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ESC Esc Amperage

Discussion in 'Electronics' started by matthias, Apr 21, 2018.

  1. matthias

    matthias New Member

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    hi i want to no what esc to buy for a 2700kv motor. Can i use a 20a or do i have to get a 30a?

    thanks
     


  2. Tony

    Tony Administrator Staff Member

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    KV is RPM per volt. There is no way of knowing what the power draw is of a motor from the KV rating.
     
  3. Smoggie

    Smoggie Well-Known Member

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    That's right. To take a stab at the ESC rating then we would need to know:
    • Motor Kv
    • Battery voltage (number of cells)
    • What prop you are using
    Kv on it's own isn't enough.
     
  4. matthias

    matthias New Member

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    kv is 2700
    battery is 11.1v 3s
    and i'm using 5'' prop

    thanks
     
  5. Tony

    Tony Administrator Staff Member

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    Steve forgot one key factor, we need to know what the wattage draw is for the motor you are using. Volts/watts=amps.
     
  6. Smoggie

    Smoggie Well-Known Member

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    I'd need the pitch of the prop to give a more accurate answer. I wrote a little calculator to work this stuff out and it's telling me that if you use a 5x5 prop (5" diameter and 5" pitch) you will pull 22A. If the prop is any lower pitch you should be well below 20A.

    This is only a guide of course and you should always test a new setup using a wattmeter. A 25 or 30A ESC would be safest as it's good practice to leave a little 'headroom' in your ESC rating.
     
  7. Smoggie

    Smoggie Well-Known Member

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    Actually for the ESC that's not required. The current the ESC sees is a result of the load on the motor which in turn is a result of the prop characteristics and the RPM. The motor will deliver whatever power it takes to spin the prop, if this is too much for the motor then the motor will burn out, so yes it is important that the motor is also rated for the amps that will be pulled, but this does not impact the ESC.

    In this case the motor should ideally to be rated for at least 25A, same as the ESC.
     
  8. Tony

    Tony Administrator Staff Member

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    This one is on you then. I ALWAYS go off of the wattage that the motor manufacturer states. rpm and prop size doesn't tell me what the motor is rated at and thus I have no idea what the safe draw is. OP, take it as you will but I would be looking at the wattage of the MOTOR to determine what ESC to go with.
     
  9. Smoggie

    Smoggie Well-Known Member

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    Attached is the calculator I mentioned. It's just a simple one, there are more accurate calculators online but this one is good for a quick check
     

    Attached Files:

  10. Smoggie

    Smoggie Well-Known Member

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    Tony,
    The wattage of the motor is only it's maximum rating when operated with it's largest prop and it's highest voltage battery. The actual watts that the motor will pull is 100% dependant on the battery and the prop that you connect (plus the motor Kv). Taking an extreme example, you can take a huge motor that's rated at 5000W but if you run it with a tiny prop it will only pull a few Watts.

    If you look at the published test specs for any motor you will see this relationship between battery and prop size which governs how much power the motor uses. For example here is the data for a Scorpion SII-2212 1850Kv which is rated for 22A/326W max.. but you will see from the table that most battery/prop combinations listed dont produce anything close to that: https://www.scorpionsystem.com/files/i2,226_data_chart.pdf

    Having said that, it's not a bad idea to always pick an ESC that at least matches the motor max current rating (if this is published). The thing you must bear in mind is that the motor WILL draw more than it's max rating if you put on a prop that's too big or a battery with higher voltage. Brushless motors are dumb, they dont know their limits, so it's important that the entire system is designed as a whole, including prop and battery.
     
  11. matthias

    matthias New Member

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    ok thanks everybody i got it
     

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