Suggestion: Try Patents at
http://www.patents.ibm.com
or
http://www.delphion.com
or
http://www.delphion.com/patquery
and type the below patent number.

us4730242: Static Power Conversion and Apparatus Having Essentially Zero Switching Losses
Inventor: Divan; Deeppakraj M., Madison, WI, USA
This item will need to engineer, design and build a rectifier from a single phase to DC according to your specifications.

I am curious. Are you are converting the motors for a variable speed function of your machine from whatever is there now (DC, multispeed AC, ?) to induction motors w/ VFD's or are you interested in changing a fixed speed function (already w/ an induction motor) to variable speed?

Anyway, to the answer you are seeking, I can't give you a specific reference (or a specific design) to help you design a single to three phase VFD from scratch. If you are interested in building one for the fun or challenge of it (more power to you!), there are "hobby" books available with cookbook designs for electronics which may include what you seek. Try Barnes and Noble (they have a website). If you just need to come up with a VFD, I can tell you that the device you are describing is commercially available and relatively cheap. For the heck of it, try WW Grainger (they have a website too), you may decide that it is not worth design/building it yourself from a cost versus hassle standpoint.     

The speed control would be used to control the machines spindle RPM which is currently controlled by changing a series of belts on different size pulleys with a 1725 RPM A/C motor.  I have looked at Grainger on what they have but the price is anything but cheap, around 600 to 800 dollars.  They also have way too many features that really are not needed for simple RPM control.  I would like to build one just to do it, have some fun, and maybe even save a few dollars in the process.  Thanks for your feedback.

Suggestion: It is probably better to shop around for a simple and less expensive AC drive since by buying parts, assembling them, testing them, etc. you would be less money efficient than over the total cost of some less expensive and still suitable ac drive that you mentioned. Also, if you build a more complex item from parts, after some time, certain parts may be discontinued without replacement. This could result in major change of your custom-built ac drive. You may like to explore the market over
http://www.thomasregister.com
under Service/Product

dmartin15
   Not knocking your idea - go for it.
   But, a collegue of minor is very into woodworking machines at home and I suggested he use an obsolete vf drive we had for his machines he said that when you are slowing the tools down you are usually looking for a lot of "grunt" and you get that by changing the belt "ratios". His experience prior resulted in a burnt out motor.
   Now if that isn't a concern to you try browsing through the local second hand machinery sellers there may be a cost saving there.
  If you go ahead I suggest browsing thru the hobby electronics mags and your local electronics hobby stores for data books.
  In Australia try www.siliconchip.com.au.
   keep us posted hey
regards Don

Great observations Don01. The different diameter pulleys do have different effective ratios. If both pulleys are the same size it would be 1:1. If the motor pulley is half the diameter of the load it is 2:1 (ie 2 revs of the motor for 1 rev of the load). Like a gearbox, power in = power out (-losses) so the change in speed results in torque multiplication. There will be a lot more torque (grunt) available to the load at lower speeds and people do use those speeds for that purpose. On a direct drive (no pulleys), providing the same power at half speed (ie 2:1 ratio example) will require a %200 (torque) overload on the original motor.  Please read on, it gets worse...

Even if you do not overload the motor (ie. never exceed nameplate current at any speed), you will probably still burn it up. The killer here is cooling, or lack of it. For a standard TEFC motor, the speed range over which you can operate at full torque may vary from less than 2:1 to as much as 4:1 (don't confuse with gear ratios/torque multiplication). This means that a 1800 rpm motor may be able to run full torque at 900rpm (a.k.a. 2:1 constant torque speed range) or even down to 450 rpm (4:1 CT speed range) before the lack of cooling causes it to exceed the insulation temperature rating (ie. burn up). The variables here are fan design, type of enclosure, and insulation temperature rating. Normally you need a motor with a separate blower (ie independent of shaft speed) to achieve speed ranges greater than 4:1. My guess is that a machine tool (woodworking, etc) motor has very little constant torque speed range (it certainly wasn't designed for one).

I don't know what type of pulley ratios you have, but I would bet that there is some serious speed reduction/torque multiplication going on. If so, the pulley system you have is the best way to do what you want. It would be quite costly and electrically ineffecient to size a motor for direct drive (ie many times bigger than what you have now) and then add the requirement for a wide constant torque speed range.

I know you were hoping for a different answer, but I hope this helps.



     
  

Check out http://www.anaconsystems.com - they specialize in the type of ac drive you seek to build.