Phase converter idea?

Can you explain in which part the heat will be lost?

Obviously, transforming 250V into 208V will use up some heat, also 2 isolation transformers will also dissipate some heat. But what else would waste energy?

i

i

I have messed up again. I assumed that you had already figured out all the complex electronics. I dont know how to design these things. I have enough trouble trying to fix them.

Did you notice the shipping charges for thet "cheap" 1 HP 3 phase motor?

I'm pretty sure you can make your idea work. And, I'd also bet that it'll keep you busy for the best part of a year learning how to get it working right. All the guys I know who are able to design devices like this had spent years and years in school and designing circuits before making complex systems like this 3 phase power generator.

Jerry

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

The biggest loss would be in the transistors. The best (theoretical) efficiency you can hope for in the method you describe is 50% just in the transistors, (class B amplifier) so if you're producing 1000 watts you are also dissipating 1000 watts. The rectifier(s) will also dissipate some.

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

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

This photo gives a clue about the amount of heat to be dissipated...

That's a 1.5kVA, 3 phase AC power source I just sold. It was really just a 200 pound, 3 channel, 45-1000Hz amplifier. Manuals are still available from California Instruments and would be helpful to anyone seriously considering building such a beast.

Ned Simmons

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

calculate power dispation in the transistors, then you will see why this isn't done very often

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

Ya know, instead of switches, you could mount a few magnets on the wheel and whizz 'em past little coils. Then use that induced current to switch your transistors. Might have to set up a cascade of transistors to step up from the wee induced currents to something that could drive your big transistors cleanly.

Hey Pete,

Worked OK for me, so here is one of the Item numbers, then do a search for Sellers Other Items.

3875488109

or maybe just try the lathe.....

Take care.

Brian Lawson, Bothwell, Ontario.

XXXXXXXXXXXXXXXXXXXXXXXXX

By the way, you can't build an inverter with SCRs. An SCR can only be turned on, not off, on command. Once you turn it on, it will not shut off until the current through it goes to zero. For building an inverter, you want power devices that can switch on _and_ off in the kHz range in order to PWM all the phases you are trying to drive. SCRs are fine for dimmers and heaters, and if you look at designs for those you will see that they rely on triggering the SCR at a variable phase angle, but it is always variable moving back from the zero crossing.

So you want to be using IGBTs probably, although you can also do it with FETs. As other people have mentioned, there are _lots_ of details to take care of in this seemingly simple system. Try building a good variable speed DC motor controller first, since that is sort of like

1/3 of what you are eventually going to get to.

-Holly

Holly, Maybe someone "lost the recipe" since SCR circuits have been used ever since they were invented with commutation circuits. There are even application notes on using SCR drives for DC motors driven from batteries. My brother used to have an inverter that took 24 volts and generated 240 volts AC and the active devices were in fact just two SCR's. That was his power source in the Alaskan wilderness. It had to be reliable.

Now, in todays modern world, there are much better solutions, but don't believe that they can't be used for inverters or commutated once turned on.

Pete