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A new type of BLDC motor drive?

Discussion in 'Sensors and Actuators' started by eem2am, Mar 7, 2014.

  1. eem2am

    eem2am

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    Aug 3, 2009
    Is there such a thing as an inverter fed BLDC motor being supplied by a current source? That current source taking the the form of eg a Current output regulated buckboost converter?
    Why would this be done?
    How do you calculate the RHPZ frequency for the buckboost given that the load is a BLDC inverter drive and the BLDC itself?

    RHPZ of CCM Buckboost = (D'^2*R)/(D * L)
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    what research have you done so far to find any information ?

    do some searching and if there is anything specific in the info that you dont understand, post your question and a link to the article

    cheers
    Dave
     
  3. eem2am

    eem2am

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    Aug 3, 2009
    I have searched all over the web on brushless dc motor driving, and I have found nothing that relates to a BLDC being supplied by a current output regulated SMPS.
    I have found loads of articles on sensorless control of BLDC's, I have found loads of stuff on CSI FED BLDC's, and loads on VSI fed BLDC's, but have found nothing on BLDC's fed by output current regulated SMPS's. I am told that the drive is a VSI type drive.

    putting the following search term into google
    "methods of driving a bldc motor"
    ...reveals absolutely nothing about BLDC's fed by current output regulated SMPS's
    http://www.ecnmag.com/articles/2009/11/brushless-dc-motor-control-techniques

    ...could any reader just confirm with a yes or no answer that driving a BLDC motor with an output current regulated SMPS is a bogus method?

    ...I have no article on it, as I can find no article on it....we have been asked to do it like this, but I think its bogus
     
  4. eem2am

    eem2am

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    Aug 3, 2009
    I have discovered the ML4425 BLDC inverter driver, so is it ok to operate it with its motor current sense input grounded, and then use an external upstream current regulated SMPS to feed the inverter/motor? (as in the attached block diagram)

    That is, please advise if we can feed the ML4425 BLDC Inverter driver with an upstream regulated current source, instead of using the ML4425 to regulate motor speed and current?

    I suspect that the way that we are doing this is bogus, and that we are all totally wasting our time at this Dutch Engineering company?

    We are going to use the ML4425 BLDC Inverter driver IC to drive a BLDC in a water pump.
    The inverter obviously needs a big supply capacitor at its input, and we do not have room for this, therefore, we will instead put a 150KHz buckboost converter upstream of the inverter, and ‘feed’ the inverter from that. We will in fact use the buckboost converter as an output current regulated power source, and so it controls the current that gets fed to the ML4425 based inverter. We will feed whatever current to the Motor/Inverter that makes it spin at 8000rpm.
    The ML4425 is capable of regulating the motor current and speed itself, using its current sense pin (pin 1), etc.. however, we will simply ground the ML4425’s current sense pin, so that the ML4425’s internal current sense and current limitation is bypassed. (since we are limiting the current with our upstream buckboost converter)

    We have a closed loop on the motor speed, whereby the motor speed is regulated to 8000rpm, however, if in so doing, the motor current goes over 20 Amps, then our current limiter kicks in and keeps the current regulated to no more than 20 Amps. The current limiter function is done by the buckboost converter and its feedback loop.

    The Vin to the buckboost is 18-32V
    The motor will be ~34V when at 8000rpm, and will draw approx 20 Amps at this speed of 8000rpm.

    ML4425 datasheet:
    http://pdf.datasheetcatalog.com/datasheet/fairchild/ML4425.pdf

    Do you agree that we are wise to act in this way? We will experience high voltage spikes at the output of our buckboost converter whenever we are in the inverter commutation dead time intervals, but should that worry us too much? We do have an overvoltage clamp there (converter shuts down till Vout falls below 35V).

    Or would we be better off making the buckboost SMPS an output voltage regulated SMPS, with output current limitation?

    Alternatively, is there a Current Source Inverter version of the ML4425? (ie something like ML4425 which drives an inverter with switch “overlap time” instead of switch “dead time”)?
     

    Attached Files:

    Last edited: Mar 17, 2014
  5. eem2am

    eem2am

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    Aug 3, 2009
    Is this a bogus motor speed regulator?

    Hello,
    I have this thread here on this,
    https://www.electronicspoint.com/new-type-bldc-motor-drive-t267675.html
    ..so I apologise for this, but I edited it and it is too far down the list for people to pick up the edits..

    The basic question is, is the attached motor speed regulator block diagram a bogus motor speed regulation/drive method?
    The inverter switches at 500Hz with 200us dead time.
     

    Attached Files:

    Last edited: Mar 17, 2014
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    eem2am: please stay within your thread and do not open a new one for the same topic. I merged the threads.
     
  7. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

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    Aug 27, 2013
    eem2am,

    This is a bit out of my wheelhouse, and you obviously have a great deal more experience in motor-drive design than I do, so if this is too far off-topic, please just ignore it.

    In CNC servo drives "constant current mode" is the most common mode of operation...as the RPM varies with load, positioning is maintained by encoder/scale based feedback. Unlike your case, RPM is more of a "target" than a rigid specification. In the RC hobby BLDC motors are typically Voltage controlled with current limited only by the driver's ability to supply it...when comparing BLDC motors to traditional three-phase motors the primary difference that emerges is how they are driven...a traditional (synchronous) line-driven three-phase motor is forced to a specific RPM by pole count and line frequency, and the current is simply a function of friction + drive requirements...With BLDCs, "commutation" is typically based on rotor position and speed is controlled by a feedback loop that increases motor voltage until the desired speed is achieved. Soooooo....to employ a current controlled Boost-buck to a BLDC where a fixed speed output is mandated seems a bit counter-intuitive. The way I see it, the boost buck either has to be configured as constant current with RPM being ignored, or constant voltage with the PWM driver controlling current in an RPM based feedback loop...

    In your diagram, the boost-buck is configured as a feedback regulated current source implying that it can provide "infinite" voltage to achieve the specific current setting...obviously there are physical limitations on the actual voltage, and there are mitigating factors involving the feedback response time...assuming the boost-buck frequency & response time are >> than the driver's switching period it would appear that when the driver switches, the voltage to the driver/inverter would spike rapidly in an attempt to overcome the new-phase inductance and this could have a very undesired affect on the other active phase-leg if there is no direct feedback to the inverter/driver....If we assume that the boost-buck's response time is << than the driver's switching period then we have exactly the opposite problem, ie, the driver cannot provide sufficient current where it is required.

    So, my thinking is that if you want the Boost-buck to regulate the speed, then it would have to be via voltage based response to the rpm feedback. In that case the Driver/Inverter works in simple commutation mode much like the RC model BLDCs, and you can focus attention there on creating a Sine Wave output during any given switching period, and leave the current consumption a function of the boost-buck's output voltage.

    ...but, as I stated, this is a bit out of my wheelhouse....and perhaps a bit more "basic theory" than "help".

    Fish
     
  8. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    @ Fish
    Nice one.

    Adam
     
  9. eem2am

    eem2am

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    Aug 3, 2009
    The problem we face is buckboost output capacitor overvoltage at the inverter's commutation dead time intervals.......if you please, to summarise, before the next question..
    We are controlling a BLDC with an inverter (voltage source inverter) which is for a 550W water pump..for a tulip field irrigation system in Holland.
    The input voltage to the system is 18-32V, and the motor voltage at our required speed (8000rpm) is 34V.
    Therefore, we need a buckboost converter upstream of the BLDC/Inverter.
    We are using the LT8705 buckboost controller.
    The Inverter switches at 400Hz with 400us dead time.
    We are using the inverter in "dumb" mode, whereby it simply switches at maximum duty cycle all the time......the motor current is thus not controlled by the inverter, but instead , will be controlled by the buckboost converter, which is upstream of the inverter.
    The buckboost converter has a feedback loop which is closed around the motor speed, so that the buckboost converter can control the motor speed. A Frequency_to_voltage_converter converts the motor speed to a voltage so the buckboost can use it in its speed feedback loop.

    Our problem is that we have very, very little room for output capacitance at the output of the buckboost converter. Thus , during the dead time intervals, the Vout of the buckboost starts skyrocketing, and is in danger of overvoltaging the 50V ceramic capacitors there….therefore, we have to temporarily stop the buckboost from supplying current during the dead time intervals…..and then resume the buckboost operation at the end of each dead time interval.
    (the dead times come round every 2.5ms)

    When we temporarily shut off the buckboost during the 400us dead times, we want the Vc pin (compensation pin) of the LT8705 buckboost controller to “freeze” at its voltage level.
    Do you believe the best way to do this is to inject a voltage above 100mV at the source sense resistor pin (CSP pin) for the 400us duration of the dead time?…..we can do this by sensing the vout overvoltage which occurs during the commutation dead times and then applying the said voltage at the CSP pin….obviously the vout will fall again at the end of the commutation dead time interval as the motor/inverter will draw current from the output capacitor bank.

    LT8705 datasheet
    http://cds.linear.com/docs/en/datasheet/8705fb.pdf
     
    Last edited: Mar 22, 2014
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    That seems to be your problem right there.

    You just need a boost converter. The extra complexity of a buck/boost design is just making your life harder than it needs to be.
     
  11. eem2am

    eem2am

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    Aug 3, 2009
    ...well, not in our case, because we need the buckboost, because we must operate the inverter at 100% duty at all times...and as such, we must therefore regulate the current into the inverter from the converter that's upstream of the inverter...it must be a buckboost because sometimes (at srart-up) the motor voltage is low (eg less than 10V) , and then when at speed, its 34v.

    here is the article of knowledge which states why it is more efficient to run an inverter at 100% duty cycle...

    So, the following
    http://www.ecnmag.com/articles/2012/08/careful-designers-get-most-brushless-dc-motors
    ..just above the waveform graph, says that BLDC inverters run cooler (less switching losses) if driven at 100% duty cycle and with the DC link voltage regulated to give the required speed, incidentally, why is this?


     
    Last edited by a moderator: Mar 23, 2014
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    If you're running your buck boost at 100% duty cycle then you can simply remove it, it's not doing anything AFAICT.

    The quote from Dave Wilson deals with increasing efficiency in the motor. It doesn't discuss the efficiency considerations of getting this variable DC voltage.

    Of course, you're the expert.
     
  13. eem2am

    eem2am

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    Aug 3, 2009
    actually we are not running the buckboost always at 100% duty...its the inverter that we are running at 100% duty.....the buckboost varies its duty to suit the required speed of the pump
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yeah, OK. That makes more sense.

    I agree with the high frequency losses in the motor, and it also probably reduces RFI.

    You're going to get switching losses somewhere, I don't think there's any real gain there. Buck/boost is not the most efficient topology in some cases. If you need boost you can't afford not to have that I guess.
     
  15. eem2am

    eem2am

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    Aug 3, 2009
    Thanks
    May I ask another question about our water pump drive (its for a Dutch tulip field irrigation system)?
    The PDF below again shows the block diagram of our setup.
    ....it goes like this....VIN...Variable output buckboost SMPS....Inverter.....BLDC.
    The point is that our DC link capacitor is only 300uF, and we have no room for more capacitance than this.
    The problem is(?), that the resonant period of the DC link capacitor, and the motor coil inductance is just 814us....and this is less than than the inverter commutation period of 1.05ms.........
    therefore, do you believe that we will suffer overly high resonant currents in our BLDC motor coils?
    When the Inverter's BLDC is permanently on 100% duty cycle like in our system, then shouldn't the DC Link capacitor be big enough such that the motor coil commutation period is much less than the resonant period of the DC link capacitor and the motor coil inductance?
     

    Attached Files:

    Last edited: Mar 29, 2014
  16. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Why don't you bloody try it instead of keep asking if this will work. It is called engineering. Oh and stop posting stupid questions when we could be helping others that really appriciate our help.
     
  17. eem2am

    eem2am

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    Aug 3, 2009
    I will try it, I am building it now, soon the Tulips will hopefully be watered. Though may we suffer the hitherto mentioned resonances?
    We are now doing the buckboost PCB, next comes the inverter pcb, then all the glue logic,etc.
    Will our switch on be a happy time, or will our tulip fields go dry due to pump malfunction.?
     
  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Go back to your text books and determine how much the output voltage will sag across your 300uF capacitor in 400uS (or whatever time period the converter is off), presumably at 20A.

    Then you tell us if that's OK.

    Do you have any formal qualifications? Surely this stuff is completely obvious.

    The mind boggles.
     
  19. eem2am

    eem2am

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    Aug 3, 2009
    Yes but that wasn't the question here.......during the dead times, the Vout actually rises as the speed feedback loop cant suddenly brake the buckboost quick enough.....we have overcome this by shutting the buckboost off temporarily when the voltage rises above a threshold. Here, I am referring to the resonant period of the DC link capacitor and the motor coil inductance, and comparing this with the commutation on time period.......................in this case, the commutation on time period is longer than the resonant period already mentioned, and this, I declare inquisitively, means we will see overly high resonant currents in our motor coil.?
     
  20. eem2am

    eem2am

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    Aug 3, 2009
    Do you believe that our modus operandi is , from a practical viewpoint..bogus?

    To summarise, we are using a buckboost converter based current source to regulate the speed of a BLDC.
    The output current of the buckboost regulates the BLDC speed.
    The Inverter that feeds the BLDC literally just alternately switches the coils of the BLDC to the output of thebuckboost......the inverter does not limit its duty cycle to achieve the required speed...the inverter just simply switches at max duty cycle all the time.

    It is the first of April today.
    Do you think that this system that I have described here is bogus?

    ...I mean..... to prevent LC oscillation in the BLDC coils, the buckboost feedback loop bandwidth would need to be 15KHz plus

    This is because the motor coil inductance is 56uH and the buckboost output capacitor is just 200uF.....due to room constraints.
    A 650W , 15KHz buckboost with 18-32Vin is not practically possible in a short time frame

    I see we are on a road to nowhere here...do you agree?
     
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