Phase converter idea?

Ned Simmons wrote:


That would have been (or is) a nice 3 phase 50 or 60 Hz power source for a big toy. - 3 phase motors, 3 phase 1.5KVA welders....
Martin
--
Martin Eastburn, Barbara Eastburn
@ home at Lion's Lair with our computer snipped-for-privacy@pacbell.net
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
--
==================================================
please reply to:
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
jim rozen wrote:

Of course, it also depends on how many sets you blow up (:
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Right. Depending on power level and voltage level, HEXFET's or IGBT's are the appropriate devices.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
jim rozen wrote:

Bipolar transistors have traditionally out performed fets. Back when I was designing switching power supplies, we used bipolars in the high power supplies because good enough fets just weren't available. A few years later I designed a supply with fet switches. It was really amazing how much easier the fet was to deal with than the bipolars. Far easier to drive, much more forgiving about over driving, etc. I think IGBT's are a best of combo, but they arrived after I (thankfully) escaped power electronics. As a general rule, when you're trying to amplify a bipolar is going to perform better. When you're trying to switch, pick a fet.
Steve
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Steve Smith wrote:

IGBTs are a dream, from the performance standpoint. Unfortunately, they are a NIGHTMARE from the tricky design details view. Yes, they definitely can be made to work reliably, but just wiring them up, without attention to transients, dv/dt and the worst characteristic, the positive temperature coefficient, will end up with a big POP! The IGBT must be driven hard into saturation, and then driven hard into cutoff, in the space of 50 - 75 nS maximum. Any period of linear operation, even for 100 nS can cause current hogging, usually to the center of the die, and the transistor fails shorted. I had a great (and REALLY frank) discussion with an International Rectifier applications engineer who told me all the stuff that they don't publish in the data sheets. It was very eye opening! After that, I did get a pretty nice 1 KW inverter running reliably.
Darlington bipolar power transistors have gone to that place in the sky with ignitrons, Mercury Thyratrons, etc. for a good reason. They can never reach the low forward voltage drop of a properly driven single bipolar or IGBT. They are simpler to drive, of course, but the power loss is a real killer.
Jon
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Thanks Jerry, the talk about SCRs exploding like hand grenades scared me a little bit, but I just might do this one day.
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
i
I like your idea of synchronizing the three output voltages by triggering them with a rotating machine. But, are you able to manufacture the three output voltage sources??
J


Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I am glad to hear it.

I do not understand the question.
The schematic is that input voltage will be converted to appropriate voltage, then rectified to DC, then rectified DC will be inverted by three inverters, according to what the rotating cylinder tells the transistors (120 degree shift). The inverted current will then be again transformed with isolation transformers, with three output legs tied with a common neutral.
Which parts of this scheme are you referring to?
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload


i
I wasnt thinking properly. I was thinking that you were intending to have three semisinewave voltage sources. Now I see it. You will have a DC voltage source that is supplied systematically to the 4 terminal three phase load.
Although *I* wouldnt have considered pulsating DC to be adequet for providing power to three phase machines, maybe it would work. Try it. I'd think the rotating source of triggering could be holes in a disc mounted on a 60 rev per second synchronous motor. LED light sources could provide triggering vlotages. If you do want to investigate the feasability of this, you could get any old 3750 RPM induction motor. It would provide a triggering rate that would remain fairly constant since its load would be constant too.
I am sure *I'd* learn something from your efforts. So, I volunteer any help I might offer.
J
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
a much simpler approach is to have a counter (base 3 would be good) so you can turn on each phase in sequence. Drive it with an oscillator and you have variable frequency. But, you will have incredibly nasty harmonics and it won't be kind to motors.
A "single phase" example of this kind of converter is the CD ignition schematic on my web page (www.wbnoble.com, look under "articles I wrote") - it works nicely, I built it because I couldn't find anything commercially, but it has a major amount of RFI - multiply the power levels by 1000 and you will seriously annoy the neighbors.
There are books written on commutating circuits and power inverters - you need to at least somewhat approximate a sine wave - 3 transistors per phase will give you 8 levels, that ought to be enough (think binary, 2**3 = 8), just add snubber circuits and logic
wrote:

triggering
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Mon, 14 Feb 2005 22:38:28 -0800, "william_b_noble"

I don't know very much about electronics but couldn't the transistors be set with epoxy to act as feet for the motor for a rotary phase convertor? ERS
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ignoramus17647 wrote:

WHAAAAAT??!!! You are going to control power semiconductors with mechanical switches?

Yes, but the first "arc" really a bouncing contact, will destroy the transistors. So, that's the end of that! You need to turn these transistors on HARD, and then off HARD, to keep the power losses within the limits. So, the base junction needs to be driven with several amps that has a rise time of no more than one microsecond, both on and off. You will never do this with mechanical switches.
A "regular small 110V motor" sounds like an induction motor, which is NOT synchronous to the power line. Your switching control would need to be synchonized to the power line to generate the 3rd phase.

How long are your mechanical switches going to last at 60 operations/second? If you can get 1 million operations, that is 277 hours, or 11 days. I'd suspect a mechanical switch will not last even that long at such a rate.
Jon
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

That's what I thought of doing, not knowing much better.
I defer to superior knowledge of more informed individuals.

Any way around this that is compatible with the mechanical approach? Ergo, place some quick electronic switch that ignores the first "arcing" instant, but then switches the current to the semicondctors HARD?

Since I am going to invert DC, synschronicity with the power line is of secondary importance (I am not going to make power line one of the legs of 3 phase)..

I am not sure why, given that I want to invert DC.

This is an excellent question. I am thinking, that what I want to do is similar to "brushes" on electric motors with brushes, and these last a long enough time. Maybe I can use same sort of brushes.
Igreatly appreciate your input, please do not hold back!
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ignoramus17647 wrote:

OK, I thought that you were going to try to make a real phase converter, and just supply the 3rd phase. That would make some sense, since you can't buy one that I know of. But, now, you are just trying to make a VFD! Thatr is totally insane. If you know where to look, you can probably find them in dumpsters! Sell your SCRs or Darlington transistors on eBay, and use the money to buy a commercial VFD! It will work, all yuou have to do is connect the wires.
Having developed power electronics myself, is it really worth a year (if you are not experienced in power electronics, make that about FIVE years) to develop this? What happenbs when you blow out all those transistors? Do you abandon the project, or do you plow ahead, buying more transistors, etc. and keep trying? Do you have a digital storage oscilloscope? You certainly need one, as you will need to record the signals as the transistors blow, so you can determine what went wrong.
When I did my 1 KW inverter, I blew somewhere in the neighborhood of FORTY $8 power transistors before I got the startup problems under control! I learned something every time one blew, too!
Jon
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Well, I hate to say it, but your opinion makes sense from a practical POV and, quite possibly, I will do what you say -- sell my transistor stuff and just buy a VFD whenever a need arises. (or perhaps buy a nice .22 gun with the money)
So, I want to thank you for your frank opinion.
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ignoramus29573 wrote:

I have another suggestion. You seem to be interested in learning about electronic design. Take a smaller bite. Your transistors might make excellent power amplifiers for your stereo. This way you're lots less likely to fry the transistors, and there will be no chance of accidentally leaning on the 300VDC buss.
Steve who still remembers how much that hurt

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.