3 phase varriable or set frequency drive for an induction motor

Discussion in 'The Projects Forum' started by alobko, Aug 28, 2012.

  1. alobko

    Thread Starter New Member

    Aug 27, 2012
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    First off let me say I'm new to working with 3 phase motors and current so don't be too angry with my not understanding. I need someones help to design a variable or set frequency drive to power a 3 phase induction motor, and from what i understand there are a few logical ways to do so. The method I see as most worth pursuing is this: [​IMG] type of circuit, obviously where the transistors are controlled by a variable or set frequency drive. What I am having trouble understanding is how to go about the later part, how to go about creating a variable frequency drive that does the 3 sets of 2 oscillations which cause the 3 phase current. Most people say that you need to use a PIC micro controller for that, but I have found it hard to understand the bits and pieces about a) which PIC's should be used b) how to use them (as in program and connect them) c)if there is any alternative to this (some sort of oscillator with a delay circuit????). I have experience programming and some other types of circuit design but that helps me little with micro controllers for which this would be the first project I ever used this type and for this type of task, so a) please don't be harsh b) help me out if you can, I can give you alot of specifics about the actual project if need be, its also not especially high current or voltage, and I don't want or need any sort of feedback or breakdown control from the motor. I literally would benefit from as close to a full set of instructions as possible, from source code to program the micro controller to how to actually program and connect it, I've never used a PIC before...

    Thanks in advance for any help you can offer!
     
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    If you are trying to do this yourself to save money, good luck. The price of a ready made VFD is so low these days, just the price of the components will be more than a finished drive. And this is not a beginners level project.

    By putting your country/location in your member information people can give more help to you.
     
  3. alobko

    Thread Starter New Member

    Aug 27, 2012
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    shortbus, Thanks for your advice, but the reason I'm asking to make this is because what I'm looking for seems to elude me. I need an extremely small VFD for this, as in something that weights less than 100 grams, and is solid state, most vfds or 3 phase converters seem to all use a 3 phase motor in them already. Also the dimensions I need this to be at must be extremely small, this is meant to power a single induction motor and at relatively low voltages and currents. Trust me, I looked buying one, but unless you can direct me to one which is 100 grams, meant for 6-12 volts, and is smaller than 5x5x5 cm^3, I need to make one. It doesn't even have to be especially good at controlling heat, it will run less than 30 seconds at a time. The fact is, I am only looking to make this because I can't find what I need. Any chance you and other people could help me construct one? This need not be anything fancy, basically as simple as it can possibly get in terms of a VFD...
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Are you committed to 3-phase motor? DC sounds more logical to me.
     
  5. alobko

    Thread Starter New Member

    Aug 27, 2012
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    I am committed. In fact, the reason it has to be such is because its going to be a very small linear induction motor... Has to be for this specific task... Unfortunately I have some very strict weight and size limitations, so I see the only solution to make one, and since I am not very experienced in this specific type of thing, I'm here asking for help/instructions/guidance, anything people can do to help... So if its not too much to ask can I ask any future posts in this thread to be not about convincing me that this is hard since I already realize such but somehow helping me get from Point A to Point instead? Thanks!
     
  6. BSomer

    Member

    Dec 28, 2011
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    When you say "meant for 6-12 volts", is that AC or DC, Input or Output?
    What size of motor are you planning on working with (HP, Voltage)?

    The smallest VFD that I have worked with was probably rated at 1/2 - 1 HP @ 240VAC. I don't think they are made smaller than that mainly because there isn't too much need for a motor that small in industrial applications. If they want something smaller they generally go to servo motors.

    What you are going to need, in basic terms, is a micro-controller with enough I/O pins and PWM capability to control some opto-isolators. The optos are connected to the gate pins of the transistors that are on the high and low side of each phase. There are IGBT modules available that have all the transistors and protection circuitry inside, these are commonly used in VFD's instead of separate transistors. This option may help to reduce the overall size of the VFD.

    As for the programming of the micro, I am of very little help. I get a headache just trying to think of how to start something like that.
     
  7. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Theres a reason that things like a VFD are the size they are. The heat from the electronics has to be dealt with and this takes mass.

    Give more information on the motor, volts, amps, HP. Just one half bridge module for a VFD weighs about the same as you want for the whole thing. And it takes three of them for three phase.

    Giving an idea of what your trying to do may get you some other ways of doing it. Some pretty smart guys on this forum. From a large area of backgrounds.

    Also sometimes you just can't get there from here, in regards to your "get from point A to point".
     
  8. alobko

    Thread Starter New Member

    Aug 27, 2012
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    More details: This is to power a linear induction motor, so i don't know how HP would be measured for that, but there is a reason I'm asking for something this small. The reason I don't care much about a heat sink nor do I need an substantial weight is because this is for a very very light weight application, this is not meant to put out much power. The reason I'm not worried about heat is because while there will be some sort of heat sink in place, this whole contraption is only meant for running in short pulses, ie less than a minute at a time, usually under 10 seconds. I'm looking for a current not exceeding 5 amps, a synchronous speed which I am unsure of but I will adjust as the task sees fix since I am unsure of the optimal one in this case, but not exceeding 900... so frequency of current should be possible to adjust. And the previously listed voltage is for DC input. I do hope I can get off with using lighter and less bulky half bridge components ie power transistors. What I'm here mostly asking help about is the micro controller and how to get it to do the correct pwm and program it to do what i'm asking. I have little understanding of how to actually implement the portion with the micro-controller, the half bridges I know what I need to do, its the micro-controller which bothers me because I'm unsure how to make the 3 separate phases and whatnot. This is primarily a microchip issue I'm asking about...
     
  9. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    try to think of a way you can put a gear track or anchor points at the end for belt drive on this device. If it is so small plastic would be a good candidate for this, or aluminum.

    Going to an existing off the shelf solution for a motor will greatly assist you in keeping this small and lightweight
     
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    OK try one more time. What is the make and model number of your motor? Do you have a link to the motor so we can see what you need to drive it?

    Three phase covers a lot of different things. When talking induction motors, your in the hundreds of volts. Then you have BLDC motors that are DC but also three phase.
    It's not realistic to expect to get 220V 3PH AC from 6-12 volts, and expect it to weigh 100 grams.

    We/I would like to help, but your not giving much information to help you.

    Edit, Kermit was typing while I was, but what he was meaning is something like this - http://www.youtube.com/watch?v=fzLz48vXPIc It would just need a stepper motor driver and programing. And ready made ones can be found on Ebay.
     
  11. alobko

    Thread Starter New Member

    Aug 27, 2012
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    Thats just it. This is not a motor I'm buying, its also being made, but by a colleague of mine who is more qualified in those areas. This is not an industrial project, this is actually more of a pet project to try out an idea. And in this case, it has some odd requirements. The motor is NOT operating at 220 VAC or anything of that sort, it is operating at the same voltage. Also the reason i'm skimpy on details is because they are not set in stone, since this is not a device being bought, it is all if need be subject to change. Sorry if i didn't make this clear to start. I hope you all can still help me, and like i said, most of the motor's specifications are subject to change except the current in it and the synchronous speed range.
     
  12. shortbus

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    Is the "motor" a true induction linear? In doing research for a motor I'm building, a switched reluctance motor, there was a lot of information on the web about "linear switched reluctance motors". And the "inverters" used to drive them. And they can work at low voltages. Google, linear SRM thesis and you will get a ton of stuff to read.

    The reason to keep after you about this is to help you. Linear induction motors are usually very large and must move at high speeds. They are whats used in the newer roller coasters and other thrill rides. Plus the 'mag-lev' type trains.
     
  13. alobko

    Thread Starter New Member

    Aug 27, 2012
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    Actually, this is similar to a sideways maglev in many ways. Thats the whole point... It is in fact necessary to have high speeds and a true linear induction motor... And while this is relatively low in terms of synchronous speed compared to most LIMs it is still quite fast. The reason that it cannot be a SRM is because there is no control over the affected surface, except the knowledge that is it in fact aluminum and of a certain dimension, but I cannot change it in any way as such... I researched all of these things before realizing that a LIM is the only possible solution... and I know they are usually large, which is why it will be built on a smaller scale than commercially available ones, and have a far smaller air gap, in the range of a millimeter, such that the magnetic field is sufficient at that distance for the induction motor to function as some of the larger ones can function with larger air gaps between the induced surface and the electromagnets. I realize this is not an easy solution, but for my project, trust me when I say it is the only one. I am here to find out just how, and what I can do to make this work. I hope I explained why it seems like we are talking about awfully small things compared to your usual LIMs and 3 phase inverters (variable frequency drives). What can I do to make this a reality, as I as I have earlier mentioned have little experience with VFDs?
    Thank you for your help in advance!
     
  14. BillB3857

    Senior Member

    Feb 28, 2009
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    Take a look at this and see if it may meet your need. A Google search of "hard drive 3 phase driver" (without qoutes) yielded many results.
    Here is one of them..... http://letsmakerobots.com/node/2876
     
  15. alobko

    Thread Starter New Member

    Aug 27, 2012
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    Ok thats seems like it could work... It does output 3 phases with an adjustable frequency, my question is if it can handle 4 amps, and 6-12v because it doesn't specify... This could very well be what i need, even though it outputs a square wave it should be enough for a linear induction motor which if i understand right does not require a sine wave? Correct me if i am wrong please... What sort of current can that chip stand? Anyone else wish to confirm that this would suffice?
     
  16. BillB3857

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    Feb 28, 2009
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  17. alobko

    Thread Starter New Member

    Aug 27, 2012
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    That might be overkill, and not exactly what I need. I need to be able to directly control the frequency and hence the synchronous speed of the LIM, also, that is slightly too heavy for what I need. Also, as I mentioned before, I doubt I need something with such a heat sink since I will only be running for a very short period of time each time. But I will look into it. I did not realize that brush-less motor power supplies are all 3 phase... Especially considering I have not dealt with brush-less motors, but is it not true that the are not always induction motors and hence do not always require 3 phase power?
     
  18. shortbus

    AAC Fanatic!

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    BLDC motors are not induction motors. They do use three phase bipolar (+ and -) DC. Most are trapezoidal wave, not a true square or sine wave. They can be run by a driver that uses no micro controller. But they do need hall sensors when used as a prime mover. The model airplane sensor less drivers are only good for something that doesn't need anything but a non-positional control. What I mean is that if you need to stop at a certain position a ESC won't work.
     
  19. alobko

    Thread Starter New Member

    Aug 27, 2012
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    Well I don't, its more like I'm trying to get the 10-30 seconds of acceleration from my linear motor and then it shuts off regardless of how far it went... It might seem strange but its what need to be done. Would a 3 phase ESC be able to run a LIM? I'm only asking because I am unsure if trapezoidal wave can actually suffice for a LIM...
     
  20. alobko

    Thread Starter New Member

    Aug 27, 2012
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    Is it possible to use a brushless ESC for a linear induction motor? Can someone tell me if a trapezoidal current is going to work? Thanks!
     
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