Werner Hugo Wölfle

Electromagnetic design of a magnetic suspension system

Magnetic levitation of a metallic sphere provides a high-impact visual demonstration of many principles in undergraduate educational programs in electrical engineering, e.g., electromagnetic design, compensation of a unstable control system and power amplifier design. This paper deals with the electromagnetic and dynamic analysis of the levitation system and it has design formulae which are derived from first principles. This analysis leads to a plant transfer function which is used to implement a proportional plus derivative (PD) compensation strategy. Issues covered include coil and wire sizing for a given temperature rise. The paper shows that the electromagnet can be optimized for a given sphere. An experimental system is described which levitates a 6 cm, 0.8 kg sphere.

PWM control of a magnetic suspension system

Magnetic levitation of a metallic sphere motivates and inspires students to investigate and understand the fundamental principles of electrical engineering, such as magnetic design, circuit design, and control theory. This paper describes the operation of a pulse width modulation converter in a magnetic suspension system. The pulse width-modulated (PWM) converter illustrates modern principles of power electronics, such as PWM control, current-mode control, averaged and linearized models of switched-mode converters, and power supply design. The reduced size of the heat sink in the switched-mode converter compared with its linear amplifier counterpart clearly illustrates improved efficiency. The experimental system described levitates a 6-cm-diameter, 0.8-kg metallic sphere.

Quasi-active power factor correction with a variable inductive filter: theory, design and practice

The mains current in an AC/DC converter contains periodic current pulses, due to the action of the rectifier and the output buffer capacitor. The high current peaks cause harmonic distortion of the supply current and low power factor. Introducing active power factor correction in the form of a boost pre-regulator can reduce the level of harmonics. Passive power factor correction in the form of an input filter inductor is less expensive but the component is bulky because it must be sized to handle the full power range of the supply. This paper investigates the use of variable inductance (the inductance varies with current), which provides adequate harmonic reduction. Three types of inductors are investigated: an inductor with a fixed air-gap operating with a saturated core, a swinging inductor which has a stepped gap and a novel inductor construction with a sloped air-gap (SAG). Results are presented for a 200 W power supply and it is shown that the SAG inductor has the best performance in terms of harmonic response and size.

A Unified Approach to the Calculation of Self- and Mutual-Inductance for Coaxial Coils in Air

This paper extends a previous formula for the mutual inductance between single-turn coils to include all coils in air with rectangular cross sections, without any restrictions on the dimensions (including overlapping coils). The formula is compared with a wide spectrum of examples from the literature and agreement is excellent in every case. Experimental results are presented to validate the formula for both solenoid and disk coils. The formula is relevant to coreless transformers, inductive coupling, wireless power transfer, and leakage inductance in resonant converters.

Optimized design of LLC resonant converters incorporating planar magnetics

The LLC resonant converter is widely applied as front-end dc-dc conversion for distributed power system and intermediate conversion for renewable energy power generation systems. A continuous miniaturization of LLC resonant converter incorporating planar magnetics is leading to higher performance, higher efficiency and decreasing costs. This paper outlines an improved design methodology for LLC resonant converters based on judicious choice of the circuit components including the resonant inductor, the transformer magnetizing inductor and the resonant capacitor as well as the dead time. Planar magnetics are playing a significant role in the low profile DC/DC converter applications. With the planar transformer employed, the parameters can be controlled precisely which is of great importance to the operating performance of the converter. Several structures of the planar transformer applied in LLC resonant converter are investigated. Finally, the comparison of the proposed structures basis on the ac resistance and the winding losses are presented.

Gapped Transformer Design Methodology and Implementation for LLC Resonant Converters

In the LLC resonant converter, the air gap is generally positioned in the core of the transformer for proper magnetizing inductance. Traditional transformer design methods assume infinite permeability of the core and no energy stored in the core. The improved design methodology for the gapped transformer is proposed with the optimum relative permeability and gap selection to meet the temperature rise and the magnetizing inductance requirements. The magnetizing current influences the magnetic flux in the core leading to the core saturation and core loss, while the resonant current contributes to the winding loss. The transformer design for a 200 W, 90 kHz LLC resonant converter is presented and experimental results validate the proposed methodology.