Dr. Andy Judge
DRS Power Technologies, Inc. (DRS PTI)
Permanent Magnet Axial Field Air Core (PAAC) Motors for
Naval Applications
ABSTRACT
Permanent magnet machines (PMM’s) offer
significant advantages in power density, torque
density, and efficiency compared to traditional
systems. In a specific type of PMM, Permanent
Magnent Axial Field Air Core (PAAC) systems,
the traditional iron stator core is replaced with a
Printed Wiring Board (PWB) and the flux path
is in an axial direction. The PWB stator uses a
mature, automated process, requiring minimal
upfront tooling costs whether quantities are in
initial prototypes or in mass production. When
length constraints drive available space, axial
field designs offer the best choice for form
factor. PAAC technology provides some
benefits in applications driven by the typical
demands for PMM’s, but where additional
benefits of low cost and superior acoustic
performance are required. Efficiencies of PAAC
systems are comparable to other PMMs, but
there are some challenges with cooling and some
limitations in power density and torque density
compared to iron core PMM’s. DRS PTI, with a
history of successful permanent magnet motor
and generator military program experience, has
designed and developed PAAC systems for
multiple Naval projects, and will present their
design details and benefits.
INTRODUCTION
PMM’s offer numerous advantages compared to
traditional field wound electromechanical
conversion systems. In traditional field wound
systems, a magnetic field is generated by
powering a set of stator coils wrapped around a
set of iron poles. This, in turn, excites a second
set of energized coils wrapped around a second
set of iron poles on the rotor, producing torque /
RPM in motor applications. For generator
applications, torque and RPM in addition to the
supplied electrical power produce voltage and
current. In both cases, electrical power is
transmitted from the rotating components
through slip rings, commutators, or “brushes”.
Traditional field wound systems suffer from
numerous inherent inefficiencies. The copper in
both the rotor and the stator have losses equal to
the square of the current times the resistance of
the wire. Slip rings suffer from frictional losses,
and are a source of wear and breakdowns.
Leakage flux causes “stray load losses”, and
occurs in both the rotor and the stator in
traditional systems. The iron in both the rotor
and the stator suffer from “eddy current losses”
which result from changing electrical fields
introducing a parasitic perpendicular reactionary
electromagnetic effect in the magnetic medium.
Eddy current losses can be reduced by providing
resistance to the perpendicular electromagnetic
flow by using laminated insulated magnetic
materials as opposed to solid conductive
structures, but these losses cannot be eliminated.
Compared to these systems, PMM’s offer the
following benefits [1]:
Higher efficiency. PMM’s do not experience
rotor copper losses and have fewer stray load
losses, with much lower rotor iron losses, which
generate 40-50% of the inefficiencies of
traditional induction machines.
Higher reliability. With a permanent existing
magnetic field inherent in the magnets, PMM’s
do not require “brushes” or slip rings to transfer
power by linking rotating electrical wires to a
stationary connection. These connections are a
constant source of wear / fatigue failures in
traditional electric motors and alternators.
Higher power density. Permanent Magnets,
especially high performance materials such as
Neodymium/Iron/Boron, have exceptionally
high magnetic flux densities, and can generate
higher magnetic fields for a given size,
improving power density.
tolerance stack-up
up and has greater displacement
in dynamic events.
Lower Size and Weight.. Replacing rotor iron
laminations, slip rings, copper windings, and
related components with a permanent magnet
system offers reductions in weight and size
compared to traditional architectures of
equivalent power and torque. This carries
additional benefits in reliability,, as lower
weight in the rotor translates to lower stress in
rotor dynamics.
High stall, starting, and low speed torque
torque.
Another benefit of the high magnetic flux
densities of high performance permanent magnet
materials is torque in starting / stall conditions,
and in the low speed range. High speed / low
torque performance can be achieved in PMM
systems through flux weakening or other means.
Flexible Geometry. As discussed in the next
section, PMM’s offer flexibility
bility in terms of form
factor, with various architectures, including
radial flux and axial flux available to meet the
needs of a given space claim.
The three most common types of motor /
generator PMM’s are axial flux and radial flux1.
In conventional PMM systems,, the magnets are
attached directly to the drive shaft,, as shown in
Figure 1, or embedded in a rotor steel
lamination stack attached
ched to the shaft
shaft, and are on
the inside of the windings. The embedded
lamination stack style, known as “IPM style
style,” is
a popular commercial choice, the most well
known application being the Toyota Prius hybrid
electric drive motor [2].
]. The IPM style offers a
low cost system to retain the magnets in high
speed applications and enables block shape
magnets, which are low in cost and “off the
shelf” compared to the custom arc shapes
required for a constant air gap in rotor surface
mounted magnetic designs. Interior magnets can
also provide a smaller air gap compared to an
exterior rotor systems, described in the next
section, as the magnet “cup” adds to the
1
Transverse-flux
flux and other less common / more exotic
designs of course exist, but, in order to limit the scope of
this report, will not be discussed in detail here. Also, this
paper assumes the windings and related iron laminations
are stationary, while the magnets rotate, although the
reverse is possible.
Figure 1: Conventional PMM (Magnets on the
Interior)
In exterior radial flux motors, or “cup type
rotor” systems, an example of which is shown in
Figure 2,, the magnets are located outside of the
windings. Designing the magnets on the exterior
of the stator allows the radius of the magnets to
be larger than the stator by the air gap distance,
distance
offering a larger surface area for the magnets
compared to interior rotors.. This provides
advantages in power density in the diametric
form factor. Also, as shown in Figure 1, for
interior styles of PMM, the windings are
encased by the stator back-iron.
iron. This leads to
challenges in winding, as automated tools, or
even hand insertion techniques, must navigate
through the limited space in the interior of the
stator. With outward facing stator poles, as
shown in Figure 2,, exterior rotors suffer from no
such limitations in space for windings
wind
and
related tooling, which provides advantages in
ease of manufacturing.
Stator
(One on each side of rotor in this [PA]
configuration)
ed
ent
Ori lux
y
l
F
ia l
Ax netic
g
a
M
Rotor Disk with Magnets
Figure 3: Axial Flux PMM
PAAC Motors
Figure 2: Exterior Magnet PMM
In an axial flux PMM, disc-shaped permanent
magnet layers rotate past stationary poles and
windings. Axial flux PMM’s are often referred
to informally as “pancake motors” due to their
distinctive shape. These designs tend to allow a
very small air gap, as the centripetal acceleration
acts in the perpendicular direction of the air gap,
resulting in no radial strain of air gap
components and magnet retention systems of
radial systems contribute to the rotor-stator gap.
The small air gap results in potentially high
efficiencies, reaching up to 98% in specialized
applications [3]. However, one should note that
gap size is limited to manufacturing tolerances,
structural requirements, dynamic events, and
magnetic attractive forces. Figure 3 shows an
example of an axial flux PMM, the DRS PA44,
a 450 HP system initially designed and
developed for commercial applications in the oil
and gas industry.
In traditional PMM designs, high grade
electrical steel is used to guide and channel the
magnetic flux to effectively couple the magnets
with the stator windings. However, stator iron is
a source of loss, and not a necessary component
of the electromagnetic circuit. An inspired
marrying of the axial-flux configuration and
high grade magnets has resulted in the DRS
Permanent magnet, Axial-flux, Air Core
(PAAC) motor.
The DRS PAAC motor, shown in Figure 4, uses
high strength permanent magnets in a dual rotor
configuration to transmit magnetic flux across a
large gap containing a printed wiring board
(PWB).
rotor assembly
PWB stator
Figure 4 – Exploded view of PAAC motor
Figure 5 shows an edgewise view of the
rotor/stator/rotor configuration of the PAAC
motor. Physically, the rotor assemblies are
typically constructed with magnet pole pieces in
pockets milled into the rotor back iron. Rotor
back iron material is typically thick magnetic
steel, designed to be rigid and resist the bending
and warping forces of the high energy magnets.
This construction of the rotor assembly has
proven to be structurally sound and robust
enough to pass MIL-S-901D high impact shock
and MIL-STD-167-1 vibration testing. With its
simple rotor design, featuring only a flat disk
with embedded magnets, accurate balancing can
be achieved in an easy, straightforward fashion.
Figure 5 – PAAC Rotor / Flux Path
PWB STATOR CONSTRUCTION
Traditional stator designs, used in either other
permanent magnet machine architectures or
“wound field” systems, feature copper wires
wound around laminated stacks of iron poles.
This system is high in weight and presents
numerous challenges in manufacturing, driving
up the cost of the system. Windings must either
be hand wound, as shown in Figure 6, incurring
large recurring costs in assembly, or be
automated, requiring a large capital expenditure.
The iron poles suffer from losses related to the
“eddy currents” inherent to the changing
magnetic field, adding to the inefficiency of the
machine and requiring additional cooling. As
discussed, in the introduction, to lower the eddy
current losses, these poles are manufactured as
laminated stacks of thin steel sheets, incurring
additional costs in manufacturing and assembly.
Wires must be wrapped around these
laminations, which can chafe and damage the
wires, causing thermal failure of the insulation,
leading to system failure.
Figure 6 – Traditional Radial Field Stator
Manufacturing
The DRS PAAC stator departs from the
traditional iron core and winding configuration
with its printed wiring board stator design,
shown in Figure 7. This printed wiring board is
constructed using standard automated PWB
equipment, which leverages the tremendous
investments made by the military and consumer
electronics industry towards making the process
reliable, repeatable, and routinely available
worldwide. The PWB manufacturing process
offers the accuracy and precision of the state-ofthe-art technology for a high quality product that
can be cost effectively produced in small or
large quantities. These processes offer the
following benefits:
•
Error-free automation for a proven
process
•
Inherent connections of windings and
phases, as opposed to the manual
processes of stripping, crimping,
soldering, etc., in the manual assembly
process of traditional systems
•
Elimination of sharp edged sheet metal
laminations, which can nick wires,
damaging insulation systems and create
electrical shorts
•
Quality assurance checking of the PWB
can be carried out by a simple resistance
and inductance procedure
•
Effective environmental sealing from
standard PWB conformal coating
•
No slot-end
end turn interface, which causes
major thermal and structura
structural challenges
in conventional systems
required, radial flux systems are typically the
superior option.
Traditionally, scaling the PAAC motor for
higher power applications was thought to be a
challenge. However, recent progress has been
made in this area. Multiple PWB / rotor systems
can exist in a single
le motor casing in a “stack of
pancakes” design, providing integer multiples of
torque and power outputs for a given diameter.
Also, a 3.3 MW PAAC system has been
developed for Wind Power Generation while
operating at 13.35 RPM.
Efficiency
Figure 7 – PAAC PWB stator
The PWB has motor coil windings that are
connected to the external electronic drive. Motor
action is achieved with the electronic drive
energizing the motor phases to set up a stator
rotating magnetic field and the magnetic field of
the rotor assemblies interacting with this stator
field.
PAAC APPLICATION
CONSIDERATIONS
Choosing the right architecture for a PMM is a
critical first step in motor / generator design.
While PAAC systems have
ave proven to be the
right choice for numerous applications, in both
Naval applications through DRS and in
commercial products ranging from power tools
to wind turbine generators,, numerous design
considerations must be taken into account.
Size, Torque Density
ity and Power Density
In general, performance of a Permanent Magnet
Machine can be given by:
Output Power = Input Power - Windage Loss Wire Resistance Loss - Bearing Friction Loss Eddy Current Loss - Other Electromagnetic Loss
(stray load losses, saturation effects, etc.)
For a generator
or application, output power is
voltage times current, while input power is
torque times rotational velocity. For a motor
application, the terms are reversed, and input
power is voltage times current, while output
power is torque times rotational velocity.
Efficiency can be defined by input power
divided by output power.
Although the system reluctance increases, by
b
eliminating the stator back iron, eddy current
losses in the stator
tator iron are also eliminated,
eliminated and
there is no saturation in the stator.
stator Also, with no
stator iron between the twin rotor magnets, air
gap magnetic permeability is near constant,
which eliminates magnet heating and rotor back
iron eddy current losses.
PAAC motors are limited by the current that can
be driven through the PWB, while traditional
motor PMM systems offer greater current
density, using low gauge wiring or formed wires
coupled with stator iron. This leads to higher
power density and torque density in traditional
PMM systems.
Offsetting the gains from fewer loss terms is the
wire resistance loss in the PWB. Copper traces
in the circuit board are smaller in cross sectional
area than formed wire in iron core stator, which
leads to increased “I2R” losses. Windage losses
are also greater in the dual rotor design, although
they typically account for a small percentage
percen
of
the total system losses.
As an axial flux system, PAAC systems are the
best choice when the available space claim lends
itself to high ratios of diameter to length. When
a small diameter / long length space claim is
Overall, the various factors combine to make the
efficiency of the system dependent on the
specifics of the design and application.
However, typically, the efficiency of PAAC
motors is superior to traditional field wound
systems, and is comparable to other PMM
architectures.
Cooling
Cooling is also a major consideration in PAAC
systems. Although the high efficiency systems
require less cooling than traditional systems,
effective heat removal from the PWB can be a
challenge. Small power systems, up to 15 HP,
are typically effectively cooled by conduction to
the chassis, and natural convection / radiation to
the ambient environment. In medium powered
applications, in the 15 to 100 HP range, internal
forced convection becomes a requirement. High
power systems, above 100 HP, typically require
liquid cooling or active control of ambient
environmental conditions for acceptable thermal
performance.
Cogging Torque
Note that with no iron stator poles,
electromagnetic cogging torque is eliminated.
Cogging torque in traditional systems occurs
when stator poles are attracted to rotor magnets.
The air between the various poles, shown in
Figure 8, with its magnetic permeability several
orders of magnitude below the steel, is seen as
“bumps in the road” to smooth movement of the
rotor. This creates efficiency losses and results
in a source of noise in the system.
Although there are several methods to reduce
cogging torque, such as skewing the stator stack
or magnets, shaping the rotor magnets for a nonconstant air gap, etc., these techniques come
with efficiency penalties, and do not completely
eliminate the issue. PAAC motors, as shown in
Figure 5, feature a constant gap magnetic
permeability, completely eliminating
electromagnetic cogging torque.
Cost
Cost is, of course, a consideration for motor /
generator selection. PMM’s can compare
unfavorably to traditional systems for initial
cost. However, the efficiency advantages and
lower weight for mobile applications, providing
savings in fuel costs, can often make PMM’s the
more cost effective solution for total cost of
ownership. Compared to other PMM’s, PAAC
systems are typically more cost effective in
small to medium production volumes, as the
automated PWB stator process comes with very
small labor costs and small capital investments
for tooling. However, in very high production
volumes for smaller motors, where stator
winding automation can be spread across tens of
thousands of units, traditional PMM
architectures can become the lower cost option.
For larger systems, above 100 kW, winding
automation becomes more of a challenge.
DRS ACTIVE NAVAL PAAC
PROGRAMS
PAAC motors are being implemented on
numerous naval programs, significantly reducing
the risk (and cost) of implementation of this type
of system for new development projects.
PAAC-25
Figure 8 – Cogging Torque
PAAC25-1 started as a commercial motor that
was developed by Core Motion of Ronan, MT.
Core Motion is the Patent holder of the PWB
stator technology, and DRS has an exclusive
license for U.S. Navy shipboard applications.
Core Motion’s motor was originally designed as
a stackable machine in which additional PWB
stators and permanent magnet rotors could be
added for additional horsepower. Core Motion’s
line ranged from a single PWB design
(PAAC25-1, shown in Figure 9), capable of
generating 6 HP continuously at 1800 RPM, to a
4 PWB design (PAAC25-4) capable of
generating 25 HP continuously at 1800 RPM.
DRS applied a “COTS Ruggedization”
approach, modifying the motor as required to
meet Naval requirements. With the DRS
changes, the PAAC-25-1, 13 inches in diameter
and 5.1” in axial length, has successfully
completed shock, vibration, and environmental
qualification testing. It is over 90% efficient at
rated load, and weighs approximately 70
pounds, including mounting frame. Delivery of
production units for a Naval application is
scheduled for Q3 2012.
designed for 3.75 hp at 3300 RPM, with > 90%
efficiency at full load. It is 10.5 inches in
diameter with a 6.125 inch axial length, and
weighs less than 45 lbs. The first PAAC19-1
prototype has been built and will undergo
qualification testing this spring.
Figure 10 – PAAC 19-1
CONCLUSIONS
Figure 9 – PAAC 25-1
AFCA
As part of a DRS corporate level program, DRS
is developing an Advanced Fan Coil Assembly
(AFCA) for Naval applications. At a system
level, the AFCA offers energy savings with a
more energy efficient HVAC module, which is
estimated to save $300k/ship/year in energy
costs for a DDG 51 class vessel. The new
system will also provide considerable
installation and maintenance savings due to its
variable speed direct drive design, which
eliminates belts, pulleys and separately mounted
motor controllers. Across the entire shipboard
HVAC system, the weight savings granted by
AFCA over the previous system add up to over 7
tons, while offering 30% space savings and 7%
increased cooling capacity.
At the heart of the AFCA System is DRS PTI’s
PAAC 19-1 motor, shown in Figure 10, which is
DRS Power Technologies, with a history of
successful programs for permanent magnet
motors, has designed and constructed numerous
PAAC (Permanent Magnet Axial Field Air
Core) motors for Naval applications. These
applications demanded the axial field form
factor and have demonstrated improvements in
cost effectiveness and electromagnetic cogging
torque compared to traditional motors, or even
other styles of permanent magnet motors.
REFERENCES / BIBLIOGRAPHY
[1] J.R. Hendershot and T.J.E. Miller. (2010)
“Design of Brushless Permanent-Magnet
Motors”, Magna Physics Publishing and
Clarendon Press.
[2] J. S. Hsu, C. W. Ayers, C. L. Coomer.
(2004) Report on Toyota Prius Motor Design
and Manufacturing Assessment, Oak Ridge
National Laboratory, ORNL/TM-2004/137.
[3] Jacek Gieras, Rong-Jie Wang, Maarten
Kamper. (2008) “Axial Flux Permanent Magnet
Brushless Machines”, Springer.
AUTHOR BIO
Dr. Andy Judge, Ph.D., is the Engineering
Manager at DRS PTI. At DRS, he is responsible
leading teams in the design and development of
permanent magnet machines. He has more than
15 years of mechanical engineering experience
including Computer Aided Design and
Engineering, FEA, CFD, Electronics Packaging,
and Permanent Magnet Machines. Recently, Dr.
Judge completed his PhD at Worcester
Polytechnic Institute, with his dissertation
focused on using ferrofluids to improve
performance in permanent magnet machines.