Brushless Motor Construction Discuss design and construction of custom Brushless motors

Micro Dan amps

Old 01-03-2019, 01:19 AM
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Jack_Hammer
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Default Micro Dan amps

I've recently installed a new Micro Dan 1875kv.
Man this think screams (speed and torque) much more than what it replaced (1300kv). The out going motor was only pushing 12oz of thrust with a 7x6 prop. (not really a 1300kv motor?) and a 3c lipo
The micro Dan only offers an additional 575kv, yet it's producing a whopping 31oz of thrust on the same battery using a 7x5 prop. According to Go brushless (micro Dan) it's continuous Amp with the prop is 26 Amp. But they say (via email) they always add 10% (+2.6 amps) to the Amp ratings for obvious reasons.

​​​​​​My question is in the prop. Is there such a huge difference in amps used between 7x5 and 7x6 props? Seems to me that it would barely be worth mentioning.
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Old 01-03-2019, 10:13 PM
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ron_van_sommeren
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Kv is not a rating, not a figure of merit.
Achieving a higher Kv is no big deal, just a matter of lowering number of winds.
Higher rpm for given voltage is supposed to happen: rpm_noload = voltage × Kv.

It's all about what the motor wants to do versus what the motor can do.
Kv, expressed in rpm/volt, matches desired rpm and voltage, nothing more. It says nothing about a motors max. power and max.current.
  1. Kv and voltage determine how fast motor wants to run.
    Kv = rpm_noload / voltage.
  2. Rpm and prop determine torque, torque in turn determines current.
    current = torque × Kv (in SI units!)
  3. Max.current and max.power determine whether that voltage/motor/rpm/prop combo can run without going up in smoke.
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Old 01-03-2019, 10:20 PM
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ron_van_sommeren
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Current and power drawn are proportional to Kv³.
E.g. double Kv and current drawn will 2³=eight-fold.
In your case current wants to increase by factor (1875/1300)³ = 3, 200% extra!

The 'full' story:
Motorcurrent is proportional to pitch¹, voltage², Kv³ and diameter⁴.
Power-drawn is proportional to pitch¹, voltage³, Kv³ and diameter⁴.

Without the exponentation
extra current with one or two cells added, simple table

So, changes in setup (and lousy Kv specifications!) can have surprisingly considerable/huge effects.
E.g. doubling voltage will four(2²)fold current, doubling Kv will eight(2³)fold current, and doubling prop diameter will sixteen(2⁴)fold current.
Even a small 10% change/difference in Kv will already lead to a 30% difference in current.

Last edited by ron_van_sommeren; 01-04-2019 at 12:40 PM.
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Old 01-03-2019, 10:24 PM
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ron_van_sommeren
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An excellent quote from
brushless motors Kv?.
Originally Posted by scirocco
While an absolutely critical part of the system ...
... Kv is actually the item one should choose last.
  1. Decide your peak power requirement based on the weight of the model and how you want to fly it.
  2. Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
  3. Pick a prop that will a) fit on the model and b) fly the model how you want - often as big as will fit is a good choice, but if high speed is the goal, a smaller diameter higher pitch prop will be more appropriate.
  4. Look for a size class of motors that will handle the peak power - a very conservative guide is to allow 1 gram motor weight for every 3 watts peak power.
  5. Then, look for a motor in that weight range that has the Kv to achieve the power desired with the props you can use - a calculator such as eCalc allows very quick trial and error zooming in on a decent choice. For a desired power and prop, you'd need higher Kv if using a 3 cell pack compared to a 4 cell pack. Or for a desired power and cell count, you'd need higher Kv if driving a smaller diameter high speed prop compared to a larger prop for a slow model.
The reason I suggest picking Kv last, is that prop choices have bounds - the diameter that will physically fit and the minimum size that can absorb the power you want. On the other hand, combinations of voltage and Kv are much less constrained - at least before you purchase the components.

So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, e.g. limited prop diameter, if it's a pusher configuration, or if you already have a bunch of 3S packs and don't want to buy more, and so on.

Minor lay-out changes by RvS
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Old 01-04-2019, 12:19 AM
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Jack_Hammer
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Thanks for the input. It's interesting to know that little diffences can offer big changes
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