Switching Frequency Of Power Devices Research Articles

Stability Improvement of Adaptive Full-Order Observer for Sensorless Induction Motor Drives in Low-Speed Regenerating Mode

In the applications of high power induction motor railway traction systems, the switching frequency of power devices is relatively low. This restriction will severely reduce the accuracy and stability of conventional discrete model for adaptive full-order observer (AFO) in extremely high-speed region. What’s worse, due to the neglect of flux error during speed estimation derivation, the conventional AFO cannot maintain stability under low-speed regenerating mode. The above two reasons limit the application of AFO in rail traction systems for full speed range. To address this problem, this paper utilizes a high-accuracy discretization which is achieved in combined reference frames and feedback gains are designed in z-domain to ensure stability and dynamic performance of AFO. Moreover, a modified speed estimation mechanism with the error of d-axis current is developed to guarantee stability in extremely low-speed regenerating mode. Rigorous simulations and experiments are performed to verify that the proposed AFO model achieves excellent sensorless control performance in full speed range.

Analytical modelling of high‐frequency losses in toroidal inductors

AbstractToroidal inductors are widely used in power electronic systems. With the increasing switching frequency of power devices, the AC losses of high‐frequency inductor windings have become an issue that cannot be ignored in design. A method with low computational time cost and high accuracy is urgently needed by designers. This paper proposes a modified analytical method for winding loss calculation of toroidal inductors based on the one‐dimensional Dowell method and the two‐dimensional Ferreira method, and proposes a compensated analytical model considering the proximity effect of inter‐turn conductors of toroidal windings. The analytical method calculates the non‐linear porosity of toroidal windings, and can accurately solve the AC losses of toroidal inductors with arbitrary porosity winding encapsulation. The analytical results are in good agreement with the finite element method (FEM) and experimental data. The experimental measurement data verifies the validity of the proposed model within 1‐MHz frequency, the calculation error of the modified analytical method is less than 25%, and the calculation error of the compensated analytical model is reduced to 10%.

Research on high-speed synchronous motor direct torque control

High-speed motor is increasingly used in ultra-high precision machining and high-performance machinery. It is necessary to achieve closed-loop control of high-speed motor in order to realize high-performance motor control. The high-frequency and high-speed characteristics of high-speed motor have brought great difficulties for its realization of closed-loop control. The performance of the hexagonal flux direct torque control (DTC) method that can reduce the switching frequency of power devices is researched for high-speed synchronous motor in this paper. After adopting this control method, the harmonics in the motor current and torque can be suppressed by adopting an appropriate filtering method. The same effect as the conventional DTC can still be achieved. It provides a basis for the application realization of the high-speed synchronous motor control system.

Low Switching Frequency Operation Control of Line Voltage Cascade Triple Converter

With the increasing power level of wind power generation system, the traditional topology of power converters can no longer meets the demand of high-power wind power generation systems due to the limitation of device performance. The line voltage cascade type multiple PWM converter (LVC-VSC) is a kind of converter that uses the traditional two-level and six-switch voltage source converter as the basic component unit, and each unit is combined with the line voltage cascade method. This type of converter is suitable for medium-voltage and high-power applications such as wind power generation and metallurgical drives because of its easy modularization, strong scalability and low number of isolated power supplies required. However, for medium-voltage and high-power applications, the switching frequency of power devices in the converter is low, usually limited to a few hundred hertz. The traditional modulation method of line voltage cascade converter has a large number of redundant states, and simply reducing the carrier ratio will cause serious degradation of control performance and system instability. To address this problem, this paper proposes a modulation strategy and a corresponding control method for low switching frequency. The modulation strategy is based on the vector relationship of finite switching states, and the optimal switching sequence is selected according to the modulation system by removing redundant states, thus ensuring the application of different modulation sequences under different modulation depths and ensuring the current quality on the basis of the minimum switching frequency, which effectively solves the control problems at low switching frequency. The experimental results show the correctness and effectiveness of the proposed modulation strategy and control method.

A novel deadbeat predictive current control of permanent magnet synchronous motor based on oversampling scheme

AbstractWhen the speed of permanent magnet motor is relatively high, the fundamental frequency of stator voltage and current will rise. However, due to the limitation of switching frequency of power electronic devices, the switching times of power devices will be greatly reduced in a fundamental cycle of stator voltage. For the traditional control method with constant switching frequency mode, the control algorithm is executed only once in a carrier wave cycle, which will result in a low control frequency operating condition for the PMSM control system. At this time, the dynamic and steady‐state control performance of stator current will be decreased. In response to above problem, this paper proposed a deadbeat predictive current control method based on oversampling scheme, which the core algorithm of current loop operated four times and updated the PWM duty twice in one switching cycle. The number of control times of the proposed method is greater than that of the conventional method when the conventional deadbeat method and the proposed method adopt the same switching cycle. Therefore, the control method does not increase the switching frequency of power devices, but improves the dynamic and stable performance of the system, especially under the high‐speed condition.

Analytical Study of SiC MOSFET Based Inverter Output dv/dt Mitigation and Loss Comparison With a Passive dv/dt Filter for High Frequency Motor Drive Applications

Fast switching characteristic of wide bandgap devices enables high switching frequency of power devices and thereby, can facilitate high fundamental frequency operation of electrical machines. However, with the switching transition times in orders of tens of nanoseconds, the high dv/dt is observed across the switching device. The high dv/dt experienced by the switches, and consequently by the machine, can degrade winding insulations or bearings over a period of time. Therefore, it is imperative to maintain the dv/dt below recommended values depending on the machine insulation. The dv/dt across the devices can be adjusted by varying the gate resistance. A high value of gate resistance, however, introduces additional switching losses on the device. Using different dv/dt filtering techniques can also help to control the dv/dt on the machine terminals. These techniques do not increase the switching losses on the device. However, it introduces additional losses in the filter resistors and also increases the cost of the system. In this paper, an analysis based on the impact of gate resistance on the dv/dt across the machine, and the corresponding losses is carried out. An analytical dv/dt filter design strategy is proposed to limit the dv/dt to a particular value. With the proposed design scheme, the value of each filter component can be easily obtained, and filter losses can be estimated accurately. Lastly, a comparison is performed on the basis of efficiency between these two techniques.

An Improved PWM Strategy for Z-Source Inverter With Maximum Boost Capability and Minimum Switching Frequency

Z-source inverter provides a competitive single-stage dc–ac power conversion with the capability of both buck and boost voltage regulation. In order to maximize voltage gain and to increase efficiency, this paper proposes an improved pulse-width modulation (PWM) strategy. By adjusting the shoot-through duty ratio of one-phase leg, it regulates the average value of intermediate dc-link voltage, which is the same as the instantaneous maximum of three-phase line voltage in one switching time period $(T_{s})$ . And the other two-phase legs maintain the fixed switching states. Thus, the equivalent switching frequency of power devices in the inverter bridge is reduced to $1/ 3f_{s}$ ( $f_{s}$ is the frequency corresponding to $T_{s}$ ). The operating principles and closed-loop controller design are analyzed and verified by simulation and experiments. Compared with the existing PWM strategies, the improved PWM (IPWM) strategy demonstrates higher efficiency under full operation range of low voltage gain (1.27-2) application. However, with the IPWM strategy, the inductor current and capacitor voltage contain six-time-line-frequency ripples, which consequently require large size of the passive components when the output frequency is very low. Thus, it is also suitable for 400–800-Hz medium frequency aircraft and vessel power supply system due to a relatively high output line frequency. Furthermore, the idea of IPWM strategy can be extended to other kinds of three-phase impedance network-based inverters.

Maximum Boost Control of Diode-Assisted Buck–Boost Voltage-Source Inverter With Minimum Switching Frequency

Diode-assisted buck–boost voltage-source inverter achieves high voltage gain by introducing a switch-capacitor based high step-up dc–dc circuit between the dc source and inverter bridge. As for the unique structure, various pulse width modulation (PWM) strategies are developed with regard to the chopped intermediate dc-link voltage. In order to maximize voltage gain and increase efficiency, this paper proposes a novel PWM strategy. It regulates the average value of intermediate dc-link voltage in one switching time period ( $T_{s}$ ) the same as the instantaneous maximum value of three-phase line voltage by controlling the front boost circuit. Then, the equivalent switching frequency of power devices in the inverter bridge can be reduced to $1/3f_{s} (f_{s} = 1/T_{s})$ . The operating principle and closed-loop controller design are analyzed and verified by simulations and experiments. Compared with existing PWM strategies, the new control strategy demonstrates less power device requirement and higher efficiency in high voltage gain applications. It is a more competitive topology for wide range dc/ac voltage regulation in renewable energy applications. Furthermore, with new control strategy, the dc-side inductor current and capacitor voltage contains six-time line-frequency ripples. To overcome the undesired influence of low frequency ripples, it is also suitable for 400–800 Hz medium frequency aircraft and vessel power supply system.

Novel Carrier-Based PWM Strategy With Zero-Sequence Voltage Injected for Three-Level NPC Inverter

To solve the issue of neutral-point (NP) balancing in the three-level NP-clamed inverter, a novel carrier-based pulsewidth modulation (CPWM) strategy of double modulation waveforms is proposed in this paper. By combining the injection of zero-sequence voltage with the decomposition of modulation waveform organically, NP balancing is achieved over full range of inverter output voltages and load angles for linear loads. Furthermore, the switching frequency of power devices is significantly reduced as an optimal selection method of zero-sequence voltage is applied for the proposed strategy, which is equal to that for the CPWM strategy of single modulation waveform. Effectiveness of the proposed strategy is validated by both theoretical derivation and experimental results.

A New Modulation Method Based on Current Hysteresis Control for Cascaded H-Bridge Inverter

This paper puts forward a current hysteresis control method suitable for cascaded H-bridge inverter. It has the following characteristics : The control method is similar to unipolar SPWM modulation for a single H bridge.It is simple for pulse generator and can effectively reduce the switching losses; The phase-shifted control pulse for multi H bridges of a phase with current hysteresis control can be generated which is similar to the phase shift SPWM by dynamically adjusting the width of hysteresis to keep power device switching frequency constant; It retains fast response, internal current limiting and with good stability for current hysteresis control.At last, the experimental results verify the correctness of the proposed method.