KV info for advanced pilots (Tutorial)

Discussion in 'Electronics' started by LukeWarm, Dec 15, 2011.

  1. LukeWarm

    LukeWarm Top Gun

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    Squishy inspired me to brush up on my motor technology so I will share the fruits of my research with you.

    With a sufficient battery and ESC, you need to know four other things to pick the correct prop size;
    1) Motor KV (RPMs per volt)
    2) The power behind it (watts, amp draw, or torque) The weight of the motor is a good indicator of this.
    3) The dynamics of the plane (the wing loading and aerodynamics)
    4) The way you like to fly the plane
    The dynamics of the aircraft deals with friction and momentum; it is information like the weight, Air flow obstructions and the aerodynamics of the air frame. Any time you change your plane’s set-up or run it in weather you're not use to flying in, land it and check the temperatures every 30seconds to a minute. Touch the ESC, battery, and motor with your bare finger and if you can't keep it there for more than a couple seconds, it's way too hot. If anything is getting too hot, sometimes you can fix it with throttle management. Do the lands and checks until you get a good sense of how hard you can push your Jet and not damage the electronics. If you have to manage the throttle more than you want to, or the problem is beyond throttle management; it’s time to upgrade a part or two. If you are trying to fix a heat problem, one of the best parts to change is the prop. Too much prop can overheat everything. More length, more pitch, both draw more power (current, amps), so you must find the right balance of the two to choose the right prop. If the motor temperature is OK, the prop length and pitch are OK. Too little prop waste energy.

    Two of the most important things that determine the KV rating of a motor is the number of turns and the thickness of the wire used in the motor windings. KV refers to the number of RPMs per volt at no load or with no prop. With a 11.1Volt battery, a 2200KV motor without a prop, will run at 24,420 RPM (11.1 x 2200=24,420). But the KV rating will tell you a lot more then what propeller you need, it tells you the temperament of the motor. The KV represents intent, and the motors amp draw represents the strength toward reaching that intent.

    A Park-Jet motor is 1800kv to 2600kv, 20amp to 65amp, 2cell to 3 cell, and a 5 to 7 inch prop with a 3 to 5 pitch. The main reason Park-Jets use high KV motors is there are a lot of advantages to the mid motor configuration, but we have to keep the prop slot as small as functionally possible. With rear mounted thrust vectoring, a high KV motor is also preferred because a low KV motor with a big prop eats the back of your plane when you turn.

    The big advantage of a lower KV motor is “it gives your plane great thrust while only using a light two cell battery”. You can substitute a high KV motor with a light three cell battery with less milliamp hours (mah). The difference you will see is shorter run times, and a little less torque due to a longer wind up time.

    The lower Kv motor is normally wound with more turns and a smaller diameter wire; because of this, it can handle less current. For this reason, a 2 cell Lipo is preferred to power it. More torque while using a smaller, lighter battery is a great combo for a 3D plane.

    The higher the KV:
    1) The less prop it will drive (less length)
    2) The longer the motor takes to wind up.
    3) The less torque per volt,
    4) The less voltage you need to get power
    5) The more speed (the higher the motor will wind)
    6) The bigger the wire with less turns in the windings
    7) The more current it will take
    8) The more voltage it will take
    9) The higher the maximum power output.(see note)
    10) The more efficient the motor is

    NOTE: More power is not necessarily more thrust, prop size and efficiency comes into play there. A lot of these are generally true, but not true in every case.
     
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  2. squishy

    squishy Top Gun

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    Great info all builders should know...higher KV is not always better, it depends on what you are building or how you want the airplane to fly and with what prop. Thanks luke for the informational post!
     
  3. LukeWarm

    LukeWarm Top Gun

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    Do you think this is for advanced pilots, or do you think everyone can understand this stuff ???
    That's one thing about this forum, we don't dumb the answers down much.
     
  4. squishy

    squishy Top Gun

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    motor and battery information should defiantly be "disseminate" better, I have never seen it explained it a truly beginner friendly way. It is a knowledge/motivational gap and should be treated as such. People need to know why they should care and then the info needs to be explained as simply as possible. It's very hard for those with no electronic knowledge. Using metaphor and visual aids is most likely the best solution, it would be time consuming to get it right and even then it may require feedback from learners to refine.

    I frequently use the water hose filled with marbles metaphor when trying to explain electronics to people, the basics of electron flow, the different measurements and the way it can be altered would be a good foundation to trying to explain motor and battery stats. "Advanced", implies to me that much of the foundational knowledge is taken for granted. Sometimes you need to take people back to basics, and sometimes a beginner who just learned or an intermediate learner, will be able to bridge the gap.

    This forum is a great example, sometimes the best people to answer the questions (and the most motivated) are those who just learned it themselves.


    but seriously, don't ask me, because I am still trying to figure this stuff out myself...lol like what the heck is the C rating actually measuring? is it a reaction within the battery? substance density? why do I have two identically rated batteries but one weighs more than the other? this is why we call it a hobby, there is no stop to the learning.
     
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  5. Learning

    Learning Rookie

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    You should factor in 80% losses in efficiency for the motor and battery so multiply by 0.8 as well to allow for losses.

    11.1 (volts) x 2200 (kv of motor) x 0.8 (losses) = 19536 rpm - (Kv of motor is rpm for one volt-2200 rpm/v)

    Best prop size for this set up is the 6 x 4 apc

    Battery max discharge in amps is mah of lipo x c rating of lipo / 1000

    E.g. 25c rating 1500mah 11.1v lipo - (C rating is how quick a battery can discharge to supply current to the motor 20 or 25 c are good enough).
    (1500 mah x 25 'c rating' ) / 1000 = 37.5 amps
    11.1v x 37.5a = 416 watts

    A cheap 2200kv motor is going to draw max 28 amps which is 311 watts so a 1500mah 25c or a 2100mah 20c is perfect for the 2200kv motor. Always have a battery which is not going to struggle to meet the draw of the motor. The battery gets very hot and will probably puff up if its to small for the motor. I destroyed a lipo using it on a much bigger motor than it could cope with, battery dead more money wasted. Best propeller size for this setup is a 6 x 4. A 7 x 5 is to big for these motors and will cause them to run too hot. 5 x 5 has no thrust at all you get about 475grams of thrust with a 5 x 5 compared to 700 grams with a 6 x 4. I use a 7 x 5 cut down to a 6 x 5.

    5 x 5 and 4.75 x 4.75 are used for 3000 kv motors running 4s,5s or 6s lipo. 4s - 35,500 rpm ; 5s - 44,400 rpm ; 6s 53,300 rpm. Crazy speeds!

    Any one else tried different prop sizes what did you think did you go back to a 6 x 4.

    If you want lipo's to last make sure they get good cooling when flying.
     
  6. LukeWarm

    LukeWarm Top Gun

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    That is some more good information “Learning”. You did a great job of explaining the c rating. I do have a few questions for you though.

    If there are 80% losses in efficiency, would not you have 20% of the RPM remaining? 24,420/100% = x/20%** x = 4884 RPM. That sounds a little too low to me.

    If there are 80% losses in efficiency, wouldn’t changing the current the motor can draw, or the prop’s length or the prop’s pitch change this number?

    I like the idea of a good rounded average loss number, but where did the “80% losses in efficiency” number come from? I figured the average loss number to be around 30%, but that is not super important to me. The KV represents intent, and the motors amp draw represents the strength toward reaching that intent.

    A prop is designed with the end tapered to focus the air flow toward the middle of each blade, it also has the extra add benefit of reducing the shockwave at the end of the prop; both things let the prop work more efficiently. The prop is designed by a computer, changing it makes it less than what it was.
    http://www.supercoolprops.com/articles/transonic.php
    http://www.rcpowers.com/forum/showt...ce-Behind-the-Slot-(for-advanced-pilots-only)
     
  7. Grey

    Grey Ace Pilot

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    He meant 20% losses for a total of 80% reliable.

    It's a commonly accepted number really.

    KV = no load. Put a prop on and the Kv goes down. Then allow for the fact that a standard LiPo will only give 85% of it's rating under sustained load. Work out it's thrust output on paper and then knock off 20% to allow for these factors to get a ballpark figure.

    It's not an uncommon theory. It's got a lot of holes but for the motors we generally play with it's about right.

    The only part I have a problem with is that he lumps all the cheap 2200kv motors together when you really can't. The motor really does matter. There's 2212-6 and 2826-6 motors out there (and minor variants of both) running the spectrum from about 25-36 amps. Move down to the MJv3 size and the amp draw goes down to 24-26 amps on average. Get something like the Eflite Park 300 2200kv and it's a 2.6 amp motor.

    I've seen too many guys buy 14 gram 2200kv motors because they are cheap and discard scratchbuilt parkjets as rubbish. I had my F-22 at a neighbouring town's club the other day and people were quite suprised by it for this very reason. The couple of guys that built them there used motors that were way too small. They struggled to get into the air let alone do much so everyone chalked them up as rubbish about 2 years ago.

    I flew mine in 25km/h winds just to prove a point, it bounced around but had plenty of power (bent the nose on landing though :()

    I'm so glad I came here and got help on this stuff up front! :)
     
  8. LukeWarm

    LukeWarm Top Gun

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    Yes, I agree:
    Anytime you have a good rounded average loss number, some people will think that it applies to all motors with any prop. Teaching that number to a new pilot is not good, there are many other things that must be learned first so shortcuts like that number are better understood.

    You came to this forum and learned, and now you are a well informed ambassador.
     
  9. rodrigo

    rodrigo Top Gun

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    one point to add at the Grey input:
    efficiency depend of the quality of the motor, only the best motors can reach 80% of efficiency and this appens at only one optimal current (or voltage)... see this graphic for MJ v3

    [​IMG]

    what it means? at 12.5A (or 7.8V with a APC6x4) the motor reach the best efficiency (near 70%), but at full throttle and 25A the efficiency decline to 60%....at 30-35A 50%, at this point 50% of energy is wasted in heat...

    ANother good point is understood the difference between same kv motors.Smaller 2200kv motors can rech higher rpm but with little props only, the same inverted situations for biger motors at same kv (supermegajet has better efficiency and can run at higher current, then a 6x4 prop will spin faster or you can choose a big prop)...
    where is the key? look the watt value or recomended maximum current and lipo battery... if a 3s lipo compatible and 12A maximum, then is a 130Watt motor no other way (11.1v x 12A= 133W)... if it doesn't exist a test prop values in Flybrushless site, use the microjet as standard and find a propeller than consume near 130w at full 10.7V (EMP 4.1x4.1) or use the motor with a 2s (8.8v) with a APC 5x5

    I hope it serve
     
  10. LukeWarm

    LukeWarm Top Gun

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    [​IMG]

    I know how to read graphs. Thank you for finding that for us "rodrigo"!!
    At full throttle, they increased the amp load to give us this graph showing this motor’s efficiency in relation to amp load.

    For an 11.1volt input, that graph tells me:
    1. The max efficiency is 70%.
    2. Between 10 to 15 amps is the best amp draw to have at the aircraft’s cruse speed.
    3. Amp draws over 15 amps will (with each step) progressively make the motor hotter.
    4. Amp draws less than 10 amps waste energy.
     
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  11. rodrigo

    rodrigo Top Gun

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    thanks for the summary .. My explanation was obviously for a rookie, not for you you're a real teacher

    for newbies, try to play with this tool (a motor/prop calculator) is really useful
     
  12. LukeWarm

    LukeWarm Top Gun

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    I gave my interpretation of the graph, and then I read what you wrote. You did a great job explain it, we had the same interpretation. I have written technical manuals for years and sometimes the way I explain things in the forum reflects that. Your way is more personable, but thank you for the complement.

    I like the tool and will play with it. Thanks!!!
     
  13. MoTheG

    MoTheG Airman

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    There are mistakes in this thread. I will sometime over Christmas come back to point them out.
    here is an ancient thread that I started.
     
  14. Grey

    Grey Ace Pilot

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    I explain it like this;

    Think of it like a car (small motor) and a truck (large motor), both capable of 100 MPH. If you put a tonne in the car and a tonne in the truck the resulting drops in speed and acceleration will be noticably different.

    So now you have a small motor and a big motor, both capable of 2200kv. If you put the same prop on each (the same load) the small one won't spin up as fast ("accelerate") or reach the same RPM ("speed") as the larger one.

    It's the explanation that gets most people in the ballpark of understanding the concept.
     
  15. erajomppa

    erajomppa Ace Pilot

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    I just remember reading a thread about EDF motors and there someone pointed out that the state-of-the-art EDF motors have relatively lower kv ratings, and that allows them to be run on higher cell count batteries. At first look this would disagree with "8) The more voltage it will take" -part, but I guess there is more than that to it?
     
  16. LukeWarm

    LukeWarm Top Gun

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    If you look at a large group of different motors in the same class, "8" generally is very true, If you find a 800KV motor that will take a 4 cell battery, it will be a very large motor. A low KV rating for an EDF is 3000 KV, but that is almost too high of a KV for a prop. Most 3000 KV motors can take a 4 cell battery. May be I will edit post 1 to read "for motors in the same class". I assumed people would know this, but you know what they say about making an assumptions. Thank you for pointing this out.
     
  17. Learning

    Learning Rookie

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    80% efficient so you are losing 20% power due to friction, heat, drag etc.

    Just a factor to include in your calculations.

    Not all 2200kv are the same, there are different size motors 300, 400, 480 (length of case on motor), 300 motor has a case length of 30mm and 480 motor has a case size of 48mm.

    motordata.gif

    The bigger the motor size the more power, torque the motor can produce. The 2200kv is going to stay the same but the torque will increase. Bigger motor more amps being drawn from battery.

    Most people will use a 300 size motor rated at max 26 - 28amps. This motor will run on 3s lipo but not 4s. If you want to run on 3s or 4s lipo then you will need 480 size motor rated at around 40amps.

    All these graphs, formulas and calculators etc are just to allow you to get a rough idea of what performance you would expect when using various components together. They just give you a ball park figure - approx thrust and approx top speed.

    You do not need to know this stuff, I like to know it because my background is engineering and I like to understand how things work.

    This thread is going to get very technical.

    Best thing if you are interested in motor, prop and battery combinations is to get a watt meter and experiment.
     
  18. rodrigo

    rodrigo Top Gun

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    this is true, but is a good point to start

    I bought my own just for curiosity and is really interesting saw the real numbers....for example my rctimer 1800kv motors work great with 6x4, but at full throttle the maximum rpm (near 16000 with 590grs of thrust) is keeped just for 10 to 20sec, then loose near 1000rpm and decrease the thrust in 30-40grs, not a real problem but another point to check
     
  19. dtribby

    dtribby Top Gun

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    OH NO!! There's something WRONG on the internet! Quick - gotta correct it!! ;) JK Rodrigo. I know how you feel.
     
  20. rodrigo

    rodrigo Top Gun

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    I knew that this was already posted... is a good idea make link to old thread, thanks for the link
     
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