Although the concept of variable-speed drives, based on utilization of multiphase machines, dates back to the late 1960s, it was not until the mid- to late 1990s that multiphase drives became serious contenders for various applications. These include electric ship propulsion, locomotive traction, electric and hybrid electric vehicles, ldquomore-electricrdquo aircraft, and high-power industrial applications. As a consequence, there has been a substantial increase in the interest for such drive systems worldwide, resulting in a huge volume of work published during the last ten years. An attempt is made in this paper to provide a brief review of the current state of the art in the area. After addressing the reasons for potential use of multiphase rather than three-phase drives and the available approaches to multiphase machine designs, various control schemes are surveyed. This is followed by a discussion of the multiphase voltage source inverter control. Various possibilities for the use of additional degrees of freedom that exist in multiphase machines are further elaborated. Finally, multiphase machine applications in electric energy generation are addressed.

Multiphase Electric Machines for Variable-Speed Applications

The area of multiphase variable-speed motor drives in general and multiphase induction motor drives in particular has experienced a substantial growth since the beginning of this century. Research has been conducted worldwide and numerous interesting developments have been reported in the literature. An attempt is made to provide a detailed overview of the current state-of-the-art in this area. The elaborated aspects include advantages of multiphase induction machines, modelling of multiphase induction machines, basic vector control and direct torque control schemes and PWM control of multiphase voltage source inverters. The authors also provide a detailed survey of the control strategies for five-phase and asymmetrical six-phase induction motor drives, as well as an overview of the approaches to the design of fault tolerant strategies for post-fault drive operation, and a discussion of multiphase multi-motor drives with single inverter supply. Experimental results, collected from various multiphase induction motor drive laboratory rigs, are also included to facilitate the understanding of the drive operation.

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Multiphase induction motor drives - : a technology status review

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Design Of Analog Cmos Integrated Circuits

The solid-state transformer (SST), which has been regarded as one of the 10 most emerging technologies by Massachusetts Institute of Technology (MIT) Technology Review in 2010, has gained increasing importance in the future power distribution system. This paper presents a systematical technology review essential for the development and application of SST in the distribution system. The state-of-the-art technologies of four critical areas are reviewed, including high-voltage power devices, high-power and high-frequency transformers, ac/ac converter topologies, and applications of SST in the distribution system. In addition, future research directions are presented. It is concluded that the SST is an emerging technology for the future distribution system.

Review of Solid-State Transformer Technologies and Their Application in Power Distribution Systems

This paper investigates the capacitor-current-feedback active damping for the digitally controlled LCL-type grid-connected inverter. It turns out that proportional feedback of the capacitor current is equivalent to virtual impedance connected in parallel with the filter capacitor due to the computation and pulse width modulation (PWM) delays. The LCL-filter resonance frequency is changed by this virtual impedance. If the actual resonance frequency is higher than one-sixth of the sampling frequency (fs/6), where the virtual impedance contains a negative resistor component, a pair of open-loop unstable poles will be generated. As a result, the LCL-type grid-connected inverter becomes much easier to be unstable if the resonance frequency is moved closer to fs/6 due to the variation of grid impedance. To address this issue, this paper proposes a capacitor-current-feedback active damping with reduced computation delay, which is achieved by shifting the capacitor current sampling instant towards the PWM reference update instant. With this method, the virtual impedance exhibits more like a resistor in a wider frequency range, and the open-loop unstable poles are removed; thus, high robustness against the grid-impedance variation is acquired. Experimental results from a 6-kW prototype confirm the theoretical expectations.

Capacitor-Current-Feedback Active Damping With Reduced Computation Delay for Improving Robustness of LCL-Type Grid-Connected Inverter

Inset PMSMs are widely used for critical high power applications demanding mechanical stability for high speed operations and simple constructional features. Even though the value of q-axis inductance differ from that of d-axis, the reluctance torque that can be extracted is minimal if the saliency is small, restricting the torque capability. However, the q-axis inductance plays a major role in the design of inset PMSMs for critical applications with a limited DC bus voltage. There may be variation in q-axis inductance of the manufactured machine from the designed value due to machine non-linearities and manufacturing tolerances, restricting the maximum torque capability at a desired speed. In this paper, a design guideline for selecting q-axis current rating and flux per pole of a machine considering variations of q-axis inductance and DC bus voltage fluctuation is presented. The effect of q-axis inductance variation on the operating point of an inset PMSM is analysed in detail.

Effect of q-axis Inductance on the Design of an Industrial Inset Permanent Magnet Synchronous Motor Drive with Restricted DC bus Voltage

The design of an isolated gate driver for a Medium Voltage (MV) Cascaded H-bridge (CHB) converter is proposed in this paper. Isolation with proper electrical insulation and a low common-mode coupling capacitance are considered as the primary design criteria while designing the gate driver. A detailed circuit analysis has been carried out to find the isolation issues in the MV CHB converters, which necessitates a 2ndstage of isolation in the gate driver circuit. Apart from improving the isolation voltage level, the additional isolation stage helps to reduce the coupling capacitance in the common mode path. As the device count for an MV CHB converter is significantly high, the complementary pulses have been generated locally in a CPLD-based delay card, which reduces the count of the optical cables and the associated components. It helps to make the system simple and cost effective. Finally, the design of the proposed gate drivers and associated delay cards are verified in a hardware prototype of 1.65kV/300V, 10kVA Solid State Transformer (SST).

Design of an Isolated Gate Driver for Medium Voltage Cascaded H-Bridge (CHB) Based Solid State Transformer (SST)

Solid State Transformer (SST) is expected to be a key component in future smart grid systems, which integrates MVAC grid with LV DC/AC grid. For a three stage SST, resonant converter is a potential candidate as the DC-DC stage due to high conversion efficiency. However, it exhibits an oscillation of 100Hz (double line frequency) in the high frequency (HF) link current, which increases device current stress and also increases conduction loss. This paper proposes an analytical model which determines the magnitude of the double line frequency oscillation in terms of input line current, input power factor, modulation index of AC-DC stage, and DC bus capacitors. The obvious solution to minimize the double line frequency oscillation is to keep a high value of capacitor at the MVDC bus. However, it reduces the power density of the system. Therefore, this paper proposes an improved solution with a combination of DC-link inductor and capacitor at LVDC bus, which reduces the double line frequency current oscillation in HF link without the need of high MVDC bus capacitors. The design is verified in a laboratory prototype of 8 kVA, 1.65kV/300V solid state transformer.

Minimization of Low Frequency Current Oscillation in Resonant Link of a Solid State Transformer by Passive Filters

The present invention relates to SiC MOSFET which has fast switching times of nearly 20ns to 50ns. To control the voltage and current overshoot of the SiC MOSFET during the switching transient an active gate driver is required. The invention presented describes an active gate driver for SiC MOSFET used in power converter topology. The proposed four step active gate driving method uses a digitally controlled switched current source to control the gate current of the SiC MOSFET. The control signals are generated by means of onboard digital control logic which can be implemented in either in a FPGA or CPLD. The voltage and the current of the device are measured to control the overshoot and ringing in the device voltage and current. The proposed gate driver has novel current sensing mechanism based on kelvin voltage measurement. The measured load current can be used for control and protection of the device from faults. Since the switching time of the SiC MOSFET is very less it is difficult to control the voltage and current overshoot in the same switching instant hence, a low speed control loop is also proposed in the invention. The low speed control loop takes its control decision based on the previous switching cycle information. The low speed control loop essentially decides the time intervals of the switching steps of the gate driver.

Digitally controlled switched current source active gate driver for silicon carbide mosfet with line current sensing

With the change in scenario of continuous thrust for the improvement of efficiency of the overall system, motors are being coupled with variable frequency drives (VFD) to achieve energy savings, reduced wear and tear on motors, reduction of maintenance cost of mechanical flow controls etc. To meet the market requirements in the low voltage segment, 1MW, 690V ac drive has been developed. The system was successfully tested for V/f control, sensor-less vector control, power on ride through and catch on fly. Various issues related to testing of the machine with open loop v/f control and sensor less vector control were addressed successfully. Load testing of the 1MW VFD also completed with back to back test facility. This paper presents the development and testing of the developed 1MW VFD with the experimental results.

Kamalesh Hatua

Papers

Effect of q-axis Inductance on the Design of an Industrial Inset Permanent Magnet Synchronous Motor Drive with Restricted DC bus Voltage

Design of an Isolated Gate Driver for Medium Voltage Cascaded H-Bridge (CHB) Based Solid State Transformer (SST)

Minimization of Low Frequency Current Oscillation in Resonant Link of a Solid State Transformer by Passive Filters

Digitally controlled switched current source active gate driver for silicon carbide mosfet with line current sensing

Development and testing of 1MW variable frequency drive