3 phase converters

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Forum topic by ClayandNancy posted 06-23-2011 03:53 PM 1632 views 1 time favorited 5 replies Add to Favorites Watch
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519 posts in 3039 days

06-23-2011 03:53 PM

I know this has probably been discussed, but this just boggles my mind.
I’m looking at getting a lathe that is 3 phase and it needs a 1-3 hp phase converter. This might be the only 3 phase tool that I would buy. (Yeah right) My question is, what’s the difference from a rotary and static converter? And is one better than the other?

5 replies so far

View crank49's profile


4032 posts in 2995 days

#1 posted 06-23-2011 05:00 PM

A rotary converter is a single phase motor running a 3 phase alternator. The output of the three phase alternator is then used to power your connected tool, lathe in this case. Expensive, crude, inefficient and durable. The lathe motor will operate at full power, but the converter itself might have a 5 hp motor in it to generate the power required to run your 3 hp lathe.

A static converter creates a simulated sort of 3 phase power using capacitors and coils. It is very load sensitive because it is like a tuned circuit. Hard to explain without getting into complicated electromagnetic theory. Just accept that it is a device intended to run one load and will probably not be able to get full power from that load. Your 3 hp lathe might only be able to operate at 2 hp without overheating.

A fairly new developement, within the last 10 years I believe, is the solid state inverter drive. These developed from industrial variable speed drives for 3 phase motors which have been around for 20 to 25 years. They work by converting AC input into DC power first. Then converting the DC back into 3 phase AC at variable hertz using fast switching solid state devices. They can make a standard single speed 3 phase motor into a variable speed motor by changing the hertz. These inverter drives are pretty efficient, small, and your motor will operate at full power. I think these drives fall in between the other two options in terms of price, but I’m not sure. I have not purchased one, just read about the technology.

View JimF's profile


144 posts in 3317 days

#2 posted 06-23-2011 06:42 PM

I purchased a used Unisaw with 3 phase motor. The seller built a VFD using a TECO FM50 controller for about $300 parts & labor. It has worked like a champ under light usage. Starts smoothly and comes up to full speed in the number of seconds I set and stops in the number of seconds I set. I like it. If you try to build your own, be sure to spend some time on the phone with the TECO tech getting it right. rick3ddd has it right about this not being the best place for how to build a VFD (unless you can make it out of wood, then none better!).

Another alternative for you is to repower with a single phase motor. From what I hear/read a good single phase motor would probably cost about the same as a VFD.

-- Insert clever tag line here

View JimF's profile


144 posts in 3317 days

#3 posted 06-23-2011 09:28 PM

OK, “building” is not the right word, but “work” must be done on the FM50 for it to be a fully functional VFD system. Apparently what was done for me consisted of at least; selection of FM50, purchasing, programming, wiring, installing new switches and mounting all to the saw. I posted above because I saw some things that were not covered in earlier posts, i.e. the price of a VFD system and the concept of repowering with a single phase motor as an alternate. I was trying to let the OP know what it might cost, not how to do it. I know enough to pay someone who knows his business to do it. If my explanation of what was done is in error, I apologize to my mechanic. I also reiterate “rick3ddd has it right about this not being the best place for how” on this subject.

-- Insert clever tag line here

View SCOTSMAN's profile


5849 posts in 3609 days

#4 posted 06-23-2011 09:55 PM

I have two rotary convertors and two static invertors .Regarding my lathes except the machine shop lathe, I.E woodworking lathes, I always fit an invertor with speed control a nice little box of tricks reasonably inexpensive .The only way to go if you want speed control,a must for modern turning especially when doing larger work,you need to keep the speed way down at first untill the project is in trued up shape have fun. Alistair

-- excuse my typing as I have a form of parkinsons disease

View ClayandNancy's profile


519 posts in 3039 days

#5 posted 06-23-2011 10:46 PM

Thanks guys I guess it’s time to ask the experts. JimF has a good point, as far as cost it might be better to find a 1 phase motor and not tax my puny brain cells.

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