Motor size constants

Motor size constants are fixed by the amount of steel and copper in a motor. No matter how the winding area is filled, size constants remain fixed.

The most important motor size constant is Km, the motor constant. It has units of torque per square root of Watts. Since torque is proportional to the current, and winding losses are proportional to current squared, Km cannot have units of torque per Watts since then can Km would change with the specific torque point. Km is useful since you can determine the minimum Km required based upon two speed torque requirement points. Km is key to motor selection

Another size constant which is much less important is Kd, which has units of torque per speed. It represents the motor’s internal speed related losses such as air friction, lamination hysteresis losses, and lamination eddy current losses. 

One last motor size constant is Tf, the motor friction constant with units of torque. Keep in mind that friction torque is not necessarily the same as the torque measured to rotate the motor. The torque required to rotate the motor may include substantial cogging torque. Cogging torque is caused by the interaction between the motors magnets and the motors laminations. It is not a loss; any energy used over coming a cogging is returned in passing the cotton point. 

Next: Defining the requirements

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