24-08-2012, 04:11 PM
Efficiency trends in electric machines and drives
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Abstract
The key challenges to increased efficiency in systems driven by electrical machines lie in three areas: firstly, to extend the application areas of variable-speed electric drives through reduction of power electronic and control costs: secondly, to integrate the drive and the driven load to maximise system efficiency: finally, to increase the efficiency of the electrical drive itself.
By adopting known, proven concepts it is possible to dramatically increase the efficiency of systems driven by electrical machines and reduce total electricity consumption by over 7%. Some of this improvement arises from the efficiency of the machine, but the majority is due to the improved system efficiency, which can be achieved with a variable-speed drive.
Almost all electricity in the UK is generated by rotating electrical generators, and approximately half of that generated is used to drive electrical motors. Hence, efficiency improvements with electrical machines can have a very large impact on energy consumption. In the short to medium term, efficiency gains within electrical machines will result from the development of new materials and construction techniques.
Introduction
Electrical machines have advanced significantly in recent years due to the introduction of new materials. New electrical steels have reduced losses and rare- earth permanent magnet materials have provided a 'lossless' source of magnetic flux. Recent advances in construction methods have reduced winding losses, so there is a continued trend to increase efficiency. For large electrical machines, efficiency is already high and so, although significant, the potential gains are limited. Greater gains are possible in smaller machines, which may be only 50% efficient.
Variable-speed drives are created when a motor is combined with a power electronic converter. By introducing variable speed to the driven load, it is possible to optimise the efficiency of the entire system and it is in this area that the greatest efficiency gains are possible.
This paper has three main sections – one covering the statistics of energy consumption and current predictions of possible savings using existing technology; a second covering the current state of the science, and a final one on future, more long-sighted possibilities.
Energy consumption
UK energy consumption statistics published by the Department of Trade and Industry) give a breakdown of energy consumption by fuel, by sector and by final end use, but do not explicitly reveal the energy consumed by electrical motor-driven systems. Studies promoted by the European Commission (De Keulenaer et al. 2004; Haataja and Pyrhonen 1988; European Commission Joint Research Centre; EU SAVE II Project. 2001) state that motor-driven systems use 65–70% of all electricity consumed by industry, whil in the US it is estimated to be 67%. It is likely that these statistics will also be representative of the UK. Walters (1999a, 1999b) further reports that more than half of all electricity consumed in the UK is used to drive electric motors.
Current state of the science
Industrial motor systems are dominated by induction motors running at effectively constant speed. Variable-speed drives, in which the speed of the machine is controlled by a power electronic converter, are taking an increasing size of the market and in 2004 accounted for 25% of new systems (de Almeida et al. 2005). The efficiency of the electrical system in isolation will first be considered, before progressing to the entire system where, with the addition of variable-speed drives, much larger energy savings can be made.
Generation
Large turbogenerators of 100–660 MW rating supply the vast majority of the UK's electricity. These are wound rotor synchronous machines whose efficiency is over 98%. The very high efficiency arises by virtue of their very large size, with designs only changing marginally in the last few decades as newer, low-loss materials emerge.
There is much more diversity in electricity generation from renewable resources. Most wind generators are doubly-fed induction machines, fed through a step-up gearbox, but direct-drive permanent magnet generators are emerging as an alternative. With direct-drive permanent magnet systems, the efficiency of the generator is increased, and the gearbox losses are eliminated, but additional power electronic converter losses are introduced, and the system costs rise due to the large mass of the low-speed generator.
Power electronic converters
Power electronic converters are used to supply a variable frequency supply to an AC motor, thereby enabling variable speed operation. Power converters have conduction and switching losses in the power devices, losses in passive components and auxiliary cooling systems. The loss is a function of device type, switching frequency, voltage and current level, but for industrial systems the converter has a typical full load efficiency, which rises with power rating from around 80% below 1 kW to over 97% at 150 kW (Rooks and Wallace 2004). Efficiency levels are rising as newer, low-loss, faster-switching devices emerge.
Variable-speed drives
For the purposes of this paper, an electric drive will be classified as the combination of a power electronic converter, electrical machine and electronic controller. The EU-funded SAVE II Programme (de Almeida et al. 2005) has identified large-scale application of variable-speed drives as the motor systems technology having the most significant energy savings potential. Savings within the electrical drive system alone are projected to be 6 billion kWh per annum in the UK (de Almeida et al. 2005).
Variable-speed drives have been adopted as standard within process control applications, where their variable speed gives greater functionality and is often essential for the application. However, for the bulk of applications, a fixed-speed drive can be employed and involves a lower initial capital cost, but generally with much lower system efficiency.
Household
There have been extensive studies of the household sector, both in the USA (Kubo et al. 2001; Little 1999) and the EU (EU SAVE II Project 2001). In Europe, the electricity consumption of a central heating pump is up to 600 kWh per annum, which is comparable to the complete lighting system of a household, or that of a fridge-freezer. Through adoption of efficiency standards and technical measures such as speed control, more efficient motors and seasonal switches, it is predicted that energy requirements could be reduced by more than a factor of three by 2020.
Japan is leading many aspects of innovation in motors and drives for household applications. In Morimoto et al. (2002), there is a focus on refrigeration and air-conditioning systems, which comprise over 40% of Japan's electric power consumption. By moving from induction machines to permanent magnet synchronous machines, a reduction in motor loss of over 60% is reported. This improvement is due to a combination of innovative construction techniques and the use of power electronic converters, which have made it possible to use the more efficient permanent magnet machine, and is in addition to the large gains a variable speed system produces.
[attachment=32657]
Abstract
The key challenges to increased efficiency in systems driven by electrical machines lie in three areas: firstly, to extend the application areas of variable-speed electric drives through reduction of power electronic and control costs: secondly, to integrate the drive and the driven load to maximise system efficiency: finally, to increase the efficiency of the electrical drive itself.
By adopting known, proven concepts it is possible to dramatically increase the efficiency of systems driven by electrical machines and reduce total electricity consumption by over 7%. Some of this improvement arises from the efficiency of the machine, but the majority is due to the improved system efficiency, which can be achieved with a variable-speed drive.
Almost all electricity in the UK is generated by rotating electrical generators, and approximately half of that generated is used to drive electrical motors. Hence, efficiency improvements with electrical machines can have a very large impact on energy consumption. In the short to medium term, efficiency gains within electrical machines will result from the development of new materials and construction techniques.
Introduction
Electrical machines have advanced significantly in recent years due to the introduction of new materials. New electrical steels have reduced losses and rare- earth permanent magnet materials have provided a 'lossless' source of magnetic flux. Recent advances in construction methods have reduced winding losses, so there is a continued trend to increase efficiency. For large electrical machines, efficiency is already high and so, although significant, the potential gains are limited. Greater gains are possible in smaller machines, which may be only 50% efficient.
Variable-speed drives are created when a motor is combined with a power electronic converter. By introducing variable speed to the driven load, it is possible to optimise the efficiency of the entire system and it is in this area that the greatest efficiency gains are possible.
This paper has three main sections – one covering the statistics of energy consumption and current predictions of possible savings using existing technology; a second covering the current state of the science, and a final one on future, more long-sighted possibilities.
Energy consumption
UK energy consumption statistics published by the Department of Trade and Industry) give a breakdown of energy consumption by fuel, by sector and by final end use, but do not explicitly reveal the energy consumed by electrical motor-driven systems. Studies promoted by the European Commission (De Keulenaer et al. 2004; Haataja and Pyrhonen 1988; European Commission Joint Research Centre; EU SAVE II Project. 2001) state that motor-driven systems use 65–70% of all electricity consumed by industry, whil in the US it is estimated to be 67%. It is likely that these statistics will also be representative of the UK. Walters (1999a, 1999b) further reports that more than half of all electricity consumed in the UK is used to drive electric motors.
Current state of the science
Industrial motor systems are dominated by induction motors running at effectively constant speed. Variable-speed drives, in which the speed of the machine is controlled by a power electronic converter, are taking an increasing size of the market and in 2004 accounted for 25% of new systems (de Almeida et al. 2005). The efficiency of the electrical system in isolation will first be considered, before progressing to the entire system where, with the addition of variable-speed drives, much larger energy savings can be made.
Generation
Large turbogenerators of 100–660 MW rating supply the vast majority of the UK's electricity. These are wound rotor synchronous machines whose efficiency is over 98%. The very high efficiency arises by virtue of their very large size, with designs only changing marginally in the last few decades as newer, low-loss materials emerge.
There is much more diversity in electricity generation from renewable resources. Most wind generators are doubly-fed induction machines, fed through a step-up gearbox, but direct-drive permanent magnet generators are emerging as an alternative. With direct-drive permanent magnet systems, the efficiency of the generator is increased, and the gearbox losses are eliminated, but additional power electronic converter losses are introduced, and the system costs rise due to the large mass of the low-speed generator.
Power electronic converters
Power electronic converters are used to supply a variable frequency supply to an AC motor, thereby enabling variable speed operation. Power converters have conduction and switching losses in the power devices, losses in passive components and auxiliary cooling systems. The loss is a function of device type, switching frequency, voltage and current level, but for industrial systems the converter has a typical full load efficiency, which rises with power rating from around 80% below 1 kW to over 97% at 150 kW (Rooks and Wallace 2004). Efficiency levels are rising as newer, low-loss, faster-switching devices emerge.
Variable-speed drives
For the purposes of this paper, an electric drive will be classified as the combination of a power electronic converter, electrical machine and electronic controller. The EU-funded SAVE II Programme (de Almeida et al. 2005) has identified large-scale application of variable-speed drives as the motor systems technology having the most significant energy savings potential. Savings within the electrical drive system alone are projected to be 6 billion kWh per annum in the UK (de Almeida et al. 2005).
Variable-speed drives have been adopted as standard within process control applications, where their variable speed gives greater functionality and is often essential for the application. However, for the bulk of applications, a fixed-speed drive can be employed and involves a lower initial capital cost, but generally with much lower system efficiency.
Household
There have been extensive studies of the household sector, both in the USA (Kubo et al. 2001; Little 1999) and the EU (EU SAVE II Project 2001). In Europe, the electricity consumption of a central heating pump is up to 600 kWh per annum, which is comparable to the complete lighting system of a household, or that of a fridge-freezer. Through adoption of efficiency standards and technical measures such as speed control, more efficient motors and seasonal switches, it is predicted that energy requirements could be reduced by more than a factor of three by 2020.
Japan is leading many aspects of innovation in motors and drives for household applications. In Morimoto et al. (2002), there is a focus on refrigeration and air-conditioning systems, which comprise over 40% of Japan's electric power consumption. By moving from induction machines to permanent magnet synchronous machines, a reduction in motor loss of over 60% is reported. This improvement is due to a combination of innovative construction techniques and the use of power electronic converters, which have made it possible to use the more efficient permanent magnet machine, and is in addition to the large gains a variable speed system produces.