Lathe/Mill 3-phase conveter

[gunsonwheels]

Maybe off-topic but anyway...

I have both a mill and a lathe that are three-phase and have run them out here in two-phase land very successfully with a conveter made from an old three-phase motor.  Other home-machinist engineers at Boeing introduced me to it, I saw a schematic of it at a California used machinery dealer and subsequently was able to fill in a "missing link" by consulting another engineer doing electric vehicles (Seattle's buses) in the '70's.  If there's enough interest I could draw it up and post a picture... if not,  PM me and I'll send out a copy.  (now where did I put that thing?? ?? ??)

George/GOW

[Soot]

I'd like to see your schematic.

[GGaskill]

I have run my machines that way since I set up the shop.  They work well.  See Method 2 on this instruction sheet from Phase-A-Matic for one way to do it.

[gunsonwheels]

It appears Method 2 is still requiring a static phase converter ($$$$$)??

Our method requires a three-phase motor for an idler, a single pole, single throw, momentary-on push button switch, a motor starting capacitor, and a fused (or breaker) two-pole switch to energize the idler.  Total cost can be less than $100 for those items and a few odd length hanks of wire.   Neither a rotary nor a static commercial "phase converter" is required.  However because it is not UL listed, it will need to be "plugged in" in lieu of "wired in" in oder to comply with most local electrical codes.

This link does the best job of explaining our set-up... with the idler starting capacitor option:

http://www.nojolt.com/how-to-build-a-rotary-phase-converter.shtml 

[keith44]

I get asked about doing this all the time, never found any affordable options for the people asking.

[GGaskill]

The commercial static convertor contains all the above-mentioned parts plus a voltage sensitive relay that switches it off when the motor has come up to speed.  They run from about $85-$170 depending on horsepower of the motor they are starting (there are more expensive ones for higher HP motors but I wouldn't use one for a large motor.) 

You can spend your money or spend your time.  You can usually find a used 3-phase motor for cheap for the idler.  With the idler, you get much better quality power than with just the static convertor.  I would not recommend the use of a static convertor only except for the lightest duty.

[Cat Whisperer]

The 3 phase idler motor RUNS on one phase.
Since it's running, the other two phases are GENERATED.
The result is that you now have 3 phase power at the connections of the idler motor.

The generated phases are about 85% of the phase hooked to the power line.  In the running motor this all averages out.

The generated phases have current and volatage shifted a bit, so capacitors are added to bring them back into alignment.  (The size of the capacitor is tuned to the size of the idler motor.)

There are numerous articles and schematics on the interned that give the details.

[Cat Whisperer]

The commercial static convertor contains all the above-mentioned parts plus a voltage sensitive relay that switches it off when the motor has come up to speed.  They run from about $85-$170 depending on horsepower of the motor they are starting (there are more expensive ones for higher HP motors but I wouldn't use one for a large motor.) 

You can spend your money or spend your time.  You can usually find a used 3-phase motor for cheap for the idler.  With the idler, you get much better quality power than with just the static convertor.  I would not recommend the use of a static convertor only except for the lightest duty.

Spot on.

Static converters are not designed to run motors that start and stop often.

[gunsonwheels]

The commercial static convertor contains all the above-mentioned parts plus a voltage sensitive relay that switches it off when the motor has come up to speed.  They run from about $85-$170 depending on horsepower of the motor they are starting (there are more expensive ones for higher HP motors but I wouldn't use one for a large motor.) 

You can spend your money or spend your time.  You can usually find a used 3-phase motor for cheap for the idler.  With the idler, you get much better quality power than with just the static convertor.  I would not recommend the use of a static convertor only except for the lightest duty

My "idler starter" consists of the capacitor, contact switch and fused knife-blade, 2-pole switch and were about $30 all together... which also brings the idler motor "up to speed".  Releasing the momentary contact switch also "turns off" the idler starter.  It's a quick "1,2,3" start sequence: 1) press momentary switch energizing capacitor to third leg of idler,  2) engage knife-blade switch to turn on 220v single phase to the other two legs of the idler, 3) release momentary switch.

Every dime I was able to spend at the time was pretty hard come by so yes... I guess I did have more time than money.  The idler motor was $75 (or $15 a pony... 5 HP).  I take pretty heavy cuts with carbide insert tooling so everything gets a good workout.  My machining is, however,  "intermitent" compared to a regular job or production shop.  Both machines' motors are 3HP. 

My first one was a rope-start version and ran a 16" South Bend lathe but I seemed always to want to make another cut right after I switched it off... and half the time didn't because of the hassle of winding the rope and having to yank it before powering the idler.  That idler motor was a larger, older 1100 RPM guy so the relatively slow rope pull would get it going.  Went with the lathe when I sold it.

[OoooOOoo1010010]

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[gunsonwheels]

Since starting this thread I joined "Practical Machinist" forum and there is a whole bunch of schematics on that board...  from the very basic and simple to the more sophisticated.  Anyone interested in this topic ought to check out what's over there.  From what I found over there I decided mine is a "get by" solution with many others giving superior performance to what I am using...
 
GOW