POWER REQUIREMENT

Power required (kW) = [Cranking speed (rpm) x Torque (Nm) x 2 x pie]/60000

*P = [120rpm (10% from idle) x 81.7Nm x 2 x pie]/60000 = 1.027kW = 1027W

From this value, the needed starter motor should be able to handle between 800 to 1200W of power delivery. This large variance is because we’re dealing with apparent power instead of a fix one. Refer previous paragraph for a bit more details.

Let us now choose a motor, it should be rated at around 1000W and somewhat low in the KV rating. The low KV rating here is to ensure the motor will have enough basic torque to begin spinning the load. Try choosing a motor that can spin large diameter propeller even if the top end speed is somewhat lacking. We don’t need the top end here. With that in mind, let us play around with a motor having a KV rating of 700.

Now if we apply 11.1V (3cell lipo) to the a motor that is rated for 700kv, the resulting unloaded rpm will be 7770.

From here, we need to reduce the rpm down to starting speed, say 120rpm therefore automatically we can increase the available torque. The reduction gear ratio should be 7770/120 = 64.75. Pause and look back for while, if we can have 1000W motor, why do we need step down? The question cropped up due to the fact of *P that was calculated. But look closely, we based our POWER calculation on a very small rpm and a very large TORQUE. In the market today, can we find such a motor that can rotate at about 120rpm yet producing more than 80Nm of torque? The answer is NO. On specialize cases maybe…

Okay back to the real world problem, the unloaded rpm of 7770 will quickly diminished as the load being applied; off hand it’ll be very difficult to predict the actual rpm of the motor under load. However from experience, the rpm will drop down to between 25 to 50% of the unloaded rpm and this translates to only between 3900 to 5900rpm of running speed.

Therefore the step down ratio can be designed to fall between 50 to 75% of the unloaded ratio. Or in this case the ratio of between 32 to 45 times of step down should be sufficed for our application.