Asymmetrical multilevel inverter with staircase modulation for variable frequency drives in fractional horsepower applications

Nowadays, the great use of small induction motors in domestic applications and other industrial applications justify the development of new servo-drives and motor designs. The main objective should be to improve the efficiency and to consume energy with a clean absorption of current harmonic from the mains. Thereby, this paper presents an application for the speed control of a small single/two phase induction motor. A new topology of cascaded multilevel inverter together with several half-bridge cells in series using a reduced number of switches and insulated gate driver circuits is proposed. Hybrid multilevel converter is controlled through a staircase modulation in order to minimize the switching losses. Waveforms generated by the system are composed in the motor windings to obtain a voltage with variable amplitude and frequency; the objective is to supply small electric power drives. The two parameters of the waveforms - amplitude and frequency - in the motor winding are easily regulated through the phase shift. The staircase modulation can be easily implemented in the multilevel inverter due to its unique structure and relatively simple control. Compared with the carrier-based PWM schemes, the staircase modulation is characterized by low switching losses since all the elements operate at the fundamental frequency. This new drive provides advantages as a straightforward control method, simplicity of the design, easily regulated starting torque and its low cost. As a whole, the design leads to a robust, compact, cheap unit, easy to manufacture. In summary, a new control type for angle phase is proposed through the staircase modulation for adjustable speed drive in fractional horsepower small single/two-phase induction motors.