no belts or anything, its all on the same shaft".
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Old 10-18-2019, 08:46:24 AM
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Default Re: building a rotary phase convertor

JB i had forgotten about the latching relay, it sounds like your setup is the same as the one i had 15 years ago. for whatever reason the man that built my first on prefered a high speed idler motor like a 3600 rpm he said a 1800 would work just not as well.
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Old 10-18-2019, 10:05:22 AM
J.B. Castagnos J.B. Castagnos is offline
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Default Re: building a rotary phase convertor

If you're not going to run a heavy load try running the motor on single phase just like the idler motor, start it with a capacitor and ad a run cap to balance the lines, should get near 2/3 capacity and do what you want to do.
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Old 10-18-2019, 10:21:48 AM
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Default Re: building a rotary phase convertor

removed post

Last edited by J.B. Castagnos; 10-18-2019 at 10:22:55 AM. Reason: duplicate post
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Old 10-18-2019, 04:41:28 PM
Jim Kennedy Jim Kennedy is offline
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Default Re: building a rotary phase convertor

Hi Tracy, If yo are not going to start it under load and if never or rarely see full load, I think start off by trying to run it as a static converter just as JB suggested, but when adjusting run capacitor values, keep the Amps below the motor name plate.

Jim
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Old 10-19-2019, 10:58:49 AM
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Default Re: building a rotary phase convertor

^ Agreed, I don't see why a static converter wouldn't work. I've built them as well as rotaries, and for a lightly-loaded 3 phase motor load, the static ought to be fine....

As JK mentions, keep an eye on the line currents for the motor and make sure they don't go over the nameplate rating. All three will differ, so make sure to measure all of them.
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Old 10-19-2019, 05:41:01 PM
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Default Re: building a rotary phase convertor

It should be noted that “static phase converter” is misleading marketing wank for “start capacitor and potential relay”. These are common parts that can be had for a few bucks. It just makes a three phase motor into a capacitor start motor, and is completely out of the circuit at running speed. No wizardry whatsoever.

“Idler” is also a misnomer. It’s a rotary phase converter, or just phase converter. Some manufacturers aptly refer to it as an induction generator. But it is defense not just “idling”.
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Old 10-19-2019, 06:59:29 PM
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Default Re: building a rotary phase convertor

I'd suggest run caps as well for a 'static' converter. If you leave the run caps out, I think you'll have trouble getting the line current under the nameplate rating.
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Old 10-19-2019, 07:32:49 PM
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Default Re: building a rotary phase convertor

Yes, without run caps, the continuous capacity would be ~2/3 of rated capacity. Have to be careful though. When power is disconnected, and the potential relay drops out, it parallels the start and run caps through its poor suffering contact. Since the caps are invariably at different voltages, that contact can be fairly well annihilated after only a few start stop cycles.

It was a two stage A/C condensing unit I observed this on. The solution was to use a two pole contactor, with one pole for the run circuit and the other for the start circuit.
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Old 10-19-2019, 07:37:31 PM
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Default Re: building a rotary phase convertor

fellows i am listening on this thread, I have nothing more than theroy how it works. what i am the most interested to learn is just how to size the capacitors, both run and start. I have the theory of how it works just dont know how to make it work. so forgive me if i dont post alot in this thread but i am here to learn! I started the thread but it belongs to the more qualified.
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Old 10-19-2019, 09:21:06 PM
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Default Re: building a rotary phase convertor

I wish I had notes from when I did a static for a 5 HP compressor. I don't think I do. There's lots written on the 'net on the topic. It takes some experimenting. If you have an old 5HP 3-phase motor, you might start tinkering at that, and then scale it up by a factor of three.

You absolutely need a clamp-on ammeter, and a bunch of run capacitors to start tinkering. These guys usually have decent prices on capacitors:

https://surpluscityliquidators.com/t...32?clear_all=1
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Old 10-20-2019, 10:59:10 AM
Tracy T Tracy T is offline
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Default Re: building a rotary phase convertor

Interesting video for anybody considering building one their self, a little more complicated than what i have seen but he does give you a idea of whats going on.
https://youtu.be/u229h7NcMxk
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