Wilfried Hofmann

Improving Operational Performance of Active Magnetic Bearings Using Kalman Filter and State Feedback Control

In this paper, the application of optimal state estimation and optimal state feedback algorithms for real-time active magnetic bearing control is treated. A linear quadratic Gaussian controller, consisting of an extended Kalman filter and an optimal state feedback regulator, is implemented. It is shown that this controller yields improved rotor positioning accuracy, better system dynamics, higher bearing stiffness, and reduced control energy effort compared to the conventionally used proportional-integral-differential control approaches. In addition, a method for compensating unbalance-caused forces and vibrations of the magnetically levitated rotor is presented which is based on the estimation of unknown disturbance forces. All results achieved in this paper are verified by means of measurements.

Semi natural two steps commutation strategy for matrix converters

A new two steps commutation strategy for matrix converters is proposed. Based on the measurement of the commutation voltage, switching patterns are presented which guarantee a safe control of bidirectional switches where separated internal conduction paths for the two current directions exist. No additional components are required. The commutation strategy was verified by simulation and experimental results.

Modeling and Ride-Through Control of Doubly Fed Induction Generators During Symmetrical Voltage Sags

Modern grid codes determine that wind generation plants must not be disconnected from the grid during some levels of voltage sags and contribute to network stabilization. Wind energy conversion systems equipped with the doubly fed induction generator (DFIG) are one of the most frequently used topologies, but they are sensitive to grid disturbances due to the stator direct connection to the grid. Therefore, many efforts have been done in the last few years in order to improve their low-voltage ride-through capability. This paper analyzes the behavior of the DFIG during symmetrical voltage sags using models in the frequency domain. A new strategy, the machine magnetizing current control, is proposed in order to enhance the system response during balanced dips. The method is derived on a theoretical basis and numerically investigated by means of simulation. Experimental results are presented and validate the proposed strategy. Finally, the practical aspects of the use of this strategy are discussed.

Speed Regulation Technique of One Bearingless 8/6 Switched Reluctance Motor With Simpler Single Winding Structure

Some types of bearingless switched reluctance motors (BSRMs) have been developed up to now. Their differences mainly focus on different stator-rotor tooth number and winding structures: two layers of windings and single layer of windings for 12/8 or 8/6 BSRM. Their driving principles are just the same: driving current and radial force current are regulated to generate torque and radial force separately. The main difference is the loading of two currents on one single layer of windings or on two separate layers of windings. Further analysis proves that single layer winding structure has more advantages, for example simpler motor and winding structure. Based on the above researches, one 8/6 BSRM with simpler single layer of winding structure has been developed furthermore. Its main characteristic is that the total winding number of motor is decreased from eight to six, which is only two more than four windings of a normal 8/6 SRM. Only six converters are demanded to achieve levitation operation. Its special driving theory and speed regulation technique of the motor by applying three phase windings have been introduced. To testify the new idea, theoretical analysis and practical test have been finished for one test motor. One initial speed of 5000 r/min of levitation operation has been achieved.

Emulation of the static and dynamic behaviour of a wind-turbine with a DC-machine drive

In this study a separately excited DC machine drive is used to emulate the mechanical drive train of a wind power station with a doubly-fed induction generator under real conditions. The static and dynamic characteristics of the wind turbine are modelled through a 3-mass oscillating system, representing the turbine, the gear box and generator moment of inertia connected by elastic shafts. This model is implemented within a software environment and generates the reference values of torque and speed to drive the DC-machine. The fluctuations of the wind speed excite the system modes and oscillations with the resonance frequencies are generated. These effects are transmitted through the DC-machine drive to the generator shaft, enabling more realistic conditions for the experimental rig. This paper shows the modelling of the system and construction of the experimental rig, as well as experimental results.

Digital control of a four leg inverter for standalone photovoltaic systems with unbalanced load

Three leg inverters for photovoltaic systems have a lot of disadvantages, especially when the load is unbalanced. These disadvantages are for example, small utilisation of the DC link voltage, the dependency of the modulation factor of the load current and the superposition of a DC component with the output AC voltage. In this paper a three dimensional space vector modulation of a four leg inverter for standalone photovoltaic systems is presented to solve these problems. A digital control strategy based on a load current observer is introduced to control the whole system with a microcontroller. Simulation and experimental results are presented to verify the performance of the proposed system.

Control of an optimized power flow in wind power plants with doubly-fed induction generators

The doubly-fed induction generator machine is one of the most widely used generator in the MW-class windmills producing a considerable amount of energy from wind. Its control through the rotor slip-rings with a back-to-back converter enables, together with the maximum active power flow, the distribution of the required reactive power between stator, rotor, and mains. This splitting can be chosen in order to minimise the system losses and avoid overloading the system components. Hence an optimised efficiency of high power windmills using this kind of generator brings a significant improvement on the yearly energy production.

New one-step commutation strategies in matrix converters

Due to the nonexistence of natural free-wheeling paths the commutation in matrix converters is more complicated than in voltage source converters. For bidirectional switches where the current flow is controllable separately for both directions this paper proposes new commutation strategies. Based on the measurement of the current direction and the signs of the line to line input voltages commutation in one step is enabled. Compared to multistepped proposals this strategy presents a much smaller deadtime near the switching time.

Torque, Power, Losses, and Heat Calculation of a Transverse Flux Reluctance Machine With Soft Magnetic Composite Materials and Disk-Shaped Rotor

In this paper, the authors introduce a type of transverse flux reluctance machines. These machines work without permanent magnets or electric rotor excitation and hold several advantages, including a high power density, high torque, and compact design. Disadvantages are a high fundamental frequency and a high torque ripple that complicates the control of the motor. The device uses soft magnetic composites (SMCs) for the magnetic circuit, which allows complex stator geometries with 3-D magnetic flux paths. The winding is made from hollow copper tubes, which also form the main heat sink of the machine by using water as a direct copper coolant. Models concerning the design and computation of the magnetic circuit, torque, and the power output are described. A crucial point in this paper is the determination of hysteresis and eddy-current losses in the SMC and the calculation of power losses and current displacement in the copper winding. These are calculated with models utilizing a combination of analytic approaches and finite-element method simulations. Finally, a thermal model based on lumped parameters is introduced, and calculated temperature rises are presented.

Loss minimization of electric drive systems using a DC/DC converter and an optimized battery voltage in automotive applications

Voltage source inverters generally feed the traction motor from a battery in automotive applications. By implementing a DC/DC converter between battery and inverter, the DC-link voltage is adjustable. This paper discusses the benefits of the variable DC-link voltage as an additional degree of freedom for a loss minimized control of electric drives in battery supplied vehicles. Inverter systems with and without DC/DC converter are compared and operating areas of improved efficiency are identified. Energy saving potentials resulting from an optimized battery voltage are discussed for different driving cycles.