Paul Thoegersen

Single current sensor technique in the DC-link of three-phase PWM-VS inverters. A review and the ultimate solution

This paper proposes an ultimate solution for reconstruction of three phase-currents in a PWM-voltage source inverter by one current sensor in the DC-link, and it gives a review of existing methods in literature and patents. The solution offers fully protection of the inverter including shoot-through of the DC-link, short-circuit of the output phases and earth faults with both low and high impedance. A special configuration in the DC-link can give such features. At the same time a new method to sample the DC-link current is proposed which both can eliminate disturbances from cables between an inverter and an induction motor, and it offers a true simultaneous sample value of all three phase-currents in the center of a switching period. The two methods are implemented in an experimental setup, and test results show that full protection is achieved and the three phase-currents are reconstructed precisely in the whole operating area.

Efficiency optimized control of medium-size induction motor drives

The efficiency of a variable speed induction motor drive can be optimized by adaption of the motor flux level to the load torque. In small drives (<10 kW) this can be done without considering the relatively small converter losses, but for medium-size drives (10-1000 kW) the losses can not be disregarded without further analysis. The importance of the converter losses on efficiency optimization in medium-size drives is analyzed in this paper. Based on the experiments with a 90 kW drive it is found that it is not critical if the converter losses are neglected in the control, except that the robustness towards load disturbances may unnecessarily be reduced. Both displacement power factor and model-based efficiency optimizing control methods perform well in medium-size drives. The last strategy is also tested on a 22 kW drive with good results.

Improved modulation techniques for PWM-VSI drives

PWM-VSI based AC motor drives have two main problems. The inverter is nonlinear which causes instability problems in some specific working points of the AC machine and it emits acoustic noise due to the switching frequency. Nonlinearities like dead-time in the inverter, load dependent DC-link voltage ripple and the voltage drop across the switches are modeled and compensated by improved modulation techniques in order to obtain an almost ideal inverter. Different feedback and feedforward techniques are proposed. The acoustic noise is reduced by using a random modulation strategy. Measurements show a significant improvement by using feedforward and feedback techniques for linearizing the inverter. An improvement in reduction of the acoustic noise emission is also achieved by using random modulation. It is concluded that a combination of a random modulation strategy and feedforward/feedback techniques gives an almost ideal AC motor drive system.

Adaptive Compensation of Reactive Power with Shunt Active Power Filters

This paper describes an adaptive method for compensating the reactive power with an active power filter (APF), which is initially rated for mitigation of only the harmonic currents given by a nonlinear industrial load. It is proven that, if the harmonic currents do not load the APF at the rated power, the available power can be used to provide a part of the required reactive power. Different indicators for designing such application are given, and it is proven that the proposed adaptive algorithm represents an added value to the APF. The algorithm is practically validated on a laboratory setup with a 7-kVA APF.

Adjustable Speed Drives in the Next Decade. Future Steps in Industry and Academia

The main trends for the next decade within the multi-disciplinarian field of adjustable speed drives are discussed, and a number of topics are especially addressed in this paper. The topics include market development over the last decade, historical development in power converter volume and weight, future drive demands, power converter architecture, interfacing to the grid, motor types, and control principles. Furthermore, some of the possibilities and trends related to decentral “intelligence” and Internet connection are discussed.

Efficiency and reliability improvement in wind turbine converters by grid converter adaptive control

This paper presents a control method that reduces the losses in wind turbine converters adaptively controlling the grid converter. The dc-link voltage adapts its reference based on the system state and therefore reduces the stored energy, and is therefore kept at the minimum necessary for the grid and generator side. Operating in this way, the electrical and thermal stress factors are decreased on the power electronic devices, increasing their lifetime. The simulation results using this method show efficiency increase and devices temperature cycles slightly decreased. Experimental results on a wind turbine power stack shows efficiency increase in the high power region.

Compact ASD Topologies for Single-Phase Integrated Motor Drives with Sinusoidal Input Current

Abstract A standard configuration of an Adjustable Speed Drive (ASD) consists of two separate units: an AC motor, which runs with fixed speed when it is supplied from a constant frequency grid voltage and a frequency converter, which is used to provide the motor with variable voltage-variable frequency needed to adjust the speed ofthe motor. The integrated motor drive concept is a result of merging the two units in order to achieve the following benefits [1–3]: reducing the design and the commissioning time in complex industrial equipments, no need for a cabinet to host the frequency converter, no need for shielded cables to reduce EMI (Electro Magnetic Interference), no need for cables for the speed transducers or for other sensors for industrial process control (e.g. pressure). This solution is currently available up to 7.5 kW being not used in the medium and high power range due to a low-density integration ofthe converter caused by the large size of the passive components (electrolytic capacitors and iron chokes) and vibration ofthe converter enclosure. This paper analyzes the implementation aspects for obtaining a compact and cost effective single-phase ASD with sinusoidal input current, investigating the physical removal of power inductors from the converter enclosure in conjunction with reducing the number of semiconductor active devices. There are two ways to do that: to integrate the inductors in the unused area of the stator yoke of the motor or to use the leakage inductance of the induction motor as a boost inductor for a PFC (Power Factor Correction) stage controlled by the inverter zero-sequence voltage component. By determining how much energy is possible to store in a corner inductor, it is proven that integrating the magnetics into the stator yoke is a feasible solution. Topologies of single-phase converters that take advantage ofthe motor leakage inductance are analyzed. The installed power in silicon active devices of these topologies is compared with a standard situation, showing that this will involve higher cost. As the iron core ofthe inductors is not suitable for high frequency operation, higher core losses will occur, but outside the converter enclosure. The advantages are: the reduction of the number of active semiconductor devices, the reduction ofthe ASD size and the better integration potential.