Efficient Control of A-C Machines in the Brushless D-C Configuration

This paper derives and analyzes equivalent circuit models of induction and synchronous motors. Optimal control algorithms and torque limits are derived for both motors. It is shown that the use of these algorithms leads to achieving high efficiency with either type of motor over a range of operating conditions that includes order-of-magnitude changes in both torque and speed. This is an important result in view of the widespread belief that these motors can only be efficient over a narrow range of operating conditions. Although the synchronous motor is shown to be slightly more efficient, especially at low speeds., it was concluded that the simplicity and ruggedness of the induction motor makes it preferable for many brushless d-c motor applications. The paper also describes a practical means of applying the optimal control functions to an induction motor brushless d-c system. Simple active networks are derived for use in generating the proper control algorithm. This type of control can be applied to any size system to achieve the maximum possible efficiency over a wide range of output speeds and torques.