2003-01-0443
Kalathur
S.Narasimhan
Hoeganaes Corporation, Cinnaminson,
New Jersey 08077
Thomas J. Miller
Chicago Powder Metals Products Co.,Schilier Park,1160176
Copyright@ 2003 SAE International
ABSTRACT
Volt system would increase the power (Watts)
availability, since W = EI, where E is the voltage and I is
The 42-Volt electrical system is being introduced in
automobiles to provide the extra power needed for
various electromagnetic devices. These paper discuses
the opportunity offered by the 42Volt for powder metal
parts and the challenges. Major opportunities are in
motors. A brief discussion of motors and the
performance requirements for the magnetic core material
used is included. Brushless motor design can benefit the
most from insulated iron powder compacts because of
the design simplicity of powder metal parts and three
dimensional flux capability which is most beneficial in
rotatingdevices.
-
1995
INTRODUCTION
The demand for fuel efficiency, pollution reduction and
creature comforts is forcing the automotive companies,
to evaluate alternate electrical systems. The most
popular choice to date is the introduction of a 42-volt
system, which uses 3 batteries in tandem with 14 volts of
peak power. The current 14-volt battery system power is
about 3000 watts, which will be exceeded by the
electroflic gadgets planned for future introduction in most
vehicles (See Figure 1). Further refinements in power
electronics may extend the wattage available using the
current 14-volt system. In a few years, the 42-volt
system may be the only option available.
The
introduction of a 42-volt system opens up an opportunity
for powder metal parts in small motors. This article
discusses the performance of powder metal parts in
various motors.
the
Figure
~:
Several
Projected
2005
V.a,
advancements
automotive
2010
are
power
in progress
2015
to
needs.
Implement 42 Volt system by the automobile companies
throughout the world. The packaging of the wires and
harness become smaller as the voltage increases
amperes of current decreases. The availability of higher
power allows several electromagnetic options which
increase fuel efficiency Major improvements are listed
below:
1 Alternator efficiency improvement can improve fuel
efficiency by 16%. An integrated starter alternator
(ISA) which connects directly to crankshaft between
engine and transmission achieves this. ISA can
accelerate engine rpm to idle speed before fuel
combustion starts, thus eliminating incomplete
combustion at idle speed. The ISA is an opportunity
for powder metal insulated iron product. This is
discussed further later on in this paper.
BACKGROUND
Figure 1 shows the limit of the existing 14-volt system
and possible extensions that could be made with
advancements in power electronics. Large cars would
exhaust the available power by the year 2005. The 42
current.
2000
2
Electric power steering will improve the fuel
efficiency by 6%. Permanent magnet motors
powered by the battery achieve this, These are
brush less D.C. motors.
Electromagnetic valvetrain (EMV) for
greater control over emissions and engine
performance.(P/M)
3. Electric water pumps. Will improve fuel efficiency by
2%. These are also electric motors that could be
fabricated using P/M. The power for the pump
comes from the battery and not from the engine.
4
Electror:nagnetic valve operation will improve fuel
efficiency by 12%. These are valves that can be
programmed to open and close and eliminate cam
lobes and camshaft. It is believed that the
electromagnetic option can provide infinite cam
profile possibility. This will have a profound impact
on P/M parts.
-
Electric water pumps with no power take off from
-
engine.(P/M)
EGR
and
-
actuation.(P/M)
Catalyst heaters for emission reduction at start
-
up.
Ignition systems and fuel systems.(P/M)
turbocharger
control
and
Chassis Systems
-
Electric power steering (EPS) "By Wire."(P/M)
IMPACT ON P/M
-
Electromechanical
The following is a discussion on the individual areas of
an automobile that can benefit from the 42-Volt system.
P/M is shown in parenthesis to indicate an opportunity or
a threat.
-
Wire."(P/M)
Active and adaptive suspension systems.(P/M)
DC/DC
-
Lighting(P/M)
36 V Battery Power Storage
-
Integrated Starter Alternator (ISA) (P/M)
-
Regenerative Braking with Reduced Break Wear
-
ISA Assisted Acceleration
Converters,
Inverters
for
14
(EMB)
"By
Thermal Systems
-
Electric air conditioning compressors with no
power take off from engine.(P/M)
-
HVAC blower motors, electrical heaters, and
electrical engine cooling fans.(P/M)
-
ijPreheat catalytic converters to work at operating
temperature for' emission control.
Power Generation and Storage
-
braking
Volt
Other Systems
-
Electric motors, wiper motors, solenoids, lighting,
and video systems. (P/M)
THREATS TO P/M
Power Distribution
-
Wires / Wiring Harness of Smaller Size and
Weight Because Amps Decrease with Voltage
Increase
The following is a list of possible negative impact that the
42-volt system would have on current P/M applications:
-
Connectors, Switches and Relays of Smaller
Size and Weight
.
Loss of P/M mechanical take off components such
as pulleys and sprockets.
-
Fuses
and
Circuit
Semiconductors
-
Loss of starter motors and alternators which contain
various P/M components.
-
Provisions for Dual and Third Voltage
.
Loss of the reverse gear in transmissions.
.
Since air conditioning unit can be operated by HVAC
motors and this will be outside the engine
compartment plastic gears which are traditionally
temperature sensitive can now become a threat to
P/~.
Protection
Devices,
Powertrain Systems
Three types of ISA motors are possible with
power take off from either in line or are belt
driven.
Induction - High efficiency with smoother
torque and is an established technology.
2
Permanent Magnet - Most efficient because
it does not require an external field
excitation.(P/M)
3
Switched Reluctance: Scalable power
output with no generated heat issues and
NVHproblems(P/M)
A
MAJOR ISSUES WITH 42VOLT SYSTEM:
At the present time arcing is a major issue with 42-Volt
system. Arcs can be self-sustaining and damaging.
Several efforts are underway to contain this. The supply
base for the components of the 42-Volt system is slowly
evolving. Electromagnetic valve closure is not reliable at
the present time. Corrosion of the connectors gets
accelerated with higher voltage. Several infrastructure
issues need to be resolved. These are issues currently
being addressed and will be resolved sometime in the
future. While all the projected devices might not happen
either due to NVH issues or performance issues, these
need to be continuously monitored.
OPPORTUNITIES FOR THE POWDERED METAL
INDUSTRY:
The major opportunities lie in the motors using either
insulated iron powders or sintered pole pieces. Since this
is an area of great interest for P/M we discuss below the
various types of motors and how P/M can benefit.
Electric motors transform electrical energy into
mechanical energy in the form of rotating shaft power.
Motors consist of four different components - stator, rotor
(armature), enclosure and shaft.
The armature is
mounted on the shaft. Operationally, the essential part
of an electric motor is the stator and armature. The stator
is made from a stack of lamination steel stamped to a
specific tooth profile, which has windings that carry
current, which induce a magnetic field on the steel.
These windings can be replaced by a permanent magnet
in certain cases. The rotor or armature also is made of
stamped lamination steel attached on a shaft with
windings. Again, permanent magnets can be mounted
on a rotor shaft directly to provide the magnetic flux and
the windings can be eliminated(1).
lamination steel and this field is opposite to the applied
field. Due to a variety of technical reasons, separation of
hysteresis and eddy current losses in a clear distinct way
is not preferred. In stead, total core loss is generally
monitored. Another important distinction that needs to
be made is that motors and transformers behave
differently. In motors, the core magnetic material is
subjected to rotating magnetic fields, which alternate in
the magnetic direction and also changes direction in
SDacewhereas in a transformer the field is stationarY.
Hysteresis losses are inherent in the processing of
lamination steel and by processing control are kept to a
minimum. The eddy current losses are reduced to a
minimum by making the electrical steel as thin as
possibl~ and isolating the individual laminations with a
coating to prevent electrical contact between the
laminations.
SOFT MAGNETIC P/M MATERIALS
The development of the powder metal process (2) for
replacing lamination steel was inspired by the fact that
core loss can be confined to an individual particle in a
compact as long as these particles are electrically
isolated by a coating. (3.4.5)
There are four general types of motors:
Synchronous motors - Alternating current (A.C.) is
applied to one winding and direct current (D.C.) to
the other field, generally the rotor.
2
3.
4.
D.C. motors - A D.C. field is applied to both rotor
and stator windings.
Induction Motors - A.C. is applied to the stator
winding and by induction to the rotor windings.
A.C. series motors - A.C. is applied to both windings,
which are connected in series.
There are number of different variations of the above
which lead -sub-classifications of innumerable number of
motor types.
The efficiency of electric motors is defined in terms of
conversion of input power to output power. There are a
number of losses that occur within the motors that result
in wastage of input power. Factors such as friction and
winding and magnetic core losses, which are, termed as
no-load losses. 12Rlosses from the stator and rotor and
stray losses due to leakage fluxes are termed as load
losses. Every motor design optimizes the efficiency,
taking into consideration ease of production, costs etc.
The losses experienced by the lamination steel (often
called electrical steel) during the magnetization by the
alternating field consists of eddy current losses and
hysterisis losses. Eddy current losses arise from the
induced field generated by the circulating current within
Figure 2: Fluidized bed powder coating process
..
~
Figure 3: Comparison of lamination steel and insulated
powder processing routes.
A powder coating process was developed to individually
coat the iron particles to maintain electrical isolation
.
between the particles after they are compacted into a
shape. The insulated iron powder compact will behave
like stacked lamination steel.
The coating process is shown in Figure 2. The coating
thickness can be varied precisely by the control of the
concentration of the plastic solution. These coated iron
powders are used successfully in ignition coils.
Powder metal has the potential to replace lamination
steel used in the stator or rotor. The advantage of
powder metal to lamination steel is shown in Figure 3.
The figure shows that manufacturing simplicity is the
best advantage that PIM offers. For instance, the powder
metal process eliminates stamping, stacking, welding
and lends itself to three-dimensional structures and can
eliminate several assembly steps.
The characteristics of the several-coated powders
manufactured by Hoeganaes Corporation are shown in
Table I. the processing of these powders into compacts
require warm compaction. The powders are heated and
fed into a die at elevated temperature. This process
allows extremely high green strength to be reached.
Green strength of nearly 276 MPa (40,000 psi) can be
achieved using this process route.
Table
Grades
Warm Compacted Insulated Iron Powder
1,000
10,000
100,000
1,OOO,<XXJ
Frequency
(~)
Figure 4:
iron.
Permeability of warm compacted insulated
100000
10000
1000
~
~
.,
.9
8
100
10
0.1
i
0.01
100
1,000 .
10,000
Frequency
100,000
1,000,000
(Hz)
Figure 5: Core loss of warm compacted insulated iron.
MOTOR APPLICATIONS
Further advances have been made to eliminate the need
for warm compaction and achieve comparable properties
shown in Table I.
However, the maximum green
strength of only 104 MPa (15,000 psi) is achievable by
this technique. The new grades are shown in Table II.
The frequency response of permeability and core loss is
shown in figures 4 and 5. Insulated iron compacts
exceed core loss performance of lamination steels at
higher frequencies. The constant permeability over a
frequency range has certain design benefits as well.
Table II: Newly Developed Insulated Iron Powder
Grades
FOR P/M MATERIALS
Various motor designs for the applicability of insulated
iron powders were evaluated. The basic premise is to
replace
the stacked
laminations
with compacted
insulated iron, called soft magnetic composite.
The
following
sections
outline
potential
P/M
motor
applications.
SWITCH RELUCTANCE MOTOR
The basic design of switched reluctance motor is shown
in figure 6. (6) This has a salient pole structure on both
its rotor and stator, but is only singly excited with phase
windings on the stator. A shaft encoder is installed in
order to synchronize the phase currents with rotor's
angular position. It is inherent in the design of this type
of motor that there be different numbers of poles on the
stator ~nd rotor. The greatest torque per ampere of
current is achieved if ttle following criteria for the core
material are met:
A large unsaturated aligned inductance achieved
using the smallest air gap, the widest poles and the
highest possible permeability.
A
small
unaligned
inductance
achieved
by
implementing design parameters such as a large
intprnnl::lr
rntnr ::Ir,.,
~
The highest possible saturation flux density
pole structure. Claw poles are stamped lamination steel
and welded together.
Using a 6:4 combination, the motor parameters (note in
the figure above the combination is 12:8) will be:
Number of poles on stator (Ns) = 6
Number of poles on rotor (Nr) 4
For a three-phase (q) current total number of steps per
revolution would be
12
qNr
For a motor of 4000 rpm (n = 67 rev/sec) the required
switching frequency of current in the phases is
268 Hz
nNr
The field frequency in the rotor is
nN"
402Hz
The insulated iron with better frequency response than
lamination steel will be a good candidate for this
application.
However, the output power of the motor is
closely related to the maximum permeability, which is
1500 for M19 lamination steel, and 500 for insulated iron
compacts.
However, the manufacturing simplicity of
insulated iron compacts makes P/M attractive for this
application even with the lower permeability.
-. -tJ~
, -
~
,"'",
'_1:
Figure 7: Claw pole motor with permanent magnet rotor
Using the conventional two-phase excitation sequence,
the fields in the stator and rotor pole sections cycle
between lower limits when the teeth are unaligned and
upper limits when they are aligned. A typical stepper
motor rtlay have 50 rotor teeth yielding 200 steps per
revolution with a stroke of 1.8 degree per step. At 480
rpm, n=8 rev/sec, 1600 steps/sec, the required switching
frequency is 800 Hz. The field in the rotor and stator
teeth cycles up and down between its minimum and
maximum at a frequency of 400 Hz. Insulated iron
compacts are likely to compete for these applications in
these types of motors due to their simplicity of
manufacture and their better frequency response.
~PIII.I
.n
ft..
,.. ..
..
Figure 8: Claw pole motor with permanent magnet rotci
~
.-
~
II
..
A.C. INOUCTION MOTORS
The A.C. induction motor has a very simple and rugged
80M
!8DH
STEPPER MOTOR
Stepper motors usually contain permanent magnets to
provide the bias field, which simplifies the excitation
windings. The rotor contains permanent magnets and is
in a known angular position, and subsequently pulsed to
another known position. The simplest version of this is
"can stack" motor.
Bonded neodymium-iron-boron
magnets on the rotor are magnetized into a multiple pole
array. These poles interact with two separate phases,
each of which is a single coil contained within a claw-
construction and is favored for a wide range of industrial
and consumer applications. The major limitation is that
its synchronous speed is determined by the A.C. supply
frequency. Induction motors use conventional windings
on the rotor or a squirrel cage rotor, which are
conducting bars shorted together at the ends by
conducting rings. These bars are embedded in the slots
in the rotor, which is a stack of lamination steel. These
lamination steels can be replaced by compacted
insulated iron powder. However, the magnetic material
properties most significantly influence the current
required to establish the flux in the magnetic circuit and
the induced m.m.f. These are represented by the
permeability and saturation induction of the core
material. Insulted iron compacts are not ideally suited for
these motors unless additional supply current is made
available to compensate for the excessive magnetizing
current.
PERMANENT MAGNET D.C. MOTOR (PMDC)
High saturation flux density and capacity to carry flux
circumferentially
PMDC motors provide the best opportunity for insulated
iron compacts. Permanent magnet motors offer better
overall performance than induction motors if the output
power is below 10kW. The basic structure of PMDC
motor is shown in figure 9. (6)
Saturation flux density and the capacity
magnetic field radially across the teeth.
to carry
LoJ core loss to provide higher efficiency machine.
High thermal conductivity to dissipate heat and allow
greater electrical loading.
~
J
Figure 9: PMDC motor layout,
In this D.C. motor, the commutator windings on the rotor
are excited
through
brushes.
There are several
disadvantages of these brushed D.C. motors also called
D.C. commutator motors:
Radio frequency interference and spark associated
with the brushes.
The accumulation of brush dust.
For comparison with the saturation flux density of
compacted
insulated
powders,
the
measured
characteristic provided for M 19 lamination steel does not
extend to the field levels that are typically used in
electrical machines. The published curve indicates that
a flux density of 13 kG may be sustained without
requiring any significant driving m.m.f. For the adoption
of insulated iron powders in a brush less D.C. motor, the
saturation flux density must not be an issue, for the
following two reasons:
1
The cross-sectional area of the stator core would
have to be increased and the overall diameter of
the motor will grow to carry the same flux.
2
The cross sectional area of the stator teeth would
have to be increased to carry the same flux
resulting in less available slot area, therefore the
overall diameter must increase or the electric
lo~ding must be .reduced to compensate for this
change.
Brush friction.
The armature has poor conduction path to dissipate
heat from the armature winding.
The space required for commutator and brush gear.
Most new D.C. motors use the brushless version wherein
the commutation is achieved by sensors located in the
stator. The essential feature of brushless motor is:
Permanent magnets on the rotor.
Field winding coils on the stator.
Electronic commutation.
Sensor coils for switching (Hall or optical sensors).
Transistors replace brushes and-commutator bars,
None of these are attractive scenarios for the insulated
iron powders, so the powder compact components
should operate at essentially the same flux density as
M19 laminations - it will just take more m.m.f. To drive
flux through these components.
The use of compacted iron powder is therefore a good fit
with the brushless D.C. motor, since its inferior
permeability (compared to lamination steel) can be
overcome simply by providing additional m.m.f. from the
permanent magnets and the trend in modern grades of
neodymium-iron-boron magnets is towards increasingly
higher coercivities. The m.m.f. can be increased either
by adopting a higher coercivity permanent magnet
material, or by increasing the magnet's length.
Linear control of speed and torque.
BRUSH LESS D.C. MOTOR DESIGN EXAMPLE
Higher efficiency over wide speed range.
A typical brush less D.C. motor will have two, four or six
permanent magnet poles. In a 2-pole machine, the flux
in the stator core will fluctuate at one cycle per revolution
of the rotor. Because of the simple, well-balanced rotor
construction, output speeds may be very high, frequently
well over 10,000 rpm.
However, the most common
applications have more modest speeds. 3600 rpm will be
considered, so that the insulated iron test data can be
utilized. Hence n = 60 rpm, and the field in the stator
core and teeth will cycle at a frequency of 60 Hz.
Insulted iron compacts can fit very well for this type of
motor application. The rotor does not experience any
measurable eddy current or hysteresis loses as the high
coercivity
magnetic
materials provide enough field
strength and are not influenced by armature reaction.
The stator could be solid steel. The stator experiences
alternating magnetic fields due both to the rotating
magnet assembly and switched phase currents in the
armature winding. The magnetic material for the stator
core should possess:
The test performed on M 19 laminations, compacted
SC120 and LCM (LCM are advanced insulated iron
powders designed to provide low core loss) powders
show that the core losses in all of these materials are
substantial
due to hysteresis,
with just a small
contribution from eddy-currents.
Each was tested at 60
Hz, and it is apparent that the core loss per unit
frequency (in units of watts/lb/cycle) is approximately
proportional to the excitation field (in Oersted).
The
constants of proportionality are shown in Table III.
Table III: Proportionality
materials
constants for insulated iron
-
0.0010
0.0022
0.0006
To achieve a flux density of 13 kG in a stator made from
each of these materials, requires different excitation
fields; which are approximately as shown in table IV; the
resulting core loss at 60 Hz is also given.
Table IV: Core loss and flux density for insulated iron
materials
Table V: Core loss and flux density for insulated iron
materials
These losses may represent a loss in motor efficiency of
-1 % using compacted LCM powder, and of -2% using
SC-120 powder.
The efficiency loss may not be significant considering the
simplicity of powder metal process and the three
dimensional flux capability of insulated iron compacts.
Lamination steels are processed to maximize flux in the
plane of the sheet. No such limitation exist with insulated
iron.compacts.
Unique Mobility developed a brushless motor replacing
stator made of Ni-Fe laminations with insulated iron
powder compact. (7) The total cost was reduced
significantly with a very small compromise in
performance. The stator segments are shown in figure
10.
Novel B~shl.essDC Stator Design"
.Se(jl)~r,
-;J'J" ,'\";
--
1 jn~./11:2.~n:IHl!lghr
-(j""1~.
.
While compacted
LCM powder offers a significant
improvement in core loss over SC-120 powder, LCM's
much poorer permeability requires a considerably higher
excitation to establish the operating flux, and this
substantially negates the core loss advantage. While the
final choice between SC-120 and LCM would be
determined by a detailed design study on an actual
motor, the view is that this data favors the use of
compacted SC-120.
It is felt that while m.m.f.
in a
brush less D.C. motor can be increased either by
adopting a- higher coercivity magnet material or by
increasing the magnet's length, this should be done to
the least extent possible in order to minimize the
additional cost in the permanent magnet materials. This
means that there will actually be a core loss penalty in
exciting compacted insulated powders to the requisite
flux levels, but it is felt this can be managed in the
detailed design of an actual motor using these materials.
For example, a typical fractional horsepower brush less
D.C. motor may have a stator core mass of 225 g (0.5
Ib), in which case the core losses at 13 kG and 60 Hz will
be as shown in Table V:
""..
FUr..Sra1or
..
-
J2r30"'.AicSegm9r1tA
-3S~~m~n:$High
-~7
inqh
:17on:! IJ
-3.2indh:21.on:rOD
'1~"i;jilH.~j*'ilv-"""1.~","~ "f,~rl(jj.lJI~
Figure10: Novel DC Stator Design using segments of
compacted insulated iron.
40
35
Nil'e Laminajitjn
30
.'
~25
~
~20:
~!
&15
.:.
!
10
5, .
0:",
0
1000
2000
."'-'
3000
4000
'A
5000
6000
7000
Snned
!fr'm )
..v"'"
.UniqueMobility-SAE
TechnicalPap&r93..1008
Figure 11: Power vs. Speed for the novel Unique
mobility motor
The segments shown in figure10 .are assembled and
tested in motor and the performance of the core material
is shown in fig11 and 12.
600
2. For brief review of Powder Metallurgy see "Recent
Advances in Ferrous Powder Metallurgy," K.S.
Narasimhan, Advanced Performance Materials, 3,727 (1996), Kluwer Academic Publishers, Netherland!
3. H.G. Rutz, F .G. Hanejko, US Patent #5,063,011
(NoJ5, 1991)
-'
NtFeLammation
500
~ 4'00
4. H.G.Rutz, C. Oliver, F.G. Hanejko ,B. Quin, US
Patent #5,268,140 (Dec7,1993)
".
,$
-..0300
~
J.
eo
ot.- 200
5. C.G. Oliverand H.G. Rutz "Powder Metallurgy in
Electronic Applications," Advances in Powder
Metallurgy and Particulate Materials Vol. 3, part 11
pp. 87-102,1995.
100
0
0
"UniqueMobility
1000
2000
3000
4000
5000
6000
7000
Speed(rpm)
-SAETechnicaiPaper93.1000
Figure 12: Torque vs. Speed for the novel Unique
Mobility Motor.
These are few of the examples among a variety of motor
designs that are being developed with insulated iron
powder compacts.
These materials offer designer
unrestricted options.
CONCLUSION
A brief review of impact of 42Volt system on powder
metal parts is presented. The major opportunity is in
motors and hence a detailed discussion is provided. The
core magnetic material performance requirements are
discussed. _Insulated iron powder developed to reduce
assembly costs and increase design flexibility provide
cost effective alternate to conventional lamination stacks
in brushless D.C. motor. Opportunity also exists for
variety of other motors including switch reluctance
motors for 42V electrical system. The threats to P/M are
also presented. There are several major hurdles that
need to be overcome in the implementation of 42 Volt
and these advancements need a constant scrutiny.
ACKNOWLEDGEMENTS
Thanks are due to Mike Marucci, Fran Hanejko and the
marketing department of Hoegana~s for valuable input
for this presentation.
Special thanks to Dr. Peter
Campbell of Princeton Electro technology Inc. for a
number of discussions and evaluations on the insulated
iron compacts.
REFERENCES
The information on 42 volt was gathered from number of
discussions with automobile manufacturers and SAE
journal articJes.
Permanent magnet motor technology, Design and
Applications, Jacek F. Gieras and Mitchell Wing,
Marcel Dekker Inc.NewYork, 1997.
6. For a good review on motor operation see "Basic
Motor Theory Operation and Applications" William H
Yeadon, SMMA 20th Exposition, October 8,1991.
7. Huang H., Debruzzi M., Riso T.,"A Novel Stator
Construction for Higher Power Density and High
Efficiency Permanent Magnet Brushless DC motors"
SAE technical paper No.931008.