Design-Oriented Analysis of Discrete-Time Current Regulators for Low Carrier Ratio High-Speed Induction Machine Drives

High speed induction machines (HSIMs) above conventional 3600 revolutions per minute (RPM) are attractive especially in compressors industry due to their high reliability, simplicity in construction, low maintenance costs, and wide speed range. However, the higher rated frequency up to 1kHz presents performance challenges while the switching frequency of conventional insulated gate bipolar transistor (IGBT) is generally less than 10kHz. The low carrier ratio (carrier frequency over operating frequency), with significant control time delays introduced by the pulse width modulation (PWM) inverter and discrete-time sampling effects, leads to severe inverter output voltage phase error, reducing the dynamic performance of the system. Furthermore, the electromechanical cross-coupling introduced by the reference frame transformation exacerbates the control performance degradation, making standard current regulators ineffective. This paper investigates these issues and compares various current regulator configurations. The discrete-time domain HSIM model is developed that includes the delays associated with PWM and sampling effects. This model is then used to design a discrete-time domain complex vector PI current regulator-based control structure that demonstrates better response compared with the other regulators studied. Theoretical and simulation studies using a 10kW, 200V, 4-pole, 300Hz, 9000RPM HSIM are conducted, and hardware-in-the-loop (HIL) results are provided to quantify and validate the performance and stability of the described current regulator design.