L. Pislaru-Danescu

Prototyping a Ferrofluid-Cooled Transformer

This paper presents the work conducted on prototyping a step-up/step-down, single-phased, low-power, and medium-voltage electrical transformer cooled by a fluid with colloidal magnetic nanoparticles. The magnetic and fluid dynamic properties and the heat capacities of the ferrofluid (magnetic nanofluid) and that of the regular coolant (UTR-40 transformer oil) were experimentally determined and comparatively evaluated. Mathematical models for the electromagnetic field and the heat transfer were defined and numerically solved to assess the capacity of the transformer to sustain the working conditions. The simulation results were utilized to improve the design of a prototype, where the UTR-40 regular coolant is replaced by ferrofluid. The numerical simulation results and the experiments evidence the superior performance of the prototype.

A novel, ferrofluid-cooled transformer. electromagnetic field and heat transfer by numerical simulation

A ferrofluid-cooled low power, single-phased electric transformer was designed and prototyped with the aim of investigating the performance that such an apparatus may exhibit. The nanometric, colloidal, super-paramagnetic fluid used as coolant has specific electric, magnetic, and thermal properties, and presents an overall better stability and capacity to withstanding electromagnetic and thermal stress. This paper addresses also the electromagnetic and heat transfer processes that occur. First, the physical, mathematical, and numerical models are introduced. Numerical simulation results suggest that the magnetization body forces may add to the thermal, buoyancy body forces in providing for better heat transfer. To outline this, several numerical models that may conveniently be treated numerically within the current hardware and software limits, while still providing for satisfactory accuracy were developed. The results may be utilized also in the design phase of the transformer.

Magnetic Nanofluid Applications in Electrical Engineering

This paper presents a superparamagnetic nanofluid (SMP-NF) and three of its applications. The SPM-NF particles were suspended in oleic acid as surfactant, and then dispersed in UTR 40 transformer oil (TO). The average particle size obtained from X-ray diffraction is 14 nm, and from scanning electron microscopy is between 10 and 30 nm. The magnetic measurements for the oleic-oil transformer acid-magnetite nanoparticles of 27 nm diameter in size and 9.78×1025 nanoparticles/m3 particle density system provide for specific saturation magnetization, Ms. The volume fraction is 1.1% and the magnetization is 62 Gs. The SPM-NF may be designed to be used either as coolant or as magnetic medium in three electrotechnic devices: a power electric transformer TMOf 2-36 kV-40 kVA, at 50 Hz, a microactuator that implements the pulse width modulation (PWM) principle, and in miniature planar spiral transformers, for galvanic separation or step-up/step-down conversion. The paper presents experimental and numerical simulation results that confirm that the SPM-NF usage open new venues in optimizing conventional electrotechnic constructions or to design novel devices.

New concept of measurement apparatus for the in situ electrical resistivity of concrete structures

This paper presents the prototype of an apparatus (AMSR) in the 2.5 accuracy class, which operates at 500 Hz, for measuring the in situ resistivity of concrete in the range 5...100 Ωm that is immune to errors due to the polarization phenomena at the interface probe/concrete sample. The direct problem of the electrical resistivity tomography, ERT, is then solved for DC and AC electric fields a sample-specific geometrical factor that provides for a convenient mean of calibrating the AMSR output against different standard concrete samples is defined. The direct problem is an important stage in solving for the inverse problem of ERT that consists in finding the electrical resistivity of the sample out of boundary (measured) data, and it makes the object of future research.

Magnetic and electric sizing of a miniature planar spiral transformer

This paper presents mathematical models and numerical simulation results for a notional miniature, planar, spiral transformer (MPST) fabricated in MEMS technology, for galvanic separation, proposed to equip an electric harvesting device (EHD). Two types of core - ferrite and (partially) magnetic nanofluid - are considered, and the lumped parameters (inductances and capacitances) of the MPST are computed. We found that magnetization body forces that occur in the super-paramagnetic nanofluid core result in complex, steady state flows consisting of fully 3D recirculation cells. The lumped parameters of the MPST were computed using 2D and 3D models and the results are in satisfactory good agreement.

New harvesting system based on photovoltaic cells with antireflexive ZnO nanoparticles coatings and DC/DC isolation conversion

In this paper we propose a new design of havesting system based on photovoltaic cells with antireflexive ZnO nanoparticles coatings and DC/DC isolation conversion. The system is based on an assembly of four photovoltaic polycrystalline cells provided with antireflexive ZnO nanoparticles coatings. The solar cells have been tested in terms of efficiency and output power for irradiance values in the range of 100 W/m2 - 1000 W/m2. The system of photovoltaic cells generates and transfers the energy to a DC/DC converter with galvanic separation. The converter uses a micro transformer with planar coils, ferrite magnetic circuit and magnetic colloidal nanofluid. The proposed model of micro transformer has been tested independently for frequencies of 100 kHz, 300 kHz and 500 kHz (which stands as the commutation frequency of the DC/DC converter), showing an efficient operation. The inductivity measurement proves that the inductivity of the primary winding is similar to the optimum inductivity of the DC/DC converter indicated by Texas Instruments manufacturing company which uses the LM 25017 integrated electronic circuit.

A novel microactuator device based on magnetic nanofluid

This paper presents the prototype of an actuator that utilizes a colloidal, super-paramagnetic, nanometric particles fluid as active medium. The experimental measurements show that magnetic properties of the fluid are adequate to the actuator working conditions. An electronic module for the control of the actuator ferrite cored coils was designed and produced; it implements the pulse width modulation (PWM) principle. The module produces train pulses of adjustable frequency and fill factor to drive the electrical currents in the actuator coils. The magnetic field produced by the actuator coils generates magnetic body forces in the magnetic fluid that add to gravitational forces to drive the fluid in a forced flow. The motion is perceived as periodic stationary surface waves, localized above the actuator coils. Mathematical modeling and numerical simulation (by the finite element method (FEM)) are used to study and evaluate these processes. The heat transfer part of the problem was also studied, although thermal stability proves to be less of a concern. The device may provide a usable actuation resolution, as the results suggests.

Improvement of polycrystalline solar cells operation under low solar radiation conditions by using ZnO nanostructured materials

The paper presents a complex process used for obtaining ZnO nanostructures for antireflective coatings. The process is suitable to silicone solar cells and can be used in order to increase their efficiency under low solar radiation, allowing the control of the morphological and optical properties of ZnO nanostructures deposited on glass through ZnO seed layer deposition process. During the research, numerous characterizations have been carried out regarding the ZnO seed layers and nanowires samples like X-ray diffraction analysis and SEM images. Also, development of the spectrophotometric analysis, consisting in the variation of optical reflection and transmission with wavelengths in the range of 400–800 nm, for the ZnO seed layers and nanowires was carried out. The photovoltaic modular system with antireflective coating was tested for different amounts of radiation starting from 100 W/m2 to 1000 W/m2 in order to evaluate the efficiency under low solar radiation conditions.

Electronic drive system of a linear magnetostrictive motor designed for outer space applications

This paper presents the driving system of a linear magnetostrictive motor (LMM) that operates in a wide range of frequencies of up to 16 kHz, which is intended for space applications. Accordingly, the design and control of the LMM differs from the classical version, causing changes in electrical driving conception. The LMM drive has two units Pulse Width Modulators (PWM) for the control of the activation coil, the magnetic bias coil, and it is also provided with a power supply block. An ensemble of three series, connected Peltier elements is powered by a third PWM modulator with the following features: f = 24 kHz, U = 28 VPP and duty cycle kPWM = 30%. The Peltier ensemble provides the cooling of LMM in the absence of terrestrial gravity.

Numerical study of the stator motion in a piezoelectric ultrasonic motor

This paper presents mathematical modeling and numerical simulation results that are part of a study concerning the dynamics of the stator of piezoelectric (PZ) traveling wave (TW) rotating ultrasonic motor. The stator is a PZ composite ring, and the rotor is a metallic ring. The rotational torque is produced by exciting the stator into a flexural traveling wave, transmitted to the rotor through the stator-rotor friction. A modal analysis provides for the structural eigenfrequencies of the stator. When the AC powering stage providing for two voltages of same amplitude and shifted in quadrature is adapted to a frequency close to the flexural resonance of the stator, the TW reaches higher amplitudes that result in higher rotational velocities because the rotor speed is proportional to the stator TW amplitude.