Phase converter idea?

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

Tim

The biggest SCRs I have are marked IR A2 88-4208. They are about 2 inch diameter and 1 inch high, hockey puck style. I am completely ignorant about the speed of these things.

Jerry

Huh? Unless he has a degree in electrical engineering, with a strong background in power conversion circuits, he will convert all his transistors back into sand, and then some! This stuff is REALLY quite tricky, and every wire (or bus bar) has parasitic inductance that will drive you crazy (and burn out components) at every turn! I go through this in designing some of my products, and still find it quite difficult. If he doesn't know how to write loop and node equations for electrical circuits, and have a foundation in AC circuit analysis, he will never get anywhere with this, except possibly hurting himself. I have had MUCH SMALLER circuits explode on me, with fragments embedded in the walls. I would really be afraid of working on gear this big without extensive safety precautions like ear protection, face shields, even protective walls.

It is insane to try to learn the intricacies of power electronics when a VFD is available off the shelf for a few hundred $.

Jon

To build a phase converter, ONLY, not a VFD, you need a 90 degree phase shift, and a Scott Tee transformer. I tried to design a very simple, all passive converter this way, but ran into a problem. It would draw a HUGE imaginary current from the mains to work. You could build the inverter stage for this with SCRs, but that is REALLY doing it the hard way. First, you'd need "inverter grade" SCRs, not standard-type ones. These turn off much faster. The commutating capacitors needed to turn these SCRs off will likely cost more than a 5 Hp VFD, brand-new, with factory warranty! That's why manufacturers stopped using SCRs in motor drives almost as soon as the first big power transistors came out in the late

60's - early 70's.

Jon

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

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

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

I wonder how long it will be before this thread gravitates to perpetual motion? That concept may be more easily grasped than some of the ideas set forth.

Bob Swinney

Sure, but how do you plan on ensuring the three inverters stay in sync?? Greg

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

Then there was the talk a friend of mine gave at work one time "How to build a power amplifier without going blind". I had the good fortune to be wearing goggles when the power fet in the circuit I was working on fried. It didn't explode, but the 5V chip driving it did when it got hit with 300VDC.

Steve

Insulated Gate Bipolar Transistor. Google IGBT for more info.

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

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

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

I have another idea. Scratch the idea about having a rotating cylinder with brushes. What I need is a small 1 phase motor (1/3 hp) connected to a small 3 phase generator (again, low power). If the 3 phase generator generates 1 or so amps, the transistors could amplify that

85 times, and I get a smooth wave on top of that. The result would be 70 amps of 3 phase, enough for more or less anything. All I need is a source of clean DC, AC motor ($15), small 3 phase generator ($???, maybe I can make it from a regular 3 phase motor), the transistor units, and 2 isolation transformers. i

i

Good idea. Make it and send some reports.

Jerry