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Tool Talk Discussion Forum

Voltage output on a phase converter

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Elmer Becker

01-19-2004 20:49:26




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Hi,
I am building a rotary phase converter out of a 240 volt 3 phase motor. I have wired the unit up and got it started with a smaller motor to spin the phase converter.
I checked the voltage on the three lines and they were. T1 & 2 205 volts, T1 & 3 215 volts, T2 & 3 235 volts.
Does anyone know what is a workable output on these 3 wires to be a useable phase converter?
I've been told, by a motor rebuilder, that by using a rotary phase converter that is double the hp of all the motors that will be running will allow the voltage and current to be off more and still work.
Any help will be appreciated.
Thanks,
Elmer (in Missouri)

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Butch

01-20-2004 19:26:30




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 Re: Voltage output on a phase converter in reply to Elmer Becker, 01-19-2004 20:49:26  
Sounds like normal converter imballance to me and should work OK. You can ballance the legs with capacitors if your load is steady but they do little good with varying loads as the imballance varies with load changes. You allready have yours going now but if you do it again try to find a 3600 RPM motor, for some reason they give more ballanced voltages than the 1750-1800 RPM ones. If your machine has magnetic starters or other single phase equipment it should be wired so the single phase 220 feed comesfrom the non-wild legs

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Elmer Becker

01-20-2004 22:15:00




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 Re: Re: Voltage output on a phase converter in reply to Butch, 01-20-2004 19:26:30  
I do intend to get a bigger 3 phase motor to run a 4 head plainer. It has four 4 hp motors and a 1/2 hp drive motor.
I like your idea about using a 3600 rpm motor. I'm thinking of starting with a 25 hp 3 phase 3600 rpm motor and backing it up with a smaller 1700 rpm motor. This might get the three phases closer to true 3 phase. Then send the power to the plainer.
I keep hearing that these new three phase motors that are being manufactured today are with closer fits that older motors. If that is true,I may need to get the phases even closer. I plan to use this same converter for my hobby shop. I have a Bridgport mill, 14" lathe, surfase gringer. drill press, 24" wood plainer, and the 4 head plainer. They are all 3 phase except the drill press. Non of the motors are bigger that 4 hp.
Thanks for the reply.
Elmer

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Big Jake

01-20-2004 07:59:41




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 Re: Voltage output on a phase converter in reply to Elmer Becker, 01-19-2004 20:49:26  
Are you utilizing run capacitors??


http://www.metalwebnews.com/howto/ph-conv/ph-conv.html



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Elmer Becker

01-20-2004 22:01:17




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 Re: Re: Voltage output on a phase converter in reply to Big Jake, 01-20-2004 07:59:41  
I haven't tried capacitors yet. Right now they are out of my ability to understand. I may have to dig into them in order to get this converter working.
I am hoping to use two motors to get the phases closer to true 3 phase.
Thanks for the reply.
Elmer in Missouri



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buickanddeere

01-20-2004 05:38:29




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 Re: Voltage output on a phase converter in reply to Elmer Becker, 01-19-2004 20:49:26  
Rotary phase convertors are an item of the past along with vacuum tube radios and hand crank started automobiles. Rather than the taking the time to build a less effient mechanical unit that will never deliever balanced power. Why not purchase the proven solid state variable frequency drives? Feed with single phase and get true three phase out. Even have the option of electrical braking and speed control.

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Doug in IL.

01-20-2004 10:40:33




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 Re: Re: Voltage output on a phase converter in reply to buickanddeere, 01-20-2004 05:38:29  
You are probably right, if you only have a machine or two. I run my 3 phase shop machines with a rotary converter. 3 lathes, 3 milling machines, 5 hp drill press, 4 grinders, a slotter, a 16 inch shaper, vertical and horizontal band saws and a couple of stand grinders. VFD's, in this kind of situation, is just cost prohibitive. With my $500 converter, I can run 2 or 3 big machines at one time. Probably 5 or 6 of the smaller ones. Works ok for me.

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Dieselrider

01-20-2004 15:16:09




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 Re: Re: Re: Voltage output on a phase converter in reply to Doug in IL., 01-20-2004 10:40:33  
Doug, I agree with you. There are some guys that really have more money than brains or live in a utopian world. Frequency drives would be great but you need one at each machine.



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buickanddeere

01-20-2004 16:13:56




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 Re: Re: Re: Re: Voltage output on a phase converte in reply to Dieselrider, 01-20-2004 15:16:09  
Why do you need a VFD at each machine? You use a single rotary convertor on muiltiple 3 phase loads. A single vfd can also be the common source for all 3 phase loads. In the units life time, the increase in effiency and reduction of heat input into the shop will pay for it's self many times over. Seems alot of people are reluctant to change.



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Dieselrider

01-20-2004 18:38:33




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 Re: Re: Re: Re: Re: Voltage output on a phase conv in reply to buickanddeere, 01-20-2004 16:13:56  
My understanding of the VFD's maybe be not up to speed. But I was understanding them to be like a static converter and you have to have them programmed to what you want to get out of that VFD. Also do you not have to size the VFD to the machine and job at hand? You can't run a 10 HP motor on a VFD that will only handle 5HP. If what your saying is that one VFD can be used while running two or several machines off of one VFD then I stand corrected and appologize.

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Doug in IL.

01-21-2004 07:02:52




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 Re: Re: Re: Re: Re: Re: Voltage output on a phase in reply to Dieselrider, 01-20-2004 18:38:33  
You are right. You cannot run multiple machines off of one VFD.



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buickanddeere

01-21-2004 08:35:14




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 Re: Re: Re: Re: Re: Re: Re: Voltage output on a ph in reply to Doug in IL., 01-21-2004 07:02:52  
A VFD rated for 50HP doesn't care if a 5,25 or 50 HP motor is connected. As long as the combined load doesn't exceed the VFD's rating, all is well. As with any motor/induction load the use of power factor correction capacitors reduce the circulating lagging currents through the rotary or a static invertor/VFD. Mills,lathes etc or any motor running at less than max rated capacity have very low power factor. Usually the capacitors are only used to circulate an out of phase magnetic field to induce the unloaded rotary convertor to rotate on startup. A three phase motor operated fully loaded with inbalanced voltage and phase relationship from a rotary invertor. Will draw high current and overheat. It can send harmonics back out the single phase line and upset sensitive equipment.

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Dieselrider

01-21-2004 17:30:16




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 Re: Re: Re: Re: Re: Re: Re: Re: Voltage output on in reply to buickanddeere, 01-21-2004 08:35:14  
What would a 50 HP VFD cost? Or even a 30 HP VFD?



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buickanddeere

01-22-2004 05:22:40




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 Re: Re: Re: Re: Re: Re: Re: Re: Re: Voltage output in reply to Dieselrider, 01-21-2004 17:30:16  
I'll check and get back to you on the latest. Any prefernce to Siemens, GE, Westinghouse etc? I assume your motors are 480V in the US rather than 600?



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DH in Carolina

01-20-2004 17:24:36




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 Re: Re: Re: Re: Re: Voltage output on a phase conv in reply to buickanddeere, 01-20-2004 16:13:56  
My understanding is that you can get three phase power out of a drive by inputing single phase power into it, but one leg of single phase power has to feed two inputs on the drive. This means that one leg of single phase is pulling double amps of the other leg. Also when I went to drive school at Allen-Bradley if you used single phase power on a three phase drive the drive would not put out full power. To the original post you can use motor run capacitors to balance the voltage between phases. There is a site on the web that has plans to build a self starting converter using only one motor. I downloaded it about a year ago and I will email to you if I can find it.DH

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VaTom

01-20-2004 05:01:58




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 Re: Voltage output on a phase converter in reply to Elmer Becker, 01-19-2004 20:49:26  
Hi Elmer,

I don't know much more than Dieselrider and have a similar situation. I've never measured my voltages. Over the last 25 yrs I've used a variety of converters, currently run a 5 hp one that has a chain drive that I kick start. As an experiment I once started every motor in the shop, one at a time, while measuring current draw until all 15hp (total) were running with a 3hp converter. Not a normal situation and I didn't have the correct wire size for that much current. Once had a 15hp converter that ate too much current and went to a new home after I saw the electric bill.

Routinely running a 5hp motor and a 3hp motor together off a 5hp converter certainly works fine for me. Now get to work!

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Dieselrider

01-20-2004 03:55:12




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 Re: Voltage output on a phase converter in reply to Elmer Becker, 01-19-2004 20:49:26  
I am no expert in the field but, you have what sounds like an exceptable voltage balance. With a rotary you will have a wild leg. The more machines you run at the same time the more it should balance out. We run a 10 hp rotary that we bought and it has a high leg also. Our 10 horse will run up to 30 horses total output but no one motor over 10. I hope this helps.



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