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  1. #1
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    Explanation of the Brushless Motor / ESC System and Capacitor Function

    Quote Originally Posted by Rocky View Post
    So no matter what I do nothing will cool the capacitors except not going full throttle on 6s lipos..... Correct?
    Not necessarily.

    I will attemp to provide a "global" overview of the electrical workings of the brushless motor systems we use in our r/c's.

    I am not an electrician or electrical engineer, but I work around industrial brushless motors controlled by variable frequency drives (VFD - similar function to ESC) and talking to electricians I have a very general understanding of how they work. I have also read up on the topic, which means much of what I am presenting here is the regurgitated knowledge of others, and not my own knowledge or experience. For this reason if any members are better qualified than I am, please correct or clarify any of the following information if I am inaccurate presenting it.

    CAPACITORS
    The function of capacitors is to control, or smooth out, spikes or ripples in the current coming from the batteries to protect the ESC. To do this the capacitors charge when the current is high and discharge when the current is lower. This charge/discharge cycle creates heat, so heating of the capacitors is a normal state of their function. Problems arise when the capacitors are over worked and the result is overheating and premature failure.

    In order to understand where these "ripples" come from we need to understand a few basic electrical concepts and the basic function/operation of the components in our systems.

    RESISTANCE vs IMPEDENCE
    Resistance is a concept used for DC (direct current) and Impedence is the AC (alternating current) equivalent.

    Resistance is due to electrons in a conductor colliding with the ionic lattice of the conductor resulting in electrical energy being converted to heat. Different materials have different resistivities (a property that defines how much resistance a given material has)

    In a DC circuit resistance is pretty straight forward, and if your wires are the correct size, your connectors are adequate size, your solder joints are sound, and your batteries have an adequate C rating, you are good to go, you have minimized resistance and the motor should get all the power it needs to perform at its peak.

    However when considering AC you must remember that it oscillates as a sine wave so the sine is always changing. This means that other effects need to be considered - namely inductance and capacitance.

    The relationship of inductance and capacitance and their affect on resistance in an AC circuit is too complex for me to try and explain. Suffice it to say they make understanding and explaining resistance in an AC circuit much more complex and confusing.

    For the sake of the discussion here what we want to remember is DC current flows constantly in one direction, while AC current consistantly changes direction of flow.

    THREE PHASE AC MOTORS
    So what do we care about AC current, our batteries are supplying DC current to our components, right?

    The brushless motors we run in our modern r/c's are true three phase AC motors. (thus the three wires on the motor, one for each phase)

    ESC (electronic speed control)
    The ESC is a trapezoidal wave generator. It produces 3 separate waves, one for each wire on the motor (i.e. it converts DC to 3 phase AC). Controlling the speed of the motor has nothing to do with voltage or amps, but instead the timing of the current fed into it. By increasing and decreasing the wave length (frequency) of the trapezoidal wave on the three phases, the ESC causes the motor to spin faster or slower.

    Since the ESC controls the motor with frequency, not voltage, when you plug 22.2 volts of battery into your power system, you have 22.2 volts going to the motor with the full amperage potential of the batteries backing that voltage.

    The ESC switches polarity of the phases to create the waves. This means the voltage through any given winding flows "alternately" one direction, then the other. This creates a push/pull effect in the magnetic field of each winding making these motors very powerful for their size and weight.

    The motor, and load placed on it determines the amp draw from the ESC and batteries.

    BACK TO CAPACITORS
    So, it is the primarily the conversion of DC current to AC current that creates the "ripple" in the current from the batteries that the capacitors need to smooth out to protect the ESC. There are other factors involved, but in the interest of keeping it as simple as possible I'm not going there. (read: I couldn't explain it if my life depended on it)

    SO WHAT DOES ALL THIS GOBBLEDYGOOK HAVE TO DO WITH CAPACITOR TEMPS??
    In my reading up on the subject a couple of key concepts kept popping up with respect to capacitors.

    1. The length of the wires from the batteries to the ESC is crucial to how hard the capacitors have to work to smooth the current flow to the ESC. The shorter the better, and the closer you are to maxing out the capacity of the ESC the more critical shorter wires become. (information varied system to system, but it seemed like 6-7 inches max length was a general rule of thumb)

    I wasn't able to find an explanation on the actual cause and effect of wire length relative to capacitor performance, but it came up time and time again in my searches. Including a confirmation by Steven Nue that this is a critical component of capacitor health.

    2. Capacitors are wear items. The reading I did pretty much confirmed something I had begun to suspect. The constant charging and discharging of capacitors, and the heat created, wears them out.

    So our capacitors are going to fail eventually, but the harder you work them, the shorter their lifespan, and if you seriously overwork them, the will fail prematurely.

    CONCLUSIONS (mine anyway, feel free to draw your own)
    1. Check the wiring length between the batteries and ESC and keep them as short as practical. I think keeping all wiring runs as short as reasonable is generally good practice.
    2. Don't overload your system. Keep prop sizing conservative, and consider running 4s or 5s on stock components. If you are wanting to push the envelope, consider component upgrades sized to handle the extra load.
    3. The capacitors aren't going to last forever. If you are confident replacing the caps, consider it a maintenance item. If not, be prepared to replace your ESC from time to time. (I have no idea what the normal life cycle of a capacitor might be)

    Hope this is helpful to some, and that I'm not too far off base with my explanations.

    Hog
    Last edited by hog; 07-21-2012 at 02:54 PM.

  2. #2
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    Hog, this is a great explanation, thank you for taking the time to post!

  3. #3
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    thanks jetnfast, glad you found it useful.

    I found an explanation as to why the wire length is important. This is a cut and paste, so not my words.
    hog

    Explanation/theory
    First a watery analogy, water running in a pipe and through a tap. Now turn off the tap quickly. You'll hear a loud knock/shock sound in the pipe. The water wants to continu flowing but it can't, for a moment the water pressure is much higher than the static water pressure. It's the same for a current that's switched off, because of the inductance it wants to keep on flowing, voltage gets higher. This is also what causes sparks (brush fire) in a brushed motor.

    The controller is like a watertap that's switched off (and on) very fast (8,16, 32kHz PWM) to get the desired current. Turning off the current, in combination with the battery wire inductance, causes voltage spikes because the current wants to continue on its course (ref. inertia of the moving watercolumn). Those voltages are higher than the battery voltage. The input capacitors (cylindrical) takes care of these spikes (they reduce the wire inductance). The longer the wires, the higher the voltage spikes induced in the wires, the harder on the input capacitors. They will get warmer, heat up and explode. This is caused by the wire inductance, not by wire resistance. Therefore, using thicker wire will not help much, it's not a bad idea either, but extra capacitors are the solution, thus reducing/compensating the wire inductance. Or longer motor wires instead of long battery wires.
    Last edited by hog; 07-21-2012 at 08:42 PM.

  4. #4
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    Perfect analogy!

  5. #5
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    I would not recommend you changing your own caps unless you know how to propery discharge them.

  6. #6
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    I would agree, send the ESC back to Castle and let them do it, even if it is out of warranty.

  7. #7
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    Did not want to reply with "quote" because of the length of hogs fine explanation of the brushless motor/esc. But I would like to add some info....

    The function of the capacitors on the speed control do not smooth out ripple from the batteries, (the batteries are pure DC).
    A device called a pulse width modulator (PWM) produces an AC waveform createing ripple. The amount of ripple produced is dependent on how much inductance there is in the brushless motor.

    Lower turn motors produce higher ripple than high turn motors. Rule of thumb (Low turn motors = higher KV motors). So the higher the ripple and voltage, the harder thoes tiny little electrolytic capacitors have to work.

    Hope this adds some helpful info the the fine thread that hog started here...It just about says it all.
    "enjoy your ride"

    Dennis

  8. #8
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    Thanks for the extra info Dennis.

    Just a clarification, for my own curiosity.

    The sources I found on this topic suggested that the "ripples" created on the AC side are "reflected back" (for lack of more technical term) onto the DC side, which creates a ripple effect in the wiring and batteries, which in turn produces extra load the input capacitors.

    This is why the length of wire between the batteries and ESC are reportedly so critical (see water pipe analogy above).

    The ripples would not be coming from, or be created, by the batteries, but rather created by the polarity switching on the AC side, and would be present on the DC (battery) side of the ESC.

    Based on your knowledge and experience, is this correct (more or less)

    hog

    PS I see where I erred in the first paragraph when I state "ripples coming from the battery" it was my understanding all along that the ripples on the DC side were caused by the polarity shifting on the AC side, not that they were created in the battery.
    Last edited by hog; 07-26-2012 at 06:53 PM.

  9. #9
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    I figured you had just miss stated about the "ripples comming from the battery".

    The ripples are actually caused because of the turns in the motor (wire wound around an iron core produces inductance and the PWM field rising and falling produce this ripple) and the capacitors attempt to absorb this, which they do, but they work the ever loving life out of the electrolyte in the capacitors. With out them the batteries would really suffer because of that ripple being induced.

    Picture a coil in and older car when they used a set of points to charge the coil, to produce the spark. The contacts on the points were protected by a small electrolytic capacitor (called condenser in thoes days) it works like a shock absorber to prevent the points from pitting when they opened and closed. .... The caps on a ESC are "shock" absorbers.

    I should add that the straight wire to the batteries does not really have measurable inductance, due to the very short length I would venture to say it would be measured in **** henrys... thats 9 0's or .ooooooo15 very very small inductance from the battery wires.
    Last edited by orca44; 07-26-2012 at 08:51 PM.
    "enjoy your ride"

    Dennis

  10. #10
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    Thank ALL of you guys for helping me out with this issue I have been having!!

    I sent in my ESC to castle and they replaced it free of charge and another one is on its way back! (My Third ESC)

    The thing is I run two 3s 11.1v batteries which equals 6s 22.2v. My question is what batteries are 2.5s that combined to equal 5s?? And how many volts are they??

    With this ESC I am definitely toning it down from 6s and actually want to enjoy my boat!

    Thanks Again!!!!

  11. #11
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    Nice to hear Rocky! As soon as I read your post I checked my email and it showed a UPS shipping notice from Castle, looks like my new one is on the way back too! So it will be interesting to see how this batch of ESCs work, since ours most likely will have come from the same batch. I plan on wiring up the castle cappack for extra capacitor insurance too. Hopefully I will have it all ready to go in the next week or so.

  12. #12
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    The Spartan is wired for series connection of the batteries which doubles the voltage but capacity stays the same. So 2 qty 11.1v 5000 mah batteries becomes 22.2 volts (nominal) but the capacity remains 5000 mah.

    There are no 2.5s batteries. You either run a 2s and 3s in series, or you run 2 qty 5s batteries in parallel.

    With a parallel setup the voltage remains the same, but capacity doubles.

    Hog

    Thanks for the clarification Dennis. Clears things up for me for sure.
    hog

    PS. I'm old enough to remember condensers and points. When I was in school working in a service station the mechanics thought it was great fun to leave a charged condenser on the bench for some poor unsuspecting night crew person doing clean up. You only ever picked up one.....
    Last edited by cooleocool; 07-27-2012 at 03:16 PM. Reason: post merge

  13. #13
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    Ya....I've been there, done that. Before I retired I instructed everyone to be sure and discharge all electrolytic caps over 1,000 mfd..... Gee...some how some of them charged them selfs

    If a person has ever been bit by a charged cap.....you will discharge them every time after that
    "enjoy your ride"

    Dennis

  14. #14
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    So Hog;
    It would be best to rewire my ESC by setting it up as a stock traxxas esc such as the vxl-3 then connecting a parallel connector with 2qty. 5s batteries?

    Would that be a good idea? If not what do you think??

    Thanks!!

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    I'm not familiar with the vxl3 so not certain what you are proposing exactly.

    I set mine up with an extra connector at the ESC and then I can plug in either a parallel wiring harness or a serial harness. This gives me easy flexibility for battery options.

    If you are planning to run the same batteries all the time, you certainly can set the ESC up with a parallel connection directly connected to the ESC.

    The extra connector is one more area where you could have problems (every connector added to a system has the potential to cause problems) but I like the extra convenience.

  16. #16
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    Thanks so much for the great explanations. Is the length of the wires between the batteries and the esc the crucial factor? I have standard length wires from the packs to the esc's but because I'm running 8s I have a fairly long, heavy guage wire (6-7") between the packs. I have also wired in a Castle CCcaps unit as close to the ESC as possible (4" +/-)

  17. #17
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    Hey I have a question one of my capacitor cable came out should I still use my car or not? Can some one help me

  18. #18
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    send it back to castle, they'll fix it. if your good with electronics then you can try fixing it your self but keep in mind that the water proofing that would be around the capacitor cable would not be water tight and you would need to re water proof it.

    good luck; Lachlan

  19. #19
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    I think Hog should go down to his local hobby store, and buy himself a bunch of boats. Then, I think he should come back on our Spartan forum, and hang out with all us guys again...yep-per, that's what I think.

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