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
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.
please reply to:
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.
IGBTs are a dream, from the performance standpoint. Unfortunately, they
NIGHTMARE from the tricky design details view. Yes, they definitely can be
made to work reliably, but just wiring them up, without attention to
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
of the die, and the transistor fails shorted. I had a great (and REALLY
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
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.
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
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 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 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
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
I am sure *I'd* learn something from your efforts. So, I volunteer any
help I might offer.
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
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
WHAAAAAT??!!! You are going to control power semiconductors with
Yes, but the first "arc" really a bouncing contact, will destroy the
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
to be driven with several amps that has a rise time of no more than one
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
a mechanical switch will not last even that long at such a rate.
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
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!
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
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
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
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
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
something every time one blew, too!
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 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.
who still remembers how much that hurt
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