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Brushless motor RPM formula

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Old 08-14-2009, 02:09 PM
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MBernhard
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Default Brushless motor RPM formula



Motor RPM = 0.8 x 3.5V x Series Cell Count x Motor kV Rating

This is at full throttle right? (No chopping by the ESC)

Mathias

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Old 08-14-2009, 06:13 PM
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Default RE: Brushless motor RPM formula

As far as I know, (and I may be wrong, it wouldn't be the first time ) the kv rating equals turns per volt. So a 1000kv motor running on a 3S LiPo, or 11.1 volts, would translate to 11.100 rpm, however, battery voltage is higher at peak charge, so you would want to take that into consideration. In other words, a fully charged 3S is actually about 12.6 volts, so a 1000kv motor on a freshly charged 3S would actually be around 12,600 rpm, and drop as the voltage decreased.
That's easier for me to calculate than more complicated mathematical formulas.
Of course, these numbers are all at full throttle.
If i'm incorrect, someone please let me know, that's how I was taught.
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Old 08-14-2009, 06:57 PM
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Default RE: Brushless motor RPM formula

Quote:
ORIGINAL: MBernhard



Motor RPM = 0.8 x 3.5V x Series Cell Count x Motor kV Rating

This is at full throttle right? (No chopping by the ESC)

Mathias


That 80% might be about right for a correctly propped motor... but there are far too many variables for this neat formula to cover all contingencies. At no-load, motor RPM might be 95+% of Kv x V, but load the motor with a prop (especially one which is too big for it) and you'll be at 75%... or 60%... or 50% of Kv x V.
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Old 08-15-2009, 03:02 PM
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Default RE: Brushless motor RPM formula

You also loose some voltage accross the motor windings themselves.
See formula (8) here:
http://www.rcgroups.com/forums/showt...ight=bergmeyer

Prettig weekend Ron
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Old 10-17-2015, 01:25 PM
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Quote:
Originally Posted by speedy72vega View Post
As far as I know, (and I may be wrong, it wouldn't be the first time ) the kv rating equals turns per volt. So a 1000kv motor running on a 3S LiPo, or 11.1 volts, would translate to 11.100 rpm, however, battery voltage is higher at peak charge, so you would want to take that into consideration. In other words, a fully charged 3S is actually about 12.6 volts, so a 1000kv motor on a freshly charged 3S would actually be around 12,600 rpm, and drop as the voltage decreased.
That's easier for me to calculate than more complicated mathematical formulas.
Of course, these numbers are all at full throttle.
If i'm incorrect, someone please let me know, that's how I was taught.

If need a full throttle RPM of about 43,000 @ 11.1v

what motor Kv should I get? approx 3800kv motor?

thanks
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Old 10-19-2015, 04:55 PM
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A 3800kV would be the minimum to get to 42,000 rpm WOT avg. 3800kV x 11.1v =42,180 max wot rpm. 4200kV x 11.1v = 46,620 max wot rpm
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