Control Issues of High Pole Count Machines at High Speed Operation

Traction AC motors for the hybrid vehicles are designed to have high pole counts to increase torque output at zero and low speed. Then maximum synchronous frequency also increases due to the high pole counts. But switching and sampling frequency of the inverter is limited because of cost and efficiency reasons, hence the ratio of sampling frequency over synchronous frequency, f_samp/f_r, is getting smaller at higher rotating speed. Instability and sampling error are two main issues of digital controllers when f_samp/f_r is significantly small. This paper focuses on the stability and sampling issues of current regulation loop implemented with a digital controller when f_samp/f_r is significantly small. Several types of current controllers are investigated in the point of view of the stability of the inner current regulation loop. And a model-based compensator is proposed to reduce the sampling error, and verified through simulations and experiments.