Setting Up Shop #4: The difference between static phase converters

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Blog entry by DylanC posted 02-17-2011 05:03 AM 4143 reads 1 time favorited 3 comments Add to Favorites Watch
« Part 3: Resurrecting a Dead Air Compressor Part 4 of Setting Up Shop series Part 5: Shop Built Router Table for under $30 »

Just a quick post to share what I learned about static phase converters while setting up my 3-phase Unisaw.

First off, there are two basic types. Each of them uses some type of capacitor circuit to energize the third phase of the motor just long enough to get the motor up to speed. After the motor is spinning the starting circuit is taken out of the loop and your three-phase motor is running on two phases of power. This was the first type of converter I looked at buying, but after speaking with a technical service representative, he informed that this type of converter is really only suited for powering motors loaded at only a fraction of their rated horsepower (see disussion of idler motors below). Any heavy loading (70 percent or greater) has the potential to cause permanent damage to your motor. (This is all based on information I got from a representative at Enco regarding a Phase-A-Matic converter.)

The second type of converter includes a second “run” capacitor circuit that engages after the starting capacitors are deactivated. This second capacitor is used to provide pulses of power to the third phase of the motor. Its not nearly a sine wave output, but its enough to allow consistent loading of the motor to 70 percent of rated capacity and intermittent loading of 90% of rated power (up to 15 minutes of 90% load with equal no load run time). (This is the type I ended up purchasing. The brand is Autogen and I bought it from Grizzly.)

Another interesting note is that if you run an unloaded 3-phase motor on 220V single phase power, the motor’s third phase will act as a generator and will actually produce the third phase of power for any other motors operating on the same circuit. Each type of static converter above is capable of operating like this and suppliers of both types recommend this configuration as a type of poor-man’s rotary phase converter. The folks who make the Autogen have a pretty good site explaining all types of converters and 3 phase power in general.

-- Dylan C ...Seems like all ever I make is sawdust...

3 comments so far

View cabmaker's profile


1537 posts in 2443 days

#1 posted 02-17-2011 05:12 AM

I certainly admire you for doing the research and all but is there a particular reason you dont want to swap the motor for a single phase ? I just hate to see anyone go into this sorta deal with high expectations. If your anticaipating adding a battery of three phase equip. you will want a rotary convertor about two times the size you think you ll need.

View DylanC's profile


200 posts in 2308 days

#2 posted 02-18-2011 05:31 AM

I ended up with this setup for a couple of reasons. I originally found a good deal on a used 3-phase unisaw and thought I could get a $120 static phase converter and have a good saw for a good price. Then I learned that the cheap phase converter I had in mind wouldn’t cut the mustard. At that point I looked into all my options with the following results:

Motor Swap, $290: That cost is for a new motor. Sure I could probably find a used one and buy a bracket, but that wouldn’t be a fair comparison. I would also have had to replace the start/stop with a single phase unit with overload protection. That switch alone is $70 from Grizzly. Total cost would’ve been over $350.

Rotary Phase Converter, $1350: Great if you need to run several different pieces of equipment, 3-phase welders, or anything with digital controls on a 3-phase power input. The bad part is I would’ve paid more for the rotary converter than I did for the saw. Not really a good option financially.

VFD, ~$300: Excellent option for motors 3 HP and smaller. For these sizes you can get a VFD that will take 220V single phase and convert to legitimate 3-phase power. Unofficially you can rig larger units to do the same thing, but the VFD now has to be roughly doubled in size. A 10 HP drive for my 5 HP motor would’ve been ~$700. Again, more than I paid for the saw.

Static Phase Converter, ~$220: Gets my saw running and allows me to load it to 3 HP continuously, 5 HP intermittently. The cost basically offset the deal I got on the saw, but I now have the ability to and operate any 3-phase equipment up to 7 HP or so without any additional investment. I don’t actually plan on purchasing any other 3-phase stuff, but if a great deal on a lathe, bandsaw or something else passes by, I can jump on it. And, by running the largest motor on the circuit (unloaded), I’ve got the ability to run smaller equipment at full power.

So, the reason I ended with a static converter was cost, basically. And it gives me some moderate flexibility on future equipment purchases. Kind of a win-win.

-- Dylan C ...Seems like all ever I make is sawdust...

View Jason's profile


5 posts in 1723 days

#3 posted 11-07-2013 07:35 PM

How is this setup with the static phase converter working for you so far. I also just got a great deal on a 5hp 3ph table saw and looking to hook it up.

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