View Full Version : Single Phase To Three Phase Converters

Dave Milosevich
02-18-2008, 08:13 AM
With the increased interest in buying one's own machine tools, the issue of three phase power comes up more frequently.

It's no big deal. There are several ways to satisfy the three phase requirement, none of which include the power company. It is highly unlikely that you will be able to get your electric utility to supply three phase power to your house if you do not live in an area zoned for industrial or heavy agricultural (read irrigation) use. Even where it is possible it is expensive.

Variable frequency drives are the slickest, most cost efficient way to go for motors up to 3 horsepower. You can buy them off the internet from any number of suppliers. Above 3HP, they get real expensive real fast.

The cheapest, off-the-shelf solution is a "solid state converter". This is not really a converter but simply a starter that gets your three phase motor spinning on single phase power where it will continue to happily run as long as you don't need it to produce more than half its rated power. That is to say, your 5 HP motor is only going to be good for 2.5 HP in this arrangement. I used one for years on a 7.5 HP motor and it worked fine. There is no gunsmithing requirement for much horsepower.

The easiest home-made solution is a rotary converter. These are also available off-the-shelf but are easy to fabricate. It is not a motor generator per-se. It is a three phase motor that is connected to single phase power and started by either a solid state converter or mechanical means like a small single phase motor or even a pull-rope. The cost is driven by how much you have to pay for a three phase motor which can be used or new.

Here's a link to get you going.


Also, Google up "three phase converter" and you will get a bunch of hits for sales and making your own.


02-18-2008, 08:54 AM
There is a lot of info on practical machinist .com just scroll down to transformers, phase converters & VFD.


02-18-2008, 09:09 AM
I recently bought a used mill with a 3 hp 3 phase motor. I bought a new 5 hp rotary phase converter from American Rotary off ebay for $340 plus shipping. Easy install and works like a charm. The 5 hp idler is made by Baldor.

Louis Boyd
02-18-2008, 09:48 AM
The disadvantages of rotary converters are they're heavy, space consuming, and the still produce only 60 hz. They can be very inexpensive if you use a surplus idler motor, but buying one with a new idler costs nearly as much as a VFD> The solid state converters give variable speed, braking (usually an option), and a compact package. My mill has a 5 hp Hitachi VFD. Got it for under half of list on Ebay. In my opinion the variable speed feature is very important on a milling machine.

A VFD isn't the only choice for variable speed motor control. My lathe has a 1.5 HP 180 VDC permanent magnet motor with a KB Electronics KBRG four quadrant controller. (available up to 5 HP). It's basicly a SCR DC power supply with voltage and current ramping and limiitng with the ability to source or sink current in either polarity. It gives similar performace to a 3 phase VFD but it can do power decelleration, not just braking.

02-18-2008, 10:37 AM
I got a quote from the power company for around $6000 to bring three phase to my shop. The three phase lines run parallel to the shop about 10' away. The transformers for a wild leg 240 3phase service are at the corner of my small city lot. In the quote the power company was going to bill me for putting up new transformers and replacing an old power pole next to the shop. This is in a residential neighborhood where I got a variance to run my business. I called many different people at the power company to try and figure what the average monthly bills would be. Finally I asked one of the supervisors why it would cost so much to power my shop? He dug up my quote and called me back. He explained that the quote was done using standard quoting procedures and he thought they could bring the three phase to my shop for FREE!!! The only stipulation was that they would only hook up to a new service that was installed by a state registered electrician. If the ran the wires overhead then I would have to pay for the copper. If they ran down the pole they would supply the copper to the service. A 10' trench was needed. The bill for my electrician to install the meter socket, dig the trench and install a new 200 amp panel was around $1500. My average power bill for my shop is less that $50. This includes running an air conditioner in the summer and a 1500W electric heater in the winter to help burn less $propane$. Most of my machines are three phase. Horizontal mill, Bridgeport style mill, 5HP Lathe, 5HP cyclone dust collector, surface grinder and 18"Grob bandsaw. Most are wired for three phase but the dust collector has an inverter wired three phase so I can use the variable speed function and the thermal overload protection. I'm pushing the limit of the inverter and the display shows how many amps it's using and I adjust if necessary. I put an inverter on my surface grinder when I generated my own three phase in my old shop. It's wired 240 single phase and I've been too lazy to wire it three phase. I wouldn't get rid of the inverter if I did because I like the soft start that the inverter can create. It doesn't jolt the wheel and lose the "dress" I also warm up the bearings on a slower setting for a while before I grind with it.

If you have many three phase machines and three phase on the pole near by, it's worth all the hassle of getting it into the shop. Even if a first quote from the power company is quite high call everyone at the power company and verify this quote. Saved me $6000.

There's many ways to generate your own three phase. My first rotary converter was just a three phase motor with a pulley and a rope to pull start it. I wouldn't recommend anybody do this. It did work and was quite cheap. A friend of mine has a shop with a little buzz box wired to his biggest machine a 7HP lathe. Then he uses this as the idler motor to start & run his mill and smaller lathe. Worked great for him for 15 years.


02-18-2008, 10:42 AM
Another thing. The VFD is a fantastic way to create three phase for ONE machine. If that's all you need then it's the option I would use. The one I put on my surface grinder cost only $150. If you want to wire up multiple machines then a rotary would probably be a much better option. Or buy an inverter for each machine.

02-18-2008, 12:24 PM
I've never opened up one of the 3 phase Lathes and checked but, if the rated voltage of the controls, lights and if used, a contactor with the same coil voltage is 120/240 3 phase, just change the motor to a 240 volt single phase unit with matching horse power. You may also have to change the forward & reverseing starter. With three phase, the third leg or "Phase" is only being used to drive the motor. The frame, pulleys, etc. would be the only issue to really deal with but would certainly be cheaper than phase converters. One would also need to check the controls etc. but, as stated the third leg is only being used to drive the motor, most of the time. Before you flame me, yes, I am a fully Licensed Master Electrician.

02-18-2008, 01:03 PM
3 phase motors are far simpler than single phase motors.
No starting capacitors and no throw out switch.
Just stator windings and a squirrel cage rotor.

They also have the special advantage of instant reversing.

A drum switch swaps two of the phases and the motor will reverse.

A single phase motor must be brought to a halt before it can be reversed.

02-18-2008, 01:58 PM
It has been since 1993-94 that I had 3 phase installed,so my comments may not be accurate today,

I called the power company and asked about 3 phase, they told me as long as the power lines were close and have a min of 5 hp It would be free.
the power line was in front of the house, they put in a second transformer and a new pole, NO CHARGE.

This is a residental area.

Maybe just lucky.

Marshall. tx

Larry Sivils

Louis Boyd
02-19-2008, 10:36 AM
Even if I had 3 phase power in my shop I'd still use electronic variable frequency drives on my machine tools. I like having speed control, torque control, and ramped acceleration and decelleration. Most three phase VFDs work with either single phase or 3 phase input since they rectify it to direct current before they convert it into three phase.

02-19-2008, 01:30 PM
It's important for anyone that's thinking of using a VFD (inverter) to convert single phase to three phase to get the correct size unit. If you're feeding the VFD with three phase it will run a bigger motor than if you are feeding it with single phase. Many are rated on the nameplate for both. Ask a VFD repair shop if you have any questions. They usually have rebuilt units for a great price as well. Always remember that VFD's are only for powering 3 phase motors and nothing else. No welders etc....

02-19-2008, 01:55 PM
You really need to have it installed on the lathe, so you can have a knob, within reach to set the speed.

On the Clausing Metosa lathe I have at work, the speed knob is on the carriage, along with a Fwd/Off/Reverse switch.

The speed knob is a little too easy to bump with your knee, so I need to move it.

The switch works great, one feature is that when you switch the lathe off, it dumps to a breaking resistor, and the chuck stops very quickly, then releases, so you can hand spin it. Put it back to the on position, and it comes right back to the same speed.

If you start to hear chatter, speed, up or slow down, and you can tune it on the fly. If you're cutting speed looks a little slow, watch your chips, and pour the coal to it, or back it off to get a good rate for your diameter/material.

Love the variable speed.

02-19-2008, 02:31 PM
The input rectifiers are only so large and have current limits. When you are pulling all the power from a single phase you need 3 times the current per phase compared to using a 3 phase input.

The output is also much cleaner when you feed 3 phase into a 3 phase bridge rectifier to make DC.
This reduces the ripple that must be controlled by filtering (larger capacitors) and electronic feedback. If all 3 phases have the same ripple on them the motor does not see it (within reason).