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Battery current = the phase currents??

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  • HarryS
    replied
    Sorry for the non relevant reply, but I like to think of this as magic. I have an EE degree from long ago and no desire to go back to my college textbooks, not that I ever learned much about 3 phase motors. Unless you're facing this question in a term paper, exam, or perhaps a job interview, why worry? A bunch of smart guys in Suzchou will sell me a controller for 50 bucks.

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  • max_volt
    replied
    If you made a device that had more power going out than in you would be rich beyond your dreams.

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  • nfmisso
    replied
    Originally posted by Edd View Post
    I know this is a real simplification as this is not how the controller works but as an illustration that i can't see how the current can be larger through the phases. I1 = I2 + I3.

    I2 or I3 can't be larger than I1.

    Click image for larger version

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    Sorry this picture is not anything like what the controller circuit looks like, even super simplified.

    Take a look at this: https://abm-website-assets.s3.amazon..._Final-web.jpg which is a simple BLDC motor driver.

    The total time averaged power going into the motor is the same as the total time averaged power coming out of the battery minus controller and wiring loses. But at any given instant the voltages accross the phases and the current going into (or out of) a phase has little correlation to battery current and output voltage.

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  • Shannon Xue
    replied
    Have you got a reasonable explanation? I am facing the same problem.

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  • AZguy
    commented on 's reply
    If they buck the voltage lower before applying it then just like a transformer the power stays fairly constant. If they don't buck the voltage then the motor current won't exceed the battery current.

    Admittedly I don't which if any controllers buck prior to applying the voltage.

  • Edd
    replied
    I know this is a real simplification as this is not how the controller works but as an illustration that i can't see how the current can be larger through the phases. I1 = I2 + I3.

    I2 or I3 can't be larger than I1.

    Click image for larger version

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Size:	1,009.7 KB
ID:	56805

    Leave a comment:


  • Edd
    replied
    hi, thanks for your response.

    i hear what you're saying re power in power out. But that applies to a transformer where voltage is boosted and the current is dropped and power is maintained. But this isn't a transformer

    The current leaves the battery and goes through one of the mosfets through one one phases and then through the other phase and back to the negative terminal of the battery. Current in a circuit cant be higher at one point than another point, unless it splits.

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  • Edd
    started a topic Battery current = the phase currents??

    Battery current = the phase currents??

    I have seen so many forum discussions talking about battery current to phase current ratio and I still cannot see how the phase currents added together does not equal the battery current.

    The dc batttery current ultimately gets split three ways via the mosfets to the phases. The current through the battery has to equal the currents in the three phases added together.

    So so how can phase current be higher than the battery current?? Where is this extra current being supplied from when it’s not coming from the battery?

    I appreciate the phases will have inductance etc. Take a simple circuit with an ac voltage a singular inductor. The current lags behind the voltage but at no point can the current through the inductor be any different to the current in the rest of the circuit.

    Am am I missing something obvious here? Any explanation would be great to hear.
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