Elecric Brushless Motors
Hubs or belt & pulleys, they are using brushless motors that have common properties that are interesting to know when using them:
- number of poles
- kv rating
- maximum voltage
- maximum amperage
- maximum power
The KV rating is an important property of a motor as it determines the rotation speed of the motor when a certain voltage is applied to it. The higher it is, the faster the motor spins for a given voltage. If you want to learn more about it, read this blog article.
This is important as in the electric skateboards world, we don't need extremely high rotation speeds, although we need good torque.
For simplicity, we will say that KV = RPM per volt. That's actually false but it gives a good approximation of how KV works.
The KV allows us to calculate another important property: the eRPM. If you're planning on using a VESC (what's this?), you will need to calculate the eRPM of your motor with the battery you're planning to use.
eRPM = pole pairs x voltage x kv
If you don't know the pole pairs of your motor, most of the motors are 14 poles, so that's 7 pole pairs. The voltage must be the voltage of your battery at maximum charge.
Let's calculate it for a 190KV motor with a 10S battery (37V nominal voltage, 42V at full charge): 7 x 42 x 190 = 55860 eRPM. So all good for the VESC and its 60000 eRPM limit.
Of course, the maximum voltage, amperage and power need to be taken into account too. The brushless motor must support the battery voltage. The ESC (or VESC) must limit the amperage going to the motor. And finally, as power (wattage) = voltage x amperage, calculations need to be made to respect that limit too.
On a hub drive setup, the electric motor is inside the wheel. This offers a limited choice of options, manufacturers are all building all-in-one solutions. You buy the whole wheel assembly, there's no choosing between motor size and urethane sleeve.
There are a few sizes options and different KV ratings though, to allow builders to use different batteries and provide different amount of power.
But the number of factories building them are limited, that's why most of the commercial electric skateboards using hub motors are often actually using the same motors.
Belt & pulleys