Basic Motor question.

I am looking at various motor types for a new airframe that I am working on, and just want to get feedback on the KV level.

Usually I run 1000 kv motors but I am seeing on a couple of motor suppliers that when the KV goes down,the price goes up.

 So what is the scaling for the kv?

Higher the KV the more powerful the motor? Or is is the lower the KV the more powerful the motor?

 

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    Moderator

    Michael Pursifull

    Higher Kv, faster the motor spins per volt.
    Lower speed per volt means it's pulling more current, more torque, generally. That usually means the motor has to tolerate higher currents, which speaks to the quality of parts, thickness of wire, etc.

    In general, the lower the Kv the larger the prop it can spin, assuming it can handle the power required, because you need to spin large props slower (so, basically I'm saying it all backwards)

    What you want to know is the power (in watts) required to turn a prop at a specific rpm that delivers the thrust you require. Then, you want to achieve that rpm (which needs to be below the max rpm for that prop, lest it break) at a power input that the motor can handle, and that is within the efficient range for that motor. Most motors start at low speeds as very, very inefficient, then within about 1/3rd of their power range they peek out around 80%, then begin to fall off in efficiency gently, so that at around 50% power they are around 70-60% efficient.

    Now, here is where the Kv really comes in.... You need to spin the motor at a specific rpm. The motor is rated by Kv, meaning that it spins that many times per volt. You have another contraint, besides max rpm of the prop, and that is how much current you can push through it. It has a max wattage, sure, but if the voltage is too low, the current will be very high, which results in more heat generated, until your motor shorts out. Melt down, my good man.

    So you can increase the voltage, to achieve the same power (wattage) for fewer amps (current) so that your motor doesn't become molten slag (dramatic). But when you do that, since the motor is speed controlled by voltage, it ties to spin the prop faster. So you also swap to a lower Kv motor. Same power to spin the same prop at that same speed, but as you try to turn it that fast, the motor draws more current, because spinning a prop faster required more power. If the prop was flat or there was no air, it would just achieve that rpm with little power/current draw.

    Hope that helps... So lower Kv motors allow you to: use more power without burning out your motor, spin the prop slower while using the same/more power, ergo, you can spin a larger prop (slower, but with more torque)

    Cheers.