Adjustable voltage DC link PWM induction motor drives

This work involves an investigation of the feasibility and consequences of Pulse Amplitude Modulation (PAM) or adjustable DC link voltage operation in PWM motor drives by simulation studies and prototype development. A detailed comparison is made with the industry standard adjustable speed drive (ASD) comprised of a diode bridge rectifier and PWM inverter. The primary advantages of such a control strategy are the following: use of standard three-phase inverter, a reduction of ripple current in the DC bus capacitor, reduction of switching harmonic motor losses, reduction of common-mode voltages and decreased inverter switching losses. An overview of viable front-end converter options to control the DC link voltage is given along with considerations necessary to choose an appropriate converter for a particular application. Some possible applications include large compressor pump and fan drives, traction drives, electric crane drives and multi-level inverters.; A detailed design analysis is made of the modulation strategies and passive components needed for the current source rectifier (CSR) in the hardware prototype including the effects of the input filter on near unity power factor (UPF) operation. A high bandwidth current regulator in conjunction with an outer voltage control loop is also developed and implemented. Performance trade-offs in terms of current and voltage response time versus the capacitor bank size and rectifier device ratings are addressed. It is shown that PAM/PWM operation can lead to a significant reduction in capacitor power loss resulting in increased capacitor lifetime or decreased capacitor bank size. Experimental results are provided to verify the analytical results. An analysis of the overall drive losses and efficiency is performed with an induction machine model including the fundamental iron losses. The rectifier and converter losses are estimated based on curve fit models to laboratory measurements and manufacturer data. The added parasitic resistive losses in the passive components of the converter are included as well. It is shown that the overall losses are increased despite a small reduction in the inverter switching losses. In addition experimental results showing reduced motor losses are shown which may offset the added rectifier losses.