Permanent magnet synchronous motors (PMSM) are widely used in industrial manufacturing, transportation and household appliances because of their high power density and excellent performance. In consideration of reducing the size of the control device, cost reduction and system operational reliability, it is necessary to abandon the position sensor and realize the sensorless control of permanent magnet synchronous motor. At present, the magnetic chain observer method is one of the most commonly used and easy to implement algorithms for sensorless control of permanent magnet synchronous motors. However, there is a pure integration link in the magnetic chain observer method, which will bring a certain zero drift due to the sensor error or the dead time. The zero-drift compensation technique based on the chain observation method is investigated in this book. In addition, the maximum torque to current ratio control (MTPA) has the feature of high operating efficiency, so the permanent magnet synchronous motor often uses MTPA control at low speed operation, and often uses weak magnetic control at high speed.