Understanding Servo Motor Time Constants

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For all Compumotor servo motors, three kinds of time constant specifications are evaluated:

Motor Thermal Time Constant
Electrical Time Constant
Mechanical Time Constant

Understanding what these constants are can help in selecting the best motor for a given application.

The motor thermal time constant is an indicator of the heat capacity of a motor's windings and outer case; a measure of to what degree a motor stores internal heat. A lower thermal time constant denotes a motor that builds up internal heat quickly, but also can dissipate that same heat quickly. A higher value denotes a motor that requires more time to shed some given amount of heat. This constant will tend to be higher in larger motors, but in general - the lower the value of this constant, the better.

The electrical time constant is the winding inductance of a given motor divided by the motor's resistance. The electrical time constant tells the amount of a motor's current ripple for a given switching frequency. The higher the electrical constant, the lower the peak power. The lower the value of this constant, the higher the switching frequency required for the motor.

A motor's mechanical time constant is a NEMA mandated measurement. It is the time required for a given motor to reach 63.2% of its maximum rated speed in a no-load condition. The mechanical time constant is basically a measure of a motor's responsiveness.

updated 11Feb2015 nmc

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