Coil Voltage Research Articles

Position estimation accuracy improvement based on accurate modeling of self-sensing active magnetic bearings

The Self-sensing Active Magnetic Bearings (SSAMBs) which feature for the simplification of independent displacement sensors, contribute to both cost reduction and convenience in control and assembly. The key point of this technique is to accurately estimate rotor position signals directly from coil voltages or currents, thus the actuator itself also serves as a virtual displacement sensor. Although several feasible approaches have been proposed to realize the self-sensing process, there still lie limitations in estimation accuracy, robustness and dynamic performance of the system. In fact, due to the nonlinear property of the self-sensing process, few accurate models are available for quantitative analysis of the estimation accuracy, and this restricts further development of this technique. Focusing on the modulation type SSAMBs, an accurate analytical model of the self-sensing in frequency domain is firstly established in this paper. Eddy current effects and filter properties are also considered for better accuracy. Based on this model, we evaluate possible estimation error sources during the self-sensing process, and investigate if the estimation accuracy can be improved fundamentally by selecting main system parameters properly. In this way, the estimation accuracy gets further improved under existing compensation methods, and better system robustness and dynamic performance can be achieved. Finally, the analytical model and associated conclusions are validated through static experiment, 4-Degree of Freedom (DOF) self-sensing rotor levitation experiment and 0–5000rpm rotating operation experiment on a 4-DOF rigid rotor-radial SSAMBs platform.

Linear Control of Switching Valve in Vehicle Hydraulic Control Unit Based on Sensorless Solenoid Position Estimation

Accurate linear brake-pressure control in a hydraulic control unit (HCU) of the vehicle dynamics control system, such as the electronic-stability control (ESC) system, can improve the control performance while significantly reduce noise and brake paddle vibration during the transient high-frequency brake-pressure control process. To minimize system cost, only simple switching solenoid valves and a master cylinder pressure (MCP) sensor are used in the ESC, thus an accurate real-time sensorless spool-position estimation is the key to the linear pressure controller. In this paper, a new sliding-mode controller (SMC) integrated with a nonlinear sliding-mode observer (SMO) is proposed to achieve a real-time linear controller for the switching valve in the HCU. First, a nonlinear SMO is used to observe the spool position based on the voltage and current values of the solenoid coil, from which the flow rate of brake fluid in the HCU of the ESC and the wheel cylinder pressure (WCP) can be deduced based on a specific hydraulic model of the solenoid valve and the braking system. Then an SMC is employed to adjust the spool position for accurate linear pressure control, taking into consideration the nonlinear characteristics of the electromagnetic force, the dynamic flow force, and the flow rate of the solenoid valve at different openings. Simulations and experiments were conducted, and the results supported the tracking ability of the sensorless linear pressure controller in different conditions. This study may provide a useful method to realize real-time linear control of high-frequency switching valve comparable to a high-performance proportional valve.

Experimental- and numerical-based studies for magnetically impelled arc butt welding of T11 chromium alloy tubes

Magnetically impelled arc butt (MIAB) welding is a pressure welding process used for joining of pipes and tubes with an external magnetic field affecting arc rotation along the tube circumference. Past studies based on experiments and finite element analysis (FEA) suggested that the weld quality can be significantly improved by proper adjustment of its input parameters such as current, voltage, flux density, etc. Experiments are useful for understanding the physics behind the process, but the quantification and optimization of the MIAB process needs a thorough numerical investigation. Therefore, the present work presents a combined experimental and numerical study for investigating the arc speed indicator of the T11 grade chromium alloy tubes of the MIAB process. Firstly, experiments on the MIAB process are conducted, and the arc speed is measured based on the four inputs (welding current, welding voltage, magnetic coil current, and voltage). This is followed by introduction of a numerical approach of multi-gene genetic programming (MGGP) to formulate a functional relationship between arc speed and the four design variables. The statistical error metrics based on the square of correlation coefficient, relative error (%), and root mean square were used to validate the model performance against the experimental data. The proposed model can be utilized to select the best combination of input variables to optimize the arc speed, which has a vital role in governing the weld quality. In addition, 2-D analysis conducted reveals that the welding current influence the arc speed of the clay the most followed by coil current and coil voltage.

Quench Protection System for a 13-T Magnet With a 700-mm Bore

A 13-T test facility with a 700-mm cold bore is being prepared for the research of high-field and high-current superconductors. Considering reuse of the existing cryostat, we select a pool-cooled and closely wound coil for the background field magnet to attain a large bore with the restricted outer diameter. The magnet is divided into two in the longitudinal direction and six in the radial direction. Rectangular monolithic conductors of Nb 3 Sn and NbTi are selected for the inner six coils and the outer six coils, respectively. To reduce the highest voltage of the Nb 3 Sn coil below 1.5 kV, the protection circuit of the Nb 3 Sn coils is separated by a circuit breaker from that of the NbTi coils. In addition, the half of protection resistors for the Nb 3 Sn coils is connected between the upper half and lower half of the Nb 3 Sn coils. The quench protection system consists of eleven 1-kV dc circuit breakers, ten protection resistors, and two diodes. According to the quench analyses with taking account of heat transfer between layers and the effect of resistance of a propagating normal zone on the current, the highest temperature is considerably lower than the adiabatic hotspot temperatures.

Experimental Study on Thermal Behavior of HTS Coils With Quasi-Insulation Winding Method at Overcurrent Operation

Recently, superconducting power applications by using the high-temperature superconducting (HTS) tape have been widely researched due to the development of the HTS tape. In particular, the HTS coils cowound with insulation materials have been dominantly used for the power applications. However, the insulated HTS coil has low thermal stability at overcurrent operation, which causes much likely to be damaged. To relieve this issue, other insulation methods such as insulation-free or partial insulation have been investigated. These methods have good thermal stability; however, they have weaknesses such as magnetic field saturation and charge-discharge delay as well. In particular, charge-discharge delay is a significant obstacle for superconducting magnetic energy storage due to a frequently variable current operation. Therefore, a quasi-insulation (QI) coil that compensates the disadvantages has been proposed. In this paper, two QI coils according to the width of the insulation tape were fabricated since the performance of this winding method is affected by the exposure area to the coolant. In addition, insulation-free coil and partial insulation coil were fabricated and tested as the control group. The terminal coil voltage and center magnetic field were measured to compare the characteristics of each coil. The experimental result shows the QI coil has good thermal stability at overcurrent operation without magnetic field saturation and degradation. In addition, cooling effect is proportional to the exposed area to the coolant and adjustable with a different width of insulation tape.

Influence of turn-to-turn resistivity and coil geometrical size on charging characteristics of no-electrical-insulation REBCO pancake coils

High temperature superconductor (HTS) no-electrical-insulation (NEI) coils demonstrate great advantages in thermal stability and self-protection features. However, an intrinsic delay is observed in the charging process and as a result there maybe a possible settle-out problem. It becomes more critical for large HTS coils with more turns, such as the magnets for the accelerator system and DC induction heater applications. This paper presents detailed studies on the charging characteristics of NEI coils. Firstly, two different no-electrical-insulation coils are wound: the first is directly wound using only REBCO tapes with brass lamination, which is called a no-insulation (NI) coil. The other one is co-wound with stainless steel (SS) strips and REBCO tapes whose copper stabilizer is electroplated, which is called a metallic insulation (MI) coil. Fast discharging tests are performed on the two coils and their equivalent turn-to-turn resistivity is calculated. A similar discharging delay is observed on both coils, but the turn-to-turn resistivity of the SS co-wound coil is much higher than that of the first coil. Then the resistivity data is directly applied to an equivalent circuit network model which is developed to predict the charging behaviours. The model calculates coil voltage, currents along the azimuthal and radial directions, as well as the induced magnetic field. A practical charging time is defined to characterize the field ramping process considering the charging delay between field ramping and current charging. The charging behaviours are extensively analyzed and compared in terms of three primary factors: equivalent turn-to-turn resistivity, coil size and ramping rate. The results show that the charging time increases dramatically with the coil size and may be too long to be practical for large-scale applications using HTS coils with low turn-to-turn resistivity. Increasing the turn-to-turn resistivity enables one to accelerate the charging process effectively. Therefore, the SS co-wound coil with higher turn-to-turn resistivity shows a much shorter charging time, which indicates that it may be more suitable for large-scale HTS magnets.

A study of the electromagnetic characteristics of no-insulation GdBCO racetrack coils under an external magnetic ripple field

Here we report the effect of an external magnetic ripple field on the electromagnetic characteristics of GdBCO racetrack coils being operated with a constant DC current. Two types of GdBCO racetrack coils, one wound without turn-to-turn insulation (NI) and the other wound with Kapton tape (INS), were examined under external ripple fields generated by a permanent magnet mounted on a rotor, which was driven by a separate AC motor. The voltage fluctuations and magnetic field variations were measured with respect to the external ripple field intensity (BERF), rotating speed, and the operating condition. When the INS and NI coils were exposed to an external ripple field (herein, Iop = 80 A, BERF = 2 mT, and 5 rpm), a voltage fluctuation occurred because a time-varying magnetic field interacted with an electric circuit creating an electromotive force. The peak-to-peak voltage (Vpp = 0.29 mV) of the NI coil was ∼1.86 times lower than that (0.54 mV) of the INS coil, because the voltage response of the NI coil lagged behind dB/dt due to the existence of turn-to-turn contact. Furthermore, the Vpp of the INS coil increased with increasing BERF and rotating speed, while those of the NI coil were barely affected due to the delay of electromagnetic induction. In excessive current and ripple field conditions (Iop = 1.125 Ic, BERF = 8 mT, and 50 rpm) the INS coil eventually quenched while the NI coil did not, implying that the electromagnetic stability of the NI coil in excessive time-varying field conditions was superior to that of the INS coil.

Self-protection mechanisms in no-insulation (RE)Ba2Cu3Ox high temperature superconductor pancake coils

No-insulation (NI) high temperature superconducting (HTS) coils possess much higher thermal stability than similar traditionally insulated HTS coils. Some NI coils are self-protecting in the sense that they fully recover after a quench without any external protection mechanism to dissipate the stored energy. The underlying mechanisms that make NI coils highly stable or even self-protecting, however, remain unclear. To answer this question, a numerical multiphysics quench model for NI pancake coils is built to study the electrical, thermal and magnetic behavior of NI coils subjected to local heat disturbances. The multiphysics model is built from an electric network model, tightly coupled to a two-dimensional thermal coil model and a three-dimensional magnetic field coil model. The results show that when heat disturbance initiates a local normal region on a turn, the transport current is redistributed not only from the local normal region, but also along the entire turn. The redistributed current flows in the form of radial current across the turn-to-turn contact resistance along the entire turn to the neighboring turns which are still in the superconducting state, driving these turns to an overcurrent state. This full-turn current sharing and overcurrent operation accelerate the redistribution of current away from the hot-spot, reducing localized Joule heating that would otherwise cause a sustainable quench. The results also show that the magnetic field generated at the coil center drops rapidly and the coil voltage changes dynamically during the early stage of normal zone formation. These phenomena can be utilized as effective methods for quench detection in NI coils by monitoring the magnetic field and coil voltage.

Cell Balancing Method in Flyback Converter without Cell Selection Switch of Multi-Winding Transformer

This paper presents a cell balancing method for a single switch flyback converter with a multi-winding transformer. The conventional method using a flyback converter with a multi-winding transformer is simple and easy to control, but the voltage of each secondary winding coil might be non-uniform because of the unequal effective turn-ratio. In particular, it is difficult to control the nonuniform effect using turn-ratios because secondary coil has a limited number of turns. The nonuniform secondary voltages disturb the cell balancing procedure and induce an unbalance in cell voltages. Individual cell control by adding a switch for each cell can reduce the undesirable effect. However, the circuit becomes bulky, resulting in additional loss. The proposed method here uses the conventional flyback converter with an adjustment made to the output filters of the cells, instead of the additional switch. The magnitude of voltage applied to a particular cell can be reduced or increased according to the adjusted filter and the selected switching frequency. An analysis of the conventional converter configuration and the filter design method reveals the possibility of adequate cell balancing control without any additional switch on the secondary side.

Accuracy Improvement of Low‐Invasive Temperature Measurement for Hyperthermia Treatment Using a Ferromagnetic Implant with Low Curie Temperature

SUMMARYWe investigated a hyperthermia treatment method in which a ferromagnetic implant with a low Curie temperature (FILCT) is implanted into a malignant tumor and then a high‐frequency magnetic field is applied outside the body. In earlier studies, we confirmed that the target temperature of the tumor can be noninvasively monitored by detecting changes in the magnetic field caused by changes in magnetic permeability. It was very difficult, however, to detect the FILCT temperature if the applied magnetic field was in an unstable state. Additionally, if the initial bias is higher, it is more difficult to detect the FILCT temperature because the resolution of the lock‐in amplifier is reduced. In order to improve the accuracy of low‐invasive temperature measurement, we formulated a method that reduces the drift in the pickup coil voltage caused by instability from the applied magnetic field. In addition, we experimentally confirmed that the influence of fluctuations in the applied magnetic field can be reduced by adjusting the initial bias value to close to 0 V.

Mode transition and hysteresis in inductively coupled radio frequency argon discharge

This contribution presents experimental results about the mode transition of an inductively coupled radio frequency (RF) (13.56 MHz) argon discharge at different total gas pressures. In particular, the positive ion saturation current and the line integrated electron density are measured by Langmuir probe and 160 GHz microwave interferometer, respectively. The mode transition strongly depends on the total gas pressure and can appear stepwise or continuously. The space resolved positive ion saturation current is separately shown for the E- and H-mode at different total gas pressures. Therewith, the pressure dependency of the RF sheath thickness indicates a collisional sheath. The hysteresis phenomenon during the E-H and the inverse H-E transition is discussed within the framework of the matching situation for different total gas pressures. The hysteresis width is analyzed using the absorbed power as well as the coil voltage and current. As a result, the width strongly increases with pressure regarding the power and the coil voltage in the E-mode and remains constant in the H-mode. In addition, the phase shift between the coil voltage and current shows a hysteresis effect, too.

RETRACTED ARTICLE: Magnetic Studies of Tokamak Plasma Equilibrium Based on Magnetic System Materials and Characteristics

In this contribution, we have studied the magnetic system of ITER, and plasma equilibrium using the semi-empirical technique. Tokamak magnet systems consist of four main sub-systems: Toroidal field coils, Central solenoid coils, Poloidal field coils, and Correction coils. The plasma horizontal position is calculated from the vertical field coil characteristics. The calculation is made focusing on the vertical field coil current and voltage changes due to a horizontal displacement of plasma column. The results are compared and discussed.

Force balanced transducer based on electromagnetic lever with improved linearity

The elastically sensitive element nonlinearity and the signal–noise ratio are the limitation of the traditional force transducer, which makes the precision is rarely higher than 0.02%. This study proposes a force transducer with electromagnetic force balance lever structure. The transducer utilized a controllable electromagnet that generated the controllable electromagnetic force to offset the load while the load value is obtained by the coil voltage, which solves the signal–noise ratio problem. Analyzing the electromagnetic force curves under different temperatures by the piecewise linearization method acquires the high linearity electromagnetic force, which solves the nonlinearity problem. Test results indicated that the transducer linearity was 0.001%, and its precision was 0.003%.

分散制御型多点加振による音響固有振動の計測

This paper describes measurement of natural vibration of acoustic space by multi-point excitation using decentralized control. A set of acoustic excitation system consists of a speaker, a microphone, and a local feedback controller. The acoustic excitation system handles the frequency range from 100Hz to 500Hz higher than a mechanical vibration system does. So the phase lag of an actuator, the dead time by propagation of sound, and the dead time by sampling of a controller influence the local feedback control. First, the various phase lag in the acoustic excitation system are identified. The speaker has the second vibration property derived from mechanical resonance, and the first delay property derived from induced voltage of coil. The propagation of sound from the speaker to the microphone causes the dead time. Sampling time of the controller also causes the dead time from the input signal to the output signal. Second, the way to compensate these phase lag in the acoustic excitation system is proposed. Resonance of the speaker makes the phase decrease 180 degree at the resonant frequency. To suppress a change in the phase in the frequency range from 100Hz to 500Hz, the resonant frequency moves to less than 100 Hz. The phase lead and the phase lag after the resonant frequency movement are compensated by the second order phase lag-lead element. The natural vibration of one-dimensional/two-dimensional acoustic space is self-excited by the acoustic excitation system with the phase compensation. By closing the feedback loop, the first mode natural vibration is excited. By adding sinusoidal signal from outside, all natural vibration in the frequency range from 100Hz to the 500Hz is self-excited (forced entrainment).

Analysis of Damage by Quench and Improvements in Rewinding for a 9.4-T Superconducting NMR Magnet

A NbTi/Cu superconducting magnet for a nuclear magnetic resonance spectrometer has been developed in our laboratory. The magnet reached the designed magnetic field of 9.4 T after several trainings in a test Dewar. However, after the magnet was installed in the final Dewar, the charge of the magnet failed after two trainings. After the magnet was dissembled and two coils were unwound, it is found that local winding close to the former flange is burnt and melted. It is also found that insulating coating on the flange near the burnt area had been damaged. Quench simulation is performed, and the simulation results are analyzed. The two coils are rewound, and some measures are taken to improve the insulation and lower the coil voltages during quench. The magnet is reramped in the final Dewar, and the operating current is reached after five trainings.

E‐H Transition in Argon/Oxygen Inductively Coupled RF Plasmas

AbstractThis contribution presents results of comprehensive investigation of the E‐H transition in an inductively coupled radio frequency argon/oxygen discharge at 13.56 MHz. For the characterization of the discharge arrangement, the spatial magnetic and electric field components of the planar coil in vacuum are calculated and provide information about the electron heating region. The used double coil leads to a radial symmetric field distribution. Further, voltage and current probe measurements reveal the inductivity of the coil and the phase shift between the coil voltage and the current. As a result, the coil current and the RF voltage are directly proportional. The phase shift is nearly constant in the E‐mode and decreases slightly in the H‐mode. The positive ion saturation current as well as the line integrated electron density are measured by means of Langmuir probe and 160 GHz microwave interferometer, respectively, to study the mode transition in argon with different oxygen contents. The positive ion saturation current and the electron density decrease with increasing oxygen admixture at fixed pressure and RF power. The onset and the way of the E‐H transition strongly depend on the oxygen content. The mode transition of a pure argon discharge is step like and changes to a continuous mode transition with the admixture of oxygen. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Discontinuity of mode transition and hysteresis in hydrogen inductively coupled plasma via a fluid model**Project supported by the National Natural Science Foundation of China (Grant Nos. 11175034, 11205025, 11305023, and 11075029).

A new type of two-dimensional self-consistent fluid model that couples an equivalent circuit module is used to investigate the mode transition characteristics and hysteresis in hydrogen inductively coupled plasmas at different pressures, by varying the series capacitance of the matching box. The variations of the electron density, temperature, and the circuit electrical properties are presented. As cycling the matching capacitance, at high pressure both the discontinuity and hysteresis appear for the plasma parameters and the transferred impedances of both the inductive and capacitive discharge components, while at low pressure only the discontinuity is seen. The simulations predict that the sheath plays a determinative role on the presence of discontinuity and hysteresis at high pressure, by influencing the inductive coupling efficiency of applied power. Moreover, the values of the plasma transferred impedances at different pressures are compared, and the larger plasma inductance at low pressure due to less collision frequency, as analyzed, is the reason why the hysteresis is not seen at low pressure, even with a wider sheath. Besides, the behaviors of the coil voltage and current parameters during the mode transitions are investigated. They both increase (decrease) at the E to H (H to E) mode transition, indicating an improved (worsened) inductive power coupling efficiency.

EFFECT OF INNER ELECTRODE LENGTH OF 2.2 KJ MATHER TYPE PLASMA FOCUS ON FOCUS ACTION

This paper presents results of experimental studies carried out with a Mather type plasma focus device of 2.2 kJ energy. A critical effect of plasma focus inner electrode, IE length on the plasma focus action is detected for IE length has been changed to 9.5, 10.5 and 11.5 cm respectively; also for different Argon gas pressure ranging from 0.2 to 1.8 Torr. To verify this effect, it is important to investigate the plasma focus characteristics such as, a focusing time, voltage spike amplitude, focus action intensity, focus duration time, plasma focus inductance, and radius of plasma focus column as well as pinching ratio.The diagnostics used in the experiments include a Rogowski coil and resistive high voltage probe to measure the discharge current and voltage signals respectively. All the results illustrated that, the best focus action has been achieved for IE length of 10.5 cm and at filling Argon gas pressures varied from 0.8 to 1.2 Torr.

High strain-rate magnetoelasticity in Galfenol

This paper presents the experimental measurements of a highly magnetoelastic material (Galfenol) under impact loading. A Split-Hopkinson Pressure Bar was used to generate compressive stress up to 275 MPa at strain rates of either 20/s or 33/s while measuring the stress-strain response and change in magnetic flux density due to magnetoelastic coupling. The average Young's modulus (44.85 GPa) was invariant to strain rate, with instantaneous stiffness ranging from 25 to 55 GPa. A lumped parameters model simulated the measured pickup coil voltages in response to an applied stress pulse. Fitting the model to the experimental data provided the average piezomagnetic coefficient and relative permeability as functions of field strength. The model suggests magnetoelastic coupling is primarily insensitive to strain rates as high as 33/s. Additionally, the lumped parameters model was used to investigate magnetoelastic transducers as potential pulsed power sources. Results show that Galfenol can generate large quantities of instantaneous power (80 MW/m3), comparable to explosively driven ferromagnetic pulse generators (500 MW/m3). However, this process is much more efficient and can be cyclically carried out in the linear elastic range of the material, in stark contrast with explosively driven pulsed power generators.

High Voltage Coil Current Sensor for DC-DC Converters Employing DDCC

High Voltage Coil Current Sensor for DC-DC Converters Employing DDCC