A Higher Level Inverter Fed Induction Motor Drive: Its Operational Challenges and PWM Solution

In this article, a modulation mode (MM) switch-based pulse width modulation (PWM) technique is proposed to mitigate the floating capacitor (FC) voltage control challenges in a higher level inverter fed induction motor (IM) drive. Although this inverter uses a single dc source (which is good for reverse power flow) and optimal number of switching devices, the lower speed operation of the constant v/f drive is not possible due to unacceptable FC voltage variation. The proposed PWM solution is used to tackle this issue by providing a smooth operation of IM for any power factor. Thus, a full speed range operation of the drive is possible, which also exploits enhanced dc bus voltage utilization capability of the inverter at higher speed operation. The design of FCs is also done for optimal selection of their capacitance values. Further, owing to three-level neutral point clamped front end structure of the inverter, a neutral point voltage balancing algorithm is derived to ensure high quality output voltages. Simulation results of indirect vector control based closed loop operation are presented to analyze the performance of the inverter fed IM drive with varying modulation index and load conditions. The effectiveness of the proposed modulation technique is also validated through experimental results.