Asynchronous Torque of Line-Starting Permanent-Magnet Synchronous Motors

A modified theory and a proposed analytical approach clarified asynchronous torque characteristics of line-starting permanent-magnet (PM) motors, taking into consideration the mutual effect among the fields due to armature current, cage-bar current, and PMs. Theoretical and analyzed results revealed that cage torque had an oscillating component with slip frequency due to magnetic saturation. This component cannot be seen in the conventional theory. It was also found that the coupling effect between PM flux and cage-bar flux caused on magnet torque larger oscillation and more significant negative dc component, compared with the case in the conventional theory. The theoretical and analyzed results were validated on the basis of experimental approaches. Results are given for a two-pole prototype motor with P_N = 5 kW, n_N = 3000 min^-1, V_N = 200 V, and Y-connection.