HIGH EFFICIENCY ELECTRICAL MOTORS STATE OF THE ART AND CHALLENGES

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1 Rev. Roum. Sci. Techn. Électrotechn. et Énerg. Vol. 62, 1, pp , Bucarest, 2017 HIGH EFFICIENCY ELECTRICAL MOTORS STATE OF THE ART AND CHALLENGES VERONICA MANESCU (PALTANEA) 1, GHEORGHE PALTANEA 1, HORIA GAVRILA 1, GHEORGHE SCUTARU 2, IOAN PETER 3 Keywords: Energy efficiency regulations, Electrical motors, Life cycle cost, Motor power losses, Non-oriented electrical steel. Since 2007 there was adopted a new standard classification for the energy efficiency of electrical motors, in order to gradually limit the impact on the environment. In this paper, we present some studies regarding the environmental savings that can be obtained through the production of more energy efficient machines, barriers in imposing the improved motors, which are met on a worldwide manufacturing market, and a motor power loss analysis in view to establish the methods to reduce them. 1. INTRODUCTION The electrical machines cover today almost 70 % of the total electrical energy consumption and the producers want to reduce this amount of energy, by using high efficiency machines. Unfortunately, the absence of a global efficiency classification leads to an impossibility for all the countries to adopt the high efficiency electrical machines. It was also found [1, 2] that if high efficiency or premium class electrical machines replace the standard ones; an important reduction in the environmental impact will be obtained. In Europe, the electrical motors are the biggest load in the industry. The impact of the electrical motors on the climate is nowadays a very challenging issue, because according to an existing report of the intergovernmental Panel on Climate Change IPCC, the increase of the average temperature is due also to the greenhouse gas concentration. The presence of humans has a negative impact in many other aspects of the climate such as ocean warming, continental average temperature and wind patterns. In Canada and in the United States of America the scientists want to reduce the CO 2 emissions up to 90 % below today s levels till 2100 and this trend will be followed worldwide [1, 3]. The main source for the carbon dioxide emissions is the energy consumption in the industry, due to the electrical motors. In Europe, the electric motors need today almost half of the total energy consumption. In all the industrial sectors are used electrical machines such as low voltage squirrel cage induction motors, thus even small efficiency improvements will have an influence on the energy savings and an important contribution in the reduction of greenhouse gas emissions. The limited number of the natural resources and the global warming phenomenon, added to the increased price of the electrical energy make the subject of energy efficiency of the electrical machines a number one objective of the industry and for the environmental saving polices [3]. A new international standard IEC [2, 4, 5] was proposed and has approved in 2008, in order to be applied to the line-fed, three-phase, squirrel cage induction motors. In this standard are defined four efficiency classes: Standard Efficiency (IE1), High Efficiency (IE2), Premium Efficiency (IE3) and Super Premium Efficiency (IE4). It is well known that IE1, IE2, IE3 are normative and IE4 is just informative. Some producers consider that, to achieve the IE4 level, is harder in the case of squirrel cage induction motors (SCIMs). This problem was not a challenge for the transition from IE1 to IE3 efficiency of the same type of motors. The European legislation interdicts the use of low efficiency electrical motors as it follows: Until November 2011, all the electrical machines had to comply with the IE2 standards; Starting from 2015 all the motors with a nominal active power between 7.5 kw and 375 kw had to be of IE3 efficiency; And from 2017 the electrical machines with the active power between 0.75 kw and 5.5 kw have to be modified according to IE3 standards. In every squirrel cage induction motor the absence of the mechanical friction is due to the absence of a direct contact, between the stator and the rotor and the mechanical losses are lowered, thus, this kind of electrical machines has the lowest price of all. The biggest part of losses is due to the copper and iron losses [6]. It is well known that the speed of an IM machines is a function of the frequency of the power supply and of the number of poles. Its speed is reduced with a few percent when the motors work from open circuit to full load operation [6, 7]. The efficiency of an electrical machine depends on the following parameters: motor efficiency, motor speed controls, power supply, mechanical transmission, maintenance practices, load management and cycling and efficiency of the end-use device [7]. 2. PERSPECTIVES IN WORLDWIDE HIGH EFFICIENCY MOTOR STANDARD The companies that choose to use high efficiency electrical machines should make such an investment, which is usually profitable due to the savings from reduced energy costs. In the real market in spite of this profitable condition there are several market deficiencies, which are a big impediment in adopting the high efficiency electrical machines, such as [8]: lack of important information, if the consumers know less about the energy performance of the device than producers, then they tend to make the acquisition on low price basis; different opinions in the company management regarding the pay-back period of an investment (an high efficiency motor has a longer pay-back interval), different voltages and frequencies in different region of the world differences in standardization (MEPS Minimum Energy Performance Standards, efficiency classification and testing). 1 Politehnica University of Bucharest, 313 Splaiul Independentei, Romania, gheorghe.paltanea@upb.ro 2 Transilvania University of Brasov, 29 Eroilor Avenue, Brasov, Romania 3 Electroprecizia Electrical Motors SRL, 18 Parcului Street, Sacele, Romania

2 2 High efficiency electrical motors 15 The electrical machine manufacturers sell and keep stock of best bought products, for a rapid delivery and an immediately replacement of a damaged motor. For this reason, the number of motor types and models, which are sold, must be kept to a minimum and generally, the high efficiency configurations are not always included [8, 9]. In real system applications with electrical motors, in order to avoid production, interruption of the manufacturing process, the end-users choose oversized devices, to ensure a safe and continuous operation. Based on this consideration they will give a more importance to the purchase price and not for the life cycle costs, in which the electric energy consumption is very important and represent almost 90 % [9, 10]. The complexity of the electrical machine driven system is another important barrier, because it is usually not enough to replace only one component (e.g. electrical motor), but to analyze the energy efficiency of the entire system. The research for the new technical solutions requires considerable time and effort, not only from the engineers, but also from the management of the company to sustain such an investment [11]. IE1 Fig. 1 Minimum required efficiency for IE class in the case of 50 Hz/ 4-pole motors according to IEC /2014 [12]. 3. ENERGY EFFICIENCY STANDARDS The efficiency of an electric motor is defined as the ratio between the output mechanical power and the input electrical power. According to the Regulation 640/2009, an electric motor is a single speed three-phase 50 Hz or 50/60 Hz, squirrel cage induction motor that [12, 13]: has 2 to 6 poles; has a nominal voltage up to 1 kv; has a nominal active power between 0.75 kw and 375 kw; is used on the basis of continuous duty operation. The induction motors work usually a large number of hours and they have long lifetimes. Its most important environmental impact is in the use-phase, because the reducing of the energy consumption leads to an increase of the operation cost and to a decrease of its environmental impact. It has been demonstrated [1, 12, 13], that for a worked hours/year the energy consumption of a motor is about 97 % of the entire life cycle cost (LCC) and an initial higher purchase price for an efficient motor will generate higher savings through its life span. Only in the case of motors that operate under hours/year the energy costs will be below 95 % of the entire LCC. A voluntary agreement, European Committee of Manufacturers of Electrical Machines and Power Electronics CEMEP/EU [12, 13] from 1999, introduces the first efficiency classes for the electrical motors with active power range between 1.1 kw and 90 kw. Follow that there is also developed, in the USA, the National Electrical Manufacturers Association (NEMA) classifications, which is different from the EU version. Since 2008 the International Electrotechnical Commission (IEC) introduces an international classification of energy efficiency for the electric motors with a power range between 0.75 kw and 375 kw. To extend the initial IEC :2008 standard in 2014 there was adopted the IEC :2014, that increases the power range between 0.12 kw till 1000 kw (Figs. 1 and 2) [12]. Fig. 2 IE4 efficiency levels for motors at 50 Hz in the case of different pole numbers [14]. In order to improve and to regulate the standard, there were introduced the Regulation 640/2009 and 4/2014, which set the Ecodesign requirements and timeline implementation for motors in the European market. These regulations refer to the 2-, 4-, 6-poles single speed three-phase inductions motors and exclude the following working conditions [12, 13, 15]: operating wholly immersed in a liquid; integrated completely into a product, and the motor energy performance cannot be tested independently; operating in one of the following cases: at altitudes above m, ambient air temperature exceeding 60ºC, at temperature above 400ºC, ambient air temperature below 30ºC, explosive atmospheres as defined in Directive 94/9/EC. The IE4 Super Premium Efficiency motors are manufactured and commercialized only by some important producers, although there is not available an existing regulation. This type of electrical machines works at two efficiency bands above IE3 motors. This is equivalent to a 20 % reduction of energy losses. In the case of a 0.75 kw motor the efficiency level for a super premium efficiency electrical machine with a nominal speed of 1800 rot/min is 85.5 % versus 83.5 % for an IE3 motor. For a 75 kw motor, the IE4 performance requires an efficiency of 96.2 % versus 95.4 % for a Premium Efficiency motor. It can be noticed that efficiency gains are higher for the small motors [16 18]. Today premium efficiency IE3 has a low value of the energy losses and it is also widely available on the market.

3 16 Veronica Manescu (Paltanea) et al MOTOR POWER LOSSES In the existing Standard of the Institute of Electrical and Electronics Engineers IEEE 112-B [19] the total power losses are divided according to: ( ) Δ = Δ +Δ +Δ +Δ. (1) Pstr Pin Pout Pel1 Pel2 Pcore Pmech Usually the electric input power P in is measured with a power analyzer and the output power P out is provided by a torque meter. In order to have a high efficiency motor, it is necessary to reduce its power losses, which are: resistive losses in the stator ΔP el1 and the rotor ΔP el2 windings, magnetic losses ΔP core (hysteresis, classical and excess losses), mechanical losses ΔP mech due to bearing, friction and ventilation of the motor and stray load losses ΔP str (Fig. 3). The resistive losses are due to the electric currents, which flows trough the stator windings and rotor bars of a SCIM. In order to obtain low resistive losses, it is necessary to decrease the current density in the conductors, by increasing their diameters and using copper instead of aluminum rotor bars [20, 21]. The magnetic losses are generated by the hysteresis and eddy currents phenomena. The hysteresis power losses can be reduced by using improved magnetic grade steels with low percent of non-magnetic impurities. The eddy currents losses can be lowered, through the use of thinner lamination sheets. Also, the magnetic losses can be further improved, by introducing non-conventional cutting technologies such as: water jet and electro-erosion methods [22, 23]. The mechanical losses are generated by the bearings friction and cooling fan air resistance. They can be improved by using low friction bearings and a more efficient air flow design. The leakage flux, induced by the load current, the nonuniform current distribution and the imperfect geometry of the air gap, determine the stray load losses, which can be reduced through design optimization and improved cutting methods. The more direct and effective method to obtain an energy efficient motor is to use high quality magnetic materials for the cores and copper bars in the rotor cage. This approach will increase the final cost of the machine, with 15 % to 30 %. [27, 28]. At this time, introducing non-conventional cutting methods is not a favorable step, because these technologies are not so adequate to mass production of the electrical machines, due to the fact that they are low speed and expensive [24 26]. Controlling the parameters of the electrical machines (e.g. magnetic material nonlinearities, slot harmonics etc.) is a very important aspect, because they may determine additional/parasitic energy losses on the grid, whose estimation implies the introduction of specific power quantities, concepts and methods to operate with [29 31]. The quality of non-oriented alloys is the key solution in the promoting of a good motor efficiency. Due to the fact that motor efficiency is higher in the case of low loss electrical steels and torque increases direct proportional with the magnetic flux density, the motor producers intent to use in the manufacture of the IE3 and IE4 machines only non-oriented alloys, which have a low value of energy losses and a high value of the magnetic flux density. Supplementary a very good blanking property, dimensional accuracy after blanking, increase of the electrical resistance through coating, corrosion resistance and strict sheet thickness are also necessary in the production of the electrical machines. An optimal metallurgical process that combines small decreases of silicon and aluminum content, followed by the improvement of the crystallographic texture, through hot band annealing in non-oxidizing atmosphere, is necessary to be implemented, in order to improve the magnetizing ability of the final electrical steel sheets. To modify the microstructure of the hot band it can be used a coiling temperature around 700ºC or a gamma-alpha phase transformation. In Fig. 4 and Fig. 5 are presented some magnetic properties of three electrical steel grades used in the magnetic cores of IE2 and IE3 class motors. The data were determined, using a single strip tester, on samples with an area of mm 2, at the frequency f = 50 Hz. Fig. 3 Induction motor loss distribution [24]. 5. MATERIALS FOR MAGNETIC CORES Non-oriented electrical alloys are used in the manufacture of the high efficiency electrical machines. Many factors that occur during the production of the magnetic materials are important for the final magnetic properties. In the mass production of the high efficiency motors a big magnetic permeability and low magnetic losses are ones of the most desired characteristics. Fig. 4 Total energy losses versus magnetic polarization for three non-oriented industrial steel grades at 50 Hz.

4 4 High efficiency electrical motors 17 because of practical issues (i.e. large scale implementation of punching devices). The electrical motor design has to take into account also the production conditions for electrical steel strips [22, 23, 25, 26]. ACKNOWLEDGMENTS Fig. 5 Relative magnetic permeability versus magnetic polarization for three non-oriented industrial steel grades at 50 Hz. This work was partially supported by a grant of the Romanian National Authority for Scientific Research and Innovation, CNCS/CCCDI UEFISCDI, Project Number 10PTE/2016, within PNCDI III Brushless servo-motors series utilizing soft magnetic composite materials and by a grant of Romanian Ministry of National Education, UEFISCDI, project number PN-II-PT-PCCA Received on June 11, MEASURING STANDARDS In 1972 and updated in 1997 the IEC introduces the standard IEC in Europe that is based on an indirect efficiency measurement and it imposed a fixed value of the stray losses at 0.5 % of the input power. On the other hand, the IEEE proposed the IEEE 112-B standard in 1984 with an update in 2004, which is based on a direct measurement method of the energy efficiency and the stray losses are calculated with relation (1). To introduce the new efficiency classification the IEC publishes in 2007 a revised standard IEC and in 2010 the IEC for large electrical motors. In 2013 it was adopted the IEC that takes into account the variable speed drive motors [27, 28]. All these standards are based on the IEEE 112-B methods for measuring the mechanical output and electrical input power. 7. CONCLUSIONS The energy efficiency classifications are an important step in the production of worldwide standard electrical motors and with mandatory future implementation of the improved IE4 standard, the energy savings are considerable. Fully processed steel strips are used for the magnetic cores of the electrical machines and they are not annealed after being mechanically cut. 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