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Publication# A contribution to energy saving in induction motors

EPFL thesis

Abstract

Electric motors consume over half of the electrical energy produced by power stations, almost the three-quarters of the electrical consumption in industry and almost the half of commercial electrical consumption in developed countries. Motors are by far the most important type of electric charges, and so constitute the main targets to achieve energy saving. Owing to their simple and robust construction, the asynchronous motors and especially those of squirrel-cage types, represent about 90-95% of the electrical energy consumption of electric motors, which is equivalent to about 53% of total electrical energy consumption. They are widely used as electrical drives in industrial, commercial, public service, traction and domestic applications. Owing to the importance of induction motors, this thesis is aimed at contributing to energy saving efforts, more specifically in the field of low power induction motors. A contribution is kept in perspective by taking into consideration the energy saving potential during the motor design stage as well as during its operation. Every effort to save energy in motor application can be made by always attempting to use energy only as much as what needed during its operation. The best way is to exploit the saving potential during motor design, while at the same time taking into account its intended application. It can be achieved either through the improvement of motor design or through the reduction of its input electrical energy when the motor has already been built. These two efforts are studied, elaborated and worked out thoroughly in this thesis. To attain this objective, a synthesis has been started with the description of how to model an induction motor. To obtain a better model, an improvement is proposed by using the Schwarz-Christoffel mapping to calculate the slot leakage inductance in induction motor. With such method, slot-leakage inductance can be determined more precisely, resulting in more accurate prediction of motor characteristics. It is based on the stored magnetic energy calculation using two-directional field distribution in the slot. The air gap influence can be observed easily, so that a reasonable slot leakage definition can be adopted. Unlike the conventional method, which is only suitable for rectangular slots (otherwise empirical corrections are required), the proposed general slot form can be extended to any desired polygonal slot form. Consideration of saturation is also indispensable because ignoring it could result in inaccuracy in motor performance prediction. Considering the saturation is essential owing to its important role in self-excitation phenomenon to establish voltage build-up in induction generator. However, the self-excitation phenomenon is undesirable in certain group of capacitor motors as it may hinder the switching-off process and mechanical braking at a desired moment. The undesirable switching-off failure condition is to be avoided by properly designing the capacitor motor. Like in this capacitor motor special application, where a proper design is useful from the point of view of operation safety, designing properly a motor is also very important in energy saving efforts. Motor design and optimization to minimize losses as well as to make possible wide speed-range motor operation are some of the efforts. However, when induction motor has already been built, saving energy is only possible by managing its supplying electrical energy. Various strategies are possible and a particular emphasis on the use of triac to reduce motor input voltage is presented. Besides, a brief economic saving evaluation is given to draw attention to the energy saving potential.

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Related concepts (31)

Energy conservation

Energy conservation is the effort to reduce wasteful energy consumption by using fewer energy services. This can be done by using energy more effectively (using less energy for continuous service) or

Electric motor

An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric

Induction motor

An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field

Related publications (21)

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Usually, each phase of the direct reluctance motor is fed separately by its own inverter producing a squarewave phase current depending on the rotor position. In the present work, a star-connection of the three stator phases is used and the direct reluctance motor is fed by a three-phase switched inverter. The space phasor theory shows that three-phase sinewave currents can produce a constant electromagnetic torque. In this case, the rotating reference frame rotates at half of the rotor speed and in the opposite direction. This command strategy allows to reduce the stator deformation due to the electromagnetic radial force which is very high in this type of motor and which is mainly responsible for the acoustic noise. The electromagnetic torque control is realised by a sliding mode control of the three phase currents. In the rotating reference frame, it is possible to show that the eight commutation states of the switched inverter are placed around an ellipse in the complex plane of the derivatives of the space phasor of the phase current. A study of this ellipse has allowed to determine the maximal torque as a function of the speed for a given continuous voltage. It is important to insure that the torque reference will never exceed this limitation in order to avoid a pullout phenomenon, highly unwanted in the speed control. Despite the fact that the resolver inside the casing of the direct reluctance motor produces a small periodic error, it is accurate enough for the position measurement and to produce the three current references. However, the derivative of the position measurement provides a speed evaluation with such a ripple that the speed control cannot be outstanding. So, for the speed measurement, an additional laser rotary encoder is used. The experimental study in the steady-state mode of the speed control shows the presence of an electromagnetic torque ripple. This is a residual ripple caused by the dissimilarity between the stator poles teeth-shapes and the magnetic saturation effect. Although this torque ripple is rather low compared with the usual command strategy, two methods are proposed in order to compensate it. The first method is based on the use of a correction factor depending on the tooth angle. First, in steady-state conditions, the correction factor is stored off-line in a table. Then it is used in-line as a feedforward disturbance compensation. The second method of reducing the residual torque ripple consists in implementing an observer of variable disturbance. This is a disturbance observer able to determine the frequency content of the torque ripple. For instance, in order to observe the continuous component, the fundamental wave as well as the harmonics 2, 3, and 6, the observer must have ten poles. The main difficulty to deal with such an observer is the fact that each pole moves quickly as a function of the motor speed. So, in order to guarantee the observer stability over a large range of speed, the poles are located dynamically regarding to the motor speed by the way of a discontinuous adaptation of the feedforward coefficients. The theoretical study and the numerical simulation have allowed to formulate some criteria about the way of locating the poles. Experimental results confirm the efficiency of the observer of variable disturbance in order to reduce the electromagnetic torque ripple.

In recent years the use of solid state frequency converters is rapidly increasing in the industrial and power plants where electric machines are installed, since it allows variable speed operations for the electric system, thus becoming a key factor which is capable of increasing the overall efficiency, as well as the global plant flexibility. The benefits that can be reached when electric motors are under investigation can be so enticing, in particular from the perspective of the operating machine coupled to the motor, that drawbacks take a back seat and can be overcome. On the contrary, power electronic for variable speed is generally not used in power generation, since the gains brought out by the present technology are not sufficient to attract anyone's interest. There are some exceptions in the wind- and the hydro power plant businesses, where there are some cases of variable speed already introduced. However in the former applications the output power is limited to some MW, while in the latter the converter is sized only to a fraction of the whole generating power, since it feeds the machine's rotor.This dissertation investigates on a possible electronic converter for power generation in the 40 MW range and above, which is also attracting from both the cost's and the efficiency's point of view, as well as from the operation reliability perspective and thus may be proposed as a breakthrough in this field. Among the possible frequency converters, those ones have been selected which perform natural commutations only, i.e. those that employ thyristors as the semiconductor devices, since the efficiency, the robustness and the relatively high performances are actually the key success factors to be addressed from the beginning. Well known matrix converter topologies will be referred to, as well as new matrix converter arrangements will be brought out and analyzed: their behaviours will be targeted by the dissertation, in particular when different strategies are adopted for controlling the operations. Specifically comparisons will be carried out when the same converter topology is used, but driven by either the well-known cycloconverter strategy or the newly brought out "active generator" one. In addition several generator winding arrangements will be proposed and analyzed, with the aim of increasing the actual feasibility of the proposed solution. Finally some test results on a sized down experimental rig will be analyzed and compared to the simulation outcomes.

Electrical machines consumed the amount of 9’346 TWh in 2019, corresponding to more than 40% of the total global electricity consumption. With the growing demand for automation of production lines and electrification of the transport industry, this value is likely to increase in the next years. Improving the performance of electrical machines will and already plays a key role in better managing our society’s energy consumption. In order to achieve this, in this work I develop the premises of a new Topology Optimization (TO) framework based on the method of moving morphable components (MMC). It allows to automatically design the best shapes of the motor’s components. Initially developed for structural mechanics, TO directly investigate the ideal distribution of material in space. This novel method often results in organic shapes. The goal of this project is to adapt an already existing TO framework from [9] adapted and written in Python, to design the winding of a linear motor.

2022