How To Calculate Brushless Motor RPMs

Brushless Motor RPM's and How to Calculate them.

Brushless Motor and ESC's are the latest and greatest things to hit the Electric RC world.

When paired with the latest in Lithium Battery Technology they offer unparalleled power and longer run times compared to their Brushed cousins.

When trying to calculate your Brushless motor max RPM's their are a few numbers you need to know first.

Your Brushless Motors KV Rating. Some motor manufactures assign turns to there motors but the KV rating is almost always on the manufactures website.
How many Volts are you getting from your batteries. Since LiPo batteries are normally run with Brushless Motor/ESC systems we will do our calculations with the standard 3.7 Volts per cell. So a 2S Lipo will be putting out 7.4 Volts and a 3S Lipo will be putting out (3x3.7=11.1) 11.1 Volts.

My first example will be using a Team Novak Havoc Brushless ESC with a Team Novak SS Pro 21.5 Turn Motors.

Since Team Novak assigns Turns to their brushless motors we have to go to Team Novak's website and get the KV rating for the SS PRO 21.5 Turn.

Click here for the Team Novak Brushless Motor spec chart.

So now that we know the KV spec for our 21.5 turn motor is 1800 kv we just need to find out how many volts we are using.

The easiest way to find out how many Volts your LiPo Battery is to look on the battery Most manufacturers have labels with cell counts MAH ratings and safety warnings on them.

The battery I am using is a 2S battery so it is 7.4 Volts.

Now its time for some simple math.

KV * Volts= Max Unloaded RPM's
or
1800 * 7.4= 13,320 RPM's

Pretty Easy.

Here is another example a HPI Flux HP Brushless System for the Savage Flux.

The KV rating for the Motor is 2200

I am running two 2S batteries in series so it is basically like having a single 4S battery (4 * 3.7= 14.8 Volts)

so the math would be.

2200 * 14.8= 32,560 RPMS

Why is this information useful? It will help you choose the correct spur gear and pinion gear combo and even help you calculate your theoretical top speed.

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