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Old 08-20-2010, 05:44 PM
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3 Phase Converter

Well, my jackstands are still on the floor, lets go for question number 2.....

I bought a Powermatic Model 74 Tablesaw for an incredibly good price...but no good deed or buy goes unpunished, I need to get a 220 3 phase converter. Did some research and the static converter appears to be the most economical way to go. The saw has a 7.5 HP motor and the conventional wisdom is that a static phase converter will allow it to run on 2 phases and develop about 5 HP which is plenty for any woodworking I will do.

Are there any downsides to using a Static Converter vs a Rotary or Variable Converter? Safety ? Motor Life? Any electrical wizards out there that can enlighten me on this subject ?

Thanks.. Mike B
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Old 08-20-2010, 06:04 PM
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Most electric motors can be wired for either 3 phase or single phase. Pull the cover plate and see if there is a diagram.
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Old 08-20-2010, 07:38 PM
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You can wire some motors to run on different voltage but you can not re-wire a three phase motor to run on single phase however the static converters do work quite well. If you can get by with about a 30% drop in power then the static converter is all you need and it is simple to wire in, when I installed one on my 14"X40" 3 HP metal lathe I actually could tell almost no difference in the way it runs as compared to the way it was when connected to three phase. If you need full power then you can still use a static converter along with another 3 phase motor to be used as an idler in a configuration very similar to a rotary phase converter. This too works quite well as long as that is the only machine it will be powering.
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Old 08-20-2010, 09:18 PM
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Quote:
Originally Posted by oldred
You can wire some motors to run on different voltage but you can not re-wire a three phase motor to run on single phase however the static converters do work quite well. If you can get by with about a 30% drop in power then the static converter is all you need and it is simple to wire in, when I installed one on my 14"X40" 3 HP metal lathe I actually could tell almost no difference in the way it runs as compared to the way it was when connected to three phase. If you need full power then you can still use a static converter along with another 3 phase motor to be used as an idler in a configuration very similar to a rotary phase converter. This too works quite well as long as that is the only machine it will be powering.
Not true, my family owned a print shop and several of the presses had motors that could be setup for either 3 phase or single phase. We bought some equipment that needed to be changed over because we didn't have 3 phase power. The wiring diagrams were inside the motor wiring cover. Im sure not all can be but some can
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Old 08-20-2010, 10:46 PM
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The motors that can be "switched over" are very rare at this point in time. I am dealing with the same issue with an air compressor motor. If you can find one as I have, expect to pay a good 6 months morgage for one.
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Old 08-21-2010, 07:50 AM
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Switching from single phase to three phase is not as simple as re-wiring the motor, that's what phase converters are sold for. Static phase converters allow a motor to run on about 2/3 power and a rotary type will provide full power, or nearly so, and lately the hot item has been the VFDs or variable frequency type converters that allow full power and in addition allow variable motor speed along with the ability to reverse directions with the controls. A hybrid type motor that can run on either single or three phase would be a rare animal and you can bet your bippy he has no such thing. The phase converter would be the only real option and in this case the static converter would probably be the best bet since rotary converters in that range will cost around $1000 or more and a variable device could be even more and would be of no practical advantage on a saw, a static converter for a 6 to 8 HP motor is less than $200. If it is as simple as switching the wiring there is a hell of a lot of people wasting a lot of money on those converters.


A three phase motor is simply a different animal than a single phase and are built differently, they are usually somewhat smaller, lighter and quite a bit cheaper to buy than a single phase. They are simply a different design and about the only practical way to run one on single phase is to modify the power supply not the motor.

Last edited by oldred; 08-21-2010 at 07:57 AM.
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Old 08-21-2010, 12:03 PM
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Hi Red I agree, there would be tons of folks wasting the money,

I drug my compressor out of an old shop that had three phase power for the compressors as well as the welders, it is most likely a few years, perhaps a decade older than I am, I am in my late 30's.

Three phase is a much more efficeint source of power, but a very unlikely source in todays world.

I can purchase a single phase 5 -7.5 hp motor at a local electrical repair place for around $300.

They are old school and actually rebuild and service any and all types of electrical motor from light duty to heavy industrial.

This might be the best option for anyone looking to replace a 3 phase motor, there have to be more than just one place like this left in the world.
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Old 08-21-2010, 12:31 PM
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Replacing the motor is quite often a better way to solve the three phase problem than trying to make the motor run on the wrong current. Once the motor has been changed there is no more problems with power and no more compromises, this can usually be done a lot cheaper than a rotary converter or VFD, either of which can cost a great deal more than a new motor.



Actually I said "New" motor but a new 7 HP single phase 220 volt motor might cause a heart attack when the buyer sees the price! If I were to consider a single phase replacement motor, and I will if I have a problem with my 3 phase converter, in that size range I would not even check on a new one. As you suggest a rebuild shop will often have a motor that may be as good as new but at a fraction of the price.
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Old 08-21-2010, 12:58 PM
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Phase Converter

Thanks for the info....I think if the 7.5 HP motor can develop 2/3 of that HP on a staic converter I will be all set...my old saw has a 1.5 horse motor so I suspect I can make a lot more sawdust with 5..several static converters on ebay for under 200 that will do up to 7.5 so I should be good to go, I attach a picture of the Saw Beast....70's vintage Powermatic made in the USA !!!!
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Old 08-21-2010, 01:49 PM
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Like I said I use a static converter on my lathe and it works just fine, I have yet to notice the 1/3 decrease in power and starting it up is as simple as turning on the switch. Might I suggest the Phase-a-Matic converters?

www.phase-a-matic.com/StaticApplicationNotes.htm


These probably have about the best track record and are quite popular, it is the one I am using now and we had several at my shop in a building that was not wired for three phase and they ran flawlessly for several years even though they were used daily except for weekends.
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Old 08-21-2010, 05:21 PM
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before you buy any phase converter, check out craigslist.net for single phase motors.
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Old 08-21-2010, 06:54 PM
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3 phase power

We just installed a 3 phase powered system to irrigate one of the farms, $ 4 K to the power co for the transformers, 2 K to the electrician and we have to pay $1200 a year to the power Co whether or not we use any power for the next 5 years, It's too far away, A friend wanted to sell me a Big 3 phase lathe he has in his shop, he never hooked it up because of the cost when he moved.
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Old 09-10-2010, 06:51 AM
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You can build a good rotary phase converter for next to nothing.
1. Idler @ 2-3 X hp of the motor you want to operate
2. Disconnect to start the idler
3. 240 V single phase to disconnect, three phase out of idler.
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Old 09-10-2010, 09:01 AM
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Quote:
Originally Posted by 930dreamer
You can build a good rotary phase converter for next to nothing.
1. Idler @ 2-3 X hp of the motor you want to operate
2. Disconnect to start the idler
3. 240 V single phase to disconnect, three phase out of idler.


Not quite all the parts there, you forgot the capacitor bank (it is necessary for proper operation and certainly would be for any high start load) which can be fairly expensive plus the cost of a motor, etc so even building one yourself can get a bit costly. With an idler motor a static phase converter can be used in place of the capacitors but that would in most cases limit use to just one machine unlike the regular rotary type that can power several motors at once, there is a good reason those commercially built rotary converters have those capacitors!
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Old 09-11-2010, 12:10 AM
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Quote:
Originally Posted by oldred
Not quite all the parts there, you forgot the capacitor bank (it is necessary for proper operation and certainly would be for any high start load) which can be fairly expensive plus the cost of a motor, etc so even building one yourself can get a bit costly. With an idler motor a static phase converter can be used in place of the capacitors but that would in most cases limit use to just one machine unlike the regular rotary type that can power several motors at once, there is a good reason those commercially built rotary converters have those capacitors!
Not needed if you pony start with a small 120v motor.
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