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What is needed to drive brushless motor?

  1. Feb 2, 2012 #1
    http://homepages.enterprise.net/jayjay/cdrom_motors.htm [Broken]

    So this is modified brushless motor.

    It says that it uses jeti06 ESC.

    But frankly I am not ready to sneeze 25€ for a ESC.

    How do you drive this in principle?
    You send pulses into those 3 windings, one after another, something like a synch machine?

    Can somebody give me an explanation what is happening, in a nutshell? I will chase the math later, I need the idea how it works.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Feb 2, 2012 #2

    jim hardy

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  4. Feb 2, 2012 #3

    So its driven by sine waves?

    But those microcontrollers are expensive, and they cost money too :(
  5. Feb 2, 2012 #4
    This is from that site, I provided.

    I thought it was controlled by PWM or something :/
  6. Feb 2, 2012 #5

    jim hardy

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  7. Feb 2, 2012 #6
    We went all through this before. So it is in fact a synchronous motor, inside out. Huh.

    Well, thank you mr. jim. As always only the from you.
  8. Feb 2, 2012 #7

    jim hardy

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    A hobbyist named Don Lancsater at Tinaja dot com back in the eighties used to drive old car alternators as stepper motors to make CNC machines for his workshop, called them "Flutterwhumpers".
    So it is possible .

    TI shows their Piccollo micros for just a few bucks apiece
    but how is a hobbyist to make such circuit boards?

    I'm way behind in this field. Serious hobbyists are always ahead of industry -

    try a google on "flutterwhumpers", or go here:
  9. Feb 2, 2012 #8
    Thanks, I will check it out. Reason I need this ESC is that I am building a quadrotor. And I need 4 of them, so a lot of money. This might get me started on building and designing my own ESCs. Thank you again :)
  10. Feb 3, 2012 #9
    I have built a BLDC motor controller from scratch. There are a lot of application notes from various microcontroller companies (Atmel, TI, microcontroller, NXP, freescale, etc.) on how to build them and how they work, and they're all about the same strategy. Some microcontrollers have specialized timers with DTI (dead time insertion to reduce shoot through currents when you drive a bridge) hardware that was made with BLDC applications in mind. How you drive it depends on which feedback sensors you have available to you and what you know about the motor.

    BLDC motors are very similar to stepper motors, except you usually have a lot less coils and you don't drive it in open-loop. The coils are generally in 3-phases, call them X, Y, and Z, 120 degrees apart, and have a trapezoidal EMF waveform due to the way they wind the coils (while an AC motor generally is winded to have sinusoidal EMF). You will control the current direction in each phase independently to determine the net rotation vector. Generally each phase gets the same amount of magnitude during each revolution.

    The general idea is you have sensing devices that tell you where along 360 degrees your motor is rotated with reference to the coils in the motor. There is a commutation sequence that you know during your design that says, "when the motor is between this angle and that angle, apply current in this direction for phase X, that direction for phase Y, and that direction for phase Z". Then when the angle is changed, you change your combination. The current is DC, and this is why its called a DC motor, even though its not really built like one. The angles that you commutate at depend on the number of poles and what sensors you're using, but in my case I switched the combination of 3 phases of DC current every 60 degrees, which meant that I had 6 different phase current combinations (0 and 360 degrees being the same combination).

    Since the currents are DC, you need a DC voltage source. To commutate, you need to be able to switch direction of current in all 3 coils, and so 3 pairs of bridge MOSFETs are generally the strategy used. The DC current is pulse width modulated at these bridges to give an "average" current between 0 and 100% of the max current, which controlls the RPM of your motor. The modern way to commutate these bridges and drive the PWM signal is with a microcontroller.

    You also need feedback of the rotor position to know what angle its at to give it the proper commutation combination. This can be done with 3 hall sensors that are placed 120 degrees around the motor that give a digital signal low or high when one of the coils passes by it. The other more complicated method is to use an ADC to sense the back EMF of the coils to determine their angle, but this does not work when the motor is stationary (because no EMF is generated). For a first time design, a motor with hall sensors is much easier to work with. The 3 hall sensor inputs can go to a microcontroller to interrupt it when to change the current directions. All of this could be done with discrete chips and comparators too, but a microcontroller makes it much easier and more flexible.

    The good thing about using hall sensors is they not only help you commutate, but they also give you position feedback and you can calculate RPM with them too, which makes for a cheap encoder. They are very poor resolution though, so you may need an external encoder if you're doing high resolution position control with no gear reductions.
    Last edited: Feb 3, 2012
  11. Feb 3, 2012 #10
    Also, the microcontroller approach is not that expensive. They cost a couple dollars at most, and you can buy a development kit for USD 20 or less.

    You only need to design the voltage regulator, gate drives, and the bridge gates circuit, and that might cost you 10-20 dollars, but you might need spare parts if you blow up some. This is the only part that it might get more expensive than a brushed DC motor since that only needs a single H-Bridge to control direction.

    You could also do it all with some comparators, a decoder, and a simple PWM chip.

    I can give you some documentation on BLDCs or share my design with you if you want help.
  12. Feb 3, 2012 #11
    I would certainly appreciate it because, I cannot find anything on internet regarding actual circuits.

    I am building a quadrotor.

    So I will need 4 ESCs. I am planning to control them from arduino(if its possible).

    I just need to have an idea what is happening, and what I must do.

    I think my custom build brushless motors will be rated 20 amps max, and will work at 20k RPM.
  13. Feb 3, 2012 #12
    After I posted, I realized you might not mean brushless DC motors, but you could be using AC motors. I can't help too much on AC motors, since I don't have experience using them.

    There is tons of stuff online how to build BLDC controllers and how they work. Here are some to get you started:

    20 amps is a lot, I take it you're planning this to carry a person? I'm trying to imagine a motor spinning at 20k RPM with that amount of current. The motor you described would be vastly different from the CDROM motor. I guess it would have to be a rather big and well designed motor. You're also going to need more heavy duty bridge FETs if the voltage and current is high, and either a high power regulator or just run off of batteries and adjust your PWM to their voltage level.

    I looked this one up as a reference to compare what kind of specs you're talking about.

    From the RPM/torque graph, you can see at 22k RPM, it is capable of 47mNm of torque, and this correponds to 109 watts of power. Using its torque constant and EMF constant, you can see at this speed and load, its drawing 33V and 3.2 Amps. This one weighs over half a pound. Consider the size and power of this motor with this specs and gauge how good of a motor designer you are compared to them. Where did you come up with the 20Amp spec?

    For example, look at this giant (17 lbs) motor that only goes up to 3000 RPM at 20Amps:
    http://www.anaheimautomation.com/manuals/brushless/L010749 - BLK42 Series Spec Sheet.pdf

    I have not built brushless motors either; I've only studied their construction of which I can give you documentation and references on. I've built the controller/electronics though.
  14. Feb 3, 2012 #13
    See the link I provided in my first post. You might get your answer there.

    He says he uses Jeti06 for his brushless motor control.
    Last edited: Feb 3, 2012
  15. Feb 3, 2012 #14
    Thank you for this info. I got better insight now. I will research this through more, to differentiate types of motors.

    Currently I am opening an old floppy disk.

    Next on the list is old HDD.

    I will see what I will find, and revive this thread when needed.

    You have been very helpful.
  16. Feb 3, 2012 #15
    http://pokit.org/get/43f6feadcd05727c6acf1e5217c7f508.jpg [Broken]

    Ok this is what I found in floppy.

    I belive its the same brushless motor find in cd roms just bigger in diameter.

    From physics point of view, what Can I do with motor "huge" like this, I mean those in cd-roms are pretty small.

    This one has 4 leads coming out.

    4th one is probably common.
    Last edited by a moderator: May 5, 2017
  17. Feb 3, 2012 #16
    To answer where I got the idea of 20 amps:

    I saw other brushless motors, and they say they are rated 20 A, so I figured.

    I still don't have a feeling for this. I am reading physics behind motors like this and alike, trying to understand how it works.
  18. Feb 3, 2012 #17

    jim hardy

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    here's a three phase controller that's very cheap

    not enough power for your big motor but good for experimenting with small motors on long crawl up the learning curve.

    Microchip has some good AN's on mmotor control...

    looks like a 555 based VCO could make it go...

    you guys have outpaced me now.

  19. Feb 3, 2012 #18
    I will actually try this. I am familiar with 555. Very much. This might be something to get me started. Thanks! I have 10 days before next semester, a lot of work!
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