Showing papers on "Power factor published in 2010"

Analytical Expressions for DG Allocation in Primary Distribution Networks

Abstract: This paper proposes analytical expressions for finding optimal size and power factor of four types of distributed generation (DG) units. DG units are sized to achieve the highest loss reduction in distribution networks. The proposed analytical expressions are based on an improvement to the method that was limited to DG type, which is capable of delivering real power only. Three other types, e.g., DG capable of delivering both real and reactive power, DG capable of delivering real power and absorbing reactive power, and DG capable of delivering reactive power only, can also be identified with their optimal size and location using the proposed method. The method has been tested in three test distribution systems with varying size and complexity and validated using exhaustive method. Results show that the proposed method requires less computation, but can lead optimal solution as verified by the exhaustive load flow method.

A High-Efficiency Dimmable LED Driver for Low-Power Lighting Applications

Abstract: This paper presents a dimmable light-emitting diode (LED) driver with adaptive feedback control for low-power lighting applications. An improved pulsewidth modulation dimming technique is studied for regulating the LED current and brightness. Under universal input voltage operation, high efficiency and high power factor can be achieved by a coupled inductor single-ended primary inductance converter power factor correction (PFC) converter with a simple commercial transition-mode PFC controller. The operation principles and design considerations of the studied LED driver are analyzed and discussed. A laboratory prototype is also designed and tested to verify the feasibility.

Accurate Power Loss Model Derivation of a High-Current Dual Active Bridge Converter for an Automotive Application

Abstract: An accurate power loss model for a high-efficiency dual active bridge converter, which provides a bidirectional electrical interface between a 12-V battery and a high-voltage (HV) dc bus in a fuel cell car, is derived. The nominal power is 2 kW, the HV dc bus varies between 240 and 450 V, and the battery voltage range is between 11 and 16 V. Consequently, battery currents of up to 200 A occur at nominal power. In automotive applications, high converter efficiency and high power densities are required. Thus, it is necessary to accurately predict the dissipated power for each power component in order to identify and to properly design the heavily loaded parts of the converter. In combination with measured efficiency values, it is shown that conventional converter analysis predicts substantially inaccurate efficiencies for the given converter. This paper describes the main reasons why the conventional method fails and documents the different steps required to predict the power losses more accurately. With the presented converter prototype, an efficiency of more than 92% is achieved at an output power of 2 kW in a wide input/output voltage range.

Correction to “Experimental Design of a Nonlinear Control Technique for Three-Phase Shunt Active Power Filter”

Abstract: This paper presents a nonlinear control technique for a three-phase shunt active power filter (SAPF). The method provides compensation for reactive, unbalanced, and harmonic load current components. A proportional-integral (PI) control law is derived through linearization of the inherently nonlinear SAPF system model, so that the tasks of current control dynamics and dc capacitor voltage dynamics become decoupled. This decoupling allows us to control the SAPF output currents and the dc bus voltage independently of each other, thereby providing either one of these decoupled subsystems a dynamic response that significantly slower than that of the other. To overcome the drawbacks of the conventional method, a computational control delay compensation method, which delaylessly and accurately generates the SAPF reference currents, is proposed. The first step is to extract the SAPF reference currents from the sensed nonlinear load currents by applying the synchronous reference frame method, where a three-phase diode bridge rectifier with R-L load is taken as the nonlinear load, and then, the reference currents are modified, so that the delay will be compensated. The converter, which is controlled by the described control strategy, guarantees balanced overall supply currents, unity displacement power factor, and reduced harmonic load currents in the common coupling point. Various simulation and experimental results demonstrate the high performance of the nonlinear controller.

A Unity-Power-Factor IPT Pickup for High-Power Applications

Abstract: This paper describes the design of a new unity-power-factor inductive-power-transfer (IPT) pickup using an LCL tuned network for application in high-power systems. This new topology has the potential to increase the efficiency and reduce the cost of high-power pickups by minimizing the reactive currents in the pickup coil and the reflected VAR loading on the power supply. In a practical system, the rectifier and associated processing circuitry distorts the current waveforms, adding an effective inductive loading to the pickup circuit. A series compensation capacitor is added to correct this loading. A design strategy is developed for the new topology, and two example circuits are constructed and compared experimentally with a traditional parallel-tuned (LC) pickup operating on a monorail-based IPT system.

Predictive Direct Power Control of Three-Phase Pulsewidth Modulation (PWM) Rectifier Using Space-Vector Modulation (SVM)

Abstract: In this paper, we present a direct power control (DPC) of three-phase pulsewidth modulation rectifier with constant switching frequency using space-vector modulation (SVM). The developed DPC scheme is based on the predictive control strategy to achieve direct control of instantaneous active and reactive power of the converter. For this purpose, at the beginning of each switching period, the required rectifier average voltage vector allowing the cancellation of active and reactive power tracking errors, at the end of the switching period, is calculated by means of predictive control algorithm in the sense of deadbeat control. The main advantages of the proposed control, compared to the works published in this subject, are that no need to use predefined switching table and voltage vector or virtual flux position, PI-based active and reactive power control loops are not necessary and constant-switching frequency. The proposed predictive direct power control was tested both in simulations and experimentally and compared with DPC using switching table. Results have proved excellent performance, and verify the validity of the proposed DPC scheme, which is much better than conventional DPC using switching table.

A Method of Reducing the Peak-to-Average Ratio of LED Current for Electrolytic Capacitor-Less AC–DC Drivers

Abstract: This paper proposes a concept of electrolytic capacitor-less light-emitting diode (LED) driver, which converts the commercial ac voltage to a pulsating current with twice the line frequency driving high-brightness LEDs. As no electrolytic capacitor is used, this driver possesses the unique advantage of long lifetime to match with that of LEDs. A method of injecting the third and fifth harmonics into the input current to reduce the peak-to-average ratio of the output current is also proposed. While ensuring that the input power factor is higher than 0.9 to meet regulation standards such as ENERGY STAR, the proposed method allows the peak-to-average ratio of the output current to be reduced to 1.34 theoretically, which is beneficial for the safe operation of the LEDs. As an example, a flyback-based electrolytic capacitor-less LED driver is proposed, and its operation is analyzed. In order to inject the third and fifth harmonics into the input current, the function of the duty cycle in a half-line cycle is derived. It is then simplified to a fitting function, which can be easily implemented with the input voltage sensing. A 25 V, 0.35 A output prototype is built and tested in the laboratory, and the experimental results are presented to verify the effectiveness of the electrolytic capacitor-less LED driver and its control method.

Enhanced power quality control strategy for single-phase inverters in distributed generation systems

Abstract: Power electronic converters are commonly used for interfacing distributed generation systems to the electrical power network. This paper deals with a single-phase inverter for distributed generation systems requiring power quality features, such as harmonic and reactive power compensation for grid-connected operation. The idea is to integrate the DG unit functions with shunt active power filter capabilities. With the proposed approach, the inverter controls the active power flow from the renewable energy source to the grid and also performs the non-linear load current harmonic compensation keeping the grid current almost sinusoidal. The control scheme employs a current reference generator based on Sinusoidal Signal Integrator (SSI) and Instantaneous Reactive Power (IRP) theory together with a repetitive current controller. Experimental results obtained on a 4 kVA inverter prototype demonstrate the feasibility of the proposed solution.

Multistring Five-Level Inverter With Novel PWM Control Scheme for PV Application

Abstract: This paper presents a single-phase multistring five-level photovoltaic (PV) inverter topology for grid-connected PV systems with a novel pulsewidth-modulated (PWM) control scheme. Three PV strings are cascaded together in parallel configuration and connected to a five-level inverter to produce output voltage in five levels: zero, +1/2V dc, V dc, -1/2V dc, and -V dc. Two reference signals that were identical to each other with an offset that was equivalent to the amplitude of the triangular carrier signal were used to generate PWM signals for the switches. DSP TMS320F2812 is used to implement this PWM switching scheme together with a digital proportional-integral current control algorithm. The inverter offers much less total harmonic distortion and can operate at near-unity power factor. The validity of the proposed inverter is verified through simulation and implemented in a prototype. The experimental results are compared with a conventional single-phase multistring three-level grid-connected PWM inverter.

A 6- $\mu$ W Chip-Area-Efficient Output-Capacitorless LDO in 90-nm CMOS Technology

Abstract: An output-capacitorless low-dropout regulator (LDO) compensated by a single Miller capacitor is implemented in a commercial 90-nm CMOS technology. The proposed LDO makes use of the small transistors realized in nano-scale technology to achieve high stability, fast transient performance and small voltage spikes under rapid load-current changes without the need of an off-chip capacitor connected at the LDO output. Experimental result verifies that the proposed LDO is stable for a capacitive load from 0 to 50 pF (estimated equivalent parasitic capacitance from load circuits) and with load capability of 100 mA. Moreover, the gain-enhanced structure provides sufficient loop gain to improve line regulation to 3.78 mV/V and load regulation to 0.1 mV/mA, respectively. The embedded voltage-spike detection circuit enables pseudo Class-AB operation to drive the embedded power transistor promptly. The measured power consumption is only 6 μW under a 0.75-V supply. The maximum overshoot and undershoot under a 1.2-V supply are less than 66 mV for full load current changes within 100-ns edge time, and the recovery time is less than 5 μs.

Frequency modulation type wireless power supply and charger system

Abstract: A frequency modulation type wireless power supply and charger system includes a power supply base unit consisting of a first microprocessor, a power circuit, a power switch driver circuit, a first resonant circuit, a first coil, a detection module and a power input interface, and a wireless power supply and charge receiver unit consisting of a secondary coil, a second resonant circuit, a rectifier filter circuit, a detection and protection module, a second microprocessor, a temperature sensor, a charging module and a power output interface and adapted for receiving electrical power from the power supply base unit wirelessly for charging an external electronic device. The detection and protection module of the wireless power supply and charge receiver unit detects the supply of the electrical power, and the second microprocessor sends a feedback signal to the power supply base unit through the signal generator circuit and the second resonant circuit subject to the detection of the detection and protection module for enabling the power supply base unit to control the power switch driver circuit to regulate its output power automatically, thereby minimizing power transmission loss and the load of the internal components of the wireless power supply and charge receiver unit.

A STATCOM-Control Scheme for Grid Connected Wind Energy System for Power Quality Improvement

Abstract: Injection of the wind power into an electric grid affects the power quality. The performance of the wind turbine and thereby power quality are determined on the basis of measurements and the norms followed according to the guideline specified in International Electro-technical Commission standard, IEC-61400. The influence of the wind turbine in the grid system concerning the power quality measurements are-the active power, reactive power, variation of voltage, flicker, harmonics, and electrical behavior of switching operation and these are measured according to national/international guidelines. The paper study demonstrates the power quality problem due to installation of wind turbine with the grid. In this proposed scheme STATic COMpensator (STATCOM) is connected at a point of common coupling with a battery energy storage system (BESS) to mitigate the power quality issues. The battery energy storage is integrated to sustain the real power source under fluctuating wind power. The STATCOM control scheme for the grid connected wind energy generation system for power quality improvement is simulated using MATLAB/SIMULINK in power system block set. The effectiveness of the proposed scheme relives the main supply source from the reactive power demand of the load and the induction generator. The development of the grid co-ordination rule and the scheme for improvement in power quality norms as per IEC-standard on the grid has been presented.

Power transmission method of high-power wireless induction power supply system

Abstract: A power transmission method used in a high-power wireless induction power supply system consisting of a power-supplying module and a power-receiving module is disclosed. The power-supplying module regulates its output energy by means of frequency modulation and driving power adjustment, enabling the energy to be received by the power-receiving module and transmitted through a power-receiving coil array and a primary resonant capacitor and a secondary resonant capacitor of power-receiving resonance circuit, a synchronizing rectifier, a low-power voltage stabilizer, a high-frequency filter capacitor, a first power switch, a low-frequency filter capacitor and a second power switch of a filter circuit for output to an external apparatus.

Power system with power converters having an adaptive controller

Abstract: A power system having a power converter with an adaptive controller. The power system is coupled to a load and includes a power system controller that receives a signal indicating a system operational state of the load and selects a power converter operational state as a function thereof. The power system also includes a power converter with a power switch that conducts for a duty cycle to provide a regulated output characteristic at an output thereof. The power converter also includes a controller that receives a command from the power system controller to enter the power converter operational state and provides a signal to control the duty cycle of the power switch as a function of the output characteristic and in accordance with the command, thereby regulating an internal operating characteristic of the power converter to improve an operating efficiency thereof as a function of the system operational state.

Load impedance decision device, wireless power transmission device, and wireless power transmission method

Abstract: A load impedance decision device, a wireless power transmission device, and a wireless power transmission method are provided. At least one of a distance and an angle between two resonators may be measured. A load impedance may be determined based on at least one of the measured distance and the measured angle. When the distance between the two resonators changes, a high power transfer efficiency may be maintained without using a separate matching circuit. Where the load impedance is determined, a test power may be transmitted. Depending on a power transfer efficiency of the test power, the load impedance may be controlled and power may be wirelessly transmitted from the source resonator to the target resonator.

Predictive Torque Control of an Induction Machine Fed by a Matrix Converter With Reactive Input Power Control

Abstract: This paper presents a new control method for a matrix-converter-based induction machine drive. A discrete model of the converter, motor, and input filter is used to predict the behavior of torque, flux, and input power to the drive. The switching state that optimizes the value of a quality function, used as the evaluation criterion, is selected and applied during the next discrete-time interval. Experimental results confirm that the proposed strategy gives high-quality control of the torque, flux, and power factor with a fast dynamic control response. The key implementation issues are analyzed in depth to give an overview of the realization aspects of the proposed algorithm.

Nonisolated High Step-up Boost Converter Integrated With Sepic Converter

Abstract: For a nonisolated high step-up converter, the combination of a boost converter with a series output module is investigated in this paper. As a solution to supplement the insufficient step-up ratio and distribute a voltage stress of a classical boost converter, a sepic-integrated boost (SIB) converter, which provides an additional step-up gain with the help of an isolated sepic converter, is proposed. Since the boost converter and the sepic converter share a boost inductor and a switch, its structure is simple. Moreover, the SIB converter needs no current snubber for the diodes, since the transformer leakage inductor alleviates the reverse recovery. The operational principle and characteristics of SIB converter are presented, and verified experimentally with a 200 W, 42 V input, 400 V output prototype converter.

Local Control of Reactive Power by Distributed Photovoltaic Generators

Abstract: High penetration levels of distributed photovoltaic (PV) generation on an electrical distribution circuit may degrade power quality due to voltage sags and swells caused by rapidly varying PV generation during cloud transients coupled with the slow response of existing utility compensation and regulation equipment Fast-reacting, VAR-capable PV inverters may provide the necessary reactive power injection or consumption to maintain voltage regulation under difficult transient conditions As side benefit, the control of reactive power injection at each PV inverter provides a new tool for distribution utilities to minimize the thermal losses in circuit We suggest a local control scheme that dispatches reactive power from each PV inverter based on local instantaneous measurements of the real and reactive components of the consumed power and the real power generated by the PVs Using one adjustable parameter per circuit, we balance the requirements on power quality and desire to minimize thermal losses The performance of the proposed control scheme is evaluated via numerical simulations of realistic rural lines in several generation/consumption scenarios Simultaneous improvement of both the power quality and the magnitude of losses is observed for all the scenarios, even when the renewable generation in excess of the circuit own load

A Novel Passive Offline LED Driver With Long Lifetime

Abstract: This paper describes a patent-pending passive offline light-emitting diode (LED) driver that has no controlled semiconductor switches, electrolytic capacitors, auxiliary power supply, and control board. It can provide a fairly smooth current from the ac mains to drive LED strings. The new circuit has the advantages of high input power factor, high energy efficiency and luminous efficacy, long lifetime, stable luminous output, and high robustness against extreme weather conditions. In addition, over 90% of the driver material is recyclable, leading to reduction of electronic waste. It is particularly suitable public LED lighting systems, such as road lighting systems. Experimental results based on a 50-W system are included in the paper to confirm the validity of the proposal. Due to the circuit simplicity, an energy efficiency exceeding 93.6% has been achieved.

Design-Oriented Analysis and Performance Evaluation of Buck PFC Front End

Abstract: In universal-line ac/dc converters that require power factor correction (PFC), maintaining a high efficiency across the entire line and load ranges poses a major challenge. Typically, a boost PFC front end exhibits 1%-3% lower efficiency at 100-V line compared to that at 230-V line. It is shown in this paper that a buck PFC front end with an output voltage in the 80-V range can maintain a high efficiency across the entire line and load ranges. A thorough analysis of the buck PFC converter operation and performance along with design optimization guidelines are presented. Experimental results obtained on a 90-W notebook adapter are provided. A loss analysis based on SIMPLIS and PSPICE simulations is also included. Major factors that contribute to the improved efficiency of the buck PFC versus the boost PFC are briefly explained.

Zero-Current-Switching Multilevel Modular Switched-Capacitor DC–DC Converter

Abstract: This paper presents a quasi-resonant technique for multilevel modular switched-capacitor dc-dc converter (MMSCC) to achieve zero-current switching (ZCS) without increasing cost and sacrificing reliability. This ZCS-MMSCC employs stray inductance distributed in the circuit as the resonant inductor to resonate with the capacitor and provide low dv /dt and di /dt switching transition for the device. The ZCS-MMSCC does not utilize any additional components to achieve ZCS and meanwhile solves the current and voltage spike problem during the switching transition, thus leading to reliable and high-efficiency advantages over traditional MMSCC. Furthermore, the ZCS-MMSCC reduces the capacitance needed in the circuit to attain high efficiency. In this case, the bulky capacitor bank with high capacitance in a traditional MMSCC to reduce voltage difference and achieve high efficiency is no longer necessary. A 150-W four-level ZCS-MMSCC prototype has been built. Simulation and experimental results are given to demonstrate the validity and features of the proposed soft-switching switched-capacitor circuit.

An Interleaved Totem-Pole Boost Bridgeless Rectifier With Reduced Reverse-Recovery Problems For Power Factor Correction

Abstract: An interleaved totem-pole boost bridgeless rectifier with reduced reverse-recovery problems for power factor correction is proposed in this paper. The proposed converter consists of two interleaved and intercoupled totem-pole boost bridgeless converter cells. The two cells operate in phase-shift mode. Thus, the input current can be continuous with low ripple. For the individual cells, they operate alternatively in discontinuous current mode and the maximum duty ratio is 50%, which allows shifting the diode current with low di/dt rate to achieve zero-current switching off. Zero-voltage switching is achieved in the MOSFETs under low line input. Furthermore, the merits of totem-pole topology are inherited. The common-mode (CM) noise interference is rather low. And the potential capacity of bidirectional power conversion is retained. In brief, the conduction losses are reduced, reverse-recovery process is improved, and high efficiency is achieved. The interleaved totem-pole cell can also be applied to bidirectional dc/dc converters and ac/dc converters. Finally, an 800 W, 100 kHz experimental prototype was built to verify the theoretical analysis and feasibility of the proposed converter, whose efficiency is above 95.5% at full load under 90 V.

A Modified SEPIC Converter for High-Power-Factor Rectifier and Universal Input Voltage Applications

Abstract: A high-power-factor rectifier suitable for universal line base on a modified version of the single-ended primary inductance converter (SEPIC) is presented in this paper. The voltage multiplier technique is applied to the classical SEPIC circuit, obtaining new operation characteristics as low-switch-voltage operation and high static gain at low line voltage. The new configuration also allows the reduction of the losses associated to the diode reverse recovery current, and soft commutation is obtained with a simple regenerative snubber circuit. The operation analysis, design procedure, and experimental results obtained from a 650-W universal line power-factor-correction prototype of the proposed converter are presented. The theoretical analysis and experimental results obtained with the proposed structure are compared with the classical boost topology.

Digital Control for Improved Efficiency and Reduced Harmonic Distortion Over Wide Load Range in Boost PFC Rectifiers

Abstract: This paper focuses on efficiency improvements and harmonic distortion reductions in digitally controlled single-phase boost power-factor-correction (PFC) rectifiers operating over wide load range. Based on a discontinuous conduction mode (DCM) detection circuit, a predictive current control law is modified to improve current shaping in both DCM and continuous conduction mode (CCM). Adaptive switching and adaptive frequency control techniques are introduced to reduce switching losses and improve efficiency at light loads. Experimental results are shown for a 300 W boost PFC rectifier.

A review of single-phase PFC topologies based on the boost converter

Abstract: The need for solid-state ac-dc converters to improve power quality in terms of power-factor correction (PFC), reduced total harmonic distortion at input ac mains, and precisely regulated dc output have motivated the proposal of several topologies based on classical converters such as buck, boost, and buck-boost. Additionally, novel control techniques dedicated to PFC have also been introduced, motivating the manufacturing of commercial integrated circuits to impose sinusoidal currents in the front-end stage of switch-mode converters. Boost converters operating in continuous current mode (CCM) have become particularly popular because reduced electromagnetic interference (EMI) levels result from its utilization. Within this context, this work deals with a comprehensive review of some of the most relevant ac-dc singlephase boost converters for PFC applications. The evolution of the conventional boost converter is demonstrated in terms of improved characteristics achieved by other boost-based topologies. Besides, it seeks to establish a fast and concise guide on ac-dc boost converters to researchers and experts in power electronics by comparing the topologies.

Distributed Voltage and Frequency Control of Offshore Wind Farms Connected With a Diode-Based HVdc Link

Abstract: This paper introduces a new technique for the distributed voltage and frequency control of the local ac-grid in offshore wind farms based on synchronous generators. The proposed control technique allows the connection of the offshore wind farm using a diode based HVdc rectifier. The use of microgrid control techniques allowed the system comprising the wind farm and the diode HVdc rectifier to be operated in current or voltage control mode. Fault response to on-shore voltage sags of up to 80% has been shown to be comparable to that of thyristor rectifiers. The proposed control technique has been shown to be robust against load changes in islanded operation, capacitor bank switching, diode-rectifier ac-breaker tripping and wind turbine power limitation due to slow wind speeds. PSCAD simulations are used to prove the technical feasibility of the proposed control techniques both in steady state and during transients.

Direct-Matrix-Converter-Based Drive for a Three-Phase Open-End-Winding AC Machine With Advanced Features

Abstract: This paper describes how matrix converters (MCs), one at each side of a three-phase open-end-winding ac machine, achieve the following features simultaneously: 1) machine phase voltage up to 1.5 times the input phase voltage in the linear modulation mode, therefore extending the rated torque operation region to 150% of the rated speed of the machine; 2) peak voltage stress across the slot insulation which is limited to the peak of input phase voltage, i.e., a factor of at least √3 lower as compared to the conventional back-to-back converter; 3) controllable grid power factor to be leading, lagging, or unity; and 4) elimination of the instantaneous common-mode voltage at the machine terminals, therefore eliminating the bearing current due to switching common-mode voltage and reduction in the conducted electromagnetic interference. To simultaneously achieve the aforementioned capabilities, a space vector pulsewidth modulation technique is described in which the MCs are modulated using only rotating space vectors. A hardware prototype of the drive system is built. Experimental results from this hardware prototype verify the operation and claims of the drive system.

Power distribution apparatus with input and output power sensing and method of use

Abstract: Power distribution apparatus with input and output power sensing and a method of use. A power distribution unit includes a sensor that senses power parameters of power outputs and a power input, a processor, and a communication circuit. A power management system includes a power manager, a user interface, and a plurality of power distribution units that may be located in one or more equipment cabinets and data centers. The system may compute apparent power, RMS power, power factor, energy usage over time, power usage history, or environmental history for any or all of the power distribution units. The system may identify an under-utilized server connected to one of the power distribution units and initiate a shut-down of that server.

Hybrid PWM Strategy of SVPWM and VSVPWM for NPC Three-Level Voltage-Source Inverter

Abstract: Neutral-point (NP) voltage drift is the main technical drawback of NP-clamped (NPC) three-level inverters. Traditional space vector pulsewidth modulation (SVPWM) is incapable of controlling the NP voltage for high modulation indexes and low power factors. Virtual SVPWM (VSVPWM) is capable of controlling the NP voltage under full modulation indexes and full power factors. However, this modulation strategy is more complex than SVPWM, increases the switching frequency, and deteriorates the output waveforms of the inverter. A novel PWM concept that includes NP voltage-balancing conditions is proposed. Based on this concept, a hybrid modulation scheme that uses both SVPWM and VSVPWM is presented for complete control of the NP voltage in NPC three-level inverters. The performance of this new modulation approach and its benefits over SVPWM and VSVPWM are verified by simulation and experiments.

Wireless power apparatus and wireless power-receiving method

Abstract: A wireless power apparatus includes: a power receiver coil which receives power, as magnetic field energy, from a power transmitter coil by magnetic field resonance produced between the power transmitter coil and the power receiver coil; a power pickup coil which derives power from the power receiver coil by electromagnetic induction; a detector which detects current flowing through the power pickup coil; and a controller which controls a coupling strength between the power pickup coil and the power receiver coil based on the current detected by the detector.