Search

Reduced switch active phase converters (APC) are cost-effective solutions for running three-phase loads from single-phase grids. This paper proposes a novel auxiliary-capacitor-based APC (AC-APC) for this application. The combination of the topological location of the auxiliary capacitor and the control of the AC-APC reduces current stress on the semiconductor devices. In this paper, a selection criterion for the optimal auxiliary capacitor value is formulated to reduce the device currents in such a way that the overall efficiency is enhanced. This paper presents a comparative analysis of converter volt-ampere (VA) rating and device power loss for conventional H-bridge active front-end and inverter-based topology, APC, and AC-APC. To evaluate the efficacy of the proposed AC-APC, an experimental study is conducted on the APC and the AC-APC feeding a 3-hp three-phase induction motor. At a full load of $\text{2.5}\,\text{kW}$ and $\text{0.8 } \text{power factor}$ , the measured maximum efficiency of the AC-APC is $2\%$ higher than that of the APC. Analytical evaluation of converter currents and device power loss are in close agreement with experimental observations. [ABSTRACT FROM AUTHOR]

For power converters with inductive loads, a freewheeling path is needed for the current due to reactive power. The MOSFET synchronous rectification (SR) is widely used to reduce the conduction loss during the freewheeling period. Due to the wide band gap of silicon carbide (SiC), the intrinsic body diode of SiC MOSFET exhibits a high voltage drop. Hence, an antiparallel SiC Schottky diode is normally implemented to eliminate its conduction. However, the external SiC Schottky diode is not fully utilized as it only works during the dead time. In this paper, the hard-switching SR is investigated in an SiC three-phase inverter and compared with a conventional inverter using freewheeling diode (FWD). An improved power loss model for the two inverters has been developed. It is found that the inverter using SR has higher efficiency due to the smaller switching loss. A 7-kW prototype of SiC three-phase inverter is built, which achieves a peak efficiency of 98.8% ($\pm$ 0.15%) and 98.5% ($\pm$ 0.15%) at 40 kHz using SR and FWD, respectively. This paper confirms that the SiC MOSFET is an ideal candidate for the SR. [ABSTRACT FROM PUBLISHER]

This paper presents a three-phase open-end winding induction motor drive. The drive consists of a three-phase induction machine with open stator phase windings and dual-bridge inverter supplied from a single dc voltage source. To achieve multilevel output voltage waveforms, a floating capacitor bank is used for the second of the dual bridges. The capacitor voltage is regulated using redundant switching states at half of the main dc-link voltage. This particular voltage ratio (2:1) is used to create a multilevel output voltage waveform with three levels. A modified modulation scheme is used to improve the waveform quality of this dual inverter. This paper also compares the losses in the dual-inverter system in contrast with a single-sided three-level neutral point clamped converter. Finally, detailed simulation and experimental results are presented for the motor drive operating as an open-loop v /f controlled motor drive and as a closed-loop field-oriented motor controller. [ABSTRACT FROM PUBLISHER]

Conventional two-level as well as multilevel voltage-source-inverter topologies with a hexagonal voltage-space vector structure generate low-order harmonics in the machine terminal voltages when operated beyond the linear modulation range in variable speed drive application. Progressing from hexagonal to a higher sided polygonal voltage-space-vector structure (VSVS) suppresses significant low-order harmonics throughout the modulation range. In addition, the linear modulation range will also be enhanced very near to the base speed. In this paper, for the first time, the generation of a multilevel 24-sided VSVS using a single dc source for induction-motor drives is proposed. A cascaded topology with a dc-source-fed flying-capacitor inverter and two low-voltage capacitor-fed H-bridge cells forms the power circuit. The features and advantages of the proposed scheme are validated through experimental results and discussions. [ABSTRACT FROM AUTHOR]

Microinverters operating into the single-phase grid from new energy source with low-voltage output face the challenges of efficiency bottleneck and twice-line-frequency variation. This paper proposed a multilevel inverter based on bridge modular switched-capacitor (BMSC) circuits with its superiority in conversion efficiency and power density. The topology is composed of dc–dc and dc–ac stages with independent control for each stage, aiming to improve system stability and simplify the control method. The BMSC dc–dc stage, which can be expanded to synthesize more levels, not only features multilevel voltage gain but also partially replaces the original bulk input capacitor and functions as an active energy buffer to enhance power-decoupling ability between dc and ac sides. In dc–ac stage, the control strategy of optimized unipolar frequency doubling sine-wave pulse width modulation is proposed to improve the quality of output waveform. Meanwhile, the multilevel voltage phase has been optimized to further reduce the power loss. Finally, a prototype has been built and tested. Associated with the simulation, the experimental results validate the practicability of these analyses. [ABSTRACT FROM PUBLISHER]

This paper presents a comparison of two modulation methods for the seven-level switched-capacitor (SC) inverter using series/parallel conversion (SCISPC). The SCISPC is a multilevel inverter which has the less number of switching devices than the conventional multilevel inverters. The level-shifted pulsewidth modulation (PWM) (LS-PWM) was applied to the seven-level SCISPC in the previous research. In this paper, the level- and phase-shifted PWM (LPS-PWM) and the LS-PWM are applied to the seven-level SCISPC, and LPS-PWM is compared with LS-PWM for the seven-level SCISPC. The LPS-PWM has both the characteristics of the phase-shifted PWM (PS-PWM) and the LS-PWM. The voltage reduction during the discharging term of the SCs is reduced by means of the shorter discharging period compared to LS-PWM. The voltage ripple of the capacitors is also reduced, which leads to higher power conversion efficiency. The difference of the two modulation methods is confirmed by the theoretical approach, simulation with PSIM, and circuit experiment. [ABSTRACT FROM PUBLISHER]

This paper presents an optimized topology of 3-$\phi$ , multilevel inverter (MLI) configuration for five or higher level operation with reduced switch count. The proposed solution consists of two basic units (BUs) of multilevel converter and a T-structured 3-level inverter (3-LI). Both BUs are equally shared by the three phases maintaining symmetry among the phases. Moreover, for a higher level operation of the proposed MLI, the modifications required are at two BUs only and not at three legs of the inverter. It further helps in reducing the requirement of extra components for higher level operation compared to the conventional solutions. Furthermore, the topology also has the benefits of reduced switching and conduction loss. An algorithm is also devised to generate the switching pulses for the BUs and the 3-LI, using a carrier-based space vector modulation technique. Presented topology is compared with other existing topologies to prove its advantage. All the details regarding the operating modes and pulse width modulation techniques employed for the proposed system are given and supported by the simulation and experimental results. [ABSTRACT FROM AUTHOR]

Adding the auxiliary switches to the conventional H-bridge inverter is an effective way to eliminate the leakage current for transformerless PV systems, such as H5, H6, etc. Inspired by the newly developed embedded-switch H6 topology, a novel embedded-switch inverter (ESI) is proposed in this paper for three-phase transformerless PV systems. First, the operation principle and characteristics of the proposed ESI are analyzed. Second, the common-mode model of the three-phase ESI is established, based on which the main factors that affect the leakage current are discussed. The finding reveals that the switching states with conventional modulation strategy result in high-frequency common-mode voltage, which is the source of leakage current. In order to solve the problem, a new modulation strategy is presented for ESI to eliminate the high-frequency leakage current. Finally, the time-domain simulation and the experimental tests are carried out. The results verify the effectiveness of the proposed solution. [ABSTRACT FROM AUTHOR]

In this paper, dual-buck structured single-stage buck–boost inverters that use power MOSFETs to achieve high efficiency are presented. The proposed inverters require fewer switches, and half of the switches have reduced current stresses. They have no shoot-through problem; therefore, high system reliability can be obtained. The dead-time in PWM signals can be minimized or eliminated, which improving the quality of the output ac voltages and increasing the efficiency. In the proposed inverters, MOSFETs can be used without reverse-recovery issues of their body diodes to boost the efficiency and increase the switching frequency. To further improve the efficiency and stability of the proposed single-phase inverter, PWM strategies combining high- and low-frequency modulation are presented. A hardware prototype of the single-phase inverter was built and tested using resistive, inductive, and nonlinear loads. Experimental results for a 300-W prototype inverter show a 97.4% peak efficiency with a 25 kHz switching frequency. [ABSTRACT FROM AUTHOR]

The parallel three-phase grid-connected converters sharing dc and ac bus can increase the system power level effectively. However, the zero sequence circulating current (ZSCC) will occur inevitably under space vector pulse width modulation or discontinuous pulse width modulation (DPWM). In this paper, the mechanism and the mathematical model of ZSCC are studied. Then, a hybrid PWM (HPWM) strategy is proposed to suppress the ZSCC based on the same zero sequence voltage injected, which is dependent on the common grid information. The dc voltage utilization of HPWM can be extended compared with sinusoidal PWM (SPWM). The number of parallel modules can be infinity theoretically by using HPWM. It is noted that there is no internal commutation among modules (expect the dq current reference). Finally, the feasibility and the effectiveness of the proposed method are verified by experiments. [ABSTRACT FROM AUTHOR]

Several applications such as power supplies for electric precipitators, ozone generation, partial discharge, etc., require ac superimposed dc (ACsDC) voltage supply. Most of such applications use intransigent systems for producing ACsDC voltages. In this paper, three different kinds of flexible power electronic converters are proposed for producing ACsDC voltages. These converters have two power conversion stages: input and output. In the input stage, two isolated dc voltages are obtained by using an isolated three-port dc–dc converter or two isolated two-port dc–dc converters. In the output stage, one of the input-stage dc voltages is used to generate an ac voltage and the obtained ac voltage is added to the remaining input-stage dc voltage to obtain an ACsDC voltage. Based on the converters used in the input stage, the proposed ACsDC converters are named as a dual active bridge (DAB) converter-based ACsDC converter (DAB-ACsDC), triple port active bridge (TAB) converter-based ACsDC converter (TAB-ACsDC), and dual transformer-based asymmetrical triple port active bridge (DT-ATAB) converter-based ACsDC converter (DTATAB-ACsDC). A detailed analysis of these ACsDC converters and their verifications using simulations and experimental studies are presented, which depict that the proposed converters can be used effectively for producing ACsDC voltages. [ABSTRACT FROM AUTHOR]

In this paper, a new nonisolated high-voltage-gain boost converter is proposed. The proposed converter is constructed by adding an additional inductor to a conventional three-level-boost (TLB) converter. When compared with the conventional boost and TLB converters, the proposed converter can achieve higher voltage conversion ratio with reduced voltage and current stresses in the switches. In particular, the proposed converter can reduce the losses of active devices by reducing the current of the switch compared to conventional converters, thus, can achieve high efficiency. In addition, the proposed converter automatically balances the output voltages for an unbalanced load without the need for any additional control strategy or auxiliary circuit. A 2-kW prototype converter was built and tested to verify performances of the proposed converter. [ABSTRACT FROM PUBLISHER]

The impedance-source network converter, utilizing a unique LC network and previously forbidden shoot-through states, provides the ability to buck and boost the input voltage in a single stage. However, the inrush shoot-through current (STC) in startup or transient process might cause undesired current stresses on converter devices. This paper focuses on the STC sensing for an effective inrush current limitation by the combined current sensing technique in the quasi-Z-source inverter. STC and phase currents for the inverter control strategy are obtained simultaneously. No extra hardware is needed and the effects of current sensor bandwidth and duty cycle on the sensing accuracy are analyzed mathematically. The voltage spike gets avoided by integrating the stray inductance into the impedance network. Finally, an STC control loop based on proposed method are embedded in the field-oriented control strategy. The inrush STC and the device current stress in the transient process get suppressed. Simulation and experimental results from a quasi-Z-source inverter validate the feasibility of the proposed methods. [ABSTRACT FROM PUBLISHER]

Grid-connected power conversion systems for renewable energy sources must fulfill several requirements, e.g., the high efficiency, the reduced cost and complexity, and, quite often, the boost capabilities that is usually achieved using a front-end dc–dc boost converter before the inversion stage, leading to a two-stage architecture. Meanwhile, single-stage power conversion systems, which perform the boosting operation within the inversion one, offer some potential advantages, in terms of reducing the complexity and the volume of the whole system. Among several proposed options, the split-source inverter (SSI) has been recently proposed by Abdelhakim et al. as an alternative option with some interesting features to the commonly used Z-source inverter. Taking into account that the SSI is controlled by a single parameter, i.e., its dc and ac sides are controlled by the modulation index, it is of paramount importance to investigate its control scheme in grid-connected mode, which has never been studied yet. Hence, this paper models the SSI dc side and proposes a modified modulation scheme combined with the commonly used synchronous reference frame control technique to achieve a decoupled control scheme of the SSI in grid-connected mode, i.e., the dc and the ac sides of the SSI can be controlled independently, which is convenient for many applications. The introduced control scheme is analyzed and simulated using a MATLAB/Simulink model, where a reduced scale 1-kVA grid-connected SSI is designed and simulated for the sake of experimental validation. Finally, the designed system is implemented experimentally to validate and verify the reported analysis and simulations. [ABSTRACT FROM PUBLISHER]

This paper presents a new high-frequency modulation method for multilevel converters. The proposed method provides a broad linear operating range and can be digitally implemented with minimal computational effort. This modulation method creates a phase voltage composed of a rectangular component superimposed on the top of a quasi-square-shaped reference function. The reference functions are defined such that the utilization of the dc-link voltage is maximized in any modulation index, while the dv/dt of the switches is always the minimum possible value. In order to implement the proposed method, an update time that is much shorter than the fundamental period is defined in the algorithm for updating the rectangular components of the reference voltage. The high-frequency rectangular component can then be imposed on the reference function to generate the final switching function by switching between two voltage levels during the time between two update instances. Several experimental results are provided to evaluate the performance of the proposed method and to compare its operation to conventional methods. [ABSTRACT FROM PUBLISHER]

This paper presents a sampled-time-domain analysis to investigate the sampling effect on instantaneous switching frequency of a digitally implemented hysteresis current control (HCC) in a voltage source inverter (VSI). The generalized expression of the instantaneous switching frequency with the consideration of sampling frequency has been derived. The effect of variation in sampling frequency on effective hysteresis band and switching frequency has been analyzed. The difference between maximum to minimum switching frequency of a fixed-band HCC VSI depends on the sampling frequency of the digital controller. The sampling also generates a steady-state tracking error exceeding the fixed-band limit at the fundamental frequency for fixed hysteresis band operation. The analysis has been done to achieve fixed switching frequency with variable hysteresis band and with varying sampling period. The frequency spectrum of the load current is presented to show the effect of a digital controller. The sampling effect on the switching performance has been demonstrated for a single-phase VSI under different loading conditions. To verify the analytical results, the simulation and experimental results are obtained with the current controller implemented on a field programmable gate array-based digital controller. [ABSTRACT FROM PUBLISHER]

This paper presents a new hybrid phase-shift modulation technique which is able to provide soft switching for the entire operating conditions of dc/dc converters. The proposed phase-shift modulation technique does not require any extra active/passive circuitry to guarantee soft-switching. In addition, the proposed technique can be applied to the general category of power circuit topologies for dc/dc converters, which use a full-bridge inverter in their power circuit. The proposed hybrid phase-shift modulation technique includes two distinct modulation strategies based on the operating condition of the converter. Thus, it has a variable structure that can optimize the performance for various operating conditions. The main advantages of the proposed technique are the achievement of soft-switching independent of the load condition, and optimized performance. Theoretical analysis and simulation/experimental results demonstrate the superior performance of the proposed hybrid phase-shift modulation technique over the conventional one. [ABSTRACT FROM AUTHOR]

Cascaded converters are adopted in many power systems to manage sources and loads at different voltage levels simultaneously. DC bus capacitor is one of the key components in these converters due to its energy storage and fast response capabilities. However, the ripple current through the capacitor will influence its lifetime and the overall efficiency of the converter. Previous studies showed that the capacitor current of ac systems is optimized when the duty cycles are synchronized under in phase or antiphase condition. In this paper, the bus capacitor ripple current of cascaded two-stage converters for dc systems with different combinations of duty cycles is analyzed. It is found that the optimum capacitor current ripple varies depending on the values of individual duty cycles and the phase-shift angle between them. By properly adjusting the phase shift, the capacitor RMS current value can be reduced without affecting the converter normal operation or the need for extra sensing circuits, hence improving the energy conversion efficiency and prolonging the capacitor lifetime with relatively simple implementation. To illustrate, a case study of a two-stage dc/dc converter is carried out. A laboratory prototype converter confirmed a maximum efficiency improvement of 2.8% if the phase shift is adjusted properly when compared to the same converter with the worst case phase-shift angle. [ABSTRACT FROM AUTHOR]

The aim of this paper is to present a new structure for switched-capacitor multilevel inverters (SCMLIs) which can generate a great number of voltage levels with optimum number of components for both symmetric and asymmetric values of dc-voltage sources. The proposed topology consists of a new switched-capacitor dc/dc converter (SCC) that has boost ability and can charge capacitors as self-balancing by using the proposed binary asymmetrical algorithm and series–parallel conversion of power supply. The proposed SCC unit is used in new configuration as a submultilevel inverter (SMLI) and then, these proposed SMLIs are cascaded together and create a new cascaded multilevel inverter (MLI) topology that is able to increase the number of output voltage levels remarkably without using any full H-bridge cell and also can pass the reverse current for inductive loads. In this case, two half-bridge modules besides two additional switches are employed in each of SMLI units instead of using a full H-bridge cell that contribute to reduce the number of involved components in the current path, value of blocked voltage, the variety of isolated dc-voltage sources, and as a result, the overall cost by less number of switches in comparison with other presented topologies. The validity of the proposed SCMLI has been carried out by several simulation and experimental results. [ABSTRACT FROM PUBLISHER]

Modular multilevel converter (MMC) is a promising new topology for high-voltage applications. The MMC is made of several identical submodules. For proper operation, each submodule can be considered as a controlled voltage source where capacitor’s voltage should be maintained at a certain level. Besides, the minimization of the circulating current, which does not flow to the load, is crucial for achieving stable and efficient operation of the MMC. The interrelations among the load current, circulating current, and capacitor voltages complicate the MMC control. This paper aims to achieve stable and balanced voltage and current control with reduced circulating current in various operating conditions. The proposed control uses weighted model predictive control based on a normalized cost function to select the inverter switching patterns, which control the load current, while minimizing voltage fluctuation and circulating current. The weighting factors were selected based on minimizing the load-current total harmonic distortion (THD) and circulating current. The analysis is conducted on a low-power case study of single-phase four-cells MMC with possible extension to higher number of cells. The low-power three-level prototype is designed and built to validate this proposed method. Theoretical analysis, simulation, and experimental results are presented and compared. Parameter sensitivity analysis was also conducted. They all confirm the effectiveness of the proposed control method. [ABSTRACT FROM PUBLISHER]

Back-to-back converters have been typically used to interconnect microgrids. For a back-to-back current-source converter, the dc-link current ripple is one of the important parameters. A large ripple will cause electromagnetic interference, undesirable high-frequency losses, and system instability. Conventionally, with a given switching frequency and rated voltage, the current ripple can be reduced by increasing the dc-link inductor, but it leads to bulky size, high cost, and slow dynamic response. To solve this problem, this paper reveals that the current ripple can be significantly reduced by adjusting the gate patterns of space vector modulation between the rectifier and the inverter in a back-to-back converter. The experimental results verify the effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

In this paper, a simplified pulsewidth modulation with switching constraint control scheme is proposed to eliminate the circulating currents for bidirectional paralleled ac/dc converters in grid-tied system. The proposed control scheme can reduce the circulating currents and does not need additional current sensors and communication device among paralleled converters compared with conventional methods. Therefore, the paralleled system cost can be reduced. In addition, the current shaping and sharing between the paralleled converters can be well accomplished so that the overall performance of the paralleled converter system can be raised. Furthermore, the dc, ac, and self-generated circulating currents are clearly analyzed by graphics, and the synchronous circulating currents are first explored. Finally, a prototype system is constructed, and the proposed control scheme is implemented using a Spartan-3E XC3S250E FPGA. Both simulation and experimental results verify the validity of the proposed theory and control scheme. [ABSTRACT FROM PUBLISHER]

This paper presents a novel microinverter for a single-phase grid-connected photovoltaic (PV) system. The proposed microinverter consists of a step-up dc–dc converter using an active-clamp circuit with a series-resonant voltage doubler and a high-efficiency inverter with single-switch-modulation step-down converters. The active-clamp circuit provides zero-voltage switching (ZVS) turn-on, recycles the energy stored in the leakage inductance of the transformer, and limits switch voltage stress. Moreover, to remove the reverse-recovery problem of the rectifier diodes, a series-resonant voltage doubler is used. Thus, this whole process provides high efficiency. To improve efficiency and reliability in the proposed inverter, only a single switch is modulated at the switching frequency without a shoot-through problem. A modified controller is also adopted to achieve fast output control. Thus, the proposed PV microinverter has the structure to minimize power losses. The experimental results show a maximum efficiency of 96.2% for a 400-W microinverter. [ABSTRACT FROM PUBLISHER]

This paper presents an experimental implementation of a predictive control strategy for a system made up of two induction machines fed by a six-leg indirect matrix converter. The objective is to evaluate the practical feasibility of this method, considering the large computational cost, and to incorporate available market technologies. This strategy achieves tracking of torque, speed, and flux references on both machines, minimizing the instantaneous input reactive power present in the system. A theoretical explanation of the main concepts that are used in the predictive control strategy is presented, along with comments on the experimental implementation, experimental setup, and results obtained under both balanced and unbalanced grid conditions. [ABSTRACT FROM PUBLISHER]

Power converters using deterministic switching frequency schemes have switching noise concentrated at harmonic frequencies, resulting in enhanced electromagnetic emission. To suppress the harmonic spikes in three-phase multilevel voltage source converters, a digital control scheme based on dithered sigma–delta modulation (SDM) is proposed in this paper. Introducing a dithered sequence in an SDM varies the switching frequency randomly, resulting in the suppression of the spurious harmonic spikes in the output spectrum even with a regular control input. Although the switching frequency varies randomly, the minimum pulsewidth of the proposed scheme is the sampling time period, avoiding the minimum pulsewidth problem. Instead of a conventional scalar quantizer, the quantizer in the proposed dithered sigma–delta converter uses the principle of space vector quantization. The proposed scheme is experimentally verified on a constant $v/f$ open-loop induction motor drive for a three-level inverter realized by cascading two two-level inverters. The performance of the proposed scheme is compared with different pulsewidth modulation schemes in the entire modulation index, including the overmodulation region. [ABSTRACT FROM PUBLISHER]

Grid-connected power converters play a key role in several applications such as the integration of renewable energy sources and motor drives. For this reason, the development of high performance control strategies for this particular class of power converters has increasingly attracted the interest of both academic and industry researchers. This paper presents the predictive optimal switching sequence (OSS) direct power control (DPC) (OSS-DPC) algorithm for grid-connected converters. The OSS-DPC method belongs to the predictive-DPC family and provides the desired power references by calculating globally OSSs. To address computational and implementation issues, an efficient control algorithm, named reduced OSS-DPC, is introduced. The implementation of the proposed control strategy in a standard DSP is evaluated on a two-level power converter prototype working as a STATCOM. Experimental results show the algorithm's potential to provide high performance during both transient and steady states. [ABSTRACT FROM PUBLISHER]

Three-level topologies are widely used in back-to-back converters of wind turbine generation (WTG) systems. In comparison with a conventional two-level topology, three-level topologies have more advantages, particularly for the high power. Neutral-point clamped (NPC) and T-type are typical three-level topologies. An open-switch fault of a three-level rectifier that is connected to a permanent-magnet synchronous generator (PMSG) leads to the current distortion and torque vibration in WTG systems. Furthermore, WTG systems can break down in the worst case. Therefore, a tolerance control for open-switch faults is required to improve the reliability of WTG systems. WTG systems by applying the tolerance control can be stopped safely and prevent additional problems. Depending on the situation, it can keep the operation of WTG systems under open-switch fault. The effects of outer open-switch faults of three-level rectifiers are analyzed by considering PMSG specifications in this paper. Moreover, a tolerance control for outer open-switch faults of the NPC and T-type three-level rectifiers are proposed with existing tolerance controls for inner open-switch faults. The proposed tolerance control is to inject the exact $d$-axis current value to eliminate current distortion. The simulation and experiment are conducted to prove the feasibility of the proposed tolerance control and to identify its performance. [ABSTRACT FROM PUBLISHER]

The industrial use of inductive power transfer (IPT) systems is becoming widespread, ranging from monorail systems, motors, people movers, and battery charging applications. This paper proposes a 3-D IPT system driven by a multiple-phase power converter. This is aided by spice circuitry simulation of the power converter, finite-element-assisted software magnetic frequency analysis, and via implementation of the proposed system. The proposed system consists of a cubic power transfer primary window generated by a rotating magnetic flux flow path. This rotating field is loosely coupled via magnetomotive force (MMF) induction into a secondary power pick-up. The system has been demonstrated as a low-power battery charging system. [ABSTRACT FROM PUBLISHER]