Minimum switching loss pulse width modulation for reduced power conversion loss in reactive power compensators

Advanced bus-clamping switching sequences, which employ an active vector twice in a subcycle, are used to reduce line current distortion and switching loss in a space vector modulated voltage source converter. This study evaluates minimum switching loss pulse width modulation (MSLPWM), which is a combination of such sequences, for static reactive power compensator (STATCOM) application. It is shown that MSLPWM results in a significant reduction in device loss over conventional space vector pulse width modulation. Experimental verification is presented at different power levels of up to 150 kVA.

A VARIABLE SWITCHING FREQUENCY PWM TECHNIQUE FOR INVERTER-FED INDUCTION MOTOR TO ACHIEVE SPREAD SPECTRUM

Voltage Source Inverter (VSI) fed induction motors are widely used in variable speed applications. For inverters using fixed switching frequency PWM, the output harmonic spectra are located at a few discrete frequencies. The ac motordrives powered by these inverters cause acoustic noise. This paper proposes a new variable switching frequency pwm technique and compares its performance with constant switching frequency pwm technique. It is shown that the proposed technique leads to spread spectra of voltages and currents. Also this technique ensures that no lower order harmonics are present and the current THD is comparable to that of fixed switching frequency PWM and is even better for higher modulation indices.

Intermittent electrical stimuli for guidance of human mesenchymal stem cell lineage commitment towards neural-like cells on electroconductive substrates

In the context of the role of multiple physical factors in dictating stem cell fate, the present paper demonstrates the effectiveness of the intermittently delivered external electric field stimulation towards switching the stem cell fate to specific lineage, when cultured in the absence of biochemical growth factors. In particular, our findings present the ability of human mesenchymal stem cells (hMSCs) to respond to the electric stimuli by adopting extended neural-like morphology on conducting polymeric substrates. Polyaniline (PANI) is selected as the model system to demonstrate this effect, as the electrical conductivity of the polymeric substrates can be systematically tailored over a broad range (10(-9) to 10 S/cm) from highly insulating to conducting by doping with varying concentrations (10(-5) to 1 M) of HCl. On the basis of the culture protocol involving the systematic delivery of intermittent electric field (dc) stimulation, the parametric window of substrate conductivity and electric field strength was established to promote significant morphological extensions, with minimal cellular damage. A time dependent morphological change in hMSCs with significant filopodial elongation was observed after 7 days of electrically stimulated culture. Concomitant with morphological changes, a commensurate increase in the expression of neural lineage commitment markers such as nestin and PI tubulin was recorded from hMSCs grown on highly conducting substrates, as revealed from the mRNA expression analysis using Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) as well as by immune-fluorescence imaging. Therefore, the present work establishes the key role of intermittent and systematic delivery of electric stimuli as guidance cues in promoting neural-like differentiation of hMSCs, when grown on electroconductive substrates. (C) 2014 Elsevier Ltd. All rights reserved.

Experimental Investigation on Switching Characteristics of High-Current Insulated Gate Bipolar Transistors at Low Currents

Insulated gate bipolar transistors (IGBTs) are used in high-power voltage-source converters rated up to hundreds of kilowatts or even a few megawatts. Knowledge of device switching characteristics is required for reliable design and operation of the converters. Switching characteristics are studied widely at high current levels, and corresponding data are available in datasheets. But the devices in a converter also switch low currents close to the zero crossings of the line currents. Further, the switching behaviour under these conditions could significantly influence the output waveform quality including zero crossover distortion. Hence, the switching characteristics of high-current IGBTs (300-A and 75-A IGBT modules) at low load current magnitudes are investigated experimentally in this paper. The collector current, gate-emitter voltage and collector-emitter voltage are measured at various low values of current (less than 10% of the device rated current). A specially designed in-house constructed coaxial current transformer (CCT) is used for device current measurement without increasing the loop inductance in the power circuit. Experimental results show that the device voltage rise time increases significantly during turn-off transitions at low currents.

On-line Estimation of Fundamental and Ripple Components of Line Currents in a Voltage-Source Inverter Operated at Low Switching Frequency

High-power voltage-source inverters (VSI) are often switched at low frequencies due to switching loss constraints. Numerous low-switching-frequency PWM techniques have been reported, which are quite successful in reducing the total harmonic distortion under open-loop conditions at such low operating frequencies. However, the line current still contains low-frequency components (though of reduced amplitudes), which are fed back to the current loop controller during closed-loop operation. Since the harmonic frequencies are quite low and are not much higher than the bandwidth of the current loop, these are amplified by the current controller, causing oscillations and instability. Hence, only the fundamental current should be fed back. Filtering out these harmonics from the measured current (before feeding back) leads to phase shift and attenuation of the fundamental component, while not eliminating the harmonics totally. This paper proposes a method for on-line extraction of the fundamental current in induction motor drives, modulated with low-switching-frequency PWM. The proposed method is validated through simulations on MATLAB/Simulink. Further, the proposed algorithm is implemented on Cyclone FPGA based controller board. Experimental results are presented for an R-L load.

An Improved Dead-Time Compensation Scheme for Voltage Source Inverters Considering the Device Switching Transition Times

Inverter dead-time, which is meant to prevent shoot-through fault, causes harmonic distortion and change in the fundamental voltage in the inverter output. Typical dead-time compensation schemes ensure that the amplitude of the fundamental output current is as desired, and also improve the current waveform quality significantly. However, even with compensation, the motor line current waveform is observed to be distorted close to the current zero-crossings. The IGBT switching transition times being significantly longer at low currents than at high currents is an important reason for this zero-crossover distortion. Hence, this paper proposes an improved dead-time compensation scheme, which makes use of the measured IGBT switching transition times at low currents. Measured line current waveforms in a 2.2 kW induction motor drive with the proposed compensation scheme are compared against those with the conventional dead-time compensation scheme and without dead-time compensation. The experimental results on the motor drive clearly demonstrate the improvement in the line current waveform quality with the proposed method.

Gray to transmissive electrochromic switching based on electropolymerized PEDOT-ionic liquid functionalized graphene films

A supporting electrolyte based on lithium perchlorate has been functionalized with graphene (ionic liquid functionalized graphene (IFGR)) by facile electrochemical exfoliation of graphite rods in aq. LiClO4 solution. Poly(3,4-ethylenedioxythiophene) (PEDOT)-IFGR films were prepared by electropolymerization of EDOT monomer with IFGR as supporting electrolyte in ethanol at static potential of 1.5 V. The Raman, SEM, and XPS analysis of PEDOT-IFGR film confirmed the presence of functionalized graphene in the film. The PEDOT-IFGR films showed good electrochemical properties, better ionic and electrical conductivity, significant band gap, and excellent spectroelectrochemical and electrochromic properties. The electrical conductivity of PEDOT-IFGR film was measured as about 3968 S cm(-1). PEDOT-IFGR films at reduced state showed strong and broad absorption in the whole visible region and remarkable absorption at near-IR region. PEDOT-IFGR film showed electrochromic response between transmissive blue and darkish gray at redox potential. The color contrast (%T) between fully reduced and oxidized states of PEDOT-IFGR film is 25 % at lambda (max) of 485 nm. The optical switching stability of PEDOT-IFGR film has retained 80 % of its electroactivity even after 500 cycles.

Experimental Investigation on Switching Characteristics of High-Current Insulated Gate Bipolar Transistors at Low Currents

Insulated gate bipolar transistors (IGBTs) are used in high-power voltage-source converters rated up to hundreds of kilowatts or even a few megawatts. Knowledge of device switching characteristics is required for reliable design and operation of the converters. Switching characteristics are studied widely at high current levels, and corresponding data are available in datasheets. But the devices in a converter also switch low currents close to the zero crossings of the line currents. Further, the switching behaviour under these conditions could significantly influence the output waveform quality including zero crossover distortion. Hence, the switching characteristics of high-current IGBTs (300-A and 75-A IGBT modules) at low load current magnitudes are investigated experimentally in this paper. The collector current, gate-emitter voltage and collector-emitter voltage are measured at various low values of current (less than 10% of the device rated current). A specially designed in-house constructed coaxial current transformer (CCT) is used for device current measurement without increasing the loop inductance in the power circuit. Experimental results show that the device voltage rise time increases significantly during turn-off transitions at low currents.

Experimental Study on Switching Characteristics of an Inverter Leg Consisting of IGBTs of Dissimilar Makes

Usually the top and bottom IGBT devices in an inverter leg are of the same make (i.e. from same manufacturer). At low power level, these two devices even may be contained in the same module. However at high power levels the top and bottom devices are in separate modules. Sometimes, in the event of device failure, device of particular make may be replaced by one of another make, but of same ratings (on account of non-availability of the original make). This paper investigates the effect of such intermixing of two different makes of high power IGBTs in an inverter leg on the switching characteristics. The switching transitions between IGBT and diode of similar make and those of IGBT and diode of dissimilar make are compared experimentally at various DC link voltages and currents. The comparisons are made in terms of, IGBT peak turn-on di/dt, IGBT peak turn-off di/dt, peak diode reverse recovery current (I-rr), peak IGBT voltage overshoot and switching energy losses.

Experimental characterisation and performance evaluation of a coaxial current transformer for measurement of insulated gate bipolar transistor switching current

Measurement of device current during switching characterisation of an insulated gate bipolar transistor (IGBT) requires a current sensor with low insertion impedance and high bandwidth. This study presents an experimental procedure for evaluating the performance of a coaxial current transformer (CCT), designed for the above purpose. A prototype CCT, which can be mounted directly on a power terminal of a 1200 V/50 A half-bridge IGBT module, is characterised experimentally. The measured characteristics include insertion impedance, gain and phase of the CCT at different frequencies. The bounds of linearity within which the CCT can operate without saturation are determined theoretically, and are also verified experimentally. The experimental study on linearity of the CCT requires a high-amplitude current source. A proportional-resonant (PR) controller-based current-controlled half-bridge inverter is developed for this purpose. A systematic procedure for selection of PR controller parameters is also reported in this study. This set-up is helpful to determine the limit of linearity and also to measure the frequency response of the CCT at realistic amplitudes of current in the low-frequency range.

Percolative switching in transition metal dichalcogenide field-effect transistors at room temperature

We have addressed the microscopic transport mechanism at the switching or `on-off' transition in transition metal dichalcogenide (TMDC) field-effect transistors (FETs), which has been a controversial topic in TMDC electronics, especially at room temperature. With simultaneous measurement of channel conductivity and its slow time-dependent fluctuation (or noise) in ultrathin WSe2 and MoS2 FETs on insulating SiO2 substrates where noise arises from McWhorter-type carrier number fluctuations, we establish that the switching in conventional backgated TMDC FETs is a classical percolation transition in a medium of inhomogeneous carrier density distribution. From the experimentally observed exponents in the scaling of noise magnitude with conductivity, we observe unambiguous signatures of percolation in a random resistor network, particularly, in WSe2 FETs close to switching, which crosses over to continuum percolation at a higher doping level. We demonstrate a powerful experimental probe to the microscopic nature of near-threshold electrical transport in TMDC FETs, irrespective of the material detail, device geometry, or carrier mobility, which can be extended to other classes of 2D material-based devices as well.

Evaluation and Minimization of Low-Order Harmonic Torque in Low-Switching-Frequency Inverter-Fed Induction Motor Drives

A low-order harmonic pulsating torque is a major concern in high-power drives, high-speed drives, and motor drives operating in an overmodulation region. This paper attempts to minimize the low-order harmonic torques in induction motor drives, operated at a low pulse number (i.e., a low ratio of switching frequency to fundamental frequency), through a frequency domain (FD) approach as well as a synchronous reference frame (SRF) based approach. This paper first investigates FD-based approximate elimination of harmonic torque as suggested by classical works. This is then extended into a procedure for minimization of low-order pulsating torque components in the FD, which is independent of machine parameters and mechanical load. Furthermore, an SRF-based optimal pulse width modulation (PWM) method is proposed to minimize the low-order harmonic torques, considering the motor parameters and load torque. The two optimal methods are evaluated and compared with sine-triangle (ST) PWM and selective harmonic elimination (SHE) PWM through simulations and experimental studies on a 3.7-kW induction motor drive. The SRF-based optimal PWM results in marginally better performance than the FD-based one. However, the selection of optimal switching angle for any modulation index (M) takes much longer in case of SRF than in case of the FD-based approach. The FD-based optimal solutions can be used as good starting solutions and/or to reasonably restrict the search space for optimal solutions in the SRF-based approach. Both of the FD-based and SRF-based optimal PWM methods reduce the low-order pulsating torque significantly, compared to ST PWM and SHE PWM, as shown by the simulation and experimental results.

Low-temperature electrical conductivity of LaNi1-xFexO3

We report the electrical conductivity between 2 and 300 K for LaNi1-xFexO3 across the composition-controlled metal-insulator (m-i) transition. Using a method first suggested by Mobius, we identify the critical concentration x(c) to be 0.3 for the m-i transition. The negative temperature coefficient of resistivity observed at low temperatures in the metallic phase follows a temperature dependence characteristic of disorder effects. The semiconducting compositions (x greater than or equal to 0.3) do not show a simple activation energy but exhibit variable-range hopping at high temperatures confirming that the m-i transition in this system is driven by increasing disorder effects.

Minimization of Torque Ripple in PWM AC Drives

A pulsewidth modulation (PWM) technique is proposed for minimizing the rms torque ripple in inverter-fed induction motor drives subject to a given average switching frequency of the inverter. The proposed PWM technique is a combination of optimal continuous modulation and discontinuous modulation. The proposed technique is evaluated both theoretically as well as experimentally and is compared with well-known PWM techniques. It is shown that the proposed method reduces the rms torque ripple by about 30% at the rated speed of the motor drive, compared to conventional space vector PWM.

Electrical Conductivity and Magneto-Resistance Studies on Polyaniline-Pmma Blends at Very Low Temperature

D.C. electrical conductivity of polyaniline (33%,40%) blended with PMMA was measured from 5K to 300mK. The conductivity behaviour is consistent with fluctuation induced tunneling. Magneto-resistance (MR) was measured between 300K and 2K. From 20K to 2K, a large positive MR was observed. At 2K, for low magnetic fields (<1 Tesla), a deviation from the normal H-2 behaviour was observed.