Hmm, won't sticking ferrous metals into the coil change the resonant frequency? Increasing L means decreasing Hz. Thus if you're not careful this thing my over run the transistors with no load.
IGBT = insulated gate bipolar transistor. They are a hybrid of bipolar transistor and field effect transistor. They are used because of the high current, high voltage ratings and fast switching.
Yea, I know what you mean about not finding the information you are looking for on the web. The web has its limits. Induction inverters are a more obscure technology for the hobbyist. Lots of simpler circuits available with good explanations. But for something that is more complex, difficult to build, hard to implement, high power, prone to failure, more expensive... the web has its limits. It could be the people building induction inverters on the web have little knowledge themselves, or are just too lazy to detail everything or assume other people know what they are talking about.
I spent some time looking for a design for a variable frequency drive. VFD shares some characteristics (in a rough sense) with induction inverters. But there really aren't may people on the web building VFDs and publishing all the details. I still want to build a VFD, so the only thing I can do is research the basic theory and start designing. If a "canned" solution for what you want is unavailable, you have to design and build it yourself. If you are really interested in researching, then visiting a good college library would help.
At some point you will have to know how to design something and have a good sense for what is reasonable. At least have enough knowledge to get in the ballpark. A 5 Henry coil is a huge amount of inductance, for example. Maybe useful as a 60hz power supply ballast.
One idea for you: Not sure if it would work, but there are cheap induction cooktops, ~1500 watts, made in China, of course. You could probably buy one of these for about $100. Hack it to your own purposes. Rewind the coil, if necessary, to a shape that can heat your iron rods. I won't say this would work, or that it would be easy, or if 1500 W is enough power, or it won't fail. Its one possibility.
If you can NOT buy it or can NOT afford it then you have to build it. I bought a VFD to run a stretch wrap machine at work. The VFD only cost about $125. it runs a 1/2 HP single phase motor. I would not attempt to build a VFD for that price unless I just was building it for fun and a learning experience.
Here is a circuit that would probably work for me. What is matching inductor and how many turns does it have? No power supply voltage listed either can I put a bridge rectifier on 120 VAC and power it with 169 VDC? What about that capacitor that is in parallel with the DC power supply? How many turns is a 1.5 uh coil? This is a good example of circuit drawings with MISSING information.
Right, I could buy a VFD for some cheap amount. But most of them are way over-rated for what I want to do. I want to power a 70 watt motor at 400 hz, 120v. Besides, I take an interest in building these types of things. I think a small design, to power a small motor, is within my abilities as a hobby. Maybe you have the same idea: its interesting to build and learn.
Grab an irs2453d and for $2 you have a full bridge driver. I have used them through multi kilowatts at 200 khz with external boot strap diodes and external complementary pnp + npn gate drivers. However you don't need any extra parts if you keep to single digit nF mosfets and below double digit Khz. (cheaper than winding gate transformers.)
With 100uf on the boot strap capacitors you should have no problem running that chip all the way down to single digit Hz if you wish, just feed a square wave into the CT pin but don't let the ct pin go below 1/6th of vcc. To drive it i just use a couple transistors and two resistors to get me a level shifted square wave going from vcc to 1/4th vcc.
If you can ensure the duty cycle stays within .5% of 50% duty cycle then you won't need a capacitor in series with the H bridge for small motors.
All the 400 hz motors i have are 3 phase, so i haven't tried to drive them with a square wave single phase inverter. 60hz induction motors don't seem to mind, but i haven't tried driving them beyond 50% nameplate ratings either.
Three phase:
I do recall there's a circuit that uses three opamps to make three phase ac, but it only works at one frequency and the three opamps make a loop.
If you were to take three 400hz sine waves from three opamps, feed them into three comparators, each receiving a 1-10khz square wave, you could get three 0-100% pwm signals which could then be sent to three half bridge drivers, pipe the output straight into a motor, or alternatively into an LC filter to kill the high frequency.
I suppose the challenge is that you really want the pwm to stay between 20% and 80% to keep everything linear; and all three sinewaves need to have exactly the same voltage to kill any dc bias from working its way in thtere.
Aside from the 6 switches needed, the parts count for a fixed frequency driver such as I just described could probably come in under $20.
But, the frequency generator is fixed in frequency and duty cycle. Looks like it could be useful for the OP who wants to build an induction system and is considering a full bridge circuit.
This 400 hz motor I have is single phase, but needs a run capacitor. Probably designed to run off aircraft power. One experiment I tried was to use a "modified sinewave" DC-AC inverter. The type that plugs into a 12v cigarette lighter and provides 120v 60hz. I found that my inverter uses a 556 timer to generate 60 hz timing pulses. It was pretty easy to modify it to produce 400 hz. Just change the timing capacitors. This worked, but I don't know how well a motor of this type can tolerate the rectangular pulse waveform. I was thinking of building a small VFD mostly as a learning experience.
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