How to choose the motor KV?
Choosing the correct KV is very important, it’s critical for the motor operation and high motor performance which is critical for the project.
There are few things that determine the top speed: Battery voltage, gear ratio (if any), wheel size ( if any) and the motor KV.
Sometimes you just need to choose (or change) the gear ratio (if there is) or even the wheel size (if there is at your application) or just changing the voltage.
Sometimes you can’t change (or it’s not easy to change) other things that affect the rpm and you got to have only the correct KV motor, for example: direct drive propeller at certain battery voltage.
In order to determine the correct KV (when you know the other parameters) - we advise first to calculate the max motor rpm that you need, for example, say it’s an ebike application:
The top speed that you want to ride is 50mph (80km/hr), battery voltage is 60V, wheel size is 26”(include the tire) and the gear ratio is 1 to 4.
First you need to know the wheel speed: 50mph (80km/hr) for 26” wheel is about 640rpm.
The calculation of the wheel speed (metric system):
80km/hr are 22.2 meter per sec, wheel diameter is 0.660m (26” wheel) and the extent of the wheel 2.074m (just multiple 0.660m by π).
So, the wheel travel 2.074m per 1 revolution, in order to travel 22.2 meter within 1 sec it need to spin 10.714 times per sec.
10.714 rotations at 1 sec is about 640rpm.
If you don’t know (or sure) how to calculate the wheel speed you can use an online wheel rpm calculator.
Now we know the wheel rpm, in order to know the motor rpm we just need to multiple by the gear ratio, our gear ratio is 1 to 4 reductions so the motor rpm is 2560rpm.
The speed that we calculated is the under load speed, but the KV value refer to the no load speed, so we are assuming that the under load rpm is about 90% of the no load speed (which is reasonable for most applications).
The max motor no load speed should be 2844rpm (2560/ 0.9 = 2844rpm)
The max motor rpm that we need is 2844rpm.
We need 47.4KV motor to get to 2844 rpm at 60V (2844/60= 47.4 KV).
It is important to say that the 60V voltage is nominal voltage and in reality the voltage changes, usually in between +10% to -10% of the nominal battery voltage (+10% for full battery to -10% for empty battery) so does the motor (and bike) speed.