To calculate the wattage of the motor power that gets converted into usable mechanical work you would need to subtract back EMF from the voltage applied to the motor.
UsableMotorPower = MotorCurrent * (AppliedVoltage - MotorBackEMF) - ElectricalLosses - MechanicalLosses.
Where MotorBackEMF is proportional to the velocity of rotation of the motor, Torque generated by the motor is proportional to the Current, which is a function of the difference of the applied voltage and backEMF divided by the electrical resistance in the motor circuit.
Essentially, as you apply voltage to the motor it will generate max torque at rest and then its rotation velocity will increase and torque decrease until the point of equilibrium.
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