Flemming Abrahamsen

On the energy optimized control of standard and high-efficiency induction motors in CT and HVAC applications

This paper contains an analysis of how the choice of energy optimal control of induction motors is influenced by motor construction, standard versus high-efficiency motor, and by application, constant torque (CT) and heating, ventilation, and air conditioning (HVAC) (interpreted as vector and scalar motor drives). The analysis is made with a 2.2 kW voltage-source-inverter-fed squirrel-cage motor drive as an example throughout the paper, but through statistics on the use of motors and their efficiencies, the conclusions are widened to a broader range (0-50 kW). Energy optimal control strategies are reviewed and cos(/spl thetav/) control, a model-based control, and a search control are implemented in the laboratory in a vector and a scalar motor drive. The convergence speed for the strategies and their ability to reject disturbances are investigated by experiments. It is also shown experimentally that, for both standard and high-efficiency motors, motor energy-efficiency improvement is achievable by energy optimal control below 60% load torque. The energy savings using energy optimal control strategies are measured on a pump system with a certain load cycle. Model-based control is recommended for CT applications and cos(/spl thetav/) control for HVAC applications.

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.

Adjustable speed drive with active filtering capability for harmonic current compensation

A new solution is proposed to solve the problem with harmonic current pollution using diode rectified adjustable speed drives (ASD) in the mean power range. A fully controlled voltage source ASD acts as an active filter on the network side, being able to compensate harmonics of several conventional ASDs or other loads in parallel. Current harmonic identification strategies, control aspects and the limit of harmonic current compensation are studied. Simulations verified by experimentation show excellent results. This new solution is economic compared to previous proposed filtering techniques in ASD applications.<<ETX>>

Energy Optimal Control of Induction Motor Drives

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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.

Efficiency Improvement of Variable Speed Electrical Drives for HVAC Applications

A large part of the produced electrical energy is consumed by ventilators, pumps and compressors, the so-called HVAC applications. A lot of this energy can be saved by speed control, but even with the large saving obtained alone by introduction of variable speed, it is still essential to optimize the control of the variable speed drive and to optimize the electrical machine with respect to efficiency. Experiments are made with energy optimal induction motor control on a 2.2 kW variable speed pump system. It is demonstrated that 10 % of the consumed energy can typically be saved by energy optimal motor control compared with constant V/Hz control. In a comparison of induction motors and permanent magnet synchronous motors for a variable speed pump application it is shown that for 2.2 kW motors an investment in high efficiency or PM motors are typically paid back within 2.5 years and 7 years respectively. For a 90 kW PM motor the pay-back time would be 24 years. It is today not profitable to use PM motors for variable speed HVAC applications above 2 kW rated motor power. A further study is required to determine this limit in power rating more precisely.

Dynamic Performance of Grid Converters using Adaptive DC Voltage Control

This paper investigates a controller that ensures minimum operating dc-link voltage of a back-to-back converter system. The dc-link voltage adapts its reference based on the system state, reference given by an outer loop to the dc-link voltage controller. The operating dc-link voltage should be kept as low as possible to increase the power conversion efficiency and increase the reliability of converters. The dynamic performance of the proposed controller is investigated by simulations and experiments.