The IR compensation method is used to provide the speed regulation in our DC motor speed controls. When the load of a PMDC motor increases, the amount of current that the motor draws also increases. Per the performance characteristics of permanent magnet DC (PMDC) motors, an increase in load will cause a decrease in speed, because of the negative correlation between speed and torque (see speed-torque curve below).
IR compensation increases the voltage going to the motor, thus compensating for changes in the load, in order to maintain a constant speed at the end-user’s setting. As a reminder: In a PMDC motor, speed is proportional to the applied voltage, torque is proportional to the available current.
The “I” represents the current (in Amps) drawn by the motor, and the “R” represents the armature resistance (in Ohms).
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