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Originally Posted by zeropointbug
LOL, okay now I know... are you sure you know what your saying?
The physical resistance is only PART of the heat output. Most of the power loss is the design of the motor, and how well it transforms the power input to it.
What is 2.7Watt? That would be barely warm to the touch....
The esc I am guessing is somewhere between 92 - 97% efficient. Couldn't tell you, it's probably about 10 - 25 watts of heat, which is alot for these little FET's to dissipate, that's why they can get so hot because of insufficient heatsinking. With a proper heatsink, 25 watts of heat is a piece of cake to keep the temps down (~15C Delta).
Is there a misunderstanding here about motor efficiency.
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Let's start by saying that motor efficiency has very little to do with ESC heat. The only motor spec that might effect ESC efficiency is the inductance value due to the phase angles produced, which can be more difficult to drive.
Assuming an FET goes rail to rail, the
only time an FET dissipates power is when there is a voltage drop across the device AND current flow. If the output of the ESC were perfect square waves (0 rise time), then the ESC would be 100% efficient no matter what the motor is doing with that power. At the 0 point of the square wave, there is full voltage across the FET but 0A (0 watts). At the max point of the square wave, there is 0v dropped but there is max current (0 watts). But during the ramp up (the total time it takes to do this is the slew rate), there is voltage dropped AND current flow, which creates the heat. The longer the ramp, the more power loss and more heat. Higher frequencies have more pulses per second and therefore more "ramp up and downs" which can make them an ESC heat up more. However, too low of a switching frequency will cause the motor to not act correctly.