3 phase or bust?

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Forum topic by jeffb33200 posted 01-02-2014 11:07 PM 2651 views 0 times favorited 42 replies Add to Favorites Watch
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15 posts in 1633 days

01-02-2014 11:07 PM

Topic tags/keywords: question tablesaw

I’m considering buying a used Delta Unisaw that has a 7HP, 220V, 3ph motor. This would be for my home use so I wonder if it’s more than I should be getting. I see inverters/VFDs for sale that will convert 220V, 1ph to 3ph but what about the wiring needed for a 7hp motor. I don’t know if 7hp is what the motor is capable of delivering or if that much power will always be drawn. Anyone have some ideas about this?



42 replies so far

View patcollins's profile


1685 posts in 2891 days

#1 posted 01-02-2014 11:23 PM

I would avoid it if you had to use a converter. Those converters can not create power only convert it. In the conversion you lose some because they are not 100% efficient and then you end up with less available power than you would with a single phase.

View JustJoe's profile


1554 posts in 2064 days

#2 posted 01-02-2014 11:32 PM

A 7hp 3-phase motor draws approx 20-22 amps at 220V. It won’t kill your wiring if you have one decent sized ciruit.
Depending on what you have on hand, it might hurt your pocketbook. The price of a VFD goes up fast once you get over 5hp, or you can buy/build a rotary phase controller (RPC). Building one would mean buying another 3-phase motor higher than 7hp for the controller. You could also build a static phase controller (SPC) but that doesn’t provide full 3-phase power so you wouldn’t get everything that the 7hp motor has to offer.

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View jonah's profile


1727 posts in 3324 days

#3 posted 01-02-2014 11:46 PM

You’d almost certainly be better off either:

a) replacing the motor with a ~3HP single phase 240V one


b) looking for a different saw that comes with a single phase motor. They are definitely out there on the used market.

View unbob's profile


810 posts in 1929 days

#4 posted 01-03-2014 12:08 AM

For what its worth. I ran a 7.5hp saw with one of these Autogen static converters for a year and could not stall the 14” saw. I used the CD14, the CD 15 might have been better for spec. These are cap run converters, basically turns a 3phase motor to run like a single phase with the cap run feature. The specs are here, also Grizzly sells these.

There are those that will claim the motor will burn out, your hair will turn orange, the paint will peel off your house using such a device, all BS.

View bigblockyeti's profile


5140 posts in 1746 days

#5 posted 01-03-2014 12:29 AM

You can usually get a 1ph 3hp motor for around $500, sometimes less. You could get one for a lot less if you happened upon a used one. Problem is almost all of the cheap used unisaw motors are 3ph for the very reason you’re encountering.

View MedicKen's profile


1615 posts in 3488 days

#6 posted 01-03-2014 01:15 AM

A VFD for a 7hp motor will be cost prohibitive. One alternative would be a rotary phase coverter. An RPC will run about $700. If this is your ONLY 3ph machine and you are not considering any more I would opt for a new motor. My shop is almost complete 3 phase and I do have a few VFD’s and an RPC. American Rotary makes a good panel, CNC compatible, for under $400 and can be purchased on ebay. American is a great company and they have been building RPC’s for along time and have a great customer service and technical department.

A VFD will give you 100% output a static converter will NOT. Friends don’t let friends use a static converter.

-- My job is to give my kids things to discuss with their

View REO's profile


928 posts in 2100 days

#7 posted 01-03-2014 01:49 AM

for a regular set up you only need to have a RPC 1/3 the nameplate power of the motor you are running. Capacitor balanced “static converters will give you full power IF they have been balanced for the power being used the trouble with a table saw is that the power being used is extremely variable dependent on its use. for an rpc generating 22 amps per leg out you would have to supply in the neiborhood of 45 amps per leg 220 single to the RPC side. You dont get into the high current draw very often and not for long term so IMHO you could get by on a 2 hp rpc properly protected with #10 feeds for 30 amp primary side properly protected. if you want every ounce of the power go for a 5 hp rpc and feed it with #6 a 45 amps.

unbob the paint on the house turns orange, your hair falls out, and the motor becomes over-unity so you can sell the extra power back to the utility for three times the price! where do you get your

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2387 posts in 3573 days

#8 posted 01-03-2014 03:43 AM

I have some experience with RPC as we have a 10 hp RPC. I put it together with an older 10 HP Baldor motor that came off an older air compressor that was junked or sold to me for parts. The RPC box I bought off of ebay. This said, we run our 7.5 hp shapers using our RPC but I will say it is a bit of a struggle. The motors do not seem to be getting enough power as they tend to struggle with some cuts our previous 3 hp single phase shapers would cut like butter. At this time, we are very close to getting into our new 40X80 so after the move I plan on upgrading our RPC to 20 hp.

I will say that our RPC runs other things great such as our power feeders, our Blum minipress, a 3 phase unisaw (3hp), a past air compressor that was a 5 hp quincy (this has since been converted to a single phase motor)...

Not to sound negative, but while I would not hesitate to go for the Unisaw purchase, even with the 7 hp 3 phase motor, I suspect you would have some challenges to overcome getting it to run at full power on single phase.

We run our 10 hp RPC with #8 on a 50 amp breaker. I am no ‘pro’ when it comes to these things, I typically self teach myself and much of the time I learn by doing. As I tell my guys, ‘if I tell you something, it is based on life experience and not some book I read, it is because I have been there, done that and already screwed up in the past…’

Not sure if my personal experience is of any help. If you are only looking at one machine to run such as just the 7 hp Unisaw, a brand new Leeson single phase 5 hp replacement can be had for around 500.00 and that is the route I would go with, unless you want to add other 3 phase equipment in the future.

-- .

View unbob's profile


810 posts in 1929 days

#9 posted 01-03-2014 06:08 AM

My experience with the Autogen static converter is from this 7.5hp saw here I found on CL, the CL photo above.
The converter sitting on the floor. Also, there is a 7.5hp idler motor on the floor to the right. The previous owner had the converter start the idler motor then the saw would be started. He decided at one point to run the saw with the converter only. His son said he found the idler was not needed, and took it out of the line. When I got this saw in place, I arranged it so as with a wall switch, the converter would start the idler, then start the saw. However I used plugs so as I could disconnect the idler and run the saw with the converter only. I found as the previous owner the idler was really not needed for this application. I could not stall the saw or notice any excessive heat build up without the idler. I did take the time to use balance caps for the third or wild leg for the idler. Having used the old Phase O Matics in the past, this autogen is a lot different, for example it has a soft start. Because I purchase another machine that even a rotary converter will not run “regenerative direct current variable speed drive”, I had to go with a Phase Perfect digital converter. Now I run everything off of that. But the saw has no noticeable change at all with that ultimate in conversion$$ over the Autogen.

On the other note, I have only seen woodworkers do the burn up freak out over static phase converters, metal workers just complain about loss of power.
Metal working machines produce really bad finish if the motor bogs, so perhaps machinist tend not to burn things up bogging down machines. That is, I just don’t hear much about burnt motors from that direction.

View unbob's profile


810 posts in 1929 days

#10 posted 01-03-2014 06:21 AM

Some other 3phase stuff, I just have not had the problems of cooking 3phase motors some claim. So, show me what ya got.

View TopamaxSurvivor's profile


18290 posts in 3702 days

#11 posted 01-03-2014 06:26 AM

You are going to need a breaker close to 100 amps 1 ph 240 to start that thing.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View TopamaxSurvivor's profile


18290 posts in 3702 days

#12 posted 01-03-2014 06:31 AM

Just had another thought; if you were in this area in WA, the power company would charge you to install a transformer big enough to start that without affecting your neighbors. Their lights will be blinking when you turn it on ;-)

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View Loren's profile (online now)


10476 posts in 3674 days

#13 posted 01-03-2014 06:33 AM

The price on a VFD in that range could be a deterrent. Smaller
ones (3hp and under) aren’t too costly. Also consider
a VFD is electronic and vulnerable to dust and moisture. They
may not last as long as a static or rotary converter.

A Unisaw with a 7hp motor is not a common thing. I suspect
you’re looking at a 14” saw like Unbob has.

btw – I just bought a 7.5hp wide belt sander (small one)
in an auction and I’m curious to see if my 5hp rotary
phase converter will start the thing. REO seems to
think it’s possible but most of what I’ve read otherwise
would imply that I’d need to upgrade to a 10hp
rotary converter.

View TopamaxSurvivor's profile


18290 posts in 3702 days

#14 posted 01-03-2014 06:42 AM

Loren, that will be interesting to see if it starts. I’ve never tried to run a 3 phase motor on a too small roto-phase.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View Loren's profile (online now)


10476 posts in 3674 days

#15 posted 01-03-2014 06:51 AM

I’ll let you all know.

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