A door operator for controlling operation of a door, the door operator having a motor to open the door against a spring force, the door operator further comprising a door position sensor for transmitting a signal indicative of door position; and among other things, calculates a door moment of inertia based on a net torque and the time for the door to reach a predetermined angle from the closed position. Also provided is a door operator that compares door speed to a desired door speed based on a door speed-position profile and generates a door speed error signal and minimizes the door speed error signal by adjusting the braking load resulting from charging a chargeable battery using the motor as a generator.