Application Note

DC Motor/Generator Technical Note: Practical Use Of The Motor Constant


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Technical Note: Practical Use Of The Motor Constant

Frequently a dc motor or generator specification will include the value of the motor constant KM, which is the torque sensitivity divided by the square root of the winding resistance. Oftentimes even seasoned dc motor applications specialist will view this intrinsic motor property as a somewhat esoteric figure of merit useful only to the motor designer. The feeling is that motor constant KM has no real practical value in selecting and dc motors and generators in the real world.

Nothing could be further from the truth! Utilizing this figure of merit properly will substantially reduce the iterative process in selecting a dc motor or generator since it is generally winding independent in a given case or frame size motor or generator from a given vendor.

Even in ironless dc motors, where the KM may vary somewhat depending on winding due to variations in the copper fill factor, it remains a solid tool in the selection process. Particularly if there is a listing with Motor Constant in ascending or descending order, like there is in Appendix III at the end of this document.

This methodology limits the "winding hunt" where one winding satisfies the current requirements but not the voltage requirements and vice versa. You "can't get there from here" if the motor does not have the intrinsic electromechanical ability to satisfy the application requirements. This is NOT a thermal issue, but the intrinsic ability of the motor to transform electrical power to mechanical power or the generator to transform mechanical power to electrical power.

Since the KM of an electromechanical device does not address the losses in that device in all circumstances the minimum KM must be larger than calculated to address those losses. This method may not get you to exactly the correct "house" but it will put you on the correct "street".

This methodology is also a good "sanity check" since it forces the user to compute both the input and output power. This was an omission the author made more than a quarter century ago in attempting to address a requirement from a University PhD's need for a motorized prosthesis device that simply defied the Laws of Physics. It was also the first and last time such an error was ever made!

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Technical Note: Practical Use Of The Motor Constant