How do I build a converter to run a 3-phase 1/2hp motor from a single-phase residential mains?

I got an old industrial sewing machine cheap. Cheap because it has a 3-phase motor, which made some other hobbyists reject it. The other option at ~150$cdn would be to replace the motor with something more domestic.

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berserk (author) 1 year ago

Wow, thanks for all the answers over the years... I had an RSS feed set up to let me know when anything arrives but it turns out that has been broken for some time. I ended up replacing the motor with an electric drill and this has worked well enough for me.

tima2 years ago


1. Don't believe anything I say--I don't.

2. Proceed with these instructions at your own risk. I will not be held responsible for injury, death and/or destruction of property. However, if none of those things happen but your new device works admirably, have kind thoughts of me.

3. Please exercise good safety practices. Know the meaning of the words: CAUTION, WARNING and DANGER and how they relate in operating and repairing electrical equipment. DO NOT work on equipment that is energized and discharge any capacitors which may have been energized recently on shut down equipment.

4. If you understand and agree with this disclaimer and will follow it precisely, please proceed with the instruction and have fun:

If you pick up a 3-ph motor with not less than 1/4th the HP of the sewing machine motor, you may use it to drive it. You will need to mechanically couple the smaller motor with a single phase one to get it to start to spin on single phase. it doesn't matter which direction the converter goes, but you will want to make sure that your sewing machine is going in the right direction! Reverse the polarity on any two outputs of this rotary converter that you have just built to make your sewing machine go the correct direction. Once the converter is spinning, the single phase starter motor may be turned off. However, if the sewing machine hits a tough situation, the converter may stall out. Then it will need restarting.

I've started converter motors with dc motors and sometimes just by pulling briskly on a rope curled around the shaft several times!

With some adjustments, you can somewhat tame the wild legs of the configuration with a capacitor *across* two of the legs and an inductor *through* the other. Look up RUN capacitors as opposed to STARTING capacitors that are rated at well above the voltages you are working with and at least 250µF. The inductor might be the primary winding of a 1000 watt power transformer.

Rotary converters perform at their best when they are well-loaded. A 1hp motor used as a converter will handle 4hp.

Normally I use delta-configured motors for this. Star-configured I haven't tried.

I presume that you need 208vac 3-ph?

The output of your machine will power delta -or- star motors.



try and they are called VFD's for the ebay crowd.... consider buying something like a teco-westinghouse, toshiba, or etc. also you can add a potentiometer for remote speed control, etc.
I hope that you are still looking for an answer.  The best choice for your application would be a Variable Frequency Drive (VFD).  You can find many that will accept 120vac in and put out 220/230/240 3 phase.  They are often called inverters.  They take in ac, convert it to dc and then convert that to 3 phase ac.  They control the motor speed by varying the frequency of the output and are generally capable of outputting up to 200 Hz.  Don't be tempted to overdrive your motor, though - it's probably not really rated for that use and you don't want to burn it out.  Stick to 60 Hz as a maximum. 
The drive will have many user-adjustable parameters, but keep it simple, at least in the beginning.   Call the manufacturer and explain your application - they will be quite helpful.  You might call Automation Direct, though there are plenty of other sources.
<a href=""></a> sells several books on th' subject and they are affordable
Buy a static motor inverter drive from Siemens, Mitusbishi or whoever,or change to a single phase motor. 3 phase motors start nicer though,which a sewing machine needs.

The inverter would be sweet because you'd get free speed control of thething too.

berserk (author)  steveastrouk8 years ago
A search at Siemens and Mitsubishi's sites on "static motorinverter drive" only brought up partial matches. Same for amazon orebay.
Is any part of the term optional? Or, better yet, do you have a link toa product?
<br />Try this !<br /><br /><br /><br />Steve<br /><br />
Re-design8 years ago
You have to build a converter and then you still need another motor to spin it up to a speed to where it can take over on it's own.  I've never seen a 3 phase motor that would start itself on single phase.
And here's a <a href="">good link</a> for some instructions on how to work it out.<br /><br />I might be wrong about needing a starter motor. I've never seen it done without but I've not seen very many 3 ph. running on 1 ph.<br /><br />If this doesn't fill you with knowledge, google "3 phase converter plans" and you'll get lots more.<br /><br />With all the work and cost to convert the existing motor, I think I would just put the effort into changing out the motor to a single phase motor. But I'm not there you are.<br /><br />Good luck.<br /><br />
berserk (author)  Re-design8 years ago
I came across the metalworking link, but I could not figure out to whatdegree their specs would work for motors. I almost got the impressionthat they were dealing with welding. Would it make a difference?
The other solution would be to use a one-phase motor to spin a three-phase generator.... but it'd probably be simpler, and certainly more efficient, just to swap motors.