Copper in energy efficient motors
The high electrical conductivity of copper is an important design factor that helps to improve the energy efficiency of motors. This is important because motors and motor-driven systems are very significant consumers of electricity, accounting for 43%-46% of all global electricity consumption and 69% of all electricity used by industry.
Inefficient electric motors waste electrical energy. Since most electricity is generated from fossil-fueled power plants, motors and motor-driven systems are indirect contributors to greenhouse gas emissions produced by these plants. Hence, there are compelling economic and environmental reasons to increase the use of energy efficient motors.
This article discusses how copper helps to improve the electrical energy efficiencies of motors. The advantages of copper as an electrical conductor in the stator and rotor are discussed, as is a new copper die-cast rotor technology that was developed specifically for premium efficiency motors. The article also introduces motor legislations implemented around the world that focus on energy savings and reduced carbon footprints that can be achieved with energy-efficient motor systems. The article focuses on AC induction motors because they are frequently specified to power industrial machinery.
Electric motors transform electrical energy into mechanical energy
An electric motor is an electromechanical device that uses magnetic attraction and repulsion to produce controlled rotational motion. In an electric motor, electrical energy delivered by a power source is converted into mechanical energy. This is accomplished when one set of electromagnets mounted on a fixed assembly (or stator) attract the opposite polarity of electromagnets on a rotating assembly (or rotor). The magnetic field produced by the stator rotates in space by the application of an electric current, thereby causing the rotor to rotate with it. In turn, the rotor drives mechanical loads coupled with it.
Electrical motor efficiency
Motors and motor driven systems are huge consumers of electricity. They are estimated to account for 43%-46% of all global electricity consumption as well as 69% of all electricity used by industry.
Since most electricity is generated from fossil-fueled power plants, motors and motor driven systems are, in an indirect sense, major contributors to greenhouse gas emissions produced by these plants.
Electric motors do not transfer 100% of the input electrical energy into kinetic mechanical energy. A certain percentage of electrical energy is “lost” during the conversion to mechanical energy. These losses, which are manifested as electrical power losses (waste heat due to the electrical resistance of the windings, conductor bars and end rings), magnetic core losses, stray load losses, mechanical losses, and brush contact losses, reduce what is known as the “energy efficiency” of motors. The electrical power losses account for more than half of a motor’s total losses.
This is a problem for several reasons. First, inefficient electric motors waste electrical energy, thereby increasing electrical demand and associated electricity costs required to power motors. Second, when electricity is generated by oil- or coal-fed power plants, the burning of fossil fuels produces carbon footprints from the usage of natural resources and the emissions of greenhouse gases. Electrical energy losses from inefficient motors, therefore, waste precious natural resources, cause increased emissions of greenhouse gases, and increase operating costs (i.e., increases utility bills). Third, waste heat from inefficient motors increases maintenance and decreases the life of the motor.
For these reasons, there are compelling economic and environmental needs to evaluate the benefits of energy efficient and premium efficiency electric motors versus their standard counterparts.
Increasing the electrical energy efficiencies of motors
Until the energy crises in the 1970s, most general-purpose motors were designed to provide rated output and operating characteristics at reasonable cost. Efficient operation was at best a secondary consideration. As energy prices began to rise, manufacturers began to develop improved motors known as "high-efficiency" and "energy-efficient".
A well-designed motor can convert over 90% of its input energy into useful power for decades. When the efficiency of a motor is raised by even a few percentage points, the savings, in kilowatt hours (and therefore in cost), are enormous. For example, it has been estimated that if all countries adopted best Minimum Energy Performance Standards (MEPS) for industrial electric motors, by 2030 approximately 322 terawatt-hours of annual electricity demand would be saved. As an additional environmental benefit, this savings in electric demand corresponds to a saving of 206 million tons of CO2 emissions.
The electrical energy efficiency of a typical industrial induction motor can be improved by: 1) reducing the electrical losses in the stator windings (e.g., by increasing the cross-sectional area of the conductor, improving the winding technique, and using materials with higher electrical conductivities), 2) reducing the electrical losses in the rotor coil or casting (e.g., by using materials with higher electrical conductivities), 3) reducing magnetic losses by using better quality magnetic steel, 4) improving the aerodynamics of motors to reduce mechanical windage losses, 5) improving bearings to reduce friction losses, and 6) minimizing manufacturing tolerances.
In addition to energy savings, other benefits of high efficiency motors over standard motors include: 1) cooler operating temperatures due to lower heat generation, resulting in lower maintenance and a longer life, 2) improved tolerance to voltage variations and harmonics, 3) extended manufacturers’ warranties, and 4) rebates and tax incentives in some regions from utilities and municipalities.
Tools to evaluate motor efficiencies and lifetime costs
As part of its initiative to enhance the efficiency of motors, the United States Department of Energy created a free online computer software tool to help motor purchasing agents make informed buying decisions over the entire lifecycle of motors under consideration. The software, called MotorMaster+, contains data on 25,000 different motors. The software helps buyers select a motor based on list price, motor efficiency, payback analysis, and return on investment. It also enables an organization to examine its motor population or any individual motor as part of an overall repair and replacement plan.
By selecting any two motors and inputting unit energy costs and usage profiles, the software calculates life cycle cost analysis and greenhouse gas emissions reductions from using premium motors versus standard motors. Older operating motors with low efficiencies can also be evaluated for replacement. These motors cannot be rewound to exceed their original electrical efficiency design standards.
Another free tool, called MotorSlide Calculator™, can help calculate approximate annual savings in choosing a National Electrical Manufacturers Association (NEMA) premium electric motor (or any level of efficiency) versus a lower efficiency model.[Note 1]
AC electric induction motors
Motors have evolved into a variety of types according to user requirements, design, and production costs. Examples include: AC motors, including AC induction motors; DC motors; and universal motors. Of these major categories of motors, there are many types (see Categorization of electric motors for a good introduction about various types of motors.)
This section will address copper in energy-efficient alternating current (AC) induction motors because these types of motors are widely used in industrial drives.
The main parts of an AC induction motor are the fixed housing body (stator), a rotating assembly (rotor), and electromagnets consisting of coils of copper or aluminium wire around a core of magnetic steel.
Copper and aluminium can both be used in the stator coils, although copper coils are the standard as they are more flexible and they enhance motor electrical efficiencies due to their higher electrical conductivity. In standard induction motors, instead of being wound in coils, the rotor conductors are die-cast in the shape of a squirrel cage within a core of magnetic steel. Aluminium die-cast rotors are the standard material but copper die-casting of rotors is an improved new technology that is increasingly used to enhance motor energy efficiency. Induction motors can be designed with wound-rotor motors instead of a squirrel-cage. In a wound-rotor motor, the rotor winding is made of many turns of insulated wire.
Other advantages to using copper rather than aluminium in AC motors include:
- Lower coefficient of expansion for copper: aluminium will creep and move approximately 33% more than copper.
- Higher tensile strength for copper: copper is 300% stronger than aluminium and thus able to withstand high centrifugal force and the repeated hammering from current‐induced forces during each start.
- Higher melting point of copper: copper can better withstand thermal cycling over the life of the motor.
Electrical conductivity in motor coils
An electric current running through a simple straight wire creates a magnetic field defined by Ampere's Law, but the field is relatively weak. Current running through insulated copper or aluminium wire wound into a helix creates much stronger magnetic fields that causes a motor to turn.
To increase the strength of the magnetic field further, the coil can be wound from a longer length of wire and/or from a thicker diameter wire. Winding the coil around a cylinder of soft iron or other ferromagnetic material can magnify the magnetic field by a factor of about 300 in common materials.
While the cylinder, commonly referred to as a “core,” magnifies the magnetic field, it is the coil that creates the field. The more wire in the coil (or coils), the stronger the magnetic field. The higher the electrical conductivity of the coil material, the stronger the magnetic field. The stronger the magnetic field, the more powerful the motor.
Electrical conductivity is a key operating parameter in determining which type of material to use in a motor’s coil. Wires made from better electrical conductors result in a more efficient transfer of electrical energy into mechanical energy. Poorer conductors generate more heat when transferring electrical energy into mechanical energy. In essence, more energy is wasted as the electrical resistance of the coil increases.
Silver has the highest electrical conductivity of all metals (6.30×107siemens/meter at 20°C). However, silver is an expensive precious metal and is therefore not considered as a coil conductor material for motors.
Copper has the second highest electrical conductivity of all metals (5.96 × 107siemens/meter at 20°C) and is much more affordable. Copper is commonly used in motors, including the highest quality motors because of its high electrical conductivity. Copper is an excellent metal to use for a motor's coils because: 1) it has less electrical resistance than almost any other non-precious metal; 2) it is easily made into wires; 3) it is not too expensive; 4) it can perform and survive at high temperatures; and 5) it can easily be recycled when the motor needs to be replaced.
The fourth highest electrically conducting material is aluminium. Aluminium has a much lower electrical conductivity than copper (3.5 × 107siemens/meter at 20°C) but is used in motors due to its lower cost.
Copper coils increase motor electrical energy efficiencies
Copper’s greater conductivity versus other materials enhances the electrical energy efficiency of motors. For example, to reduce load losses in continuous-use induction-type motors above 1 horsepower, manufacturers invariably use copper as the conducting material in windings. Aluminium, because of its lower electrical conductivity, may be an alternate material in smaller horsepower motors, especially when the motors are not used continuously.
In general, older, standard-efficiency motors have higher losses than premium motors that meet more current energy standards. One of the design elements of premium motors is the reduction of heat losses due to the electrical resistance of the conductors. To improve the electrical energy efficiencies of induction-type motors, one design consideration is to reduce load loss by increasing the cross section of the copper coils. Increasing the mass of copper in a coil increases the electrical energy efficiency of the motor.
A high efficiency motor has more copper in the stator winding than its standard counterpart. For example, a 10 horsepower premium efficiency motor uses up to 75% more copper than a similar-sized motor with a standard efficiency.
For these reasons, early developments in motor efficiency focused on reducing electrical losses by increasing the packing weight of stator windings. This made sense since electrical losses typically account for more than half of all energy losses, and stator losses account for around two‐thirds of electrical losses.
Copper die-cast rotors
The rotor is the rotating part of the motor. Rotor losses, an important form of power losses in induction motors, are largely but not entirely proportional to the square of the slip (slip is the difference between the rotational speed of the magnetic field and the actual rpm of the rotor at a given load). Thus, rotor losses are reduced by decreasing the degree of slip for a given load. This is accomplished by increasing the mass of the rotor conductors (conductor bars and end-plates) and/or increasing their conductivity, and to a lesser extent by increasing the total magnetic field across the air gap between rotor and stator.
The electrical efficiency of motors can be improved by replacing the standard aluminium electrical conductor in the motor rotor with copper, which has a much higher electrical conductivity. Until recently, die-cast motor rotors were produced only from aluminium while researchers worked on solving technological issues with copper pressure die-casting. Today, copper pressure die-casting is a proven technology and thousands of die-cast copper motor rotors are produced annually for motor applications where energy savings are prime design objectives.
The use of copper in place of aluminium for conductor bars and end rings of induction motor rotors results in improvements in motor energy efficiency due to a significant reduction in I2R losses. Motor modeling by a number of manufacturers has demonstrated that motors with copper rotors yield overall rotor loss reductions from 15 to 20% compared to aluminium.
The advantages of motors with copper motor rotors on an equivalent basis with aluminium include the following:
- Motors have longer lives: they generate less heat and reduce thermal stresses, including those on insulation, which enable them to operate longer.
- Motors are smaller: the increased electrical conductivity of the copper rotor material plus the need for a smaller volume of steel enables the motors to be shorter in length.
- Motors have 1‐5% higher energy efficiency ratings, so therefore consume less energy.
- Motors have lower overall manufacturing costs.
Currently, market penetration of copper rotor motors is mainly in low-voltage industrial motors ranging from 1 - 100 kW. There is a potential market for copper rotor motors in small fractional horsepower applications, but this has not yet come to fruition.
In the U.S., a growing number of commercially available, general-purpose induction motors with die-cast copper rotors exceed National Electrical Manufacturers Association (NEMA) premium efficiency standards and display at least 10% lower total electrical losses than an average NEMA Premium® motor of the same size, as defined by US Department of Energy’s MotorMaster+ software tool (see: Tools to evaluate motor efficiencies and lifetime costs).
For example, ultra-efficient motors up to 15 kW exceed NEMA Premium® standards. This was achieved by combining low resistive (I²R) losses of high-conductivity copper squirrel cages with optimized stator designs.
Two series of high-efficiency motors with copper rotors are offered by a German manufacturer whereas a French manufacturer produces die-cast copper rotors for a wide variety of applications and manufacturers.
Also, the U.S. Army now employs 520 volt AC induction motors with die-cast copper rotors on a hybrid drive system on each axle of its severe-duty trucks. This has resulted in fuel economy increases of up to 40%.
Several motor models with cast aluminium rotors also exceed NEMA Premium® efficiencies. These models are considered by some to be not as advantageous as copper rotor models because they require more material in the stator. Also, copper rotors enable designs with higher efficiency levels than NEMA Premium® (i.e., the so-called "NEMA Super-premium" and beyond).
To optimize the electrical conductivity of copper die-cast rotors, it is necessary to use copper alloys with very low levels of impurities. Even low levels of most impurity elements will significantly increase the electrical resistivity of copper. Alloys C10100 (99.99% Cu, 0.0003 P, 0.0010 Te) and C11000 (99.90% Cu, 0.04% O) are recommended for die-casting copper rotors. Both these alloys have electrical conductivities of 101% International Annealed Copper Standard (IACS).
|Parameter||Copper alloy C10100||Copper alloy C11000|
|Electrical conductivity||101% IACS||101% IACS|
|Electrical resistivity||17.1 nΩ•m||17.1 nΩ•m|
Energy-efficient legislation impacting motors
Manufacturers, in coordination with various manufacturing associations and in conjunction with voluntary government initiatives, have developed a wide range of motors with increased electrical efficiencies. At the same time, governmental and inter-governmental agencies seeking to achieve energy savings and reduce carbon footprints from more efficient industrial motor systems have issued increasingly stringent standards and regulations requiring users to buy high- and premium-efficiency motors (over various time horizons) instead of standard efficiency alternatives for many applications.
Initiatives exist for nations to move towards Minimum Energy Performance Standards (MEPS) for motors. In 2002, five nations adopted MEPSs. By 2011, thirty nine nations (the EU-27, as well as the U.S., Canada, Brazil, Mexico, Costa Rica, China, Korea, Taiwan, Australia, New Zealand, Israel, and Switzerland) will have adopted some form of mandatory MEPS for three-phase electric motors. Motors in these countries account for 70% of global electricity use in motor systems. If the mandatory MEPSs in these 39 countries were raised to best-practice levels, savings could approach 206 million tonnes of CO2 emissions annually by 2030.
A summary of the worldwide standards of energy efficient motor programs is available. Highlights of motor laws in the U.S. and E.U. are presented below.
Motor laws in the U.S.
EPAct 92 was the first major energy law to require minimum, nominal, full-load motor efficiency ratings for most industrial motors. It set minimum efficiency levels for electric motors. The motors, which came to be known as “EPAct motors”, are still commercially available. Their nominal efficiencies are between one and four percentage points higher than those of the so-called “standard-efficiency motors” that had dominated the market for decades.
The Consortium for Energy Efficiency (CEE) and the National Electrical Manufacturers Association (NEMA) agreed on a joint specification defining a "premium" efficiency motor. Motors meeting minimum specifications are eligible to carry the NEMA Premium® designations. Publications are available that compare NEMA Premium efficiencies versus EPAct minimums.
In 2005, the Energy Policy Act of 2005 (EPAct 2005) established NEMA Premium® efficiency ratings as the basis for motor purchases by the federal government. NEMA Premium® motor efficiency ratings are up to several percentage points higher than those of their EPAct predecessors. This law broadened the size range to include motors from 1 to 500 hp.
Based on U.S. Department of Energy data, it is estimated that the NEMA premium-efficiency motor program would save 5.8 terawatt hours of electricity and prevent the release of nearly 80 million metric tons of carbon dioxide into the atmosphere over the next ten years. This is equivalent to keeping 16 million cars off the road.
The Energy Independence and Security Act of 2007 (Public Law 110-140, usually called “EISA 2007 Act”), the most recent law regulating motors, took effect in 19 December 2010 and affects all new motors purchased after that date. Title III, Section 313 of the Act increases the mandated efficiency of electric motors in commercial and industrial applications and expands the range of motors to be regulated. Included in EISA is major provision for improving the minimum required energy efficiency in all integral horsepower motors. EISA covers:
- 1-200 horsepower motors: The law eliminates EPAct level motors and requires that National Electrical Manufacturers Association (NEMA) premium efficiency level motors must be manufactured after December 19, 2010. These motors use more copper and steel than their less-efficient counterparts. Motors made prior to this date can still be sold and installed but manufacturers cannot make new motors less than the premium efficiency standard.
- 200-500 horsepower motors: The law requires minimum EPAct efficiency motors and advises that NEMA premium motors be considered for heavy duty cycle higher power cost applications.
- Other special purpose motors (e.g., (vertical pump motors) are included in the legislation as well.
Rebates may be available for premium efficient motors, depending on the project and its location (search the database here for applicable rebates). A summary of the new EISA standards for motors can be found at: NEMA Premium Efficiency Levels Adopted as Federal Motor Efficiency Performance Standards. Further details about NEMA premium motors is available at: NEMA Premium Motors
Motor laws in the E.U.
Up to 2010, the E.U. had established voluntary programs, resulting in a significantly lower percentage of high efficiency motors on the market than in the U.S. A brief history of motor laws in the E.U. follows.
In 1998, the European Committee of Manufacturers of Electrical Machines and Power Electronics (CEMEP) issued a voluntary agreement of motor manufacturers on efficiency classification, with three efficiency classes: Eff 3 for Standard Efficiency, Eff 2 for Improved Efficiency, Eff 1 for High Efficiency. Two years later, the modern era of efficient motors in the E.U. began as the voluntary agreement between European Motor Manufacturer Association and the European Commission took effect.
In June, 2005, the European Parliament adopted a Commission proposal for a Directive on establishing a framework for setting eco-design requirements (such as energy efficiency requirements) for all energy consuming products in the residential, tertiary and industrial sectors.
The current, mandatory, efficiency level across a wide power rating range required of motors sold in Europe is embodied in the EU Minimum Energy Performance Standard (MEPS) scheme, introduced in July 2009. The EU MEPS not only raises the efficiency standard of motors sold in Europe, it also links Europe’s requirements to international standards. EU MEPS covers 2-, 4- and 6-pole single speed, three-phase induction motors in the power range 0.75 to 375 kW, rated up to 1000 V and on the basis of continuous duty operation.
High efficiency motors (Eff1) represent only 12% of the market in the EU today.
International standards for electric motor efficiency labeling
A new international standard for electric motor efficiency labeling was introduced in 2008 (and revised in 2011) by the International Electrotechnical Commission (IEC). This standard, IEC 60034-30, defines energy efficiency classes for single-speed, three-phase, and 50 Hz and 60 Hz induction motors. The standard is designed to unify motor testing standards, efficiency requirements, and product labeling requirements so that motor purchasers worldwide have the ability to easily recognize premium efficiency products. On 22 July 2009, Commission Regulation (EC) No 640/2009 implementing Directive 2005/32/EC stated that in the E.U., with a few exceptions for special purposes, drive motors shall not be less efficient than the IE3 efficiency level (premium efficiency) as of 1 January 2015.
Rotor losses in IE3 systems are considerably reduced by using copper instead of aluminium as the conductor material for the squirrel cage. The slip under load, which is proportional to the rotor losses, is significantly decreased compared with aluminium motors. Unlike aluminium motors, IE3 motors with a copper rotor do not require an increased amount of iron or need merely a moderate increase. Other measures can also be taken to save energy in IE3 motors.
There is a 3‐4% energy efficiency difference between IE1 and IE3 standard motors, but the differences, and the absolute level of efficiency, depend on the output of the motor relative to its rating.
IE3 is a new classification, but one that has been recognized by NEMA in the U.S. It generally applies to large, industrial motors. In Europe, this grade of motor will only become mandatory in some applications in 2017. In the U.S., the required introduction date was 19 December 2010 for larger motors; smaller motors will become mandated in 2017.
The U.S. and a few other countries have already introduced legislation requiring IE2 standard motors in certain applications. Many others have plans to introduce such rules. In Europe, IE2 became the obligatory standard on 16 June 2011. For some motors, this is also true of China in 2011, although for other motors a minimum IE1 standard will be introduced in that year replacing earlier, less rigorous, requirements.
|International Electrotechnical Commission / EN 60034-30||EU MEPS||CEMEP (European voluntary agreement)||US EPAct||Other, similar national regulations|
|IE3 Premium efficiency||IE3 Premium efficiency||Identical to NEMA Premium efficiency|
|IE2 High efficiency||IE2 High efficiency||Comparable to EFF1||Identical to NEMA Energy efficiency/EPACT||Switzerland, Canada, Mexico, Australia, New Zealand, Brazil, China|
|IE1 Standard efficiency||Comparable to EFF2||Below standard efficiency||Switzerland, China, Brazil, Costa Rica, Israel, Taiwan|
A time table of minimum performance standards for the various motor efficiency levels in various countries is presented below:
|Efficiency levels||"Mandatory" Minimum Energy Performance Standards (MEPS) for industrial electrical motors||"Voluntary" Minimum Energy Performance Standards (MEPS) for industrial electrical motors|
|Super premium (IEC Class 4)||Advocacy commenced||Advocacy commenced|
|Premium (IEC Class 3)||USA (2010), Canada (2010), Mexico (2010), EU (2015-2017), Japan (2015)||China (2011), Korea (2012), India (2014)|
|High (IEC Class 2)||ANZ (2006), Korea (2008), Brazil (2009), China (2011), EU (2011), Taiwan (2013)||India (2011)|
|Standard (IEC Class 1)||Available in Africa, Asia, Latin America, Europe||Available in Africa, Asia, Latin America, Europe|
|Below standard||Available in Africa, Asia, Latin America, Europe||Available in Africa, Asia, Latin America, Europe|
Once a consensus regarding the classification of the efficiency of motor driven systems is achieved, manufacturers and users can then move to create a labeling scheme. With time, legislation on standards may follow. For the present, though, legislation‐based installation of higher efficiency motors will be limited to stand alone motors.
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- Information regarding the melting and pouring of copper metal into die-casting machines for copper motor rotors from The Copper Development Association Inc.
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Cowie, J.G. and D.T. Brender (2003). Die cast copper rotors for improved motor performance; Pulp & Paper Industry Technical Conference.
Peters, D.T., J.G. Cowie, E.F. Brush, et al. (2005). Performance of motors with die-cast copper rotors in industrial and agricultural pumping applications; IEEE Conference on Electrical Machines and Drives.
Peters, D.T., J.G. Cowie, E.F. Brush, O.J. Van Son (2003). Copper in the squirrel cage for improved motor performance, IEEE Electric Machines and Drives Conference.
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- Electric Motor Systems (EMSA)
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