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Help: 3-phase sine generation lookup-table microcontroller

Discussion in 'AVR' started by abicash, Oct 15, 2013.

  1. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    It's been a long long time since i worked with three phase synth sine wave converters, but it seems that if we designed a half bridge that puts out a synth sine with a fictitious ground at 1/2 Vcc, we'd have one phase done. The next phase would be the same exact thing except the pattern offset by 120 degrees, and the next phase by another 120 degrees. But then the peaks would be limited to 1/2 Vcc rather than Vcc which we get with a single phase H bridge. So maybe we'd want to think about creating the pattern distinct from the single phase approach, unless maybe we're using an output transformer.
    Not sure how you guys are approaching this here though. Maybe you can enlighten me.
     
  2. misterT

    misterT Well-Known Member Most Helpful Member

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    True, MrAl. At one point I was saying that you need to design and spec the overall system first (before writing code). There are many ways to drive the half bridges. I also linked an article that show the waveforms of each phase if you want to get the most out of the system.
    But, at the end I went along with the "straightforward" way.
     
  3. Mike odom

    Mike odom Active Member

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    I guess I'm too used to using an assembler as opposed to a compiler... too much thinking like a micro...
     
  4. dave

    Dave New Member

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  5. abicash

    abicash Member

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    Hello misterT :)

    I have connected these drives to my IGBTs through a driver card .
    IGBTs are not loaded. I applied an external DC voltage (+50V) to the bridge and checked cold firing, and i am happy that there is no cross over conduction and everything seems to work fine.
    Currently am applying a delta bridge of 100w lamps to the outputs to check functionality on loaded conditions.
    Should work okay ;)

    EDIT : Yes it does !!!!!:D
     
    Last edited: Oct 18, 2013
  6. abicash

    abicash Member

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    Hello

    Glad to post that i tested this on a 3PH Induction motor and everything runs okay :)
    Thanks for helping me out :)
     
  7. abicash

    abicash Member

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    Hello again

    I made a final board and tested it last Saturday on a drill machine 3-phase Ind Motor.
    I tried applying some extra torque to the shaft by lightly holding it with my hand.One of the IGBT's blew in this experiment.
    I replaced it and monitored the waveform with the inverter in loaded condition.
    I see some lines out of place of the main waveform cap(s) .I presume this is the back-emf.Since they are absent when inverter is not loaded with motor load.
    I have 2us dead-band in vertical limb.(i will try capturing some waveforms.)
    Can someone point out what this is?
     
  8. abicash

    abicash Member

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    Hi
    I am attaching 2 waveforms
    Both across R-Y phases with a 3-phase loaded motor.
    I see back-emf pulse where there should not be any pulses

    I changed the PWM freq to 8KHz (from 16KHz) , still the problem persists.
    How to mitigate this issue? can anyone help? misterT?
     

    Attached Files:

  9. abicash

    abicash Member

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    Here is a better waveform.
    It sure is BEMF as i understand, or am i wrong?

    Can someone please help me on this?
     

    Attached Files:

  10. JimB

    JimB Super Moderator Most Helpful Member

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    Looks ugly doesn't it!

    My first thoughts are that something is oscillating at a high frequency, a bit difficult to be sure without having hands on the scope.

    My first thoughts would be to try some low(ish) value capacitors to kill the oscillation.

    JimB
     
  11. abicash

    abicash Member

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    Hello JimB

    Thanks for replying.
    I have 0.22uf x 3 capacitors across the bridge (across each limb) along with a 10000uf 600v electrolytic capacitor.
    What is the standard procedure or values of capacitors to be put across the bridge?

    Do you think this waveform is harmful?
     
  12. Mushi

    Mushi New Member

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  13. Mushi

    Mushi New Member

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    MisterT could you please help me out with varying the duty cycle of the three pulses that are 120 degrees out of phase?.. i know how to generate them but i don't know how to change the OCRx value to vary the duty cycle.. your help will really mean a lot! Thank you
     
  14. Mushi

    Mushi New Member

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    Abicash can you please help me out. I am working on the same project but it is not working out properly.. i want to control the speed of the 3PH induction motor too but i am failing to do it! Thank you in advance..
     

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