When the motor drive operates in the torque mode, a constant torque implies a constant current in the motor winding. But we see the supply current varies as the motor speed changes. The supply current is at the lowest when the motor stalls and the highest when the motor moves the fastest. The total energy provided by a constant voltage source is consumed by the winding resistive loss and the work done by the motor. When the armature current is constant, the resistive loss is constant. The work done by the motor is the torque multiplied by the motor speed.
How does the motor drive keep the winding current constant when the supply varies? When the load is greater than the motor torque, the motor slows down and stalls. If the drive keeps the same PWM on period, the current would increase; so the PWM duty cycle is reduced to keep the current constant by the current loop. The PWM duty cycle is proportional to the motor speed (assume the winding resistance is small). The drive circuit draws current from the supply during the on period of the PWM cycle. The peak current is same if the constant winding current is maintained. So the average supply current is smaller when the motor speed is reduced.
Therefore to infer winding current from the supply current, the PWM duty cycle has to be known.
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