Electrically excited synchronous motor has the advantages of wide speed range, strong overload capability and adjustable power factor. It is widely used in the field of high power transmission. The system structure and the neutral point voltage balance control strategy of a neutral point clamped (NPC) three-level high voltage converter are introduced for this purpose. Speed sensorless vector control strategy based on current frequency (IF) control and sliding mode observer (SMO) is used to control the variable frequency starting and speed regulating of electrically excited synchronous motor. The MATLAB simulation results show that the converter has good startup and operation performance due to the adoption of the proposed control strategy. It provides technical support for high voltage and high power transmission applications.