Medium and high voltage frequency converters has wide applications in the field of industrial frequency conversion. H-bridge cascaded multilevel frequency converter has the characteristics of modularization and can realize the expansion of voltage and power by using low voltage devices. The main circuit structure of a three-stage H-bridge cascaded converter driving squirrel cage asynchronous motor is introduced. Then, the key control technologies, such as the carrier phase-shifting sine wave pulse width modulation (PWM), the DC voltage equalization control of power unit, and speed sensorless vector control based on the voltage and current hybrid flux observer are introduced. The MATLAB simulation results show that the converter has good start-up and operation performance by using the proposed control technologies to control squirrel cage asynchronous motor. It provides technical support for medium and high voltage and high power transmission applications.