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Motors KV Rating Explained, And How To Figure Out Headspeed

Discussion in 'Electronics' started by Tony, May 2, 2011.

< Lipo Batteries Explained | Analog vs Digital Servos, What's The Difference? >
  1. Tony

    Tony Administrator Staff Member

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    What's up everyone. Well, I have been asked time and time again what the KV means on the side of a motor. Well, this video will explain it really quickly. It's a simple equasion that you can use to figure out your head speed as well.

    Pretty much, if you have a 3500kv motor, then that means it will turn 3500 rpm if only 1 volt was applied and it was running at 100% efficency. Lets face it though, nothing runs at 100% efficency, so in these, we are using 80% to be safe. If you want, you can use 90%. Below is the equasion.

    (kv*v/(m/p))*.8

    Confused yet, don't be.

    kv= the kv rating of your motor
    v= the voltage of your battery
    m= main gear teeth
    p= pinion teeth

    If you use this equasion, you can figure out what kv motor you need for the perfect head speed, and with what pinion without ever buying a motor before hand.


     
    Last edited: Feb 6, 2017
    liquid1 likes this.


  2. cobieg

    cobieg New Member

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    Is there a video for this ?
     
  3. Tony

    Tony Administrator Staff Member

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    Yup, my bad. I switched the forum about a year ago to No HTML and it messed up the link. The link above is not fixed. Let us know if there is anything we can help with.
     
  4. Serge

    Serge New Member

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    Hi,

    This is an old thread but in-line with my questions.
    I did not received clear answers on my post in the ESC category.

    The actual speed depends on the load and on the motor resistance as well.
    Is the 0.8 rule always good enough? Is there a more accurate calculation?

    If you can't find the kv you need for the speed you need, is it possible to use a higher kv (next available)?

    Thanks
     
  5. Tony

    Tony Administrator Staff Member

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    The .8, or 80% efficiency is just an estimate. Every motor is going to have a different efficiency rating and even the same motors are going to vary. I used .8 just as an example of what to expect. If you would like a higher theoretical head speed, then use a higher number (such as .9 or .95, or even 1.0 which is taking it out completely). This will give you a higher calculated head speed and keep you from over speeding the head just in case the motor is more efficient than you think.

    As for not being able to find the KV, I would opt to go higher then calculate out what the max throttle should be and lower my throttle curve to meet the correct head speed. I really should redo this article and just have everyone use 100% efficiency so that the head speed will appear higher than it really is. The calculation in this article/thread can in rare cases cause head speeds to be higher than suggested.
     
  6. Serge

    Serge New Member

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    Thanks, but your answer sound confusing to me.
    Using 100% just mean the "no load" rpm you will get. That's the definition of kv.
    That theoreical speed is useless, since as soon as you increase the current (load) a voltage drop in the motor will occur and the real max speed you get is lower.
    In the application I have in mind, target speed (full load) is mandatory. It cannot be lower.
    Then if I want to be safe, I would rather apply a lower number, not a higher in order to chose the kv.

    Going the other way, using a kv a bit too high, means that the max pwm from the ESC must be limited below 100%.
    Is it what you mean by "lower my throttle curve"?
    Is there a loss of global energy efficiency when running away from 100% ESC output?
     
  7. Tony

    Tony Administrator Staff Member

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    The reason I would use 100% efficiency is to give a theoretical 'higher' head speed so that you don't over speed the head. What I mean by this is, if your head is rated for 3500 rpm and you calculate out a throttle curve to achieve that head speed with an 80% efficiency rating and your motor has a 95% efficiency rating, your head speed is going to be over 3500 rpm.

    But yes, the speed I'm referring to is a no load speed. And unless you are running a governor, your load and head speed is going to constantly vary due to collective and cyclic pitch changes. In which case a governor is your best bet to achieve a stable head speed.

    I know this is confusing, but once you do the math a few times it will all start to make sense. when I first started learning this stuff, I was banging my head on the wall trying to figure it all out lol. But like happened to me, it will come to you one day and just 'click' so to speak.
     
  8. Smoggie

    Smoggie Well-Known Member

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    Of course the other variable that you have to use in the calculation is battery voltage. A typical LiPo is 4.2V per cell when fully charged and about 3.7V 'resting' after use, but under heavy load it can easily drop down to around 3.4V per cell. So even if you decide on an efficiency factor there is still a quite wide range of RPM depending on voltage. This is one reason why it's best to use a governor on a heli, otherwise your RPM will vary a lot through the flight.
     
  9. Serge

    Serge New Member

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    Hi,
    Another question for you.
    When a given motor has a specified stator resistance of 17 mOhm for example, this is cold (no load).
    How do we know the resistance at maximum temperature? (full load) It must be higher.
    I suppose that a hot motor maximum speed will be lower than when cold.
     
  10. Tony

    Tony Administrator Staff Member

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    The only thing that I would know to do is to measure the resistance when it is hot. I have no idea what the equation is or even if there is one for something like this. It's beyond my scope, I know that.
     
  11. Serge

    Serge New Member

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    Thanks for all these quick replies.
    I suppose there might be an approximate rule of thumb on this like a multiplying factor.
    I would not be surprised that the hot resistance be at least 50% higher.
    I am actually not even sure if the rated resistance is cold.
    Still need to search.
     
  12. Serge

    Serge New Member

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    I have found this equation:
    C = 1/[1-((T-25)*0.00393))]
    Not sure it's reliable. It would need that for a 50°C temperature increase the resistance increases only by 24%.
    Still rather low. I don't know how much hot these motor can run.
     
  13. bassslayer1960

    bassslayer1960 Member

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    Hey Serge,
    What the heck are you building that needs such a critical headspeed?
    I'm sure there are a bunch of us that would like to know.
    Robert
     
  14. murankar

    murankar Moderator Staff Member Armed Forces

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    The stator resistance is moot to most pilots out there.

    As for headspeed is concerned, the math portion is to give a rough estimate. The purpose of the math is so that you can get a ballpark for your setup. Your ideal headspeed should be achieved at around 85% to 90% with no load. This allows some head room for those "full load" situations.

    Personally on my turkey leg 570 I have MY ideal headspeed at around 50% throttle (2200) and my high head speed at around 85% throttle. This gives my Vbar neo headroom to maintain the set head speeds while under load.

    Again all the math is for is the ballpark. It saves you time and money to getting you headspeed set.
     
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