U.S. Pat. No. 9,281,772 discloses a controller for a power convertor. The controller can put, on a motor, a brake of good response. The following Non Patent Literature 1 discloses a flux observer for safe operation of a motor.
Patent Literature 1: U.S. Pat. No. 9,281,772
Non Patent Literature 1: Y. Xu, Y. Wang, R. Iida and R. D. Lorenz, “Extending low speed self-sensing via flux tracking with volt-second sensing,” 2017 IEEE Energy Conversion Congress and Exposition, Cincinnati, Ohio, 2017, pp. 1888-1895.