Variable-speed PM synchronous motors with ferrite excitation

The standard motor for variable-speed main drives is the induction motor (IM). In comparison to synchronous machines (SYM), induction motors show the disadvantage of additional ohmic losses in the rotor. Therefore, especially low-power induction motors are not able to reach energy efficiency levels as high as IE4. The transition of motor technology from induction motors to synchronous motors with ferrite excitation provides the opportunity to increase motor efficiency without using expensive rare-earth magnets. Therefore, this paper deals with the steady-state modeling, simulation and design of PM synchronous motors with ferrite excitation in respect of reaching the highest possible motor efficiency. Due to the comparatively poor magnetic parameters of the ferrite magnet, a motor design using the flux concentration effect has to be used. An analytical approach based on a magnetic network is developed to describe the steady-state operating behavior of the ferrite excited PM synchronous motor. The non-linear network equations are solved by applying the Newton-Raphson Method. Due to short calculation times and a high flexibility in respect of the design of a whole product series, the analytical approach is preferred to a numerical approach. To validate the simulation results, a prototype is designed based on a fundamental induction motor. Finally, the test of the prototype in laboratory allows a comparison between simulation and measurement as well as between ferrite excited PM synchronous motor and induction motor.