Zhengming Zhao

An Improved Direct Torque Control for Three-Level Inverter-Fed Induction Motor Sensorless Drive

A sensorless three-level neutral-point-clamped inverter-fed induction motor drive is proposed in this paper. The conventional direct torque control (DTC) switching table fails to consider the circuit limitations, such as neutral-point-balance and smooth vector switching, caused by the topology of a three-level inverter. Two kinds of modified schemes for three-level DTC are proposed to solve these problems. They also provide performance enhancement while maintaining robustness and simplicity. Fuzzy logic control and the speed-adaptive flux observer (with novel gain and load toque observation) are introduced to enhance the performance of the system. The issue of large starting current is investigated and solved by introducing the technique of preexcitation. A 32-bit fixed-point DSP-based motor drive is developed to achieve high-performance sensorless control over a wide speed range. The effectiveness of the proposed schemes is confirmed by simulation implementation and experimental validation.

Hybrid Selective Harmonic Elimination PWM for Common-Mode Voltage Reduction in Three-Level Neutral-Point-Clamped Inverters for Variable Speed Induction Drives

This paper proposes a hybrid selective harmonic elimination pulsewidth modulation (SHEPWM) scheme for common-mode voltage reduction in three-level neutral-point-clamped inverter-based induction motor drives. The scheme uses the conventional SHEPWM (C-SHEPWM) to control the inverter at high frequency (≥ 0.9 motor rated frequency) and uses the modified SHEPWM (M-SHEPWM) to control the inverter at low frequency. It also uses a scheme to ensure the smooth transition between the two SHEPWM schemes. As a result, at high frequency, the C-SHEPWM provides the required high modulation index for the motor, while at low frequency, when a passive filter is less effective for common-mode voltage reduction, the M-SHEPWM is used to suppress the common-mode voltage. Experimental results show that the proposed hybrid SHEPWM scheme could meet the modulation index need of the motor and reduce the common-mode voltage in the drive, and the two SHEPWM schemes could transition smoothly.

Frequency Decrease Analysis of Resonant Wireless Power Transfer

Recent years have witnessed the booming development of resonant wireless power transfer (RWPT). Compared with conventional inductive power transfer, the frequency of RWPT is usually much higher. To reduce the resonant frequency while maintaining the transfer efficiency constant at the same transfer distances, two solutions are proposed and realized in this letter. Two fundamental structures for RWPT are analyzed and the expressions of the transfer efficiency are deduced. It is pointed out that the transfer quality factor and the load matching factor are two important factors of achieving high transfer efficiency. The larger the transfer quality factor, the higher the transfer efficiency. There is an optimal load matching factor to reach the highest transfer efficiency. The transfer efficiency can remain constant at the same distances if the transfer quality factor is kept at the same level and the load is matched. Theoretical calculations and experimental results provide a sound basis for decreasing frequency.

A Hybrid PWM Applied to High-Power Three-Level Inverter-Fed Induction-Motor Drives

A hybrid pulsewidth modulation (PWM), combining the merits of both space-vector PWM (SVPWM) and selective harmonic elimination (SHE) PWM (SHEPWM), is proposed for three-level neutral-point-clamped (NPC) inverter-fed high-power adjustable-speed drives, which uses asynchronous SVPWM at low frequency and SHEPWM at high frequency. For SHEPWM, a novel formula is proposed to obtain the initial values of switching angles, leading to another valid solution that is different from the known ones in the literature. Furthermore, it is shown that, by eliminating the quarter-wave symmetry, unlimited groups of solutions to three-level SHEPWM can be obtained. The characteristics of the multiple solutions in terms of harmonic distribution, pulsewidth, and total harmonic distortion are investigated for the application of SHE in practical drives. Switching between SVPWM and SHEPWM is problematic if no appropriate measure is taken, particularly when the influence of the minimum pulsewidth (MPW) is considerable. A simple but effective method, taking into account the MPW, is proposed in this paper to address this problem. The multiple groups of solutions to SHE are simulated and experimentally verified on a low-voltage three-level NPC inverter prototype. Experimental results obtained from a low-voltage prototype and an industrial 6-kV/1250-kW three-level drive are presented to validate the smooth switching between SVPWM and SHEPWM.

Analysis of the Double-Layer Printed Spiral Coil for Wireless Power Transfer

As a critical part of the wireless power transfer system via strongly coupled magnetic resonances, the resonant coils must be cautiously designed for the specific resonant frequency and high quality factor. There are some issues needed to be considered and studied in the coil design, such as the coil structure, parasitic parameter extraction, and optimizing. In this paper, the double-layer printed spiral coil is used, which could fully take advantage of the limited space and make larger parasitic capacitance for lower resonant frequency. Using the simplified partial element equivalent circuit method and finite element method, the circuit model with consideration of parasitic parameters and high-frequency losses is built, and the impedance characteristic of coil is simulated, which coincides well with the measurement result. In addition, several elements affecting the high-frequency loss, including the skin effect, proximity effect, and dielectric loss, are discussed for reaching higher quality factor, which is critical for the power transfer system.

Frequency-Splitting Analysis of Four-Coil Resonant Wireless Power Transfer

Based on four-coil magnetically coupled resonant wireless power transfer, this paper demonstrates, explains, and analyzes the frequency-splitting phenomenon by using the circuit theory. First, the complete and simplified models are built to describe the whole system. The numerical formulas of the system efficiency both at and away from the resonant frequency are presented. Then the frequency-splitting phenomenon is displayed and further explained mathematically and physically. An important factor for frequency splitting is introduced. The boundary condition for frequency splitting and its splitting frequency points are deduced. The amplitude-frequency and phase-frequency characteristics of the input impedance are measured and described to verify the explanation. Three related factors are studied, and the sensitivity analysis is conducted. A solution is proposed that helps improve the efficiency when frequency splitting occurs. The theoretical calculations and experimental results provide a sound basis.

A comparative study of Luenberger observer, sliding mode observer and extended Kalman filter for sensorless vector control of induction motor drives

Observer-based sensorless techniques are becoming more and more popular and universal. This paper presents a comparative study of three kinds of observers for direct field oriented controlled (DFOC) induction motor (IM) drive: Luenberger observer (LO), sliding mode observer (SMO) and extended Kalman filter (EKF). Each kind of observer employs the dynamic full-order IM model and mechanical equation is introduced to improve the performance of speed estimation. Theoretical principles of the three observers are illustrated. Not only computer simulations, but also a series of experimental results are presented to evaluate the performances of the three observers. A comprehensive analysis and comparisons are given from several aspects, such as steady state accuracy, dynamic performance, low speed operation, parameter sensitivity, noise sensitivity and complexity. The advantages and disadvantages of each observer are summarized and a comprehensive conclusion is given.

An improved virtual resistance damping method for grid-connected inverters with LCL filters

In renewable energy generations, LCL filters are commonly used to connect in series with the output ports of the converter to smooth the currents flowing into the grid due to their enhanced attenuation ability and smaller inductance compared to single L filters. However, the inherent resonance of an LCL filter makes the control of such a system challenging, and different passive and active damping strategies have been developed till now. This paper aims to the damping problems of LCL filters and proposes a new approach to realize active damping. Based on traditional virtual resistance damping strategy, the new algorithm employs a lead compensation block in the capacitor current feedback loop so as to alleviate the impact of control delay. It is easy to be transformed from conventional virtual resistance damping method since just an additional digital filter is needed. Theoretical analyses for the essence equivalent relationship between passive damping and virtual resistance damping, as well as the impact investigation of control delay are made in details. The results show that the traditional virtual resistance active damping method is equivalent to passive damping of connecting damping resistance in parallel with the capacitor in the control block diagram, while the proposed virtual resistance active damping method is equivalent to passive damping of connecting damping resistance in series with the capacitor in the control block diagram. Both simulation and experimental results have verified the feasibility.

Selective Wireless Power Transfer to Multiple Loads Using Receivers of Different Resonant Frequencies

In multiple receivers of resonant wireless power transfer, selective power flow among the loads is an important issue. This paper proposes a new method to control power division. The two-coil structure with different resonant frequencies of the sending and receiving loops is modeled and analyzed. The efficiency is proved to peak at the resonant frequency of the receiving loop, regardless of the resonant frequency of the sending loop. Using this feature, selective power transfer can be achieved by setting the receiving loops at different resonant frequencies. The efficiency of a particular load is greatly influenced by the driving frequency. The multiple-load system with different resonant frequencies is modeled and the efficiency expression of each load is deduced. The mutual inductances of the receiving coils have a small impact on the efficiency distribution. The closer the resonant frequencies of the receiving loops, the less isolated the related loads. The calculations and the experiments confirm the analysis.

Investigation and analysis of the influence of magnetic wedges on high voltage motors performance

To weaken the pulsation of the harmonic magnetic field and improve the wave of the air-gap magnetic field in high voltage motors because of their open slots, magnetic slot wedge should be imbedded into the slot opening. In the paper, through analyzing the air-gap magnetic field distribution of large-scale motors affected by embedding magnetic slot wedges in open slots, the influence of magnetic slot wedge on the motor parameters and performance is discussed in theory. The finite element analysis (FEA) and the experiment results were given, which not only proves the validity of the analysis in the paper, also through the simulation results the proper parameters of magnetic slot wedges for best performance of motors were concluded, which provides a theoretical reference for the manufacturers when choosing or processing magnetic slot wedges.