<p>In the speed regulation system of a permanent magnet synchronous motor (PMSM), the open circuit of a power device is the most common type of fault but is not easy to detect. With the fault, the control precision and output performance of the system decrease. To reduce the risk of fault contagion and ensure the safety and reliability of the system operation, this paper focuses on open-circuit fault diagnosis and fault-tolerant control. First, based on the defects of the long diagnosis time and poor adaptability to speed and load changes of the average current method, which is commonly used in engineering practice, an online fault diagnosis strategy that utilizes changes of the sector width is proposed. Experimental results demonstrate that the diagnosis time can be shortened to one electrical period or less. Meanwhile, extra sensors are not required to assist in the fault diagnosis, greatly reducing the volume and cost of the whole drive system. After that, to verify operation with a fault, a three-phase four-leg inverter is introduced in this scheme, which uses the advantage of the hardware redundancy of the three-phase four leg to reduce the difficulty of fault-tolerant control. A reliability comparison between three-phase bridge and three-phase four-leg topology is analyzed in detail using the Markov model, and the fault-tolerant performance of the latter is verified by experimental results.</p>