CHAPTER 4. SCREENING ANALYSIS

TABLE OF CONTENTS

4.1 INTRODUCTION

........................................................................................................... 4-1

4.2 DISCUSSION OF DESIGN OPTIONS

.......................................................................... 4-1

4.3 DESIGN OPTIONS NOT SCREENED OUT OF THE ANALYSIS

............................. 4-3

4.3.1 Copper Die-Cast Rotor Cage

............................................................................... 4-3

4.3.2 Reduce Skew on Conductor Cage

........................................................................ 4-4

4.3.3 Increase Cross-Sectional Area of Rotor Conductor Bars

.................................... 4-4

4.3.4 Increase End Ring Size

........................................................................................ 4-4

4.3.5 Change Gauges of Copper Wire in Stator

............................................................ 4-5

4.3.6 Stator Slot Size Manipulation

.............................................................................. 4-5

4.3.7 Air Gap Between Rotor and Stator

...................................................................... 4-6

4.3.8 Improved Grades of Electrical Steel

.................................................................... 4-6

4.3.9 Thinner Steel Laminations

................................................................................... 4-6

4.3.10 Anneal Steel Laminations

.................................................................................... 4-7

4.3.11 Additional Stack Height

....................................................................................... 4-7

4.3.12 High-Efficiency Lamination Materials

................................................................ 4-8

4.3.13 Better Bearings and Lubricant

............................................................................. 4-8

4.3.14 More Efficient Cooling System

........................................................................... 4-8

4.3.15 Change Run-Capacitor Rating

............................................................................. 4-9

4.3.16 Summary of Technology Options Not Screened Out

.......................................... 4-9

4.4 DESIGN OPTIONS SCREENED OUT OF THE ANALYSIS

.................................... 4-10

4.4.1 Air Gap Between Rotor and Stator

.................................................................... 4-10

4.4.2 Plastic Bonded Iron Powder

............................................................................... 4-11

4.4.3 Summary of Technology Options Screened Out of the Analysis

...................... 4-11

LIST OF TABLES

Table 4.2.1 Summary List of Options from Technology Assessment

.................................... 4-3

Table 4.3.1 Design Options Not Screened Out of the Analysis

............................................ 4-10

Table 4.4.1 Design Options Screened Out of the Analysis

................................................... 4-11

4-iCHAPTER 4. SCREENING ANALYSIS

4.1 INTRODUCTION

The purpose of the screening analysis is to identify design options that improve small electric motor efficiency and determine which options the Department of Energy (DOE) will

either evaluate or screen out. DOE consults with industry, technical experts, and other interested parties in developing a list of design options for consideration. Then DOE applies the following set of screening criteria to determine whic h design options are unsuitable for further consideration in the rulemaking (1 Title 10 of the Code of Federal Regulations, Part 430, Subpart C, Appendix A at 4(a)(4) and 5(b)):

(1) Technological feasibility. Technologies incorporated in commercial products or in working prototypes will be considered technologically feasible.

(2) Practicability to manufactu re, install, and service . If mass production of a technology in commercial products and reliabl e installation and servicing of the technology could be achieved on the scale necessary to serve the relevant market at the time of the effective date of the standard, then DOE will consider that technology practicable to manufacture, install, and service.

(3) Adverse impacts on product u tility or product availability . If DOE determines that a technology will have significant adverse impact on the utility of the product to significant subgroups or consumers or result in the unava ilability of any covered product type with performance characteristics (including reliability), features, sizes, capacities, and volumes that are substantially the same as products generally available in the United States at the time, that technology will not be considered further.

(4) Adverse impacts on health or safety . If DOE determines that a technology will have significant adverse impacts on health or safe ty, that technology will not be considered

further.

This chapter discusses the design options th at DOE considered for improving the energy efficiency of small electric motors and describes how DOE applied the screening criteria.

4.2 DISCUSSION OF DESIGN OPTIONS

Several well-established engineering practices and techniques exist for improving the efficiency of a small electric motor. Improving the construction materials ( e.g. , the core steel, winding material, cooling system) and modify ing the motor’s geometric configuration ( i.e. , the core and winding assemblies, the rotor, and stat or) can make an electric motor more energy

efficient.

As discussed in the market and technology as sessment (chapter 3), there are four general areas of efficiency loss in small electric motors: I 2 R, core, friction and windage, and stray load.

4-1DOE presented an initial list of technology options used to reduce energy consumption and thus improve the efficiency of general purpose induc tion motors. Unfortunate ly, methods of reducing electrical losses in the equipment are not complete ly independent of one another. This means that some technology options that decrease one ty pe of loss may cause an increase in a different type of loss in the motor. Thus, it takes a great degree of engineering skill to maximize the efficiency gains in a motor design overall, balancing out the loss mechanisms. In some instances, motor design engineers must make de sign tradeoffs when finding the appropriate combination of materials and costs. However, there are multiple design pathways to achieve a given efficiency level.

Although I 2 R and core losses account for the majority of the losses in an induction motor, friction and windage losses and stray load losses also contribute to the total loss. In an induction motor, friction and windage losses can manifest in the bearings, bearing lubricant, and cooling fan system. Stray load losses are electromagnetic losses that generally require process changes in manufacturing. These various technologies ca n constrain the design parameters of a motor and thus limit the improvement in efficiency.

Table 4.2.1 presents a general summary of the methods that a manufacturer may use to reduce losses in small electric motors. The approaches presented in this table refer either to specific technologies ( e.g. , aluminum versus copper die-cast rotor cages, different grades of electrical steel) or physical changes to the motor geometries ( e.g. , cross-sectional area (CSA) of rotor conductor bars, additional stack height).

4-2Table 4.2.1 Summary List of Options from Technology Assessment

Type of Loss to Reduce Technology Option Applied

I 2 R Use a copper die-cast rotor cage

Reduce skew on conductor cage

Increase cross-sectional area of rotor conductor bars

Increase end ring size

Change gauges of copper wire in stator

Manipulate stator slot dimensions

Decrease air gap between rotor and stator

Change run-capacitor rating (CSCR motors only)

Core Improve grades of electrical steel

Use thinner steel laminations

Anneal steel laminations

Add stack height ( i.e. , more electrical steel)

Use high-efficiency lamination materials

Use plastic bonded iron powder

Friction and Windage Install better bearings and lubricant

Install a more efficient cooling system

4.3 DESIGN OPTIONS NOT SCREENED OUT OF THE ANALYSIS

This section discusses the technology opti ons that DOE considers viable means of improving the efficiency of small electric motors.

4.3.1 Copper Die-Cast Rotor Cage

Aluminum is the most common material used t oday to create die-cast rotor bars in small motors. Some manufacturers that focus on produci ng high-efficiency designs have started to

offer electric motors with die-cas t rotor bars made of copper. Copper offers better performance than aluminum because, per unit area, copper has a higher electrical conductivity ( i.e. , a lower electrical resistance). However, copper has a hi gher melting point than aluminum, so the casting process becomes more difficult and is likely to increase both production time and cost for manufacturing a motor.

Considering the four screening criteria for this technology option, DOE did not screen out copper as a die-cast rotor cage conductor material. Because this material is in commercial use today, DOE concluded that this material is technologically feasible and practicable to manufacture, install, and service. DOE is not aware of any adverse impacts on consumer utility, reliability, health, or sa fety associated with the use of coppe r as a die-cast rotor cage material.

4-34.3.2 Reduce Skew on Conductor Cage

In the rotor, the conductor bars are not straig ht from one end to the other, but skewed or twisted slightly around the axis of the rotor. Decreasing the degree of skew can improve a motor’s efficiency. The conductor bars are skewed to help eliminate harmonics that add cusps, losses, and noise to the motor’s speed-torque char acteristics. Reducing th e degree of skew can help reduce the rotor resistance and reactance, which helps improve efficiency. However, overly reducing the skew also may have adverse eff ects on starting, noise, and the speed-torque

characteristics.

Considering the four screening criteria for this technology option, DOE did not screen out adjusting rotor skew as a means of improving effici ency. Rotor skew is one of the variables that motor design engineers can manipulate to obtain ce rtain performance and efficiency targets. The rotor skew is part of the overall motor desi gn, which is input into automated production equipment that punches and stacks the steel to create a rotor with the desired skew. Because this design technique is in comme rcial use today, DOE considers this technology option both technologically feasible and practicable to manufac ture, install, and service. DOE is not aware of any adverse impacts on consumer utility, reliability, health, or safety associated with properly manipulating the rotor skew to obtain improved performance.

4.3.3 Increase Cross-Sectional Area of Rotor Conductor Bars

Increasing the cross-sectional area of the rotor conductor bars, either by making the diameter of the conductor bars larger or changing the cross-sectional geometry of the rotor, can improve motor efficiency. Either way, increasin g the cross-sectional area of the rotor conductor bars will decrease the resistance, increase current flow, and lower losses. Note that the shape of the rotor may affect the size of the end rings.

Considering the four screening criteria for this technology option, DOE did not screen out increasing the cross-sectional area of rotor conduc tor bars as a means of improving efficiency. Motor design engineers adjust this particular variable when manufacturing to obtain performance and efficiency targets. The rotor conductor ba rs are created by automated production equipment that have certain tolerances and allow variance in this parameter. Because this design technique is in commercial use today, DOE considers this technology option both technologically feasible and practicable to manufacture, in stall, and service. DOE is not aware of any adverse impacts on consumer utility, reliability, health, or safety associated with increasing the cross-sectional area

of rotor conductor bars to obt ain increased efficiency.

4.3.4 Increase End Ring Size

Creating an electrical connection between the rotor bars by increasing the size of the endrings can increase motor efficiency. Increasing the size of the end rings reduces the resistance

and thus lowers the I 2 R losses in the end rings.

Considering the four screening criteria for this technology option, DOE did not screen out increasing end ring size as a means of improving efficiency. As with some of the previous technology options, motor design engineers adjust this variable when manufacturing an electric

4-4motor to achieve performance and efficiency targets. Automated production and casting equipment, which allow some degree of variability, determine the end ring size. Because this design technique is in comme rcial use today, DOE considers this technology option both technologically feasible and practicable to manufac ture, install, and service. DOE is not aware of any adverse impacts on consumer utility, reliability, health, or safety associated with increasing the size of the rotor end rings to obtain increased efficiency.

4.3.5 Change Gauges of Co pper Wire in Stator

Increasing the slot fill by changing the gauge of wire used in the stator winding can also increase motor efficiency. Motor design engineers can achieve this by manipulating the wire gauges to allow a greater total cross-sectional area of wire to be incorporated into the stator slots.

This could mean either an increase or decrease in wire gauge, depending on the dimensions of

the stator slots and insulation th icknesses. As with the benefits associated with larger crosssectional area of rotor conductor bars, using more to tal cross-sectional area in the stator windings decreases the winding resistance and associated losses. However, this change could affect the packing factor of the wire in the stator slots. The stator slot openings must fit the wires so that machinery can pull (or push) the wire into the stator slots. DOE used 65 percent as a maximum peak slot fill percentage in all of the design s presented in chapter 5 of the Technical Support Document (TSD).

Considering the four screening criteria for this technology option, DOE did not screen out changing gauges of copper wire in stator as a means of improving efficiency. Motor design engineers adjust this technology option in fracti ons of a half a gauge when manufacturing an electric motor to achieve desired performance and efficiency targets. Because this design technique is in commercial use today, DOE considers this technology option both technologically feasible and practicable to manufac ture, install, and service. DOE is not aware of any adverse impacts on consumer utility, reliability, health, or safety associated with changing the wire gauges in the stator to obtain increased efficiency.

4.3.6 Stator Slot Size Manipulation

Varying the stator slot size and geometry associated with a given motor design can improve motor efficiency. Similar to changing gauges of wires, manipulating the stator dimensions can allow the motor design engineer to incorporate more copper into the stator slots. This decreases the losses in the windings and can affect motor torque and performance (including efficiency).

Considering the four screening criteria for this technology option, DOE did not screen out changing stator slot size as a means of improving efficiency. Mo tor design engineers modify this technology to achieve desired performance and effici ency targets. Because this design technique is in commercial use today, DOE considers this technology option both technologically feasible and practicable to manufacture, in stall, and service. DOE is not aware of any adverse impacts on consumer utility, reliability, health, or safety associated with changing stator slot sizes to obtain increased efficiency.

4-54.3.7 Air Gap Between Rotor and Stator

Reducing the radial air gap between the rotor and stator can improve motor efficiency. For small electric motors, the air gap is commonly set at 15 thousandths of an inch. However, reducing the air gap to 12.5 thousandths of an inch per side is still practic able to manufacture. Although DOE is aware that some stepper motors have tighter tolerances, radial air gaps less

than 10 thousandths of an inch are not achiev able for continuous duty motors, such as small electric motors, as defined by the Energy Polic y and Conservation Act, as amended, 42 U.S.C. 6291–6317 (EPCA).

Considering the four screening criteria for this technology option, DOE did not screen out decreasing the radial air gap to 12.5 thousandths of an inch as a means of improving efficiency. DOE recognizes that reducing the radial air gap is a means design engineers use to achieve desired performance and efficiency targets. Becau se this design technique is in commercial use today, DOE considers this technology option both technologically feasible and practicable to manufacture, install, and service. DOE is not aware of any adverse impacts on consumer utility, reliability, health, or safe ty associated with reducing the radi al air gap to 12.5 thousandths of an

inch.

4.3.8 Improved Grades of Electrical Steel

Losses generated in the electrical steel of an induction motor can be significant for an electric motor. Generally, these losses are classified as either hysteresis or eddy current. Hysteresis losses are caused by magnetic domains resisting reorientation to the alternating magnetic field ( i.e. , 60 times per second, or 60 hertz). Eddy currents are physical currents that are induced in the steel laminations by the ma gnetic flux produced by the current in the windings. Both of these losses generate heat in the electrical steel.

One way to reduce hysteresis losses is to incorporate a higher grade of core steel into the electric motor design ( e.g. , switching from an M56 to an M 19). Even for the same thickness ( i.e. , gauge) of core steel lamination, losses are reduced as the grain size increases, thus reducing magnetic resistance to reorientation by the alternating current.

Considering the four screening criteria for this technology option, DOE did not screen out improved grades of electrical steel as a means of improving efficiency. Design engineers use this approach to achieve desired performance a nd efficiency targets. Because this design technique is in commercial use today, DOE considers this technology option both technologically feasible and practicable to manufac ture, install, and service. DOE is not aware of any adverse impacts on consumer utility, reliability, health, or safety associated with improving the electrical steel.

4.3.9 Thinner Steel Laminations

DOE can use thinner laminations of core steel to reduce eddy currents. DOE can either change grades of electrical steel as described above, or use a thinner gauge of the same grade of electrical steel. The magnitude of the eddy currents induced by the magnetic field becomes smaller in thinner laminations, making the motor more energy efficient.

4-6Considering the four screening criteria for this technology option, DOE did not screen out thinner steel laminations as a means of improvi ng efficiency. Design engineers use this approach to achieve desired impr ovements in performance and efficiency. Because this design technique is in commercial use today, DOE considers this technology option both technologically feasible and practicable to manufac ture, install, and service. DOE is not aware of any adverse impacts on consumer utility, reliability, health, or safety associated with using

thinner steel laminations.

4.3.10 Anneal Steel Laminations

Annealing the laminations can also reduce the losses in the electrical steel. This process subjects the laminations to an extremely high te mperature to alleviate the stresses from punching and assembly. There is reduced stress, but also gr ain growth, which alleviates hysteresis losses. In addition to these benefits, the steels are oxidized to create a resistive coating that helps reduce eddy current flow and thus core losses.

Considering the four screening criteria for this technology option, DOE did not screen out annealing steel laminations as a means of impr oving efficiency. Design engineers use this approach to achieve desired impr ovements in performance and efficiency. Because this design technique is in commercial use today, DOE considers this technology option both technologically feasible and practicable to manufac ture, install, and service. DOE is not aware of any adverse impacts on consumer utility, reliability, health, or safety associated with annealing steel laminations.

4.3.11 Additional Stack Height

Adding electrical steel to the rotor and stator to lengthen the motor can also reduce the efficiency losses in steel. Lengthening the motor by increasing stack length reduces the magnetic flux density, which lowers the core watts lost overall. However, increasing the stack length affects other performance attributes of the mo tor, such as starting torque. Issues can arise when installing a more efficient motor with additional stack height because the motor becomes longer and therefore may not fit into app lications with dimens ional constraints.

Considering the four screening criteria for this technology option, DOE did not screen out additional stack height as a means of improving ef ficiency. Design engineers use this approach to achieve desired improvements in performance a nd efficiency. Because this design technique is in commercial use today, DOE considers this technology option technologically feasible. Regarding the second screening criterion–practicable to manufacture, install, and service–DOE is concerned that increasing motor length makes installation of these motors too problematic. However, DOE recognizes that many small moto r applications are not constrained by motor length. Thus, DOE believes that this technol ogy option meets the second screening criterion. DOE is not aware of any adverse impacts on consumer utility, reliability, health, or safety associated with increased stack height.

4-74.3.12 High-Efficiency Lamination Materials

Another technology option to improve the effi ciency of small electric motors is using

nontraditional laminati on materials. These materials are stee l alloys that contain boron or cobalt ( e.g. , Hiperco 50TM a , vanadium permendur, amorphous metals such as METGLAS b ), which are used instead of conventional and silicon steel when assembling a stator and rotor stack.

These materials offer lower core losses because the laminations are extremely thin and the magnetic permeability is extremely high. These steels are not typically used in small electric motors, as defined by EPCA. However, DOE is not aware of any technical impediment preventing their use, because these are commercia lly available steels used in the manufacture of electric motors. DOE is aware of some small electric motor designs that were prepared for a manufacturer using one of these high-efficiency lamination materials. Thus, DOE believes that such steels are technologically feasible and practicable to manufacture, install, and service, because of their use in mass-produced electric mo tors. However, DOE is concerned that there may not be an adequate supply of these material s to meet the manufacturing requirements of all

the small motors manufactured in the United Stat es today. DOE is not aware of any adverse impacts on product utility, product availability, hea lth, or safety for this technology option.

4.3.13 Better Bearings and Lubricant

Another technology option to improve the effi ciency of small electric motors is using better ball bearings and a lower-f riction lubricant. Using impr oved bearings and lubricants minimizes mechanical resistance to the rotation of the rotor, which also extends motor lifetime.

Considering the four screening criteria for this technology option, DOE did not screen out better ball bearings and lubricants as a means of improving efficiency. Design engineers use this approach to achieve desired impr ovements in performance and efficiency. Because this design technique is in commercial use today, DOE considers this technology option both technologically feasible and practicable to manufac ture, install, and service. DOE is not aware of any adverse impacts on consumer utility, reliability, health, or safety associated with better ball bearings and lubricant.

4.3.14 More Efficient Cooling System

Using a more efficient cooling system that circulates air through the motor is another technology option to improve the efficiency of small electric motors. Improving the cooling system reduces air resistance and associated frictional losses and decreases the operating temperature (and associated el ectrical resistance) by cooling the motor during operation.

Considering the four screening criteria for this technology option, DOE did not screen out a more efficient cooling system as a means of improving efficiency. Design engineers use this approach to achieve desired impr ovements in performance and efficiency. Because this design

a Registered trademark of the Carpenter Technology Corporation.

b Registered trademark of Metglas, Inc., a wholly-owned subsidiary of Hitachi Metals, Ltd., Tokyo, Japan.

4-8technique is in commercial use today, DOE considers this technology option both technologically feasible and practicable to manufac ture, install, and service. DOE is not aware of any adverse impacts on consumer utility, reliability, health, or safety associated with improved cooling systems for small electric motors.

4.3.15 Change Run-Capacitor Rating

Another technology option to improve the effi ciency of CSCR small electric motors is changing the microfarad rating of the run-capacitor. A motor design engineer will usually modify the run-capacitor’s rating along with the auxiliary windings of a CSCR motor to simulate a balanced two phase motor at full load. The combination of these changes helps to minimize the required current to run the motor, thereby reducing the I 2 R losses

Considering the four screening criteria for th is technology option, DOE did not screen out changing the rating of a run-capacitor in a CSCR motor as a means of improving efficiency. Design engineers use this approach to ach ieve desired improveme nts in performance and efficiency. Because this design technique is in commercial use today, DOE considers this technology option both technologically feasible and practicable to manuf acture, install, and service. DOE is not aware of any adverse impacts on consumer utility, reliability, health, or safety associated with improved cooling systems for small electric motors.

4.3.16 Summary of Technology Options Not Screened Out

Table 4.3.1 summarizes the design options that DOE did not screen out of the analysis.

4-9Table 4.3.1 Design Options Not Screened Out of the Analysis

Type of Loss to Reduce Technology Option Applied

I 2 R Use a copper die-cast rotor cage

Reduce skew on conductor cage

Increase cross-sectional area of rotor conductor bars

Increase end ring size

Change gauges of copper wire in stator

Manipulate stator slot dimensions

Decrease the air gap between rotor and stator (down to 10 thousandths)

Change run-capacitor rating (CSCR motors only)

Core Use improved grades of electrical steel

Use thinner steel laminations

Anneal steel laminations

Add stack height ( i.e. , more electrical steel)

Use high-efficiency lamination materials

Friction and Windage Install better bearings and lubricant

Install a more efficient cooling system

4.4 DESIGN OPTIONS SCREENED OUT OF THE ANALYSIS

DOE screened out the following design options from further consideration because they do not meet the screening criteria.

4.4.1 Air Gap Between Rotor and Stator

Reducing the air gap between the rotor and stator can improve motor efficiency. For small electric motors, the air gap is commonly set at 15 thousandths of an inch. Although reducing this air gap can improve efficiency, there is some point at which the air gap is too tight and becomes impracticable to manufacture. DOE believes that air gaps below 12.5 thousandths of an inch would exceed the threshold for practicability to manufacture.

Considering the four screening criteria fo r this technology option, DOE screened out decreasing the radial air gap below 12.5 thousan dths of an inch as a means of improving efficiency. DOE considers this technology opti on technologically feasible, because smaller air gaps do not present any technological barrier. DO E also is not aware of any adverse impacts on health or safety associated with reducing the radial air gap below 12.5 thousandths of an inch. However, DOE believes that this technology option fails the screening criterion of being practicable to manufacture, install, and service. Such a tight air gap may cause problems in manufacturing and service, with the rotor potentially coming into contact with the stator. This

4-10technology option fails the screening criterion of adverse impacts on consumer utility and reliability, because the motor may experience higher failure rates in service when the manufactured air gaps are less than 12.5 thousandths of an inch.

4.4.2 Plastic Bonded Iron Powder

Plastic bonded iron powder (PBIP) could cut pr oduction costs while increasing the output of electric motors. Although other researcher s may be working on this technology option, DOE is aware of a research team at Lund University in Sweden that published a paper about PBIP. This technology option is based on an iron powder a lloy that is suspended in plastic, and is used in certain motor applications such as fans, pumps, and household appliances. 1 The compound is then shaped into motor components using a centrifugal mold, reducing the number of manufacturing steps. Researchers claim that this technology option could cut losses by as much as 50 percent. The Lund University team already produces inductors, transformers, and induction heating coils using PBIP, but has not ye t produced a small electric motor. In addition, it appears that PBIP technology is aimed at torus, claw-pole, and transversal flux motors, none of

which fit EPCA’s definiti on of small motors.

Considering the four screening criteria fo r this technology option, DOE screened out PBIP as a means of improving efficiency. Although PBIP has the potential to improve efficiency while reducing manufacturing costs, DOE does not consider this technology option technologically feasible, because it has not been in corporated into a working prototype of a small electric motor. Also, DOE is uncertain whether the material has the structural integrity to form into the necessary shape of a small electric motor steel frame. Furthermore, DOE is uncertain whether PBIP is practicable to manufacture, insta ll, and service, because a prototype PBIP small electric motor has not been made and little information is available on the ability to manufacture this technology to make a judgment. Howeve r, DOE is not aware of any adverse impacts on product utility, product availa bility, health, or safety that may arise from the use of PBIP in small

electric motors.

4.4.3 Summary of Technology Options Screened Out of the Analysis

Table 4.4.1 shows the criteria DOE used to sc reen radial air gaps below 10 thousandths of an inch, PBIP, and permanent magnet el ectric motors out of the analysis.

Table 4.4.1 Design Options Screened Out of the Analysis

Design Option Screening Criteria

Radial Air Gaps Below 10 Thousandths of

an Inch Practicability to manufacture, install, and service; adverse impacts on prod uct utility or product availability

PBIP Technological feasibility

4-11REFERENCES

1 Horrdin, H., and E. Olsson. Technology Shifts in Power Electronics and Electric Motors for Hybrid Electric Vehicles: A Study of Silicon Carbide and Iron Powder Materials. 2007. Chalmers University of T echnology. Göteborg, Sweden.

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