Variable frequency drive

[desterline]

I'm about to embark on the oddessy of making a variable frequency drive for some of the tools in my machine shop. (120v, under 5 amps, 20Hz to 200Hz) <p>Does anybody have any plans / schematics / thoughts / ideas / etc that would be worth considering before I start?<p>Thanks
-Denny

[Chris Foley]

Hi, Denny. AC variable speed drives are now relatively inexpensive and really pretty reliable. When you're doing this, the price of failure is usually a minimum of a smoked motor, plus whatever you're working on, plus the possibility of damaging the machine itself. Consider looking at some of the Automation Direct product, particularly their AC Micro Drives (GS Series). I've had some success with using them for control applications. You can control them through a serial port, or through an external potentiometer or DC signal.<p>Automation Direct AC Drives Web Page<p>By the way, make sure you get reliable information on the highest frequency your AC motor can handle before you start, preferably from the motor manufacturer.<p>[ August 31, 2003: Message edited by: Chris Foley ]</p>

[Chris Smith]

What sort of motors are you using? AC 60hz? DC? <p>AC motors are best controlled by a Variac, and no need of any other devices. <p>Second choice is a chopper circuit, light dimmers. <p>They deliver the maximum amperage, with only a variable of voltage. <p>But all must run at 60 Hz?

[desterline]

Hi guy's, thanks for the replies. <p>At the moment I'm using a synchronus 120V AC single phase motor on a cheap drill press (realy cheap, like I don't realy care if I destroy it cheap). <p>The eventual goal is not only get variable speed, but actualy to increase the max RPM I can get.<p>Yeah, I know there are many limmiting factors in trying to increase the RPM, not the least of which is the posibility of destroying the spindle bearings and / or exploding the drive pullys and / or the rotor of the motor. But I'm not looking for 30K RPM here, I'm only looking for 3000 - 5000 RPM, considerably safer.<p>As to buying a pre-made unit: First, all of the comercial units I've seen (even the GS series from Automation Direct) is desinged for controling three-phase motors, not single phase like I have. Second, since this is a hobby, it's actualy more of an experiment than a cost-justified need.<p>The design seems relitivly straightforward, full wave bridge rectifier on the input, some high voltage stiffener caps to smooth out the phase transitions, and a H-brige for the output. Toss with an assortment of control circuitry (probably a PIC, but I could do it with discreets) and Voila!<p>My only stumbling block at the moment is the choice of evtive elements for the output H-bridge. My initial thought was to use some IGBTs, since they seem to be the choice for the comercial units. But I'm new to them and don't realy understand how I would drive the gates on the high side of the H-bridge.<p>Any thoughts?
-Denny

[zotdoc]

There is an excellent article in Home shop Machinist May/June 1996 by Charles Eyer on 90 volt dc motors and building a controller for them. I can't help you much if you want more speed, but these dc motors really have much better torque at low speeds than ac motors.

[cato]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by desterline:
<p>My only stumbling block at the moment is the choice of evtive elements for the output H-bridge. My initial thought was to use some IGBTs, since they seem to be the choice for the comercial units. But I'm new to them and don't realy understand how I would drive the gates on the high side of the H-bridge.<p>Any thoughts?
-Denny<hr></blockquote><p>You have to isolate the drive on the high side as you probably suspect. Check out the Ap notes from International Rectifier and the other IGBT manufactures. There are Opto Isolators and transformer based isolation schemes. This is really the hard part of your journey Grasshopper. Speed and timing are key. (You really must grab the pebble from the hand.) If you don't shut off one IGBT (and I mean slam it off to overcome internal and external stray capacitances) before you turn on the next one, you're going to let the magic smoke out of some fairly expensive parts. My memory is foggy now, but there are also issues about a parasitic diode in the IGBTs and adding a protection diode external to the IGBT to deal with one current or another...I don't remember. The Ap notes should deal with it.<p>That said, let me put my vote in for just buying one. I'm pretty sure you can get 1 , 1.5 and maybe even 2 horse power variable speed drives for single phase motors for a few hundred bucks. You'll go through that in Cracked IGBT's in no time. <p>Don't forget, the energy levels you would be dealing with, are quite deadly. Because of the frequencies and currents involved, layout is extreamly important. I don't think an hand wired prototype will ever work. Ground planes are a must...bypass caps everywhere...<p>Maybe just get a better drill...

[Crowbar]

www.eng-tips.com/viewthread.cfm?SQID=23 ... 237&page=2
Try this link for more infomation on controlling single phase motors with VFDs.

[toejam]

have you looked for pullys that can change the speed up?

[desterline]

Project is dead, this thread is dead. Move on.

[cato]

Was it something we said?