Exlar Actuation Solutions
A More Efficient Servomotor
Richard Welch Jr. Consulting Engineer
An innovative architecture can boost
performance by minimizing the number of
winding turns that are not actively doing
work.
According to the U.S. Dept. of Energy (DOE),
as much as 65% of a typical manufacturing
plant's monthly electric bill goes to pay for
all the electricity consumed by its electric
motors. Hence, if you ask plant managers
to describe the three most important words
associated with electric motors today they
will quickly respond with efficiency, efficiency
and efficiency. Furthermore, designers and
system engineers who design and build
equipment used in manufacturing plants
constantly search for motors that provide
the most bang for least buck. Thus motor
manufacturers are increasingly trying to
design electric motors having the highest
possible power density (i.e., power output
per unit motor volume) along with maximum
power efficiency.
A cross-section of a typical solid-core stator
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The stator can have either “inner” or “outer”
rotation. The one-piece lamination contains
several teeth and the multi-phase electrical
winding is inserted into the slots in between
the teeth. The illustration depicts a crosssection of solid-core, 4-pole, 12-tooth stator
lamination along with the slot locations for
each phase of the 3-phase electrical winding.
The T-Lam stator design is one result of
this quest. Exlar Corp. developed it after
extensive research and development and
now uses the stator in all its SLM and
SLG brushless dc servomotors and GSX
and GSM rotary to linear actuators. It is
interesting to review how the T-lam stator
design improves both the power density and
efficiency provided by brushless
dc servomotors.
For well over 100 years motor manufactures
have been designing and building ac
induction, variable reluctance (V-R), and
brushless dc (BLDC) motors using what's
generally called a “solid core” stator. This
solid core design uses multiple, one-piece
laminations to construct the stator's magnetic
core structure. The one-piece lamination
contains several teeth and the multi-phase
electrical winding is inserted into the slots
in between the teeth. In the accompanying
image of a solid-core stator, notice that
R-phase is inserted into slots 1-4-7-10,
S-phase into slots 2-5-8-11, and T-phase into
slots 3-6-9-12 and this winding configuration
is called a distributed winding.
Stators having identical volume: T-Lam (left)
and solid core (right)
It is interesting to compare a solid core
version with a T-Lam stator having the same
volume. Examine a solid-core stator and it
quickly becomes evident there's a significant
amount of the winding extending beyond
both ends of the stator's magnetic core
structure. These are called the end turns
and are necessary to complete the electrical
path within the winding. However, end turns
add to the winding's electrical resistance but
do not contribute to the motor torque output.
Motor torque comes only from that section
of the winding that lies within the stator's
magnetic core structure. Hence, end-turn
length, along with the percent slot fill, affects
both the motor's continuous torque output
and power density along with its power
efficiency.
One can construct the solid core stator's
distributed winding either by hand or with
automated winding and insertion equipment.
Hand winding is often limited to prototypes,
low volume production, and/or motor rewind/
rebuild. High-volume motor production
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generally employs expensive ($500K+)
automated winding and insertion equipment
along with blocking and lacing machines to
form and secure the end turns.
In addition, a “trickle” varnish machine is
typically used to impregnate the winding
with a varnish that rigidly secures the wires
so they can't move and wear through the
thin coating of surface insulation on each
wire, thereby causing premature winding
failure. The varnish also improves heat
transfer within the winding and between its
surrounding magnetic core structure. This
improves heat transfer increases that in turn
increases the motor's continuous torque and
power density.
Lower Winding Resistance
A 3-D view of a T-lam stator magnetic core
structure reveals that instead of several
one-piece laminations it contains multiple
T-shaped laminations that are mechanically
held together to obtain the individual core
segments. In turn, each core segment
is wound on a bobbin winder (typically a
$25K machine). Then these entire wound
segments are assembled together to create
the finished stator sub-assembly.
A T-Lam laminated core structure, left,
versus a wound solid core stator.
End views are above.
In contrast, the end turns in a T-Lam motor
typically amount to at most 7% of the total
wire used to wind its concentrated winding.
Thus the a T-Lam winding has much lower
electrical resistance for the same number of
total turns. Furthermore, the smaller volume
of space taken up by end turns frees up
room to make the core 3 to 5% longer. This
lets rotor magnets be longer.
Moreover, slot fill ratios tend to be higher
— Exlar normally gets 78 to 81% slot fill
for all its T-lam stators versus 55 to 60%
for its solid core stators. This higher slot fill
It is evident from a direct comparison that the generally allows T-lam stators to typically use
concentrated T-lam winding has significantly thicker wire, about one wire gauge larger.
less end turn length. Depending on core
Wire tables for copper magnet wire reveal
length, the ends turns in a solid core stator
that for every change in wire gauge, the
can amount to 30 to 60% of the total wire
wire's electrical resistance-per-foot changes
used to construct its distributed winding.
by 26%. So a T-Lam motor having the same
This end-turn waste boosts the winding's
volume as its solid core counterpart, wound
electrical resistance by 30 to 60%.
with the same number of turns, will have
on average a winding resistance 40 to 45%
lower.
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To better understand the significance of lower
winding resistance, consider the expression
for the maximum continuous torque a BLDC
servomotor can safely put out without
overheating:
Where TR = rated winding temperature,
°C; T0 = ambient temperature, °C; Rth =
winding-to-ambient thermal resistance,
°C/W; ω = motor angular velocity, radian/
sec; DM = motor viscous damping, Nm/radian/
sec; FM = motor friction, Nm; KT(TR) = motor
total torque function at TR, Nm/A; R(TR) =
winding electrical resistance at TR, Ω; TL(ω)
= maximum continuous torque motor can
deliver to its load at each velocity.
Next, consider the expression for the motor
maximum continuous power output at each
velocity, P(ω):
P(ω) = ωTL(ω) (2)
Finally, consider the expression for the
motor's percent power efficiency when
the motor operates at its rated winding
temperature:
The first point to notice in equations (1) (2)
and (3) is that by lowering winding resistance
and keeping all other parameter values fixed,
a T-lam motor can put out more continuous
torque and power for the same temperature
rise. The T-lam architecture also improves
the motor's power efficiency for a fixed motor
power dissipation. The accompanying figures
graphically show the affect of lowering
winding resistance by 40%.
It is also clear that lowering winding
resistance by 40% has another benefit. It
lets the T-lam motor put out 17% higher
continuous stall torque and power at a power
efficiency that is 2% higher than that of a
comparable solid core motor. (When both
motors are operated with the same 130°C
winding and 25°C ambient temperature and
with no heat sink.)
Furthermore, the T-Lam is up to 5% more
efficient than its solid-core equivalent when
operated at less than its maximum allowable
continuous torque-speed and powerspeed curve. This is because the winding
now operates at less than its 130°C rated
temperature and the winding resistance is
even lower.
Use of thermally conductive epoxy instead
of varnish to impregnate the winding can
also boost efficiency. For example, Exlar
uses a potting epoxy that is a recognized
component in a UL-1446 insulation system
that has a Class 180 H (180°C) temperature
rating. Actual measurement shows that
potting the stator winding using this thermally
conductive epoxy lowers its winding-toambient thermal resistance, Rth(°C/W), by
50% compared to impregnating the winding
with a typical varnish.
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A T-Lam stator potted in its aluminum
housing and a cross-section of the stator
The winding is encapsulated by the epoxy
thereby improving heat transfer between
the winding and the surrounding T-Lam
core structure. The epoxy also hermetically
encapsulates the winding.
Power output and efficiency curves make
it clear that potting a T-lam stator with
thermally conductive epoxy lowers the
winding-to-ambient thermal resistance, Rth,
by up to 50%. It also produces a motor that
has 45% higher continuous stall torque
and 63% higher maximum power output
compared to the original solid core motor.
However, the power efficiency curve is
somewhat counter intuitive. The potted
SLM90-238 motor has about the same or
slightly higher power efficiency between
0 and 8K rpm compared to the original solid
core motor. As shown, above 8k rpm its
efficiency rises above 8 rpm such that at
13 rpm its power efficiency is actually 10%
higher than that of both a solid core and a
T-lam motor impregnated with a varnish.
Furthermore, between 13.3 and 15 rpm the
potted version is infinitely more efficient. This
is because a varnished motor's velocitydependent power loss caused by both
friction and viscous damping prevents the
motor from producing continuous torque
above 13.3K rpm without exceeding its
130°C rated winding temperature. Thus the
question becomes: What prevents increased
power efficiency when you pot the stator with
thermally conductive epoxy and lower its
thermal resistance by 50%?
To answer to this question one must look
again at equations (1) and (3). Initially, the
winding resistance for the T-lam motor is
40% lower compared to the original solid
core design because of higher slot fill and
less end turn waste. Hence, for each “I”
amperes of current there is less I2R power
loss. This lets the T-lam motor put out higher
continuous torque and power with higher
efficiency because the velocity dependent
viscous damping and friction power loss
remains basically the same for both types of
motor.
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Potting the T-lam motor with thermally
conductive epoxy cuts in half the windingto-ambient thermal resistance and boosts
the maximum allowable power dissipation
inside the motor by the same 50%. Because
the winding's electrical resistance remains
the same after it is potted, the increase in
current needed to produce the higher torque
and power results in higher I2R power loss.
Thus the lower Rth does not boost power
efficiency until the SLM90-238 gets above
11 rpm. The key to more power efficiency
over the entire operating velocity range is to
lower the winding electrical resistance. The
only way to accomplish this is by operating
the motor at a lower winding temperature.
produces 17% higher continuous stall torque
and maximum power output, compared to
the original solid core motor but with the
power efficiency (dashed Brown line). The
reason: The winding temperature only rises
to 100°C with a 25°C ambient because Rth is
50% lower. It takes less current to produce
the lower torque and power, and there is
9% less winding resistance. Thus there is
less I2R power lost to heat, so windings stay
cooler and power efficiency rises.
The electrical resistance for copper magnet
wire typically used to construct motor
electrical winding depends on winding
temperature. The accepted international
standard for a copper winding's change in
resistance with temperature is:
R(T) = R(T0) (1 + .00393(T-T0)) (4)
In equation (4), T0, is the reference
temperature (typically 25°C) and T (°C) is
the new winding temperature either above
or below T0. Equation (1) shows that for a
copper winding with an electrical resistance
specified at 25°C, electrical resistance at
130°C rises by a factor of 1.4126 or 41%.
However, if the winding operates at 100°C
then the increase in resistance is only 1.294.
This is 9% less than its 130°C value.
Suppose the SLM90-238 operated not at its
maximum continuous torque-speed curve
(dashed blue in the accompanying figure)
but instead at a reduced level (solid red). It
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The continuous torque and power output
curves for the Exlar SLM90-238 T-Lam motor
compared to the equivalent solid core motor.
At right is the SLM90- 238 continuous power
efficiency curve.
The combined effect of 40% lower
winding resistance and 50% lower thermal
resistance becomes evident in the plot of the
continuous torque-speed curve for a T-lam
motor (SLM90-238). The continuous power
output and power efficiency curves are those
for a free standing motor without a heat sink
and operating at its 130°C rated winding
temperature in 25°C ambient.
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A graph of speed versus power efficiency for
T-lam and solid core motors operating at
various temperatures shows the T-lam
version gives 17% higher continuous stall
torque and power compared to the original
solid core motor. An additional benefit is
a 3 to 10% higher power efficiency and
operation at a higher velocity compared to a
varnished motor.
More info
Exlar Corp - (www.exlar.com)
Proto-lams Corp - motor laminator,
(www.protolam.com)
Thomson Laminations - motor laminator,
(www.tlclam.net)
For a basic tutorial on PM motors, see R.
Welch, “The Basics of Permanent Magnet
Motor Operation,” available from Welch
Enterprise (welch022@tc.umn.edu)
For information on the process of winding
motors, refer towww.patentstorm.us/
patents/4385435/description.html, www.
alliance-winding.com/products/windinsert/
index.html, odawara.com/products.htm,
andwww.alliance-winding.com/products/
windinsert/autoassembly.html
For more information on motor thermal
behavior, see R. Welch, “Continuous
Dynamic and Intermittent Thermal Operation
in Electric Motors,” a course offered by the
SMMA Motor and Motion College (www.
smma.org)
For more information on motor safe
operating range, see R. Welch, “How
to Measure a BLDC Servomotor's Safe
Operating Area Curve (SOAC) Without Using
a Dynamometer,” a tutorial available from
Welch Enterprise (welch022@tc.umn.edu)
For more info on motor efficiency issues, see
R. Welch, “How to Improve Electric Motor
Power Efficiency,” a course offered by the
SMMA Motor and Motion College (www.
smma.org), and R. Welch, “How to Increase
a Motor's Continuous Torque Output and
Power
Density By Potting its Stator with Thermally
Conductive Epoxy,” Proceedings of the 2006
SMMA Fall Conference, Saint Louis, Mo.
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