Phase converter idea?

Theoretically possible, but why? You can buy a speed controller with soft start, dynamic braking, and dozens of features, that also converts single phase to to 3 phase, for $200 or so. Why bother to make your own, which will take many hours, might or might not work well, might or might not fail catastrophically, and will never have all the features of a commerical speeed controller?

Richard

Ignoramus17647 wrote:

He is only a little closer to being able to make a VFD from those transistors than if he'd start with a bucket of sand..

I do not disagree with you, practically speaking, most things are easier to buy than to build, but I am curious theoretically. Will this be able to be a basis for a high current phase converter.

i

To build your own inverter type phase converter you have to start with building a big DC power supply. It's much harder than it looks. What would very likely happen is that you'd go spend $40 on big power rectifiers and as soon as you fired it up they'd go "BOOM!" and you'd be wondering why.

There is a design for an SCR-based variable frequency power supply in the back of the old GE SCR Handbook. I did a lot of design work planning to build one until I found out more about the failure mechanism of large SCRs (roughly analogous to a hand grenade) and decided I didn't want them in my house. You can pursue this if you want to and you will likely learn a great deal but I can tell you this with great certainty: you will never cobble up something cheaper and more robust than the inexpensive inverters you can buy now from places like dealerselectric.com - but do keep us posted.

If you decide to press on, make sure you google on the term "snubber" because a snubber circuit is what you'll have to design and test to keep your power rectifiers from going boom.

GWE

"Robert Swinney" wrote: He is only a little closer to being able to make a VFD from those transistors than if he'd start with a bucket of sand.. ^^^^^^^^^^^^^^ I had a friend who used to say, "Give me enough sand, and I'll build you a golden castle."

I'm working on an induction heater (just blew two more transistors, pffbt), I could just as well buy one, with all the features, temperature control, ability to run any coil, and be completely load protected, but I won't. I have more incentive because these things run upwards of $10k, but that's not my point.

Some people like to build a Gingery lathe too...

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

You have the block diagram basically right except for the microcontroller and software to provide transistor control, implement overload protection, and orderly start-up/shutdown sequences. It took industry about 10 years to get to where we are today: relatively cheap, bullet-proof, simple to use VFD's. Phase converters, rotary or static, are obsolete.

Randy

i

I really like the idea of your building that device for comverting single phase to 3 phase. I like it so much that I'd suport your efforts by supplyimg any parts that I might have. (and I have alot of stuff for projects like that). My thought is; you are a smart guy with curiousity. This sure would become a way for you to get alot of experience with electronics and learn things that will possibly benefit you in your future. This is a metalworking news group but there are guys in this group who are very knowledgeable about the circuits you will use. I hope you build the phase converter.

J

Are bipolar transistors really the correct device? Aren't some of the modern VFDs built using hexfets?

Not to discourage him of course - but today the cost of devices is

*so* small in comparison to the rest of the project, it would make sense to purchase the best transistor for the job because that would probably only add about ten bucks to the overall cost.

Jim

You mean something like this:

5 HP, AC Drive, 230VAC SINGLE PHASE, Inverter, VFD That's very interesting!

Thanks! Obviously, if it took the industry 10 years, I cannot figure it all out on my first attempt.

i

Thanks Jerry, the talk about SCRs exploding like hand grenades scared me a little bit, but I just might do this one day.

i

i

I have some IR SCRs That will handle 1,000 volts and 1,000 amps that I'd domate to a *real* project. I'd expect these SCRs to be adequet for any household power requirements without fear of exploding. I think you can build a fixed frequency unit to convert single phase to three phase. I also think there are several guys on this group who can show you how to do it. I know Don Foreman could design something like this 'single frequency converter'. There are probably many other RCMers who would help you with any design problems you might encounter. I'd supply the rectifier diodes and SCRs. I am not smart enough to help with design.

J

Of course, it also depends on how many sets you blow up (:

It took industry that long to develop cheap power semiconductors that can handle the voltages. Now they're available. Appropriate devices for such a design would be MOSFETs and IGBT's.

As Spehro says, "the journey is it's own reward" (or something like that.) If it interests you, go for it! You will definitely learn a few things along the way. I've never built an inverter, but I'd be glad to collaborate by email with such help as I can offer. I don't think you'd save any money by building one, but that isn't to say you shouldn't do it anway.

You'll also need enough DC capacitance to keep voltage up to supply the other phases while the singlephase power goes thru zero.

Right. Depending on power level and voltage level, HEXFET's or IGBT's are the appropriate devices.

When power elex are being developed, things do go POW -- but they're little POWs, more like a smallish caliber pistol than a grenade.

Eye protection at the bench is a very good idea.

Jerry, I must say that these are worth real money on ebay. Try looking into this. 300 amp ones sell like hot cakes for $25 each.

While I appreciate the donation offer, I want you to be aware that these things are not just cute garbage, that they have high value. I do not want you to not be aware of it.

I can guarantee that if I embark on this project, I will not sell any donated products on ebay, but would either use, accidentally destroy, or return them back.

Regarding the control circuits, what I am curious about is whether I can control the process with a regular small 110V motor spinning at the regular RPM. That is, the motor, and some wheel with contacts and such, would tell the system which circuits to turn on and off.

A wheel of that motor would be made of dielectric, with some copper pieces on it that would at certain points close some switches that would then supply various phases etc. I can draw a picture but you may be getting the idea.

If the control current is 3-4 amps, then arcing on the contacts should not be (I hope) too bad.

Various contact points would be touching that cylinder at different position along the spinning axis. One position may say "turn phase 1 PLUS on", another would say "turn phase one MINUS on", another one would say "turn phase 2 PLUS on", another would say "turn phase 2 MINUS on",another one would say "turn phase 3 PLUS on", another would say "turn phase 3 MINUS on". These copper strips will be positioned with 120 degree offset.

That same motor could drive a fan that would cool the transistors.

This cannot really go wrong unless, something falls off, or there is a short. (or so I think)

The issue that I see right now is that the inverted current will not be sinusoidal .-'~`-._.-'~`-._.-'~`-._.-'~`-._.-'~`-._, , but instead will be ~~~~____~~~~____~~~~____~~~~____ -- basically a combination of plus or minus voltage and not a smooth curve. Perhaps a curve can be smoothed with some capacitors and isolation transformers. My memory of relevant school physics is quite dim.

That would be the Kalashnikov style solution, simple, stupid, and more easy to understand than a complex circuitry. I would prefer that to building a sophisticated control circuitry that I would be most likely to get wrong anyway.

i

What is an IGBT?

Yep, I wondered about that also. I have 4 10,000 mF 200V (250 surge) capacitors, but they would be unlikely to handle 246 or so utility volts, so the voltage would need to step down a bit. Not sure if they are DC or AC.

If this idea moves further towards implementation, I may keep the caps, if not, I will sell them.

If I am going to go ahead with this, I will keep a project page similar to my generator restoration page

What is becoming clear is that old junk UPSes are in fact a real gold mine. Power cords, contactors, switches, circuit breakers, SCRs, IGBTs (I mined some also), all of those are quite valuable. The real difficulty is handling heavy items and throwing away what is not needed.

I will from now on bid on all large UPS lots from the military, regardless of condition.

thanks to all.

i

How fast are they? I've been curious about building an SCR inverter as Dan experimented with:

that current rating, I could pump what, a good couple of kilowatts through a coil without that thing breaking a sweat! The only problem is the capacitance needed, I'll have to drop a lot of change to get enough capacitance rated for that kind of current and voltage.

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @