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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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speedy72vega
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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

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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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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Old 10-06-2018, 05:51 PM
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If you wanted you could take this further a figure out a theoretical top speed that your gearing could put out. This is by no means and exact, accurate number. There are too many variables to get an exact number on how fast your r/c can go. There are most likely better ways to figure it out, but this way seems to get fairly close for me. This is based off of a 3s battery, so 11.1 will be a static number for one of the formulas. The formula I used for rpm gives a more realistic mph over kv battery voltage. I will explain more in the example.

Numbers you will need for this formula:
Pinion tooth count (pT)
Spur gear tooth count (sT)
Gear ratio (GR)
Tire Diameter (TD)
Approximate RPM (~rpm)
KV rating ( kV)
Amperage of battery (mah/ah)
Voltage of Battery (bV)
Percentage of cell count (%s)
Percentage of kVa (%kVa) (2s only)

Formulas:
GR = sT pT (driven gear drive gear)
%s = (3s bV - 2s bV) 3s bV (for 2s battery)
%kVa = 1 - %s
%s = (4s bV - 3s bV) 4s bV + 1 (for 4s battery)
~rpm = kV ah %kVa (2s)
~rpm = kV ah (3s)
~rpm = kV ah %s (4s+)
MPH (aka top speed) = (~rpm TD) (GR 336)

If you have multiple gear ratios, such as a f/r differential, the second () for the mph formula will look like this: (GR GR 336). This will use your overall gear ratio.

The 336 is a static number based on pi and a breakdown of distance over time converted in just distance (as far as I've been able to figure). I can give a better explanation of this if you would like.

Example using my bandit will the old GR vs new GR:

Old pT = 83t
Old sT = 24t
New pT = 33t
New sT = 76t
kV = 3800
TD = 4.25 ( This is only the rear tire, since only the rear tire sees power)
ah = 5
bV = 11.1

~rpm = 3800 5 = 19000

Old GR = 83 24 = 3.458333 = 3.46
MPH = (19000 4.25) (3.46 336) = 69.4587806221 = 69.46

New GR = 76 33 = 2.303030303 = 2.30
MPH = (19000 4.25) (2.30 336) = 104.4901656315 = 104.49

If i was to use a 2s the math would look like this (using numbers from new GR):

%s = (11.1 - 7.4) 11.1 = .3333333 = .33 or 33%
%kVa = 1 - .33 = .67
~rpm = 3800 5 .67 = 12730
MPH = (12730 4.25) (2.3 336) = 70.0084109731 = 70.01
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Old 10-06-2018, 05:55 PM
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If you do the kv battery voltage your mph numbers will turn out to be very unrealistic. When i used that with the old gear ratio, numbers were saying that I was topping out at about 172+mph, which will all know that I was nowhere near that.
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