How to Calculate Motor Torque Constant April 3, 2012

The torque constant, Kt, of a motor is a very useful parameter for sizing and controlling motors showing a linear speed / torque relationship.  Both DC brush type and brushless motors exhibit this linear performance curve relationship.

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Without getting into theoretical detail, Kt is simply the slope of the torque / current curve of a motor.   The units of the constant are found in torque units per amp. (e.g. N-m/amp, oz-in/amp, etc.) Kt can be useful in both design and application.  Looking at the current in the system the designer or control system can calculate the actual torque output of the motor during operation.  This can be used in a variety of ways.  It allows the designer to understand the actual load that is seen in the application and understand if the optimal motor for the application is being used.   On the control side, a control can be set up with a current limit to ensure that the mechanical system is not over loaded or it can be used to assure that the motor does not overheat during use.  

Additionally, Kt is used in torque mode applications to maintain a controlled amount of torque to meet the application needs.  Example:  The power source for the application allows for the motor to draw a maximum current of 10 amps. Based on the design, you need to drive a maximum load of 10N-m. What is the minimum Kt needed for this application?

It should be noted that the parameter Kt is not related to the voltage under which the motor is operated.   If you use the motor at 12VDC or 24VDC this constant will remain the same.  This attribute of the motor is very useful as the motor is used in a motion control system.   The overall torque output of the DC motor system is maintained by monitoring current while the motor’s speed output is varied by controlling the voltage into the motor.

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Warren Osak
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