Choosing new motors

Discussion in 'Cinestar Misc' started by Jean Gabriel Taboada, Apr 7, 2013.

  1. Jean Gabriel Taboada

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    Hi,

    I was wondering how does one choose new motors, i was browsing different brands and the choices are endless, is there a mathematical formula which gies you how many kv you need for an especific frame?
     
  2. Gary Kaplan

    Gary Kaplan New Member

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    Jean,

    I have an idea why no one has replied to your question. Its a bit complicated to explain, but I will try. I will pass along some of my knowledge from the R/C world in hopes that others will help me out in the future on the photo/video side of things (in which I am the "newbie"). I will try to keep things as simple as possible. My E.E. degree sure came in handy when I started flying electric R/C helicopters and airplanes. These principles apply to electric R/C airplanes too, if you ever decide to mount a camera on one.

    There are many different (important) electrical and physical parameters regarding brushless motors and batteries like those used on all electric heli's. It takes a little explaining so please bear with it. :)

    Principle 1.) Kv = The motor will turn (the propeller) 1 revolution per 1 volt (V) of the battery that you use.
    In the case of a LiPo (Lithium Polymer) battery each cell is 3.7V (volts). The term "S" is used to describe the battery pack. So, a 3S battery has 3 cells and is 11.1V (3.7 volts per cell x 3 cells). A 4S battery is 14.8V (3.7 x 4), a 5S battery is 18.5V (3.7 x 5), and a 6S battery is 22.2V (3.7 x 6). Using what Quadcopter calls their "heavy lifting motor", the QC-3328; it is rated at 700Kv (but they call it "no load speed"....I'll explain in a minute). So, if you use a 6S battery for your heli (22.2V) and a 700Kv motor you get 22.2 x 700 = 15,540RPM out of the motor under no load, meaning "without a propeller attached to the motor. Just the motor shaft spinning by itself. If you used a 4S battery instead of a 6S you would get 14.8 x 700 = 10,360RPM under a no-load (no propeller) condition.

    Principle 2.) Propellers create a load on the motor (see above)! ;-)
    Once you put a propeller on the motor everything I just said above goes out the window! The theoretical RPM you calculated using the motor Kv and battery voltage goes way down. The reason is the motor is now loaded up with the effort needed to spin the propeller. The diameter of the propeller and its pitch (the angle the blades) will affect how fast the motor can turn. Increasing the propeller diameter will make the motor work harder (which generates heat) as it tries to reach (but can't) its theoretical max RPM at full throttle/power input from the transmitter (calculated in principle #1 above). Motor manufacturers will tell you about "recommended prop size" to use because they have tested a bunch a sizes for you and found which ones give maximum RPM (thrust or "lifting power") while staying within the boundaries of voltage (and current), and heat the motors can tolerate.

    Principle 3.) Motors have different number of "poles" and "windings" (the number of turns of wire wrapped around the pole inside the motor housing). Each one of these spec.'s affect the motors torque ("horse power") or it ability to turn the propeller under load. A motor with a small number of poles (like a 6-pole motor used in a regular R/C helicopter or Jet "ducted fan") will product less torque than a 14-pole motor used in a airplane or a hex-copter. In a regular RC heli the 6-pole motor turns a gear which in turn spins the rotor shaft that spins the rotor blades. The gearing system steps up the slow turning motor RPM into higher "head speed" of the rotor blades. A 14-pole airplane motor has a propeller mounted directly to the shaft of the motor in the same way a hex-copter or oct-copter does. The motor has enough torque to directly spin the propeller by itself.

    Principle 4.) Max Current (A): is the maximum amount of current that the motor (and speed controller) can handle going through the windings of the motor. Pull too much current through the motor and the too much heat is generated and the windings burns out, the motor stops turning and your helicopter crashes. The size of the propeller affects the amount of current taken from the battery (making the battery hot) and put through the speed controller and the motor itself. If you keep putting a larger and larger prop on the motor and run it you will find this limit (something you don't want to do). So, you have to stay within the maximum current rating of the motor by using the proper size propeller for the type of motor that you have installed.

    Principle 5.) Max Voltage (V): is the maximum battery voltage the motor (and speed controller). Manufacturers will specify maximum voltage (usually. Sometimes they don't and they just give max current and power input and you have to figure out what prop to use to stay within the recommended range) for the motor. They will state something like "Voltage Range 11.1~14.8V (meaning a 3S~4S battery), or 14.8~22.2V (4S~6S).

    Principle 6.) Power into the motor (mechanical power) = Voltage of the battery pack (V) x current through the motor (A). You have to stay under this power rating of the motor to avoid burning it out. If you go up in battery voltage (like from a 4S battery to a 6S battery) the current will go down and the motor will run cooler. You can then keep increasing the propeller size until the current reaches the maximum level. Conversely, if you keep the battery pack constant (say you are using a 6S battery) you can raise of lower the current coming out of the battery and through the motor by increasing or decreasing the propeller size. So you again will see a recommended propeller listed. The smaller diameter propellers will have a smaller load on the motor and will increase the RPM and give more speed to the heli but will suffer less thrust and therefore less "lifting capability". This is good for a shoot if you want to fast and you'll be carrying a small Go-Pro or DSC. A large diameter propeller will turn slower (lower RPM) and will not go as fast but you will have more lifting and can carry heavier loads.

    Summary: So you take upgrade your motors to something that has a high pole count (like 14 pole) so you can spin a large prop, can take a good amount of input power (W) and current (A) and high voltage (such as 6S) and decent Kv for speed (RPM). Then you can use a variety of propellers to suit your needs at the time. Easy, huh? :)

    So, taking a look again at the Quadcopter QC-3328

    Technical Info
    • Lithium cell count: 3-6 (Means you can use a 11.1V 3S~ 22.2V 6S LiPo)
    • Load current: max. 30A (size of the prop will affect this number for each battery voltage. Its better to use a 6S battery to keep the current down with any prop yo use in the recommended range below)
    • Maximum load current. (15 sec): 50 A (maximum amount of current the motor can take for short 15sec. bursts of speed)
    • No-load speed: 700 rpm / V (this is the Kv rating. They should have used Kv instead of mechanical load that only engineers understand)
    • Recommended Propeller Size: 10" .. 14" (these are the propeller sizes you can use. Small prop = more speed but less lift, large prop = less speed but more lift)
    • Mechanical power: 350 W (equals the amount of power (watts) the motors can take. It is your battery voltage 11.1~22.2V x the current you take out of the battery and put through the motor based on which prop you use).
    • Thrust max.: 2200 g (thrust or 'power/lift' created by the largest propeller for this motor which is 14" while using the highest voltage battery which is 6S).
    • Dimensions: h=33.5mm, d=28mm (this is shown in the motor model number QC-3328. Its 33mm long and 28mm in diameter).

    OK, I'm exhausted!!!! School is out of session!!!

    Cheers, and good luck.
     
    Sam Baird and Scott Stemm like this.
  3. Gary Haynes

    Gary Haynes Administrator
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    There is a calculator online at www.ecalc.ch that is useful for determining whether a combination of multiple factors will give good results. But as Gary wrote there is a long list of items that you need to know in order to calculate the answer.
     
  4. Gary Kaplan

    Gary Kaplan New Member

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    Gary,

    Yeah, I found that these calculators are decent but its all based on theoretical stuff that I am sure you already have seen don't exactly come true in practice at the field. It takes a bunch of fly, measure, modify, fly, measure, modify grunt work to fine tune the optimal prop/motor/esc/battery combination. Its especially true of high speed airplanes. those calculators get you close but only in the ballpark. Its all fun stuff (to me anyway).
     
  5. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Gary K.: I take my hat off to you. That is one of the best, succinct, and well written explanations I've seen.

    Nicely done!
    Andy.
     
  6. Gary Kaplan

    Gary Kaplan New Member

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    Thanks Andy. I am happy to share all that I know on the technical and application side of RC in hopes that the people here with help me with the photo/video/cinema side of things which I know zero about.
    Looks as though I will have to wait a while for the 8-HL and Synapse to be released before I purchase my new venture.

    Gary K.
     
  7. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Once you've regained your strength, I think the forum members might be interested in your thoughts regarding selection of props in terms of optimal size/pitch/materials (e.g. 14 x 4.5 plastic versus same size/pitch in wood or carbon fiber), or swing a small prop, higher pitch, etc.

    Andy.
     
  8. Gary Kaplan

    Gary Kaplan New Member

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    Sure, I'd be happy to. Just point me towards a certain thread or maybe start a new one on the topic. I'm not that tired. :)
     
  9. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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  10. daniel clarke

    daniel clarke Member

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    That was a great read gary thanks. maybe you could help me someday to get the right throttle and pitch curves for my trex 600 clone.

    cheers.
     
  11. Gary Kaplan

    Gary Kaplan New Member

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    Daniel,

    Sure, I'd be happy to at least offer some suggestions. I've been there done that myself. Biggest issue is starting off making sure your head speed is correct. Do you fly it in throttle mode or stunt mode?

    Gary
     

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