EVALUATING PARALLEL PATH MAGNETIC TECHNOLOGYTM
(“PPMTTM”) MOTORS AND CONVENTIONAL MOTORS BY
THEIR PERFORMANCE CURVES
AND
THE PPMT VALUE PROPOSITION FOR VARIABLE SPEED AND/OR
VARIABLE LOAD SYSTEMS
C JOSEPH FLYNN AND PATRICK J PIPER
QM POWER
PRELIMINARY REPORT NOT FOR RELEASE
1. Summary
PPMT motors have significant power, efficiency, reliability and cost advantages over
conventional motor alternatives for applications that operate under varying speeds or
loads.
Table 1
Higher Power Density
Higher Efficiency
Higher Reliability
Lower Cost
PPMT Comparative Advantage over Conventional
Motors in Variable Speed and Loading Applications
Average
Minimum/Maximum
27%
0-100%+
75-109%
0-400%+
Higher
Same - Higher
10% Less
0-50% Less
Applications that typically operate under variable speeds and/or loads include but are not
limited to power tools, electric or hybrid-electric vehicles (including passenger cars,
heavy, medium and lightweight trucks, buses, other fleet vehicles, golf carts, ATVs,
marine, bikes, forklifts, locomotives and the like), non-vehicle industrial applications
(conveyors and servo motors), robotics, toys and variable speed appliances.
2. Background
One of the most common misperceptions regarding motors today is their actual operating
efficiency. It is not uncommon for a lay person or even an engineer to state that the
efficiency of motors is already in the upper 80% to lower 90% range, implying minimal
need or capacity for further optimization. Most motor manufacturers advertise their
motors by peak efficiency and peak power. However, with conventional motors, these
numbers do not coincide in application. For instance a 1 hp motor with a “peak”
efficiency of 84% may be less than 68% efficient at 1 hp. There appears to be a common
lack of understanding of motor performance especially “peak” versus “average”
efficiency and/or power in applications that have varying speeds and loads.
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In reality there is much to improve upon over the current state of the art motors. Our
objective is to provide a basic understanding of motor curves and the mathematical
constraints of these curves and present the PPMT motor solution for improving
performance in applications with varying speed and load requirements.
3. Scope
The scope of this paper is confined to comparing PPMT motors to permanent magnet and
field wound DC motors, as they are typically the most commonly used conventional
motors utilized in variable speed or load applications. PPMT also offers a value
proposition over other types of motors, to which some reference will be made but will not
be fully analyzed; this will be done in separate papers. We will define mathematically
what constrains a motor by analyzing the motor’s performance curves. This analysis will
be in simple terms, however the underlying math can be quite complex. It is not our
intention to mathematically analyze motor design parameters, which would be beyond the
scope and intention of this paper.
Since the general shape of the performance curves of Permanent Magnet DC (“PM-DC”)
and PPMT motors remain essentially the same, regardless of motor size, this is a
scientifically valid study with regards to predicting performance trends from smaller
applications like power tools to larger applications like electric vehicles.
4. The Torque vs. Speed And Power Out Curve
The torque vs. speed curve defines the constraints of the motor’s magnetic circuit. All of
the other performance curves are derivatives of this curve, especially the more important
performance curves, Power Out and Efficiency. A conventional motor’s Torque vs.
Speed, Efficiency and Power Out curve is shown in figure 1 and a PPMT Torque vs.
Speed, Efficiency, and Power Out curve is shown in figure 2. Each of these two curves
sets will have the same basic shape, whether they are scaled for a motor producing a few
watts or thousands of horsepower.
In order to conduct accurate performance comparisons of various sizes of motors (ie –
independent of scale), the curves are normalized to a percent of each of the motor’s
maximum values for each parameter. This does not change the curve shapes or motor
characteristics.
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Figure 1
Figure 2
(Power Out is normalized to Power In)
All PM-DC and PPMT motors have these basic shapes for all horsepower ratings.
Power out is a mathematical derivative of torque and speed and is given by:
Equation 1
Powerout
Where
Powerout
Torque
Speed
9.55
=
=
=
=
TORQUE⋅ SPEED
9.55
Mechanical Shaft Power [watts]
Shaft Torque [Newton Meters]
Shaft Speed [RPM]
Constant accounting for units
Motors that have a linear speed torque relationship, such as conventional PM and most
wound field DC motors will, by mathematical definition, always produce a parabolic
Power Out curve, like the one shown in figure 1. Any PM-DC motor curve that has a
different shape must be showing only a portion of the motor’s true power profile.
Motors that have a hyperbolic speed torque relationship will produce an essentially
rectangular Power Out curve, like the one shown in figure 2. To our knowledge, PPMT
is the only permanent magnet motor technology that produces a hyperbolic speed torque
curve.
The main advantage of PM-DC motors, most wound field DC motors and PPMT motors,
over standard AC-induction motors, is the ability to easily vary the motor speed by
controlling the input power. Varying the speed however does not change the torque to
speed characteristic or their mathematical relationships to Power Out. Regardless of
what value the maximum speed actually is, the performance curves will still be identical
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to those shown in figures 1 and 2. More detailed information on the shape of the Speed
Torque curve can be found in Appendix A.
In applications that have time varying loads and speeds, particularly where the variations
in speed and load cannot be predicted, such as operating electric or hybrid electric vehicle
drives over unknown terrain, one needs to consider the average power delivered by the
motor over its entire operating range.
Even though peak Power Out occurs at 50% of the maximum speed for both types of
motors, the average power delivered over the entire operating range by the PPMT motor
is about 27% greater than that delivered by the conventional PM motor. The average
power for each motor is shown in figures 1 and 2 and is given by:
Equation 2
∑ powerout i
average power
i
i
range
The average power delivered by the conventional motor over its operating range is 66.8%
and the average power delivered by the PPMT motor over its operating range is 84.7%
(84.7/66.8=1.27). At the edges of the load curve the difference is even more dramatic.
At low rpm, a PM-DC motor may only be at 40% of peak power, where as a PPMT
motor, at the same rpm, would be at 82% of peak power (more than 2x increase in
power). This is a significant advantage for applications such as power tool, robotic
actuation, and electric vehicle applications that have a wide operating range of loads and
speeds and frequently accelerate from zero rpm under load (ie – acceleration tests from 060mph).
The relatively low average power of a PM-DC motor over its operating range and low
‘low end’ torque is why many electric and hybrid electric vehicles use 3 phase induction
motors even though they require more costly variable frequency inverters to convert dc
power to 3 phase ac power. The down side to using 3 phase induction motors is they are
synchronous machines where speed is controlled by varying the frequency of the ac.
Induction machines can only be optimized to a narrow range of frequencies and when
operated outside this frequency range their efficiency drops dramatically.
PPMT motors avoid the need to sacrifice efficiency to obtain additional power. A PPMT
motor, with its high average output power, high ‘low end’ torque and high peak and
average efficiencies offers a true dc power solution for all applications requiring a wide
operating range of load and speeds.
5. Efficiency Demystified
Efficiency typically has a higher value to the end customer in applications that require
using a limited power supply (ie – a battery) or for those using significant energy (ie -
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24/7 high HP industrial). Motors servicing applications with higher efficiency value
propositions typically publish “peak” efficiencies between 60-90%. Most PPMT motors
have measured peak efficiency in excess of 95-98%. However, in order to understand the
very important role “Average Efficiency” plays in variable speed and load applications,
we have assumed for the balance of this analysis that both the conventional and PPMT
motors have the same peak efficiency. A 94% “Peak Efficiency” was chosen as the basis
for comparison at it was the highest published peak efficiency we were able to identify in
the market for conventional PM-DC motors.
The graphs in figures 1 and 2 are related to graphs captured by a dynamometer where the
measurement starts with a motor operating at no load speed and then the dynamometer
loads the motor until it reaches zero rpm. (Normal testing standard)
Analysis: Table 2
Parameter
PM**
94%
57%
54%
65%
67%
34%
22%
Peak Efficiency*
Average Efficiency
Where Peak Efficiency Occurs as Percent of Peak Power Out
Efficiency at Peak Power Out
Average Power Out as Percent of Peak Power Out
Peak Power Out as Percent of Maximum Power In
Low End Torque (at 20%)
PPMT
94%
76%
79%
83%
85%
89%
68%
PPMT
Comparative
Advantage
33%
28%
27%
162%
209%
*Baseline for comparison, identical peak efficiencies
**PM and most wound field DC
Calculations for Analysis:
Average Efficiency, Equation 4
∑ Effii
average effi
i
range
Average Power Out as Percent of Peak Power Out, Equation 5
percent




∑ powerout i 
i
range


powerout peak
5
Peak Power Out as Percent of Maximum Power In, Equation 6
percent
powerout peak
powerin max
Equalities
Both conventional PM and PPMT motors share these characteristics.
a. Power In is inversely proportional to speed vs. torque offset by ‘overhead’
b. Peak power and peak efficiency can never mathematically coincide
c. Efficiency drops as a function of the increasing distance between Power Out
and Power In, point A in figures 1 and 2.
6. Normal Operating Ranges For Variable Speed & Load Applications
In this section we will examine the advantage of PPMT motors over the specific
operational range of targeted motor users. Figures 3 and 4, show power and efficiency
curves for conventional PM-DC and PPMT motors with the operational ranges of
selected applications noted.
Figure 3
Figure 4
(Power Out is normalized to Power In)
Zero RPM to Peak Power Range
Many variable speed and load applications such as electric power tools and robotics
operate from zero rpm to peak power as shown in figures 3 and 4. This is in the lowest
motor efficiency range. A comparison of peak and average efficiencies between
conventional PM motors and PPMT motors operating in this range is shown in table 3.
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Table 3
Parameter
PM
PPMT
Efficiency at Peak Power
Average Efficiency Over 0 RPM
to Peak Power Operating Range
65%
33%
83%
69%
PPMT
Comparative
Advantage
27%
109%
Zero RPM to 75% Of Entire Operating Range
Other variable speed and load applications such as traction drives for electric vehicles,
industrial machines and similar applications operate from zero rpm to about 75% of the
motor’s entire operating range as shown in figures 3 and 4. A comparison of peak and
average efficiencies between conventional PM motors and PPMT motors operating in
this range is shown in table 4.
Table 4
Parameter
Peak Efficiency At 75% of
Maximum Speed
Average Efficiency Over 75%
Speed Operating Range
PM
PPMT
83%
86%
PPMT
Comparative
Advantage
4%
44%
77%
75%
Motor Operation At or Around Peak Efficiency
Normally the only applications that operate at or around the Peak efficiency are constant
load constant speed applications. Note that these calculations are for a conventional
motor that has the same efficiency as a PPMT motor. This would be a much more
expensive best of class PM-DC motor. In the vast preponderance of cases, the PPMT
motor starts with a 10-30% peak efficiency advantage over a PM-DC motor. At low
RPM, a BLDC motor could be operating at 10-15% efficiency, whereas the PPMT motor
would still be operating in excess of 60% or a 4x+ comparative advantage. Thus, the
actual PPMT comparative advantage in most cases is substantially greater than the value
shown in this analysis.
7. Reliability
PPMT motors use the same materials, coils and magnets and a similar rotor design as
seen in conventional permanent magnet motors. All of the increased performance is
associated with the design and arrangement of the magnetic circuit. As a result, there
should not be any new reliability issues associated with a PPMT motor. In fact, PPMT
motors are actually more reliable than brush DC motors, due to lack of brush contacts and
most likely more reliable than brushless PM-DC over longer periods of time, due to the
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fact that they do not have magnets in their rotors, which can fail over time due to
excessive centrifugal force and bond failure.
8. Cost
A PPMT motor can be produced at the same or less cost than existing motor
technologies. This is largely due to the design of the PPMT motor and recent
developments that have lowered the costs of the magnets and electronic microcontrollers
required to operate the motors. A more detailed component cost analysis is attached in
Appendix B.
Since a PPMT motor has no active components (magnets or coils) in its rotor, its design
can be simplified and its cost reduced. In addition, since a PPMT motor can operate near
its peak efficiency at its peak power, it can often be much smaller than its competitors,
which often have to be oversized by as much as three times to a much higher peak power
but then operate at a lower “targeted” power to take advantage of the higher efficiency
available at that part of the curve. As a result, the material costs can be as much as 50%+
less than that of a conventional motor in a given power class. This concept will be
further discussed in the White Paper on Fixed Speed Applications where the trade off is
more often a requirement for existing competitive solutions.
A further cost advantage of the PPMT design is a minimal cooling requirement. Since all
active components of a PPMT motor are at the periphery of the motor, they are easily air
cooled at minimal cost. This contrasts to brush DC motors, which must cool the rotor
field coils (via active cooling or oversized components). High performance brushless DC
motors also require rotor cooling, to prevent the permanent magnets from exceeding their
maximum use temperature.
PPMT’s high performance at low cost is partially made possible by recent advances in
high performance components. The strongest magnets available are Neodymium magnets
which have recently come off patent protection and fallen by about 90% in price in the
last few years. In addition, continuing advances in electronics (Moore’s Law) have
allowed for the construction of cheaper, smaller, faster, more precise and more
sophisticated microcontrollers.
Finally, it is also important to remember a motor’s efficiency greatly impacts its
operating cost and the total system cost of the end user. In heavy use applications a
motor may consume 5-10 times more energy in dollar terms than it costs to purchase the
motor up front. As a result, its efficiency is a huge cost driver for the end user. In some
applications, the efficiency advantages of PPMT could pay for the motor in less than one
year via power grid electric rates alone. In battery-powered applications, PPMT extreme
efficiency could substantially reduce the system cost by reducing the total required
battery watt-hours for a given application.
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Appendix A: How Back Electromotive Force [BEMF] Defines the Speed Torque
Curve
BEMF determines the form of the torque vs. speed characteristic, whether torque and
speed have linear, hyperbolic or some other mathematical relationship.
A conventional PM-DC motor will produce a ‘generator effect’ that opposes the input
voltage. The amount of the opposing voltage produced by the ‘generator effect’ will be
equal to the amount of voltage the motor would produce when operated as a generator at
any given speed.
The ‘generator effect’ or Back EMF in a conventional PM motor is proportional to motor
speed. The voltage available across a motor phase [coil] at a given speed is equal to the
supply voltage minus BEMF so the available current in a motor phase [coil] at a given
speed is equal to the supply voltage minus BEMF divided by the phase [coil] resistance,
equation 3.
Equation 3
Iphase
Where
Vsupply
Rphase
Vbemf
Iphase
=
=
=
=
Vsupply − Vbemf
Rphase
Voltage supplied to motor [volts]
Motor Phase resistance [ohms]
Motor Back EMF [volts]
Current in a motor phase [amps]
Motor torque is proportional to the current in the motor phases [coils] which is given by
equation 3 therefore:
When a conventional motor is running at no load speed the Back EMF is almost equal to
the supply voltage. When the motor is presented with a load, speed drops, Back EMF
drops proportional to the drop in speed and the current in the motor phase and torque
increase proportionally. These algebraically related proportional relationships will
always produce a linear speed torque characteristic, independent of motor size or
efficiency.
When a PPMT motor is running at no load speed the Back EMF is almost equal to the
supply voltage. When the motor is presented with a load, speed drops, Back EMF drops
as an algebraic power of the drop in speed and the current in the motor phase and torque
increase at an algebraic power of the speed. These algebraic power relationships will
always produce a hyperbolic speed torque characteristic, independent of motor size or
efficiency.
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PPMT
90-98+ % efficiency
Efficiency range
and contributors to I2R losses minimal due to
cool field coils (within the
that range
permanent magnet flux
range). Primary motor
losses due to windage, iron
core, and bearing friction
losses.
Primary markets would be
Primary market
replacement for superior
power and efficiency and
same or lower cost.
DC Brush
Efficiency ~40-75% for small motors
up to 90-94% for large motors.
Efficiency may be poor in small
applications due to large brush
losses, bearing losses, etc. End to
end total efficiency in some battery
powered tools may be less than 30%.
BLDC (Brushless DC)
Efficiency ~50-85% for small motors
and 70-95% for large motors.
Typically greater efficiency than AC
induction or AC syncronous for same
size.
Stepper (Hybrid PM)
Efficiency similar to PM motor. Stepper
motors are expensive, because they are
constructed to high precision. This is due
to their use in positioning equipment, etc,
which require precise control. Effectively a
PM stepper motor is nothing more than a
high precission BLDC motor with many
poles.
Toy motors, DC appliance and power Low Power: computer cooling fans etc. Industrial, precision appliance, and robotic
tool motors. Generally smaller
- High power BLDC motors are found motion control.
applications. Some electric vehicle in electric vehicles and some industrial
applications.
machinery. These motors are
essentially AC synchronous motors
with permanent magnet rotors.
Switched Reluctance
Efficiency ~40-80% for
small motors and 4095% for large motors.
Electric motor
components
Rotor (discriminator) Simple stamped steel
laminate
Rotor is wound and has commutator Rotor has magnets (+cost)
for brush contacts (+ cost)
Rotor stamped laminate and PM magnets
(+cost)
Rotor stamped laminate
(larger for same power
as PPMT)
Stator (discriminator) Stamped steel, coils, and
magnets
Stamped steel and coils, plus
Stamped steel and coils
graphite brushes with springs (+cost)
Stamped steel and coils (precision + cost)
Stamped steel and coils
(larger for same power
as PPMT)
Housing
(discriminator)
Bearings
Shaft
Controller
(discriminator)
Structural
Structural
Structural
Structural
Structural
Structural
Medium cost
Structural
Structural
Simpler
Structural
Structural
High cost
Structural
Structural
Medium cost
Simple wiring
Low specific power (more materials
required)
Simple wiring
Low specific power
(more materials
required)
Similar to slightly more
expensive than PPMT
Structural
Wiring
Specific power
(discriminator)
Structural
Structural
Medium cost. Amplifiers for brushless
motors are more complex than brush
types and generally call for two
additional solid-state power switches.
Simple wiring
Simple wiring
Simple wiring
High specific power reduces Medium specific power (slightly more Medium specific power (slightly more
materials cost per HP
materials required)
materials required)
Total
Baseline - NA
More expensive then PPMT for larger
motors
Slightly more expensive than PPMT
Substantially more expensive than PPMT
Low power density.
Primarily low cost
appliances.
Appendix B: Motor Cost Analysis
Cost analysis result: PPMT is likely to be the same or less expensive in production than all other motor types.
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