Things I don't understand, motor spec and ratings KV/RPM etc

There is no direct relationship between the kv and power. Roughly speaking, how it works is this:

The KV is how fast the motor wants to run at no load. As you load it, it slows down from that no-load speed. As it gets slower, it tries harder and harder to go faster, so it does more work as you load it more. As you load it more, the power that it is making increases. This can go on, until the point that you reach the power limit of the motor. If you exceed the power limit, you will burn out the motor. You also have to be aware that as you load it more, and thus increase the power, you also increase the amperage being used, and you can also burn out the ESC if you reach it's amp limit before you reach the power/thermal limit of the motor.

So all else being equal, meaning the same ESC, the same battery, and the same prop, the motor with the higher KV will make more power at full throttle. This is because it is trying to spin the prop faster than the slower motor does, which necessarily means that it is going to use more power. Therefore, a higher KV motor will make more power with a given prop. I have also found a tendency where a given motor construction (same manufacturer, same size, but different KV windings) tends to show that higher KV motors also can withstand higher loads, they are rated for more power. I'm not quite sure why this is.

So it would seem like the obvious answer is to use a motor with higher KV. They make more power with a given prop, and they can withstand more load/power. This is true if your goal was power/weight.

However, if your goal is efficiency, then it is more efficient to turn the prop slower. That is, thrust/watt.

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Michael Rector December 16, 2012 at 6:11am

i am just starting myself, so i don't have much to contribute at this point, but i did find this calculator yesterday that helps explain some power to kv to prop equations. you can plug in some numbers and see how it affects thrust, load, etc...

xcopter calc

http://www.ecalc.ch/xcoptercalc_e.htm
Rob_Lefebvre November 12, 2012 at 6:56am

There is no direct relationship between the kv and power. Roughly speaking, how it works is this:

The KV is how fast the motor wants to run at no load. As you load it, it slows down from that no-load speed. As it gets slower, it tries harder and harder to go faster, so it does more work as you load it more. As you load it more, the power that it is making increases. This can go on, until the point that you reach the power limit of the motor. If you exceed the power limit, you will burn out the motor. You also have to be aware that as you load it more, and thus increase the power, you also increase the amperage being used, and you can also burn out the ESC if you reach it's amp limit before you reach the power/thermal limit of the motor.

So all else being equal, meaning the same ESC, the same battery, and the same prop, the motor with the higher KV will make more power at full throttle. This is because it is trying to spin the prop faster than the slower motor does, which necessarily means that it is going to use more power. Therefore, a higher KV motor will make more power with a given prop. I have also found a tendency where a given motor construction (same manufacturer, same size, but different KV windings) tends to show that higher KV motors also can withstand higher loads, they are rated for more power. I'm not quite sure why this is.

So it would seem like the obvious answer is to use a motor with higher KV. They make more power with a given prop, and they can withstand more load/power. This is true if your goal was power/weight.

However, if your goal is efficiency, then it is more efficient to turn the prop slower. That is, thrust/watt.

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Things I don't understand, motor spec and ratings KV/RPM etc

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