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Valentin Pricop

Valentin Pricop. PhD review. Introduction. Development of advanced methods for the measurement of soft magnetic material characteristic. The ramping rate of normal conducting particle accelerator magnets is usually in the order of 1 T/s.

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Valentin Pricop

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  1. ValentinPricop PhD review

  2. Introduction Development of advanced methods for the measurement of soft magnetic material characteristic. • The ramping rate of normal conducting particle accelerator magnets is usually in the order of 1 T/s. • It is best to characterize a magnetic material at its foreseen working conditions; for the mentioned ramp rates, the measurements are referred to as quasi-static. • Relevant information during magnet design: first magnetization curve, permeability, coercivity, saturation and remanence. 1.33 T/s 0.65 T/s Valentin Pricop - PhD review

  3. Which device to be used? The most common devices used for testing electrical steel strips: Valentin Pricop - PhD review

  4. Which device to be used? The Epstein frame was selected for the following reasons: • Provides macroscopic measurements (magnet laminations are shuffled in order to obtain averaged magnetic properties across the yoke volume). • Provides measurements for a wide range of applied fields. • Provides measurements for both isotropic and anisotropic properties. • Best results for quasi-static testing of isotropic magnetic materials. • Low complexity, high reproducibility and is standardized in IEC 60404-2 Valentin Pricop - PhD review

  5. Quasi-static testing soft magnetic materials Quasi-static measurements are achieved when the influence of dynamic effects, especially eddy-currents, is low with respect to the coercive force. The value of the magnetic field strength due to eddy-currents can be approximated with the following equation [1]: When the rate of change of flux is sufficiently low, quasi-static measurements are achieved. [1] FaustoFiorillo, “MEASUREMENTS AND CHARACTERIZATION OF MAGNETIC MATERIALS”, Elsevier Academic Press, 2004 Valentin Pricop - PhD review

  6. IEC recommendation IEC standard recommendation: is to cycle the current through the excitation coil at a very low rate (30 to 60 second for a complete cycle; “some materials, eg. pure iron, may require longer”). • Depending of the coercive force and permeability of the material IEC’s recommendation may be overrated or depreciated. • Increasing the cycling time to compensate for the high ,raises the difficulty to determine the value of the polarization: the level of the induced signal decreases, the zero drift of the integrator is more difficult to handle. Optimized approach:controlling the waveform of the induction. • A maximum allowed level for of 1% of is considered. • Applying an induction waveform with as large as possible to respect the above inequality will shorten the cycling time and increase the voltage on the pick-up coils. is high constant NGO steel complete cycle time: 30 s GO steel complete cycle time: 60 s Valentin Pricop - PhD review

  7. Controlling the induction waveform - Examples Standard approach Optimized approach Would not qualify as quasi-static measurement (). The cycling time would have to be increased 13 times. For some configurations of material properties with magnet ramp rate, quasi-static measurements may not be the best choice. Valentin Pricop - PhD review

  8. Controlling the induction waveform Why: • Faster measurements, • Higher induced signals in the pickup coils, • Increased accuracy of measurements, • Lower drift after integration. But: • The process is iterative and an algorithm with quick convergence has to be developed, • Current hardware come with some limitations: control of power supply has to be made in current follower mode, resulting in increased noise from the control circuit. Valentin Pricop - PhD review

  9. The Epstein frame Primary coil; placed outside for better cooling. Pick-up coil – used to measure the induced voltage due to flux variation; placed close to the core to minimize the surface difference between the coil and core. The core is composed of the tested specimens; four strip bundles assembled in a square having double-lapped joints. Mutual inductor for air flux compensation. The samples have the following dimension: width = 30 mm ± 0.2 mm; length = 280 … 320 mm ± 0.5 mm. Valentin Pricop - PhD review

  10. The electrical circuit of the Epstein frame Valentin Pricop - PhD review

  11. Epstein frame – field equations The equation relating the applied magnetic field strength to the voltage measured on the shunt resistor is: Where: • is the magnetic field strength (A/m). • is the number of windings in the primary coil. • is the current through the primary coil (A). The equation relating the magnetic polarization to the voltage measured in the secondary winding is: Where: • is the voltage measured by AI1 (V). • is the magnetic flux in the pickup coil (Wb). • is the number of windings of the pickup coil. • is the cross-section surface of the samples (m2). • is the magnetic flux in the mutual coil (Wb). • is the magnetic flux in the pickup coil due to the magnetization of the sample (Wb). • is the length of the magnetic circuit (=0.94 m). • is the voltage drop on R (V). • is a high precision shunt resistor (=1 Ω) • is the magnetic flux in the pickup coil in the air (Wb). • is the magnetization of the sample (A/m). • is the magnetic polarization of the samples (T). Valentin Pricop - PhD review

  12. Epstein frame – air flux compensation The presence and proper operation of the air flux compensation inductor has been verified: • Any samples have been removed from the frame. • A 50 Hz sinusoidal voltage has been applied to the primary windings of the frame. • The voltage measured on the pickup coils of the Epstein frame was 2.2 V • The voltage measured between the non-common terminals of the secondary windings was 72 mV. • 72 mV represent 3.2 % of 2.2 V, above the 0.1 % value recommended by IEC standards. • Additional calibration of the mutual inductor is required to comply with IEC standards. Valentin Pricop - PhD review

  13. Measurement accuracy – magnetic field strength The magnetic field strength is determined with the following equation: • and are constants. • The tolerance of the resistor is 1 %. • The measurement of the voltage is accomplished with absolute accuracy of 1745 µV (for 5 V) and 47 µV (for 4.7 mV). • The error for determining the magnetic field strength is: Valentin Pricop - PhD review

  14. Measurement error – polarization The polarization is determined with the following equation: • The mass of the specimens is measured with an error of 0.01 % • The allowed tolerance for the length of the specimen is 0.2 % • The error for the determination of material density is 1 % • The error for the determination of the staking factor is 0.7 % • The error for determining the surface area of the samples is: • The measurement of the voltage is accomplished with absolute accuracy of 1745 µV (for 5 V) and 47 µV (for 4.7 mV) • The error for determining the polarization is: Valentin Pricop - PhD review

  15. The instruments Voltage measurement and generation of control voltage for power supply is accomplished with DAQ NI-6216. • Generation of control waveforms for fields with amplitude below 250 A/m have a relative accuracy greater than 1 %. • The specified accuracy is achieved for 32µs conversion intervals, therefore optimal sampling rate will be 30 kS/s. The power supply is KEPCO BOP 36-6ML (it is setup to operate in current mode). • Typical output ripple is 6 mA p-p. From signal-to-noise ratio point of view 13 A/m is the minimum limit to achieve signal-to-noise ratio of 15 dB. • Typical absolute error is 0.3 mA. Valentin Pricop - PhD review

  16. Epstein frame - assumptions Assumptions for the operation of the Epstein frame: • The effective length of the magnetic path () is 0.94 m. • Field is uniformly distributed in all samples (). • The magnetic induction vector () has the same amplitude in all samples and has the same direction as the applied field (). • Cross-sectional area of the samples () is uniform. • The air flux is compensated. • Air gaps between the layers are neglected. • The temperature is constant. Any gap between the layers at the corners will cause non-equal distribution of the magnetic flux between the layers. Magnetization saturation dependence on temperature for nickel. Valentin Pricop - PhD review

  17. Process outline • The samples are demagnetized. • A model for the feed-forward algorithm is determined by applying to the material an cyclic excitation field of low strength. • The normal magnetization curve is determined by cycling the polarization with a cosine waveform with increasing amplitude (an iterative procedure determines the waveform of the excitation field required to achieve a cosine waveform for the polarization). • Measurement data is saved to file and samples are demagnetized again. Valentin Pricop - PhD review

  18. Demagnetization of samples The demagnetization of samples has the purpose to bring the material in the state J=0 for H=0. The demagnetization is performed at a frequency and in a number of cycles specified by the operator with the following particularities: • In the first cycle the power supply will reach its peak current in order to confidently saturate the material and erase its magnetization history. • The waveform of the signal is sinusoidal with decreasing amplitude over time Valentin Pricop - PhD review

  19. Determining the normal magnetization curve • Usually for FEM modelling the first magnetization curve is used. • For soft magnetic materials, the first magnetization curve can be assumed to be the same with the normal magnetization curve. • The normal magnetization curve is the continuous line linking the locus points of symmetric hysteresis cycles. • Besides the first magnetization curve, relevant information during magnet design is: Valentin Pricop - PhD review

  20. Iterating algorithm In the following slides will be presented the operation of the iterating algorithm: • The material under test is M 700-65 A. • The initial excitation field will be cycled with a 1 Hz frequency and a peak amplitude of 250 A/m , randomly selected for this example. • The values for H and J are determined using the equations: Valentin Pricop - PhD review

  21. Drift of the integrated signal Because the drift had a stable value with integration time and amplitude of integrated waveform, I assumed that the origin of the drift was the errors in the data acquisition system. The drift was removed with the following equation: Valentin Pricop - PhD review

  22. Signal processing For the next operations the arrays comprising the BH curve are required to be monotonic for each branch. To achieve this requirement, the number of points are decimated by a factor 10 and a moving average filter is used. Some situations (testing GO electrical steel close to saturation) require a higher number of points. In this case the decimation factor has been decreased to a factor 2 and the moving average filter was no longer applied. Valentin Pricop - PhD review

  23. Obtaining the desired waveform for polarization To optimize the next process, the waveform of J is normalized: The normalized value of the excitaion field is determined using a table lookup operation with linear interpolation: Valentin Pricop - PhD review

  24. Determination of the gain for the new waveform To reach the new point, the amplitude of the new excitation field has to be increased: • For this example ; • The new excitation waveform will be amplified with a factor Valentin Pricop - PhD review

  25. New value for the cycling frequency is determined The same data processing as previously described is applied to the arrays and the new model is obtained: Valentin Pricop - PhD review

  26. Convergence of the algorithm After two iteration of the algorithm: The conditions to stop the iterative process are: • The form-factor of the new waveform falls within 0.5 % of the form-factor of a pure cosine signal: • The variation of the coercive force between 2 iterations falls below 1%: • If hardware limits have been reached then the process is aborted and measurements are saved only to the current point. 6 % decrease of coercive force 0.76 % decrease of coercive force Valentin Pricop - PhD review

  27. Benchmark with other induction control methods The effectiveness of this technique is shown by comparing the convergence characteristic with similar methods: • There is a scarcity of papers regarding waveform control techniques for low frequency magnetic testing. • Stan Zurek et al., in “Use of Novel Adaptive Digital Feedback for Magnetic Measurements Under Controlled Magnetizing Conditions”, presents the results for a similar control technique: • For conventional grain-oriented electrical steel, , cycling frequency 1 Hz, the time required to achieve convergence was 20 minutes. • For conventional non-oriented electrical steel,, cycling frequency 1 Hz, the time required to achieve convergence was 8minutes. Valentin Pricop - PhD review

  28. Measurement examples – M 700-65 A • Details about the samples: • Number of samples: 24 • Density of the material: 7.75 g/m3 • Mass of samples: 1084.9 g (trans), 1087.6 g (long) • Conductivity of the material: 3.54∙106 S/m • Length of samples: 0.3 m • Width of samples: 0.03 m • Thickness: 0.648 mm (trans), 0.649 mm (long) • Stacking factor: 1 • Cut mode: laser • Annealing status: as cut • Cycling frequency for last measurement: 0.52 Hz (trans), 0.41 Hz (long) Valentin Pricop - PhD review

  29. Measurement examples – M 111-35 N • Details about the samples: • Number of samples: 28 • Density of the material: 7.7 g/m3 • Mass of samples: 637.3 g (trans), 640.8 g (long) • Conductivity of the material: 1.92∙106 S/m • Length of samples: 0.3 m • Width of samples: 0.03 m • Thickness: 0.326 mm (trans), 0.326 mm (long) • Stacking factor: 1 • Cut mode: guillotine • Annealing status: as cut • Cycling frequency: 1 Hz (trans), 1Hz (long) Valentin Pricop - PhD review

  30. Limitations of the presented system • For minimizing the dynamic effects the presented procedure takes into account only the effects due to eddy-currents, additional effects may exist (ex: due to domain wall movement). • When measuring low fields, the system provides measurements with high error level and low signal-to-noise ratio. • A series of assumptions were made which may affect the quality of the provided information. Valentin Pricop - PhD review

  31. How does this work help the MNC section? • This system can provide quasi-static magnetic measurements by minimizing the dynamic effects due to eddy-currents. • The procedure provides the possibility to test materials either quasi-static, with constant cycling frequency or with imposed ramp rate. • This procedure tackles the standard measurement procedure limitation of small induced signals in the pickup coil. • It is an automated tool, easy to use, will have a friendly user interface and handbook. Valentin Pricop - PhD review

  32. Acknowledgement I would like to thank: • Daniel Schoerling, for his guidance. • Davide Tommasini, for approving my work. • The TE department, for offering valuable information. • The professors of Transivania University of Brasov. Thank you for you attention! Valentin Pricop - PhD review

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