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If a BLDC motor has a power rating of say, 500W.

Is this a measure of its power consumption at specifically full speed?

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If it's an output power rating, it won't be at full speed (which is the unloaded speed) but at rated speed, which varies, but will typically be 80-90% of the unloaded speed.

Decent motors will specify the measurement conditions and show how speed reduces as power increases for a given voltage under increasing load (torque) and current.

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A properly specified BLDC motor (or in fact any motor) will state its mechanical output power. If the motor specification says 500 watts (and it's a reliable specification) then that is the output shaft power. Input power can be a little to a lot more depending on how you drive it and what you are asking the motor to do regards speed and torque.

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Mechanical output power is determined by torque and speed.

Speed is limited by supply voltage and how much the bearings of the motor are rated for. Torque is limited by how much current the motor coils can take without overheating, which is mostly constant irrespective of speed.

Thus the actual motor power will depend a lot on the actual application: what speed the motor is used at, how much cooling it gets and what is the ambient temperature.

Any power rating offered by manufacturer is just an example at a specific situation. If the specific situation (i.e. speed and cooling used) is not given, the number is close to meaningless.

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