vvEPA
United States
Environmental Protection
Agency
Office of Atmospheric
and indoor Air
Programs
EPA-430-R-93-009
June 1993
State of the Art Survey of
Motor Technology Applicable
To Hermetic Compressors
For Domestic
Refrigerator/Freezers
Recycled/Recyclable
Printed on paper that contains
^' V at least 50% recycled fiber
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,u, * Of The Art Survey
Motor Technology Applicable To Hermetic
Compressors For Domestic Refrigerator/Freezers
Prepared for
Environmental Protection Agency
Division of Global Change
Revised: March, 1993
Fteference64128
64122
67986
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Prologue
Motor technology can play an important role in allowing potential efficiency
improvements of various refrigerator/freezer design alternatives. This review of the
motor technology applicable to small hermetic refrigeration compressors was
undertaken as part of a larger study to evaluate the options for maximizing the
efficiency of domestic refrigerators. The motor characteristics documented in the report
are being used in the overall study as part of the input database for modeling and
evaluating design options.
Findings i
1. Two kinds of approaches are possible for compressor motors
Constant speed, 2 pole, 115 VAC, 60 HZ, single phase, squirrel cage induction
motors are current practice in all refrigerators and freezers; motor speed is close to
3500 RPM;
i
Variable speed alternatives are:
- 2 speed (1750/3500 RPM) squirrel cage induction motors; and
- Continuously variable speed, electronically driven motors.
2. Smaller compressors will be needed in future refrigerators.
super insulation (either vacuum panels or thicker walls) is likely to reduce loads.
Dual loop (or staged) systems using two compressors may replace single
compressor/evaporator designs to gain theoretical energy efficiency advantages.
3. Small compressors are inefficient because of inefficient motors: these can be
improved with currently available technology.
Current small compressors are relatively inefficient, mainly due to inefficient
motors.
I
Current technology exists to make efficient motors for small compressors at a
reasonable price.
4. Variable speed technology can provide an additional and/or alternative means
to achieve superior energy efficiency
Variable speed compressor operation has the potential to reduce compressor power
requirements by virtue of more efficient heat exchanger utilization and elimination of
cycling losses.
Two speed induction motors are a low first cost, relative to electronically, driven
variable speed motors, means of reducing compressor on/off cycling.
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Two speed motors, though not currently produced for or used in domestic
refrigerator compressor applications, could be produced for this application, at a cost
approximating that of premium efficiency single speed motors, but at an efficiency
reduction of 5% at full speed and close to 15% at half speed. At these reduced motor
efficiency levels, no net improvement in R/F energy consumption can be expected to
result from their use.
The most efficient small variable speed motor/drive system alternative is an
electronically commutated permanent magnet rotor DC motor, with full speed
system efficiency comparable to the best single speed induction motor, and halt
speed efficiency within a few percent of full speed. There is some uncertainty with
respect to the cost level that would be reached after a few years of mass production
but one quarter horsepower ECM motor/drive systems are currently being produced
and sold on a modest scale OEM basis for approximately $100.
OEM unit costs of these variable speed drives could fall to $50-$55 (or less) in large
scale (hundreds of thousands of units annually) mass production.
The variable speed drive and motor can operate at higher speeds, allowing reduced
compressor cost, and replaces the existing compressor motor, for offsetting cost
reductions of approximately $20. The net increase in OEM component costs, then,
would be approximately $30-$35.
Modeled results of R/F performance with a variable speed compressor show
improvements in performance over a single speed compressor.
A substantial energy saving (10%) can be obtained by replacing the inefficient
fan/motor of typical current practice with a modest efficiency fan and motor.
With the standard efficiency fan/motor, the variable speed drive operating at steady
state results in increased total energy, with the energy associated with 100% fan run
time more than offsetting compressor power savings.
With efficient fans, the VSD compressor shows energy savings.
With variable speed fans added to the VSD, substantial energy savings are obtained.
A limitation on the compressor energy savings is the declining variable speed motor
system efficiency below half speed.
The same general observations apply to the application of VSD to a high
performance cabinet, with increased sensitivity of the variable speed options to the
fan efficiency.
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The value of these energy savings was calculated. Assuming an electric energy cost
of 80 per kWh, for real discount rates less than 10% the real, present value of the
saved energy is on the order of $75, over the projected 15 year life of the
refrigerator, greater than the incremental cost, of the variable drive.
5. Energy improvements from improved compressor performance, especially
small compressors, are possible with improved motors and' variable speed
technology. Dual loop and super-insulation systems should be economically
viable with this currently available (but as of yet) unmanufactured technology.
Motors for small compressors can be improved to efficiency levels within a few
percent of large compresors for $15, while the energy gain they would produce
would be worth $75.
Variable speed technology could add $35 (costs) to refrigerators, but save more than
$ 10 per year in electricity.
Research and improved manufacturing could improve the benefit/cost calculations
used in this report.
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Table of Contents
1.0 Introduction [[[ l~ *
1.1 Stratospheric Ozone Depletion and the CFC Phase Out .......................... 1-1
1.2 Global Warming [[[ I'2
1.3 DOE Appliance Energy Efficiency Standard Setting ............................... 1-2
1.4 Other Regulatory and Policy Initiatives [[[ 1-2
2.0 Issues in Compressor and Motor Selection ............................................... 2-1
2.1 Motor Technology Options [[[ 2-3
3.0 Single Speed Induction Motors - Current Practices in
Refrigerator/Freezer Compressors [[[ 3-1
4.0 Two Speed Motors ................ . [[[ 4-1
5.0 Variable Speed Drives [[[ 5-1
5.1 Efficiency [[[ 5-4
5.2 Variable Speed Motor/Drive Costs [[[ 5-5
5.3 Variable Speed Drives - Life, Reliability ................................................. 5-7
5.4 Description of Specific Variable Speed Motor/Drive Types ................... 5-8
5.4. 1 Electronically Commutated Permanent Magnet Rotor DC
Motors [[[ 5-8
5.4.2 Pulse Width Modulation Inverter/3 Phase Induction Motor ........... 5-9
5.4.3 Six Step Inverter - 3 Phase Induction Motor Drives ....................... 5-11
5.4.4 Switched Reluctance Motors .............................................. - ............ 5-13
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Table of Figures
Figure 2-1: Lorenz-Meutzner Refrigerator/Freezer Cycle ... 2-1
Figure 3-1: Typical Squirrel Cage Induction Motor Used in Domestic
Refrigerator Compressors (RSIR example) 3-2
Figure 3-2: OEM Cost of Single Speed Motors for Hermetic Refrigeration
Compressors, Versus Motor Efficiency and Compressor
Capacity 3.3
Figure 5-1: "Generic" Variable Speed Drive Block Diagram 5-2
Figure 5-2: Variable Speed Electronic Drive for a One-Quarter Horsepower
Electronically Commutated DC Motor 5.3
Figure 5-3: PWM Operation for Varying Output Voltage to a DC Motor 5-4
Figure 5-4: Comparison of Variable Speed Drive/Motor Efficiencies
(Fractional Horsepower) 5.5
Figure 5-5: Voltage and Current Output of a PWM Inverter (approximately
PWM frequency: 1.1 kHz) 5.10
Figure 5-6: Six Step Inverter Voltage and Current Wave Forms 5-12
Figure 5-7: Simplified Schematic of Switched Reluctance Motor
Configuration L 5.^4
Table of Tables
Table 2-1: Alternative Refrigerants [.. 2-3
Table 3-1: Refrigerator Compressor Motor Efficiency ' 3.4
Table 4-1: Potential Two Speed Motor Manufacturers i 4-2
Table 5-1: Manufacturers of ECPM Controllers , 5.9
Table 5-2: Manufacturers of PWM Inverters i 5.11
Table 5-3: Manufacturers of Six Step Inverters 5-13
Table 6-1: Summary of Calculated Energy Savings with Variable Speed
Compressor g_2
Table 6-2: Present Value of Energy Savings of Variable to Speed
Compressor g-2
Table 6-3: Present Value of Energy Savings of Variable to Speed
Compressor (Current Variable Speed Motor/Drive Technology .... 6-3
Table 6-4: Present Value of Energy Savings of Variable to Speed
Compressor (Efficient Low Speed - Future Technology) 6-3
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Acknowledgements
During the course of this work, a number of compressor, motor, and variable speed
drive manufacturers were contacted, whose assistance and cooperation we acknowledge
and appreciate:
Compressors: Tecumseh Products Co., Copeland, and Americold
Motors: General Electric, A. O. Smith, Emerson, and Baldor
Variable speed drives: Toshiba, Emerson, Mitsubishi, Hitachi, Westinghouse,
Magnetek, Lenze, Vee Arc, Ranco, Inland, PMI, Minarik, EG&G, Fasco, Boston
Gear, and Graham
VI
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1.0 Introduction
The design of the domestic refrigerator-freezer will be undergoing a significant set of
changes over the next several years, driven by interrelated developments in a number of
national energy and environmental policy areas - the CFC phase out, growing concerns
about global warming, DOE appliance energy efficiency standard setting, and others.
These developments have created the need for a comprehensive examination of the
design options for domestic refrigerator/freezers. These interrelated options include
compressors, motors, refrigerants and lubricants, the refrigeration cycle, cabinet
insulation, and other aspects of cabinet thermal design.
This report covers the technology status of the motors that are used, or potentially could
be used, in the small hermetic compressors used in the refrigeration system of domestic
refrigerators, addressing the potential for and cost of improvements in the efficiency of
these motors. The status of applicable variable speed motor technology and potential
refrigerator energy savings is examined.
The environmental and energy policy background is discussed briefly below.
1.1 Stratospheric Ozone Depletion and the CFC Phase Out
Evidence accumulated over the past 15 years indicates that fully halogenated
chlorofluorocarbons (CFCs) have caused measurable deterioration of the atmosphere's
stratospheric ozone layer, which plays a significant role in attenuating solar ultraviolet
radiation. Increased levels of ultraviolet radiation would have a large number of
undesirable effects, including increased levels of skin cancers. Over the past few years,
this subject area has received renewed attention as the result of observations in the
mid-1980s of "gaps" in the ozone layer in the vicinity of the poles. As a result of this
attention, the Montreal CFC protocols were concluded in Fall, 1987. This international
agreement was signed and ratified by the major free world industrial nations requiring a
freeze, then phased production curtailments, of CFCs. By 1998, production of CFC-11
CFC-12, CFC-113, CFC-114, and CFC-115, as well as certain "halons" were to be
reduced to 50% of 1986 levels. An additional provision provided for periodic review of
scientific evidence and adjustment of allowable levels of production accordingly. The
reassessment completed in 1990 resulted in a nearly total phase out of CFCs by the year
2000. The Clean Air Act of 1990 has codified this accelerated CFC phase out schedule
into U.S. environmental law. In April of 1991, NASA reported the results of
satellite-based measurements of stratospheric ozone levels indicating that ozone
depletion of 5% over the mid latitudes has already occurred. The likely result of this
development will be further acceleration of the timetable for CFC phase out. This has a
direct impact on R/F insulation and compressors which have been designed around the
characteristics of CFC-11 and CFC-12, respectively.
1-1
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1.2 Global Warming
Concurrent with the accumulation of scientific evidence of stratospheric ozone
depletion, increasing concerns have been developing about global warming caused by
increasing atmospheric concentrations of carbon dioxide and the trace greenhouse gases.
The most significant of the trace greenhouse gases are the CFCs and methane.
The fact that the CFCs are powerful greenhouse gases has reinforced the pressures to
accelerate the CFC phase out time table. Measures to limit or reduce CO2 emissions
have been proposed. Because CO2 is one of the basic combustion products of all of the
fossil fuels used to produce energy for heating, transportation, and electric power
generation, measures to reduce CO2 emissions require the burning of less fossil fuels.
One regulatory measure to bring this about is increasing energy efficiency standards.
1.3 DOE Appliance Energy Efficiency Standard Setting
In February, 1989, DOE issued a final rulemaking under the Energy Policy and
Conservation Act, as amended, establishing minimum energy efficiency standards for
most categories of consumer appliances (Federal Register, 1989a). Depending on the
category, the standards take effect between 1990 and 1993. The standards for domestic
refrigerators and freezers went into effect on January 1, 1990, generally requiring
efficiency levels in line with the most efficient products available in the late 1980s,
whose efficiency was nearly double the levels prevailing only 10 to 15 years earlier.
Global warming (and national energy security issues) have resulted in the recent
adoption of significantly reduced levels of allowable electric energy consumption for all
categories of domestic refrigerators and freezers, effective on January 1, 1993 (the 1993
standards reduce allowable energy consumption by approximately 30% from the levels
under the current Federal regulations that took effect on January 1, 1990) (Federal
Register, 1989b).
1.4 Other Regulatory and Policy Initiatives
States are instituting reforms in planning and rate making that put demand reductions on
an equal playing field with building additional supply capacity. Integrated resource
planning, adopted by many states, requires utilities to evaluate every "resource"
(demand reduction or supply) in terms of total societal cost.
California, Oregon, Washington, most of the New England states, Wisconsin, and New
York have adopted ratemaking processes in which utility rates or return on investment is
adjusted so that utilities do not lose profits for forgone Kwh sales, but can profit from
demand reductions. In California and several other states, the non-pollution aspect of
demand reductions has led regulators to allow shared savings of customer bill reductions
to further increase utility profits. As a consequence of this change in utility regulation,
the demand for efficient refrigerators is rising.
1-2
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The Golden Carrot/early retirement program is one concrete manifestation of this trend.
Under the Golden Carrot, utilities are banding together to pool rebates to produce an
incentive for production of a R/F that is 30% better than DOE's 1993 standard in the 18
to 22 cubic foot range. With the impetus described earlier from CFCs, global warming,
and other state regulatory reforms, the Golden Carnot will provide a strong incentive for
vast improvements in energy efficiency.
1-3
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2.0 Issues in Compressor and Motor Selection
To meet the dual challenge of designing for new blowing agents and refrigerants, while
meeting much higher efficiency standards, significant changes in the design of both the
cabinet and the refrigeration cycle are under consideration. There are many design
options, and it is important to evaluate the potential of each option to contribute
significant energy savings, without using CFCs and without increasing the cost of the
refrigerator beyond the value added by the change (with respect to energy costs or
utility) or decreasing the utility of the refrigerator to the consumer. Design options that
are being pursued in current R&D programs include:
Low thermal loss cabinets ("super-insulated boxes") reduce energy consumption by
reducing the amount of cooling that is needed. To utilize heat exchangers effectively
and minimize cycling losses, the compressor capacity should be reduced in
proportion to the thermal load. However, sufficient compressor capacity may still be
required to provide a sufficiently fast pulldown for food preservation.
Dual refrigeration loops (separate refrigeration systems for the refrigerator and
freezer compartment) take advantage of the increased COP at the higher evaporator
temperature that can provide the required cooling of the fresh food compartment.
The Lorenz cycle, shown schematically in Figure 2-1 uses a non-azeotropic
refrigerant mixture with two evaporators and an interchanger to operate the fresh
food compartment and the freezer at separate evaporator temperatures for higher
efficiency. (Lorenz, 1975)
..
Figure 2-1: Lorenz-Meutzner Refrigerator/Freezer Cycle
HX1
HX2
Condenser
-[Fresh Food Evaporatorj-
Temp
4[/XJ-g-JFreezer Evaporator
Variable speed compressor operation can save energy by allowing continuous
operation at low capacity, eliminating cycling losses and allowing more efficient
utilization of heat exchangers. Overspeed operation can provide additional capacity
for pulldown. The latter characteristic might be particularly advantageous with high
performance cabinets. \
2-1
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One consequence of these design changes is a decrease in the compressor capacity
required to best match a given size refrigerator. Improved insulation and cabinet design
will reduce cooling loads, reducing required compressor capacity. Two compressor/two
evaporator systems meet the freezer and fresh food compartment heat loads with
separate refrigeration systems, obtaining, in theory, significant improvements in the
efficiency with which the load in the fresh food compartment is met (higher evaporating
temperatures, no defrost cycle). Even less compressor capacity is needed, especially in
the fresh food compartment where nominal compressor capacities of only 200 Btu/hr
may be needed. While the Lorenz cycle does not inherently result in a drastic
compressor capacity reduction, other compressor problems, such as high starting torque
requirements, have been observed. Variable speed compressors will tend to be smaller
displacement, with pulldown requirements met by overspeed operation.
Present commercially available low capacity compressors (<600 Btu/hr nominal
capacity) have very poor efficiencies, low enough in some cases to completely negate
the gains obtained from the design options described above. To realize the efficiency
benefits of reduced loads and dual evaporator systems will require improved efficiency,
lower capacity compressors.
Regardless of the R/F cabinet and refrigeration cycle design approach taken, increases in
the efficiency level that is available in refrigerant compressors will result in proportional
increases in the efficiency of the refrigerator using the compressor.
A major issue for compressor design is the change in refrigerant from CFC-12 to a low
ozone depletion, low global warming potential refrigerant. While CFC-12 has been
shown to be a significant part of the cause of both stratospheric ozone depletion and
global warming, it is an excellent working fluid for domestic refrigerator/freezers. It has
a favorable pressure-temperature relationship, good therniodynamic efficiency, stability,
total miscibility with low cost mineral oil, and moderate temperature rise with
compression and is non-flammable, non-toxic, and low cost. Alternate refrigerants that
do not contain chlorine or have currently acceptable ozone depletion potentials do not
possess identical attributes of CFC-12. Thus, compressor modifications or new designs
will be required to adapt to the characteristics of a selected alternative refrigerant. Table
2-1 lists some of the potential alternative refrigerants and their status, including
potential substitutes for CFC-11, CFC-114, and CFC-502, as well as for CFC-12.
The major working fluid options include the near drop in replacements for CFC-12 (i.e.,
those refrigerants having vapor pressure-temperature curves close to that of CFC-12),
lower vapor pressure refrigerants such as HCFC-124, and non-azeotropic refrigeration
mixtures (NARMs). Lower vapor pressure refrigerants might be utilized in low
capacity systems (design options described above), if shown to result in higher
efficiency of low capacity compressors, by virtue of the larger displacement that would
be needed. The Lorenz cycle would utilize a NARM.
2-2
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Table 2-1: Alternative Refrigerants
Substitute
Refrigerant
HCFC-22
HFC-134a
Ternary
HFC-152a
HFC-123
HFC-124
HFC-125
HCFC-141b
HCFC-142b
NH3
Displaced
Refrigerant
CFC-12
CFC-502
CFC-12
CFC-12
CFC-12
CFC-11
CFC-114
CFC-502
CFC-1 1
CFC-12, 114
CFC-1 1
CFC-12
CFC-502
Probable
Availability
Current
Current
(w/HCFC-124)
1993
Current
1990-1991
1 993-95
Available in
limited amounts
Current
Current
Current
Description, Status, Comment
Commercially available, widely used refrigerant.
Contains chlorine, will be phased out under
Montreal Protocol Copenhagen Amendments
Commercially available, rapidly expanding
production
Available in limited amounts
Commercially produced and sold in fairly small
quantities, used primarily as a component in
CFC-500 (26%) and as a component in aerosol
propellant blends
Toxicity tests have shown sufficient toxicity to set
AEL at 10 ppm; commercially available
Co-product of HCFC-123 production. Long term
toxicity testing started
Near-term availability in blends to replace CFC-502
Commercial production began in July 1 988.
Toxicity testing underway. Possible use as a foam
blowing agent
Jsed in R22/Rl42b blends
Commercially available. Widely used in industrial
refrigeration sector. Toxic with low flammability
Source: Arthur D. Little, 1993 . -. - '
The major issues that need to be considered in adapting the compressor design to an
alternate refrigerant include the displacement required to obtain the intended capacity,
lubricant selection, and material compatibility, especially the motor winding insulation.
In summary, higher efficiency compressors are needed, especially in smaller capacities
Motor technology is an important consideration, because increasing the efficiency of the
compressor motor is a straightforward way to improve compressor efficiency. For
variable speed compressors, the variable speed motor and electronic drive represent the
major technology component and the major cost driver.
.
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2.1 Motor Technology Options
Motor efficiency impacts the overall energy consumption in a refrigerator/freezer in two
ways: 1) the compressor energy use is directly related to the motor efficiency for any
given compressor pump work; and 2) motor inefficiencies may result in additional
2-3
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compressor pump work requirements due to heating of the working fluid within the
compressor shell. Because of the combined effect of these two factors, a 10% increase
in motor efficiency could result in an overall compressor energy reduction of 13 to 16%.
The use of variable (either two-speed or continuously variable) speed compressor
operation to reduce cyclic losses and improve heat exchanger utilization, has proven to
result in significant improvements (of approximately 25% to 30%) in seasonal
efficiency in air conditioning (Cann, 1989; Sulfstede, 1989) and commercial
refrigeration applications. These improvements accrue from reduced heat exchanger
loadings, which will result in thermodynamic performance advantages. In addition,
cycling losses are reduced by longer, or continuous, compressor operation. Potential
disadvantages are additional losses associated with electronics and increased fan
energies
Improvements might be achieved in domestic refrigerators, as well. Applications to
these smaller capacity ranges have been limited due to the costs of the electronics and
the current lack of a market demand. A major requirement for energy savings is the
reduction of fan and electronics energies during the lengthened cycle time. Again,
advanced motor technology will be required to realize the potential efficiency
improvements.
In view of the importance of motor technology to the potential efficiency improvements
of various design alternatives, this review of the motor technology applicable to small
hermetic refrigeration compressors was undertaken as part of a larger study to evaluate
the options for maximizing the efficiency of domestic refrigerators. The motor
characteristics documented in the report are being used in the overall study as part of the
input database for modeling and evaluating design options.
This report covers the technology status of electric motors to drive the small hermetic
compressors used in the refrigeration system of domestic refrigerators. Two general
areas are addressed:
The potential for and cost of improvements in the efficiency of the motors currently
used in small hermetic refrigeration compressors, with particular emphasis on the
motors used in smaller (<800 Btu/hr nominal capacity) compressors.
The status of variable speed (both two discrete speeds and continuously variable
speed) motors in terms of availability, efficiency, and cost.
This report is intended to serve three functions: 1) description of the state-of-the-art of
current motors and the potential for future improvements; 2) summary of performance
and cost data as input to evaluations of refrigerator/freezer system design options; and
3) present preliminary results indicating the level of R/F energy consumption reductions
that can be obtained through the use of variable speed compressor operation.
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3.0 Single Speed Induction Motors
Compressors
Current Practice in Refrigerator/Freezer
The compressors used in all domestically produced and sold domestic refrigerators and
freezers in the U.S. are powered by 2 pole AC squirrel cage induction motors that
operate on normal household line power, i.e. 115 VAC, 60 HZ, 1 phase. The motors run
at a single speed, near 3500 RPM. In addition to running the compressor at the normal
range of loads, the motor must provide adequate starting torque, because small, single
cylinder compressors are difficult to start.
As shown in Figure 3-1, the basic arrangement of this type of motor consists of a rotor
and stator, each built up of a stack of electromagnetic grade steel laminations, as shown
in the cut-away rotor in Figure 3-1. The "squirrel cage" rotor has a series of lengthwise
aluminum bars cast into the rotor laminations. These bars are connected by rings at
each end of the stack. The stator laminations have a series of slots for the windings
which are aluminum or copper wire. Two sets of windings are provided, one being 90°
out of phase with the other. The main, or run, winding operates directly from line
current, and is always energized when the motor is running. The major design
categories of this type of motor involve the manner in which the secondary winding is
utilized for starting the motor and then running at normal speed. The basic categories of
induction motors are:
I
RSIR or Resistance Start/Induction Run;
CSIR or Capacitor Start/Induction Run; and
PSC or Permanent Split Capacitor.
RSIR and CSIR motor use the secondary winding for starting only, the capacitor start
version providing higher starting torque. As shown in Figure 3-1 (a photograph of an
RSIR rotor and stator), the secondary winding uses much smaller diameter wire, which
can be energized for only a limited period of time without overheating. The RSIR
motor is very low cost, but is inherently limited to 8 - 10 percent less efficiency than
PSC motors. In PSC motors, the secondary winding also operates when the motor is
running. A capacitor in series with this winding shifts the phase of the input voltage
approximately 90°, so that both windings together create a rotating magnetic field. The
net effect is improved utilization of both the windings and the iron in the motor,
increasing the efficiency, but at the added cost associated with the capacitor.
Motor efficiencies may be improved by the following:
Using additional material (increasing stack height and using larger diameter wire);
Using low loss steel in the laminations; and
Using thinner laminations
Figure 3-2 plots the estimated cost per motor (basis: the OEM price paid to the motor
supplier by the compressor manufacturer for production quantities of the motors) of
single speed induction motors vs. efficiency for motors sized for compressors having
nominal capacities between 200 and 800 Btu/hr. Note that for the smaller motors, the
maximum physically attainable efficiency is lower than for the Larger motors, and as a
3-1
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CO
00
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result, the cost-efficiency curves cross. Table 3-1 compares present practice in this
capacity range with estimated maximum efficiencies inherent in this motor type. From
Figure 3-2 and Table 3-1, it is apparent that: |
Figure 3-2: OEM Cost of Single Speed Induction Motors for Hermetic Ftefrigeration Compressors
Versus Motor Efficiency and Compressor Capacity
30
to
i-
JB
1 20 H
o
o>
o>
in
O
O
o 10
Induction Run
70
75 80
Peak Motor Efficiency
i
85
OEM Cost: Price each paid by compressor manufacturer to motor manufacturer for mass production
quantities. Includes cost of required relay, capacitor, etc.
Efficiency: Not accounting for any mechanical losses
Source: Table 3-1 and informal discussions on OEM cost levels with OEM suppliers of motors and
manufacturers of refrigeration compressors
High motor efficiency (peak efficiency near 85%) is technically feasible in any
compressor capacity of interest, once all of the aforementioned measures to improve
efficiency have been applied.
The cost of attaining the highest efficiency is significant, qualitatively. For the
smaller compressor capacities, the highest efficiency motors could cost up to 2 to 3
times more than the cost of the motors typically used currently.
3-3
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Table 3-1: Refrigerator Compressor Induction Motor Efficiency
Compressor
200 Btu
400 Btu
600 Btu
800 Btu
Motor
1/16
1/8
1/6
1/4
Today's Motor
RSIR*
RSIR
RSIR/PSC
PSC
Efficiency
70-73%
73-76%
78-82%
80-84%
Possible
PSC
PSC
PSC
Efficiency
86%
86%
86%
RSIR - "Resistance Start/Induction Run" Type Motor - requires relay.
"PSC - "Permanent-Split Capacitor" Motor with PTCR start-assist device - requires capacitor.
Source: Data provided by GE Motors, 1/12/90
Only a few manufacturers produce compressor motors for R/F applications. Americold,
Matsushita, and Embrace make them for their own compressors; Americold supplies
compressors primarily to White Consolidated Industries. GE makes these motors for
sale to all compressor manufacturers. Tecumseh manufactures most of the motors for
their small compressors. A.O. Smith and Copeland make motors in the 1/2 and integral
horsepower sizes; Copeland provides them only for their compressors. Industrias
Copreci of Spain makes compressor motors, but they have not entered the U.S. market.
3-4
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4.0 Two Speed Motors
Two speed motors provide a method for potentially improving system performance
because frequent on-off cycles can be largely replaced with long periods of half speed
operation, with reduced evaporator and condenser coil loading. There are two basic
arrangements of two speed motors, consequent pole and arrangements with separate sets
of two pole and four pole windings. The former arrangement is simpler, more compact,
and lower cost, but inherently lower in half speed efficiency in a fractional horsepower'
motor.
Based on the limited information that is available relative to two speed motors for this
application, the performance and cost characteristics of a two speed motor for an 800
Btu/hr compressor, optimized for low speed efficiency, are (reference 5):
80% full speed efficiency
70% half speed efficiency
Cost approximately that of a maximum efficiency single speed motor
(Figure 3-2, Table 3-1)
These efficiency numbers represent the estimated highest level of efficiency that might
be attained for this type of motor, in the output needed to operate an 800 Btu/hr
compressor. It should be emphasized that no 2 speed motors are actually in production
at this time for this application. Cost and efficiency estimates are the result of presales
preliminary design studies of a major supplier, and are subject to considerably more
uncertainty than the cost-efficiency curves in Figure 3-2 for single speed motors.
Table 4-1 shows some two speed motor manufacturers and the availability of two speed
motors. Two speed motors are not available from many U.S. motor manufacturers in
this size range. A.O. Smith and Copeland have made 1/2 and 1/3 horsepower
compressor motors but these are not in production. Baldor has made a 1/3 horsepower
compressor hermetic motor but it is three phase. Other manufacturers, including
Emerson, G.E., and potentially Americold are working on the development of two speed
motors at this time. The technology exists to make two speed motors, but there is not a
large enough market for them now. Two speed motors are most frequently used for fans
and pumps in integral horsepower sizes. Some residential central air conditioning
applications in the 1 1/2 to 5 horsepower range and larger use two speed compressors,
including Bristol (produces the 2 speed compressor), Copeland (produces the 2 speed'
compressor), Lennox, Goodman (Janitrol), and Carrier.
4-1
-------
Table 4-1: Potential Two Speed Motor Manufacturers
A. O. Smith
Copeland
Baldor
GE
Americold
Emerson
Smallest Size (hp)
1/2
1/3
1/3
Status
not in production
not in production
3 phase only
some development
possible development
some development
Source: Americold, 1990; Baldor, 1990; Copeland, 1990; Emerson Electric, 1990; GE Motors, 1990; A.O.
Smith, 1990.
4-2
-------
5.0 Variable Speed Drives
Variable speed drives allow motors to operate over a continuously variable speed range
with the maximum speed not limited to the 3500 - 3600 RPM maximum speed of a 60 '
Hz induction motor. In domestic appliance applications, the variable speed drive is an
electronic device that converts input line electric power (115 VAC, 60 Hz, single phase)
to a multiple phase, adjustable output voltage and frequency, driving the motor at
variable speed. Potential benefits of variable speed drive of the compressor include:
Follow refrigerator loads closely
Allow for quick pull down
Reduce evaporator loading
Potential for better matching of individual evaporator loads in a cycled two
evaporator system
Maximum speed can be greater than 3600 RPM, allowing reduced compressor
displacement, size, and cost
Reduce cycling losses
Reduce frosting
Variable speed drives are either commercially produced, or under development, to
operate three general categories of motors that are potentially applicable to domestic
refrigerator/freezers:
Three phase induction motors
Electronically commutated permanent magnet rotor DC motors (ECPM)
Switched reluctance motors
The basic electronic hardware configurations of the variable speed drives used to
operate each of these motor types are very similar to each other. Figure 5-1 is a
simplified block diagram of a "generic" variable speed drive, highlighting the
similarities. After passing through an EMI filter (to minimize the transmission of
electronic noise back onto the AC line), the input AC is rectified and filtered to DC.
Output transistors switch the DC to the individual motor windings, to suit the
characteristics of the motor being driven. Figure 5-2 is a photograph of a one quarter
horsepower variable speed drive containing these major subsystems, that has been
developed for a specific appliance application. In general, to drive any of the three
motor types over a wide speed range requires that the motor input voltage be varied,
and, for induction motors, that the DC supply be switched to synthesize an
approximately sinusoidal A/C output. Pulse width modulation (PWM) is the technique
most commonly used to effect this voltage variation. Figure 5-3 illustrates the operation
of PWM, to vary the input voltage to a DC motor. An output transistor is switched on
and off at a much higher frequency than the motor rotation frequency. The output
voltage is varied from 0 to the DC supply voltage by varying the length of the on time of
the output transistor from 0 to the full period of the square wave. The inductance of the
motor winding stores energy in excess of the average DC voltage during the pulse on
time, and releases the energy during the pulse off time to maintain current flow,
5-1
-------
smoothing the electric current to a nearly constant level (or, in an induction motor,
nearly sinusoidal wave form). During the off period of the pulse, current passes through
a diode "bypassing" the power transistor.
Figure 5-1: "Generic" Variable Speed Drive Block Diagram
(2% Loss) (Minimum 2% Loss)
* J-
115 VAC EMI Filter ^c Rectifier DC >-p
. *. * * -L'
Power
Transistor/
Diode
Bridge
i
Piltor '
Capacitor
Source: GE Motors, 1990; Toshiba a,b; Lloyd, 1982
t
r
Control
Circuits
f Motor j
I -V^y
(1-2% Loss)
There are a variety of other techniques for varying motor speed electrically or
electronically, that are in use or under development. Those in use are generally used in
specialized industrial applications, usually at power levels in excess of 25 horsepower.
The three variable speed drive/motor combinations covered in this section represent
those technologies that have been developed, or are undergoing development for mass
market appliance applications and have the best potential for low cost production.
In addition to controlling the output transistors as required to control the speed of the
motor, the control section often performs other functions such as: fault detection,
protection, and reporting; status monitoring; driving panel displays; controlling
additional operation functions, such as reversing the rotation direction and dynamic
braking; overload protection; torque limiting; and others. Many general purpose
inverters include a large selection of these features, adding considerably to the cost. In a
cost driven appliance application, only those functions that are actually useful would be
included.
As the number of variable speed drives in service increases, regulations are likely to be
imposed requiring low frequency harmonic filtering (in addition to higher frequency
EMI filtering) on the input line. This type of regulation is already in place in Europe for
electronic motor drives drawing more than 200 watts from the AC line. This type of
filtering adds components and cost to the variable speed drive, with little effect on the
efficiency.
5-2
-------
o
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c
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P- CJ
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- c
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ex
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"§
CO
E
S
O
LLJ
Q.
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TO
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in
o>
_
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-------
Figure 5-3: PWM Operation for Varying Output Voltage to a DC Motor
Voltage
On Portion_
(typical)
DC Supply
Voltage
Average DC
Output
Off Portion
(Typical)
Current
Average
DC Current
Ripple
5.1 Efficiency
The overall efficiency of a variable speed motor and drive (shaft power output * AC line
electric power input) is the product of the electronic drive efficiency and the motor
efficiency. Electronic drive efficiency for the three types of drive is nearly identical,
because the configuration of the power electronics (EMI filter - rectifier - filter - output
transistor/diode bridge) is essentially identical for each of the three drives. The
efficiency decreases with decreasing output voltage (and motor speed) and, to a lesser
extent, with decreased load (torque).
For small motors, the inherent efficiency of the three motor types are ofset from each
other by small increments:
ECPM's have the highest efficiency, approaching 95% (motor only), because the
field is supplied by the permanent magnet rotor, with no electric resistance losses.
ECM's incur only minor ripple loss from PWM operation at intermediate input
voltages and speeds.
5-4
-------
Three phase induction motors are approximately 5 percentage points lower in
efficiency than ECPM's. The PWM waveform causes an additional 3-5% efficiency
degradation, at a reasonably high PWM frequency (10 KHz) and 5 - 10%
degradation at low PWM frequency (1-2 KHz), due to the content of higher
harmonics of the pulsed waveform.
Switched reluctance motor efficiencies are intermediate to the other two types, but
require very high quality iron in the laminations, and very precise timing of
energizing and deenergizing the stator poles, to attain the intermediate efficiency
level.
Figure 5-4 plots overall motor/drive efficiency vs. motor speed for each of the three
basic motor types (applicable to between 1/10 and 1/3 horsepower output).
5.2 Variable Speed Motor/Drive Costs
This subsection addresses prospective mass production costs of variable speed drives for
refrigerator compressor application. Sections 5.4.1 through 5.4.3 present current costs
for commercially available drives.
Estimates of eventual mass production costs of variable speed drives are subject to
considerable uncertainty, because there is no U.S. mass production (on the appliance
industry scale) and corresponding OEM sales upon which to base cost estimates. In
general OEM prices and manufacturing costs are commercially sensitive information
which manufacturers are reluctant to divulge to the public at large. General Electric is
marketing variable speed, electronically commutated motors, targeting mass market
applications - appliances, air conditioning, and automotive - and currently have total
motor/drive system sales for all applications approaching 50,000 units annually
Applications range from 5 horsepower heat pump compressor drive motors to one half
horsepower indoor air blower motors for central air conditioning systems. The latter
(packaged one half horsepower, 1200 RPM motor and drive) are being sold to OEM
customers for approximately $125 each, with specific prices depending on volume and
other commercial arrangements (GE Motors, 1990). In Japan, PWM inverters are mass
produced for small "mini-split" heat pump applications, reportedly at a manufacturing
cost of the inverter of $25/horsepower (at average rated motor power output of about
11/2 horsepower) (Greenberg, 1988 p. 7). Others place the large volume purchase
direct material cost of the complete inverter in the Japanese mini-split heat pumps (the
major blocks shown in Figure 5-1 plus low frequency harmonic filtering on the input) at
approximately $70 per horsepower (GE Motors, 1990). With assembly costs, overhead,
and profit margin added in, full OEM pricing would be on the order of $100 to $125 per
horsepower.
It can be expected that the ultimate mass production OEM cost of a one quarter
horsepower motor and stator plus a drive on a printed circuit board (see Figure 5-2) will
be well under the $100 level. The cost of the motor will be similar to the cost of
premium efficiency single phase AC induction motors (Figure 3-2), on the order of $25
5-5
-------
Figure 5-4: Comparison of Variable Speed Drive/Motor Efficiencies (Fractional Horsepower)
100 -
90
U!
I
I
80
70
60
t
25 SO 75
Speed (Percent of Maximum)
Source: Arthur D. Little estimates based on data from references 5, 13-15.
100
varying with output in the same fashion as induction motors. The OEM cost of the
variable speed drive (either PWM inverter or ECPM drive with PWM speed control)
after several years of mass production can be expected to fall to between $25 and $40,
and be relatively insensitive to the power output level, as the control portion of the
circuit cost becomes a significant portion of the total. To reach this cost level requires a
very high level of integration of control circuitry.
The cost of the controller could be offset, to some extent, by operating the motor and
compressor at higher speeds, up to approximately 6000 - 7000 RPM, at nominal
capacity. Operating speeds of this magnitude are common practice for the variable
speed rotary compressors used in mini-split, room sized air conditioning systems, and
should be compatible with the smaller compressors used for R/Fs. This range of
operating speed has also been demonstrated in R/F capacity reciprocating compressors,
on a laboratory project basis (GE, 1990). In either case, the normal range of compressor
design optimization and durability issues wo.uld need to be addressed prior to
commercial production based on these higher speeds, but the higher speeds are clearly
feasible.
5-6
-------
The resulting reduction in compressor displacement and motor size would allow some
reduction of the cost of these components, on the order of $5 to $10 at the OEM cost
level. The variable speed drive and motor replace the standard induction motor whose
OEM cost is on the order of $ 15. The total of these offsetting cost reductions is on the
order of $20 - $25; the net increase in OEM component cost to the R/F manufacturer
associated with the VSD/motors, in mass production, can be expected to be on the order
or !|>25 - $30.
Total applied costs will include, in addition to motor and drive cost, the cost of the
refrigerator temperature controller, whose function is to determine the required
compressor speed and generate a speed control signal to the motor drive based on
cabinet interior temperatures and a control algorithm. These components replace the
mechanical thermostats used in current refrigerators. In production, the costs could be
expected to be comparable, with no net effect on the manufacturing cost or retail price
ot an R/F. ^
5.3 Variable Speed Drives - Life, Reliability
Domestic refrigerators typically operate for periods on the order of 15 years and do so
at a very high reliability level. Electronic variable speed drives have been available for
about two decades; drives technologically similar to todays state of the art drives have
been produced for well under 10 years. The latter time period is insufficient to establish
life and reliability relative to the 15 year expected lifetime of the refrigerator In
general, however, electronic variable speed drives have established a good reliability
track record, consistent with solid state electronics in general. Key to achieving
longevity is provision of adequate cooling for the power electronics components and
control of environmental factors such as moisture condensation.
Simply by virtue of adding another component having a finite possibility of failure the
addition of a variable speed drive might be expected to reduce the overall reliability of
the appliance. However, a number of factors related to the variable speed drive are
favorable to overall reliability:
Electromechanical contactors, such as the relay used to switch the start windings in a
conventional induction motor are replaced with the more reliable solid state
electronics of the variable speed drive.
Control circuits will tend to be exclusively highly reliable solid state devices
replacing the more failure prone electromechanical defrost timer, and thermostat.
The already high level of compressor mechanical reliability will be improved by
reduced frequency of stop-start cycles.
Overall, given the high reliability of solid state electronics, it is reasonable to expect that
variable speed drives could be used in the domestic refrigerator without significantly
impacting the overall reliability of the appliance.
5-7
-------
5.4 Description of Specific Variable Speed Motor/Drive Types
The following subsections present brief descriptions of the operating principle of each
of these motor types with a variable speed drive, with a brief summary of the current
commercial status of each of the three VSD-motor technologies.
5.4.1 Electronically Com mutated Permanent Magnet Rotor DC Motors
Electronically commutated permanent magnet rotor DC motors, also commonly referred
to as "brushless DC" motors, have a permanent magnet rotor and (usually) three sets of
stator windings. As the rotor rotates, the stator windings are commutated, i.e. switched
in phase with the permanent magnet poles on the rotor. To control commutation timing,
rotor position is sensed and fed back to the ECPM controller and used as the basis for
timing the switching of the current to the motor windings by the power transistors in the
controller. In many brushless DC motor product offerings, rotor position is sensed by
Hall effect sensors in the motor. An additional set of lead wires is required to connect
the sensors to the controller. An alternate technique, developed and used by GE, senses
the back EMF in the off winding, and commutates on an initial rise of the EMF. The
advantages of this technique are the obvious cost savings of eliminated Hall effect
sensors and lead wires and, for hermetic compressors, the reduction of the number of
wires (to three) that must penetrate the hermetic shell of the compressor, reducing cost
and improving reliability.
The basic operational characteristic is identical to a conventional brush-type permanent
magnet DC motor (speed is proportional to DC supply voltage, torque is proportional to
current). To provide for variable speed operation, the ECPM controller must vary the
DC supply voltage to the motor, as well as providing for correctly timed commutation.
Pulse width modulation is a cost effective and efficient means of varying the DC voltage
supplied to the motor (Figure 5-3), because the high speed PWM switching is done at
the same output transistors used for commutation. The effect of the high speed PWM
switching on the motor is a low level of ripple in the DC voltage and current to the
motor, which results in a modest increase in I2R losses (decreasing the motor efficiency
by on the order of one percent) in the stator windings.
The high efficiency of electronically commutated DC motors is attributable to two basic
attributes of the motor:
The permanent magnet rotor supplies the field, without requiring any input power, as
required with induction motors and wound field type DC motors.
The placement of the windings on the stator allows room for more winding wire
cross section, allowing lower I2R loss than in a typical brush type DC motor.
Permanent magnet materials that are used for ECPM rotors are:
' Ferrite;
Rare earth - cobalt (samarium-cobalt); and
Neodymium - Iron - Boron (e.g. "Magnequench")
5-8
-------
Ferrite magnets are relatively low in magnetic field strength and cost. The latter two
materials have much stronger fields, resulting in more compact motors and more power
output from a given size stator. The cost, however, is considerably higher, so that
currently the least costly motor uses a ferrite rotor. There is a general expectation that
the Neodymium - Iron - Boron material will decrease in cost to the point where it is cost
effective. Table 5-1 lists suppliers of electronically commutated DC motors.
Table 5-1: Manufacturers of ECPM Controllers
Manufacturer
General Electric
Emerson
Inland Motors
Magnetek
PMI
Minarik
EG&G
Fasco
Available
Size
(hp)
1/5 - 8 1/2
1
1/4
custom
custom
custom
custom
custom
Present Cost
(quantity 1)
250 - 300
1,700
Comments
in production
starting production
custom
custom
custom
custom
custom
custom
Source: Telephone conversations and/or product literature of the manufacturers listed.
5.4.2 Pulse Width Modulation Inverter/3 Phase Induction Motor
PWM inverters convert fixed frequency and voltage input electric power (e.g. 115 volt,
60 Hz, single phase residential electric power) into 3 phase variable frequency and
voltage output to an essentially conventional 3 phase induction motor. The PWM
inverter is a specific case of the "generic" variable speed drive of Figure 5-1, where the
PWM output of the output power transistors is switched to approximate the sinusoidal
voltage required to drive the induction motor efficiently. Figure 5-5 shows the voltage
and current waveforms that are typical of a PWM (with a relatively low PWM
frequency) inverter driving an induction motor.
In comparison with electronically commutated motors, PWM inverter driven, fractional
horsepower, induction motors are inherently limited to efficiencies roughly 10-15
percentage points lower. The major reasons for this are:
The induction motor maximum efficiency is a few percentage points lower than the
ECPM, for the reasons discussed in 5.4.1.
i
The PWM sinewave approximation (Figure 5-5) results in some degradation of the
motor frequency, because of ripple and harmonics associated with the PWM square
waves. In addition, the PWM frequency is generally on a fixed carrier that is not
precisely synchronized with the timing of the output waveform. This results in some
asymmetry to the waveform, adding additional harmonic content to the waveform.
The net result is lower motor efficiency, by 3 to 5% for high PWM frequency, and
by 5 to 10% for low PWM frequency.
5-9
-------
Figure 5-5: Voltage and Current Output of a PWM Inverter (approximate PWM frequency: 1.1 kHz)
VOLTS
CURRENT
V
Source: Toshiba product literature
"X
V
5-10
-------
Partially offsetting the efficiency difference is a small advantage in induction motor cost
in comparison with permanent magnet rotor motor cost, attributable to the difference in
materials cost between a squirrel cage rotor and a permanent magnet rotor.
PWM inverters are widely used in the U.S. for 3 phase motor speed control in industrial
applications, primarily in integral horsepower and larger output. Table 5-2 is a partial
listing of sources of PWM inverters and current selling prices, which reflect low volume
production, small lot sales requiring significant application engineering assistance, and
the inclusion of various control, fault detection, and motor protection features beyond
those required to simply run the motor.
Table 5-2: Manufacturers of PWM Inverters
Manufacturer
Toshiba
Emerson
Mitsubishi
Hitachi
Westinghouse
MagneTek
Lenze
Vee-Arc
Ranco
, Size
(hp)
1/4
1/2
1/2
3/4
1
3/4
1/4
3
3
Model
E Series
Prism
Freqrol-F2, K
VWE
AccuFlow JR
FHP 402
PWM 7030
Present Cost
(quantity 1 )
750
600:
550
585
610
I
1575
500
Comments
AC applications
AC applications,
starting production
Source: Telephone conversations and/or product literature of the manufacturers listed.
5.4.3 Six Step Inverter - 3 Phase Induction Motor Drives
Single phase voltage is first rectified and filtered, with SCR's in the rectifier to provide
a controlled, variable DC voltage. This voltage is then switched six times per cycle
(twice per phase times three phases) in order to emulate stepwise sine waves. The line
current will rise in an exponential manner and resemble a scalloped sound wave, since
motors are inductive devices. Typical voltage and current signals from a six step
inverter are shown in Figure 5-6.
Six step inverters are an essentially obsolete alternative to PWM inverters for driving
small, 3 phase induction motors at variable speeds. 10 to 15 years ago, when PWM
frequencies were limited to levels well under 1 KHz, the PWM approximation of a sine
wave was poor, and the higher harmonic content due to the fixed timing of the PWM
carrier was significant. At that time, the six-step technique, which is quite simple to
control and requires power transistor switching rates equal only to twice the motor
rotation frequency (times the number of motor pole pairs), were highly competitive with
PWM inverters with respect to performance and cost. As PWM frequencies have
increased and the control sophistication of PWM inverters has improved, the
competitive position of PWM has improved considerably.
5-11
-------
Figure 5-6: Six Step Inverter Voltage and Current Wave Forms
a) Voltage output of the three
Inverter phases
b) Motor connections
c) Voltage wave form acros
each winding
Source: Lloyd, 1982
d) Current wave form
each winding
Efficiencies for six step inverters are about the same as for PWM inverters, perhaps one
to two points lower. Efficiency of the drives decreases by one to two efficiency points
as the size of the drive decreases because the control signal losses remain about the
same. The output of these drives decreases the efficiency of the motor by about 10%
compared to across the line AC voltage because the output current is noisy, and the six
step waveform is only a crude approximation to a sinusoidal waveform and has
significant higher harmonic content.
Table 5-3 shows current manufacturers, sizes, efficiencies, and costs of six step
inverters. They are available from several sources, including Emerson, Lenze, and
Boston Gear. They are available in sizes as small as 1/4 horsepower. Most six step
inverters are marketed for fan, pump, and other industrial applications and provide
features that appeal to this market.
5-12
-------
Table 5-3: Manufacturers of Six Step Inverters
Manufacturer
Lenze
Emerson
Boston Gear
Graham
Size
(hp)
1/2
1/2
1/4
Model
(least power output)
611
Horizon I
AFA 21 00 C
custom
Present Cost
(quantity 1 )
470
3400
1,000
1,000
Source: Telephone conversations and/or product literature of the manufacturers listed.
5.4.4 Switched Reluctance Motors
Switched reluctance motors are a relatively new technology, enabled by continuing
improvements in power and control electronics technology. As shown in Figure 5-7, the
motor consists of a rotor and stator, each having a different number of equally spaced
discrete poles. The rotor consists of a stack of iron laminations with individual poles
formed as shown in the Figure. There are no conductor bars and therefore no induced
current in the rotor. The stator is formed from a stack of iron laminations, with
individual, wound poles. The rotor position is sensed by an encoder in the motor
housing and pulses of current are applied to the stator poles to match the rotor angle.
The pulses are timed, as indicated in the Figure, so that each stator pole is energized as a
rotor pole approaches it, and deenergized when the rotor pole is aligned to it. The
"generic" drive of Figure 5-2 is applicable, with PWM used to match the input voltage
optimally to the speed and torque of the motor.
Switched reluctance motors have several unique attributes that are potentially
advantageous, depending on the application:
Very high low speed torque;
j
The rotor uses no conductor bars or permanent magnets, and the potential cost of the
motor is correspondingly lower;
l
Overall motor/drive efficiency can approach ECPM motor/drive efficiency.
However, to realize high efficiency requires very precise timing of energizing and
deenergizing of the stator pole windings and very low loss (and high cost) lamination
materials, limiting the potential cost benefits.
I
These features are not the distinguishing advantages for a refrigerator compressor motor
that they are for vehicle traction drives or direct drives of washing machine agitators.
The technology was developed at Leeds and Nottingham Universities in England.
Switched Reluctance Drives, Ltd. in Leeds, England licenses the technology. No U.S
manufacturer is in production of SRM's, but several development programs have been
initiated in the U.S. SRM's are being investigated for use in electric vehicles for
traction drives, where the characteristics high torque at low speed, (as needed for good
acceleration), and good efficiency at higher speed and low torque (cruising conditions)
5-13
-------
Figure 5-7: Simplified Schematic of Switched Reluctance Motor Configuration
At rotor position and rotation direction shown:
Winding A has just been switched off
Winding B is off
Winding C was just switched on
of switched reluctance motors, as well as potentially lower motor costs, are key
advantages. The Japanese are currently developing this technology for use with "quiet"
washing machines. EPRI is also interested in this application; they are starting a project
with Whirlpool and Emerson. In general, this technology is at an earlier stage of
development and commercialization than the preceding three technologies.
5-14
-------
6.0 Potential Value of Improvements
This report has examined three areas of motor technology
Constant speed induction motors and the potential for their efficiency improvement
Two speed motor technology
Variable speed motor technology
The value of the efficiency improvements obtainable for single speed induction motors
are considered in the compressor technology report (Dieckmann, 1991). The
combination of increased cost and degraded efficiency associated with two speed
induction motors in the fractional horsepower ranges leaves this a irather unattractive
option for this application.
The potential system performance advantages of a variable speed compressor were
examined parametrically, at the DOE closed door energy test condition, for the "typical"
18 cu ft. automatic frost free refrigerator/freezer with a top mounted freezer, as
described in the Appendix. The cases considered were:
1993 standards level cabinet thermal performance, as a baseline, and a high
performance cabinet (with 30% lower thermal loads).
Current, fans and fan motors, and ECPM fans/motors. i
For the high performance cabinets, the basis for the single speed compressor capacity
and maximum variable speed capacity was the capacity of the compressor in the
typical 1993 R/F, reduced by half the decrease in cabinet load from current thermal
performance, thereby maintaining pulldown capability.
For each of the above cases, the performance with a single speed compressor was
compared with the performance with a current technology variable speed motor -
compressor and with a variable speed compressor having an efficient low speed
range. The efficiency vs. speed characteristic of current technology variable speed
drives was taken to be the upper, ECPM curve from Figure 5-4. The efficient low
speed range drive was assumed to have a system efficiency of 84% at the low speeds
where continuous steady state compressor operation would occur (30% of maximum
speed; an efficiency of 84% is comparable to the one half speed efficiency of current
technology drives).
Table 6-1 summarizes the energy savings for these cases that are attributable to
replacing the single speed compressor with a variable speed compressor (current
variable speed drive technology). The detailed underlying assumptions are presented in
the Appendix.
i
Table 6-2 summarizes the energy savings for these cases that are attributable to
replacing the single speed compressor with an efficient low speed variable speed
compressor, as described above. The detailed underlying assumptions are presented in
the Appendix.
6-1
-------
Table 6-1: Summary of Calculated Energy Savings with Variable Speed Compressor (Current
Variable Speed Motor/Drive Technology)
Cabinet
Typical 1993
Low Thermal
Loss w/Constant
Pulldown
Energy
Consumption Single
Speed kWh/yr
668
604
604
452
402
402
Fans
Fan A
FanB
FanC
Fan A
FanB
FanC
Energy Consumption kWh/yr
Variable Speed
665
510
485
479
351
329
Saving vs. Single
Speed
3
94
119
-27
51
73
Fan A Standard fans, 1 speed
Fan B ECPM fans, 1 speed
Fan C ECPM variable speed condenser and evaporator fans
Table 6-2: Summary of Calculated Energy Savings with Variable Speed Compressor (Efficient Low
Speed - Future Technology)
Cabinet
Typical 1993
Low Thermal
Loss w/Constant
Pulldown
Energy
Consumption Single
Speed kWh/yr
668
604
604
452
402
402
Fans
Fan A
Fan B
Fan C
Fan A
FanB
FanC
Energy Consumption kWh/yr
Variable Speed
638
486
464
460
334
313
Saving vs. Single
Speed
30
118
140
-8
68
89
Fan A Standard fans, 1 speed
Fan B ECPM fans, 1 speed
Fan C ECPM variable speed condenser and evaporator fans
The value of these energy savings is tabulated in Tables 6-3, and 6-4 assuming an
electric energy cost of 80 per kWh, for a range of real discount rates between 2 and 10
percent, over the projected 15 year life of the refrigerator. The discount rates cover a
range between real, after tax returns to savings accounts at the low end of the range, to
after inflation credit card interest rates at the high end of the range. For real discount
rates in the range that consumers would rationally choose for safe investments (at the
low end of the range), the present value of the saved energy is on the order of $75
greater than the incremental cost, at retail, of the variable speed drive. The efficient low
speed variable speed drive saves approximately 20 kWh/yr over current variable speed
drive technology; the present value of the savings is approximately $20.
6-2
-------
Iable,i6.~;?: Pl£f?nt yalue of Ener9V Savings of Variable to Speed Compressor (Current Variable
bpeed Motor/Drive Technology)
Cabinet
Typical 1 993
Low Loss
W/constant
pulldown
Fan
A
B
C
A
B
C
Savings kWh/yr
3
94
119
-27
51
73
Present Value for Discount
Rates
2%
3
98
124
53
76
5%
3
81
102
44
63
10%
2
60
76
32
46
. ~
Over 15 year life of appliance
Fan configuration: Table 6-1 footnotes
V 'Ue °f Ener9V SavingS °f Variable to sPeed Compressor (Efficient Low Speed
Cabinet
Typical 1 993
Low Loss
W/constant
pulldown
Fan
A
B
C
A
B
C
Savings kWh/yr
30
118
140
-8
68
89
Present Value for Discount
Rates
2%
31
123
146
71
93
5%
26
101
120
58
7(5
10%
19
75
89
43
57
Over 15 year life of appliance
Fan configuration: Table 6-1 footnotes
6-3
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7.0 Areas for R & D
R & D resources could be applied usefully in two broad areas in motor technology: the
development of higher efficiency induction motors for use in compressors, especially
smaller compressors; and improvements in low speed performance of variable speed
motors and drives while reducing costs.
The techniques for designing higher efficiency induction motors are well known in the
hermetic motor industry. The major issues involved from the industry viewpoint are
product engineering and marketing issues of cost vs. market size, and integration with
the compressor. Because the motor is an integral part of the compressor package, high
efficiency motor development is best carried out in the context of high efficiency
compressor development. This subject, therefore, is discussed in greater length in the
compressor technology report (Dieckmann, 1991) that was prepared under this program.
Early development of small, high efficiency compressors would provide important
support to current programs to demonstrate the performance potential of R/F's with dual
loop systems and/or super insulated cabinets.
The results of the performance and payback analysis discussed in the Appendix and in
Section 6, respectively, clearly indicate that significant energy savings can be obtained
with variable speed compressors and fans. However, the savings are limited by the
rapid degradation in variable speed drive (VSD) - motor efficiency below 50% of
maximum speed. At current projected VSD - motor costs, the present value of the
energy savings, at $0.08/kWh over the 15 year life of the R/F is only marginally greater
than the likely increase in retail cost that would be needed to cover the incremental cost
of the motor and drive. R&D efforts, therefore, should be targeted to:
Increase the low speed (1/4 to 1/3 of maximum) VSD - motor efficiency,
maximizing the savings that can be obtained with a variable speed compressor.
Further efforts to reduce potential mass production costs, through increased
component integration and utilization of advances in component technology.
Prototype development of a variable speed compressor based R/F could be pursued,
based on current technology, with detailed performance measurements providing a'
basis for further assessment of the potential of this approach and for identifying areas
for improvement.
7-1
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8.0 Conclusions
8.1 One Speed Motors (Present Practice)
2 pole AC squirrel cage induction motors running close to 3500 RPM and operating on
115 volts, 60 Hz, single phase AC are used universally in domestic refrigerator/freezers
and freezers. In the small capacity compressors (< 600 Btu/hr nominal capacity)
currently used in very small refrigerators, but potentially of interest for two
compressor/two evaporator system designs, low cost, low efficiency motors are
currently used.
Motors whose efficiency is very close (within 1 or 2 percentage points) to that of the
motors used in the best domestic refrigerator compressors, with outputs matching the
needs of smaller capacity compressors could be developed and manufactured fairly
readily. The costs would be considerably higher than that of presently used motors -
cost benefit analyses are needed to determine the optimum efficiency. The
cost-efficiency-power output data plotted in Figure 3-2 is reasonably accurate, and can
be used in these trade-offs.
8.2 Two Speed Motors
As with one speed motors, the technology to design and produce two speed motors is
well known. Two speed motors are inherently lower in efficiency than one speed
motors of comparable cost, on a comparable mass production basis -- by several
percentage points at maximum speed, and by close to 15 percentage points at half speed.
8.3 Variable Speed Drives
Inverter drives of induction motors and variable speed electronically commutated
motor/drives are both well developed and commercialized technologies for providing a
motor with continuously variable speed operation and control, although sales and
production in the U.S. have not yet reached mass production levels for any application
or class of applications. The latter is inherently higher in overall motor/drive efficiency
by close to 15 percentage points, for the reasons discussed in 5.4.1 and 5.4.2. In mass
produced configurations for domestic appliance applications, the costs of the electronic
drives can be expected to be equivalent, while the permanent magnet motor is slightly
higher in cost. The higher system efficiency of the electronically commuted motor and
drive is well worth the small differential in cost.
Currently there is no large scale mass production of either type drive in the U.S. GE is
marketing electronically commutated motors and drives targeted for mass market
applications - appliance and automotive. Total unit sales of GE ECPM motor/drive
systems are approaching 50,000 units annually, with rapid growth in unit sales coupled
with steady decreases in unit costs. Inverters are mass produced in Japan, for
applications such as small residential heat pumps. Manufacturing costs of these mass
produced inverters are on the order of $100 per horsepower (over a range of 1 to 3
horsepower). As U.S. production quantities increase, costs can be expected to fall
toward Japanese mass production cost levels.
8-1
-------
The analyses in the Appendix and Section 6 indicate that at current real energy prices,
the energy savings resulting from variable speed compressor operation are sufficient to
payback the likely retail price differential that would result from the incremental
manufacturing costs of applying a variable speed compressor to a typical R/F. The
payback period is long, however; 5 to 10 years. One factor limiting the energy savings
is the degradation of motor and system efficiency at the reduced speeds (about 1/4 to 1/3
of maximum) that would be typical of steady state operation.
-------
9.0 References
Americold, 1990 (January), private communication.
Arthur D. Little, Inc., 1990 (August), "Impact of a CFC Ban on the Cost and
Performance of Household Refrigerators, Centrifugal Chillers, and
Commercial/Industrial Refrigertion Systems," Technical Memorandum to U.S. DOE.
Baldor, 1989 (Novembef), private communication.
Cann, P., 1989 (November 28 - December 1), "Integrated Heat Pumps", Meeting
Customer Needs with Heat Pumps. Atlanta, GA., Sponsored by EPRL
Copeland, 1990 (January), private communication.
Dieckmann, J.T., 1991, State of the Art Survey of Hermetic Compressor Technology
Applicable to Domestic Refrigerator/Freezers, report to EPA Global Change Division,
prepared by Arthur D. Little, Inc., Cambridge, MA.
Electric Power Research Institute, 1987, "ASD Directory," Second Edition, Palo Alto,
California.
Emerson Electric, 1990 (January), private communication.
Federal Register, 1989a (February 7), pp. 6077 - 8.
Federal Register, 1989b (November 17), pp. 47916 - 47938.
GE Motors, 1990a (January 12), Telefax summarizing induction motor efficiencies.
GE Motors, 1990b (January), private communication.
Greenberg, S., et al., 1988. Technology Assessment: Adjustable Speed Motors and
Motor Drives (Residential and Commercial Sectors). LBL-25080.
Lloyd, J. D., 1982, Variable - Speed Compressor Motors operated on Inverters
AHSRAE Transactions V88, pt 1.
I
Lorenz, A., Meutzner, K., 1975, "On Application of Non-azeotropic Two-component
Refrigerants in Domestic Refrigerators and Home Freezers," IIR.
Lovins, A. B., et al., 1989 Edition (April), The State of the Art: Drivepower. Rocky
Mountain Institute, Snowmass, CO.
i
Smith, A.O., 1990 (January), private communication.
Sulfstede, L., 1989 (November 28 - December 1), "Residential Variable Speed
Technology." Meeting Customer Needs with Heat Pumps. Atlanta, GA., Sponsored by
EPRI.
9-1
-------
Toshiba(a) International Corporation, Houston, Texasn Toshiba Instruction and
Maintenance Manual - Tosvert - 130E Transistor Inverter. Bulletin No. IMVT130E230.
Toshiba(b) International Corporation, Houston, Texas, Toshiba Inverter Seminar (a 20
page brochure published by Toshiba).
9-2
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Appendix: Value of Variable Speed Motors for Reducing Cycling Losses, and
Otherwise Improving R/F Efficiency
One basic premise underlying the variable speed motor state of the art survey is that the
potential exists to obtain significant energy savings with an efficient variable speed
compressor. The primary sources of the savings are the elimination of losses associated
with on-off cycles, improved utilization of heat exchanger capacity, and, possibly,
savings in fan power if fans were operated at reduced speed (taking advantage of the
speed cubed fan power law) when the compressor speed is reduced. To examine this
basic premise, a series of comparative performance calculations were carried out, at the
standard DOE closed door energy test conditions, using the EPA refrigeration analysis
(ERA) model. As indicated in Table A-l, the main points of comparison are:
Constant speed vs. continuously variable speed (e.g. cyclic operation vs. continuous)
Effect of (importance of) fan efficiency,
For a variable speed compressor, constant speed ECPM fans vs. variable speed
ECPM fans operated at reduced speed and substantially reduced power.
Table A-1: Matrix of Constant Speed vs. Variable Speed Cases
Cabinet
Baseline
High Performance
(Thermal load 2/3 of baseline)
Fans w/Single Speed
Standard Efficiency
ECPM, 1 Speed
ECPM, 1 speed
Standard Efficiency
ECPM, 1 Speed
ECPM, 1 Speed
Fans v//Variable Speed
Standard Efficiency
ECPM, 1 Speed
ECPM, Variable Speed
Standard Efficiency
ECPM, 1 Speed
ECPM, Variable Speed
To obtain meaningful results on the comparative effects of variable speed and different
fan efficiency levels, the following assumptions were followed for all cases:
I
The baseline R/F is an 18 cubic foot refrigerator freezer with automatic defrost and a
top mounted freezer, having freezer wall insulation 2 3/8 inch thick and fresh food
compartment insulation 1 7/8 inch thick (R value 8.0 °F/in per Btu/hr-ft2). The
annual DOE test energy use is approximately 20 kWh/yr over the 1993 standard
level (668 kWh/year vs. a 1993 standard level of 690 kWh/year). The freezer
volume is 4.6 cubic feet and the refrigerator volume is 13.4 cubic feet; the adjusted
volume is 20.9 cubic feet.
The baseline compressor has a nominal EER of 5.3 and a nominal capacity of 865
Btu/hr. i
Losses associated with on-off cycling of the constant speed compressor are assumed
to add 2% to the compressor input power at a 50 percent duty cycle.
A-1
-------
The ECPM compressor motor has about a 13% efficiency advantage over the
baseline unit, including electronics losses, at constant speed.
The current technology variable speed motor and drive system efficiency-speed
relationship is assumed to be as shown in Figure 5-4 for electronically commutated
permanent magnet rotor (ECPM) motors. At maximum speed about 1/3 of the losses
(3% of the total input power) occur in the drive electronics, and the remainder (7%
of the total input power) occur in the motor. At lower speeds, a higher proportion of
the losses occur in the electronics.
The "efficient low speed" variable speed drive is assumed to have an overall system
efficiency of 84% at 25% to 30% of maximum speed, comparable to the half speed
efficiency of current technology variable speed drives. 25% to 33% of maximum
speed is the steady state speed range that results when the compressor is sized to
provide pulldown performance comparable to current R/F. This level of low speed
variable speed drive performance is not currently available, but could potentially be
achieved through R&D.
At 30% speed, assumed to be the lowest practical speed that will ensure proper
lubrication, the compressor motor drops efficiency 10% from the maximum speed
value, including electronics losses.
The baseline steady state cabinet loads are 134 Btu/hr (fresh food compartment) and
135 Btu/hr (freezer), including the effect of mullion heaters and controls, but not the
power input to the evaporator fan, which varies with duty cycle and fan efficiency.
The baseline condenser fan moves 90 ACFM through the condenser, and its motor
consumes 12 watts.
The baseline evaporator fan moves 50 ACFM through the evaporator, and its motor
consumes 9.4 watts.
ECPM constant speed fans are assumed to move the same air flow as the baseline,
with 3.6 watts power input to the motor.
The effect of variable speed refrigerant mass flow and fan air flow on heat exchanger
performance is estimated using the heat exchanger routines within ERA.
The "high performance" cabinet is assumed to have 70% of the steady state
conduction load as the baseline cabinet. This level of cabinet performance,
combined with refrigeration cycle improvements, would result in energy
consumption approaching the DOE "Level 5". The design measures to achieve this
reduction aren't specified for this study, but could include thicker walls, improved
door gaskets, carbon black insulation, and reduced anti-sweat heat. Pulldown
requirements are assumed to be identical. Therefore, the nominal compressor
capacity (maximum capacity for variable speed) is taken to be 85% of the baseline
cabinet compressor capacity.
A-2
-------
Fan powers and air flow rates are unsealed, i.e., equal to the baseline cabinet case.
Tables A-2 and A-3 summarize the results of the efficiency comparison, for the baseline
cabinet and for the high performance cabinet. The major observations relative to these
results are:
A substantial energy saving (10%) can be obtained by replacing the inefficient
fan/motor of typical current practice with a modest efficiency fan and motor.
With the standard efficiency fan/motor, the variable speed drive operating at steady
state does not result in energy savings since the energy associated with 100% fan run
time more than offsets the compressor power savings.
With ECPM fans, the VSD compressor results in a substantial net energy savings.
A limitation on the compressor energy savings is the declining variable speed motor
system efficiency below half speed (using current technology). The last three cases
of Tables A-2 and A-3 are based on an assumed variable speed motor drive system
efficiency of 84% at 30% of nominal speed (vs. 90% at maximum speed and 84% at
one half speed, with current technology). For this improved level of low speed
motor performance, energy use is reduced by approximately 25 kWh/year over the
comparable, current variable speed technology case.
The same general observations apply to the high performance cabinet, with increased
sensitivity of the variable speed options to the fan efficiency.
A-3
-------
Table A-2: Calculated Results - Standard Cabinets
Case
Baseline
Constant Speed and
ECPM Fans
ECPM at Full Speed,
Standard Fans
ECPM at Full Speed,
ECPM Fans
Variable Speed and
Standard Fans
Variable Speed
ECPM Fans
Variable Speed
Variable sp. Fans(4)
Efficient Low Speed
Variable Speed/Std
Fans
Efficient Low Speed
Variable Speed and
ECPM Fans
Efficient Low Speed
Variable Speed and
ECPM Fans
Fan Watts
Evap.
9.4
3.6
9.4
3.6
9.4
3.6
2.0
9.4
3.6
2.0
Cond.
12.0
3.6
12.0
3.6
12.0
3.6
2.0
12.0
3.6
2.0
Cabinet
Load Inc.
Fan BtuH
280
273
280
273
299
279
274
299
279
274
Duty Cycle
% Speed
.38
.37
.37
.36
.98
30%
.90
30
.91
30%
.97
30%
.90
30%
.90
30%
Energy Inputs, kWh/day
Comp.'2'
1.49
1.44
1.28
1.25
1.17
1.09
1.09
1.10
1.03
1.03
Fans
.20
.06
.19
.06
.50
.16
.09
.50
.16
.09
Aux.(3>
.15
.15
.15
.15
.15
.15
.15
.15
.15
.15
Total
1.83
1.66
1.62
1.46
1.82
1.40
1.33
1.75
1.33
1.27
Total
Energy
kWh/yr
668
604
591
531
665
510
485
638
486
464
(t) Base cabinet load except fans = 268 Btu/hr
(2) Incl. 2% for on-off cycling loss constant speed
(3) Aux: controls, defrost, anti-sweat heaters
(4) Fans at 83% speed, 83% air flow
A-4
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Table A-3: Calculated Results - High Performance Cabinet
Case
Baseline
Constant Speed and
ECPM Fans
ECPM at Full Speed,
Standard Fans
ECPM at Full Speed,
ECPM Fans
Variable Speed and
Standard Fans
Variable Speed
ECPM Fans
Variable Speed
Variable sp. Fans'5'
Efficient Low Speed
Variable Speed and
Standard Fans
Efficient Low Speed
Variable Speed and
ECPM Fans
Efficient Low Speed
Variable Speed and
Variable Speed Fans
Fan Watts
Evap
9.4
3.6
9.4
3.6
9.4
3.6
2.0
9.4
3.6
2.0
Cond.
12.0
3.6
12.0
3.6
12.0
3.6
2.0
12.0
3.6
2.0
Cabinet'1'
Load Inci.
Fan, BtuH
200
194
200
194
216
199
195
216
199
195
Duty Cycle
% Speed'2'
.30
.29
.30
.29
.81
30%
.74
30%
.74
30%
.80
30%
.74
30%
.73
30%
Energy Inputs, kWh/day
Comp.'3
1.01
0.98
.87
.84
.82
.76
.76
.78
.72
.71
Fans.
.16
.05
.15
.05
.42
.13
.07
.41
.13
.07
Aux.(4)
.07
.07
.07
.07
.07
.07
.07
.07
.07
.07
Total
1.24
1.10
1.10
.97
1.31
.96
.90
1.26
.92
.86
Total
Energy
kWh/yr
452
402
400
353
479
351
329
460
334
313
(1) Base cabinet load less fans = 190 Btu/hr
(2) Baseline case compressor capacity 735 Btu/hr
(3) Inci 2% for on-off cycle losses of 1 speed compressor
(4) Auxiliary: controls, defrost, anti-sweat heaters
(5) Fans at 83% speed, 83% air flow
A-5
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