Motor constants

Motor constants fall into two groups: motor size constants and motor winding constants.

Size constants are constants that are fully defined by the motor size.  These constants are the peak torque, thermal impedance and the motor constant, Km, which has units of torque per square root of watts.  Km is an important constant in sizing motors and will be discussed later.

Winding constants depend on how the motor is wound, specifically, the gauge of the wire and the number of turns.  Winding constants are Kt, the torque constant, Ke, the back EMF constant, L the inductance, and R the resistance.

Size constants are fixed by the motor since each motor has a fixed amount of lamination steel, magnet and volume for the copper winding.  No matter how you fill the available winding area, motor size constants do not change. On the other hand, winding constants change depending upon how the motor is wound.

A motor manufacturer could have in stock a single design of lamination stack and rotor for brushless DC motors waiting to be wound for customers. When properly wound these motor components could be assembled into motors that have a wind range of back emf constants.  Because laminations require special tooling it’s typical for motor manufacturers to have a number of different lengths of lamination stacks using the same lamination and thereby providing different size motors. Each different lamination stack size provides a different size motor and

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