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Variable frequency drive

Hello there instructables,
I have another question discussing maglev theory and i was directed that the true way to build this project was to use 3-phase power!
So this question, although related, is also an entirely new project in its own right!
Now to the point, i have been researching single to 3-phase conversion, and there are a few options available like rotary or static phase converters, but i have been intruiged by a device called a variable frequency drive!
Okay so what i understand so far (i think) is that these convert ac to dc, run that dc through a H bridge rectifier using igbt's and then convert the 3 outputs back to ac...... okay assuming my analogy is correct so far, to my question,

1. How are the igbt's triggered in the h bridge? do they use a function generator with altering timesteps to create the seperate phases?

2. Also how are these kept in sequence with eachother?

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steveastrouk5 years ago
1.) They're triggered by a microcontroller.
2.) The program in the microcontroller is perfectly adequate for locking the phases together.

In a VFD, things are also complicated by measurements of phase angles to ensure the motor isn't running close to slipping.

Xmortx114145 (author)  steveastrouk5 years ago
Would the slip be as much of a problem as ill be using it to run a linear induction motor?
There was me thinking that i could use 555 timers to trigger the igbt's! Couldnt be that simple or everyone would do it instead of buying!
5 years ago
Slip's an issue in all induction machines, but ultimately here, it limits the mass of the payload.

And no, you can't use 555s You really need sine waves to get good results and not waste huge amounts of power.
Xmortx114145 (author)  steveastrouk5 years ago
Okay i understand the issue of slip, but in my first prototype ill be using low power with no load whatsoever merely for proof of concept and then building apon that, but in the interest of the discussion i also want to find out as much as i possibly can so i have the knowledge base to build on my prototypes in the first place, where would you suggest the best place, book or rescource to find the information i need to do this? I understand the H bridge but i thought the igbt's would be triggered by dc waves then the outputs would be changed back to ac?
5 years ago
VFD design is an extremely complex topic, and not one for a casual amateur to just knock up easily. Microchip have a paper introducing some of the design complexities From that you'll see the kind of things that are involved. If you wanted to study VFD design, sure, go for it. If you want to get round to making a linear induction motor, then buy a VFD.

THE guru on linear motors was Professor Eric Laithwaite. You'll have to hunt for his books, but he knew his stuff.
Xmortx114145 (author)  steveastrouk5 years ago
Oh i did buy one as you suggested and have had a bit of success with the linear motor, in term that i made an aluminium tube move around 4 feet before my track decided to self destruct as the homemade coils werent up to the task, thats not what this is about at all! The vfd will be used for this purpose but im trying to design the track to be transportable with as few external components as possible, now i could hack apart the one i did buy and integrate it but it seems such a shame to destroy something like that when i could learn the science of how they work and have an endless supply of frequency converters for future projects that may need them, keep in mind this is my idea of fun and strange as it might seem i enjoy learning the science aswell as playing with the new gadget after iv completed it! As for laithwaite, i wish i could afford to spend upwards of £40 on a book but my better half wont allow that because i think it may be on the cards as a xmas pressie :-)
5 years ago
The science of how it works is predicated by a degree in electrical and electronic engineering. If you have one, then the Microchip paper is an Aide-memoire, and you can use it to block out an arbitrary real system, without it, its like a book with pages missing.
Xmortx114145 (author)  steveastrouk5 years ago
Yeah some pages are indeed missing but i have found the code there very useful :)