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 |  موضوع: مقالة بعنوان Reasons for Turning to Slotless Technology السبت 24 أكتوبر 2020, 1:44 am | |
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مقالة بعنوان
Reasons for Turning to Slotless Technology
PITTMAN
www.haydonkerkpittman.com
و المحتوى كما يلي :
When first introduced, brushless DC motors, despite their many advantages,
were cast as a costly alternative to brush commutated motors and were
typically only specified for low power applications where long life was the
primary desired requirement.
Without the mechanical brush commutator mechanism that would wear and
eventually result in motor failure, brushless motors could be relied upon to
deliver performance over time. As for other advantages, conventional wisdom
held that brushless motors provide high speed and fast acceleration, generate
less audible noise and electromagnetic interference, and require low maintenance. On the other hand, brush commutated motors would afford smooth
operation and greater economy.2
Reasons for Turning to Slotless Technology
In the past decade, brushless motors have gained broader appeal and
greater acceptance in industry for a wider range of applications previously dominated by brush commutated products, due in part to dramatic
reductions in the cost and size of electronic components and advances in
motor design and manufacturing. At the same time, manufacturers have
further sought to challenge conventional wisdom by improving brushless
motor design in an effort to combine the traditional advantages of brush
commutated and brushless types. A noteworthy example of how far these
innovations have progressed involves the slotless (instead of slotted)
construction of the brushless motor’s stationary member, or stator.
The slotless stator design originated with the goal to deliver smooth
running performance and eliminate cogging, which is an unwanted
characteristic especially in slower-running applications (less than 500
rpm). The absence of cogging is, in fact, the most-often cited reason for
selecting a slotless brushless motor.
Slotted Motor Construction
Most brushless motors (slotted or slotless) use electronic commutation,
usually Hall effect sensors and magnets, in place of brushes. The
motor’s rotor consists of a steel shaft with permanent magnets or a
magnetic ring fixed around the circumference of the shaft. The magnets
are responsible for producing torque. As the flux density of the magnet
material increases, the amount of torque available from the rotor
assembly increases.
In traditional slotted brushless motors, the stator features a group of
slotted steel laminations (0.004 in. to 0.025 in. thick), which are fused
to form a solid uniform stack and create a series of teeth. Wound copper
coils, which produce electromagnetic fields, are then inserted into each
of the slots. Together the laminated stack and wound copper coil form the
stator assembly. The return path completing the magnetic circuit consists
of the laminated material outboard of the copper windings in the stator
and the motor housing.
Brushless slotted motors are especially powerful, because the teeth around
which the copper wire is wound place the iron closer to the magnets, so the
magnetic circuit is completed more efficiently. As the air gap between iron
and magnets is reduced, the torque available for the motor is increased.
However, slotted stators are known to cause cogging, which is attributed
to the teeth in their construction. Cogging occurs when the permanent
magnets on the rotor seek a preferred alignment with the slots of the
stator. Winding copper wires through the slots tends to increase this
effect. As magnets pass by the teeth, they have a greater attraction to
the iron at the ends of the teeth than to the air gaps between them. This
uneven magnetic pull causes the cogging, which ultimately contributes
to torque ripple, efficiency loss, motor vibration, and noise, as well as
preventing smooth motor operation at slow speeds. A slotless stator
offered a solution to the problems experienced with cogging in slotted
brushless DC motors.
Slotless Motor Construction
Instead of winding copper wires through slots
in a laminated steel stack as in conventional
slotted brushless motors, slotless motor
wires are wound into a cylindrical shape
and are encapsulated in a high temperature
epoxy resin to maintain their orientation with
respect to the stator laminations and housing
assembly. This configuration, which replaces the
stator teeth, eliminates cogging altogether and results
in desired quiet operation and smooth performance.
The slotless design also reduces damping losses related to eddy
currents. These currents are weaker in a slotless motor, because the
distance between the laminated iron and magnets is greater than in a
slotted motor.
Slotless motors are typically designed with sinusoidal torque output that
produces negligible distortion, rather then a trapezoidal voltage output.
The sinusoidal output reduces torque ripple, especially when used with
a sinusoidal driver. Because the slotless design has no stator teeth to
interact with the permanent magnets, the motor does not generate detent
torque. In addition, low magnetic saturation allows the motor to operate
at several times its rated power for short intervals without perceptible
torque roll-off at higher power levels.
Compared with slotted motors, slotless construction also can significantly
reduce inductance to improve current bandwidth. The teeth in a slotted
motor naturally cause more inductance: the coils of copper wire around the
teeth interact with the iron in a slotted motor, and this interaction tends to
send the current back on itself, resulting in more damping (or dragging) and
impacting negatively on slotted motor response and acceleration.
In terms of delivering power, conventional slotted motors used to enjoy
the advantage over slotless types, due (as noted) to the proximity of iron
and magnets and the reduced air gap.
However, this advantage has virtually evaporated, in large part due to the
utilization of high energy, rare-earth magnets (such as samarium cobalt
and neodymium iron boron). By incorporating these magnets, manufacturers of slotless brushless motors have been able to routinely compensate for the greater air-gap distance. These more powerful magnets
effectively enable the same (or better) torque performance for slotless
products compared with slotted. Eliminating the teeth and using stronger
magnets both serve to maximize the strength of the electromagnetic field
for optimum power output. Rare-earth magnets, along with the fact that
fewer coils, or “turns,” of the wire are required in slotless motors, also
help contribute to low electrical resistance, low winding inductance, low
static friction, and high thermal efficiency in slotless motor types.3
Reasons for Turning to Slotless Technology
One more important difference between slotless and slotted designs
is the rotor diameter. Slotless motors have a larger rotor diameter
than slotted construction for the same outside motor diameter and will
generate a higher inertia, as well as accommodating more magnet material for greater torque. For applications with high-inertia loads, the slotless
product is more likely to be specified.
Slotless Motor Applications
In general, brushless motors are usually selected over Brush commutated motors for their extended motor life. (While motor life is application
specific, 10,000 hours are usually specified.) Other reasons for specifying
brushless motors include a wide speed range, higher continuous torque
capability, faster acceleration, and low maintenance.
In particular, slotless versions of brushless DC motors will suit those
applications that require precise positioning and smooth operation.
Typical niches for these motors include computer peripherals, mass
storage systems, test and measurement equipment, and medical and
clean-room equipment.
As examples, designers of medical equipment can utilize slotless motors
for precise control in machines that meter and pump fluids into delicate
areas, such as eyes. In medical imaging equipment, slotless brushless
DC motors decrease banding by providing the smoother operation at
low speeds. Airplane controls supply smoother feedback to pilots. And,
by eliminating cogging and resulting vibration, these motors can reduce
ergonomic problems associated with hand-held production tools. Other
appropriate applications include scanners, robots for library data storage,
laser beam reflector rotation and radar antenna rotation equipment,
among many others.
Customization Options
Slotless brushless DC motors, as with most motors today, feature a
modular design so they can be customized to meet specific performance
requirements. As examples, spur gearheads can be integrated on motors
for an application’s specific torque and cost requirements; planetary
gearheads offer a higher-torque alternative. Slotless motors can further
be customized with optical encoders, which provide accurate position,
velocity, and direction feedback that greatly enhances motor control and
allows the motors to be utilized in a wider range of applications. Choices
for optical encoders include having either two- or three-channel output,
line count options, and the availability of built-in commutation tracks. As
a low cost alternative to optical encoders, rotor position indicators can be
specified.
When using optical encoders, differential line drivers can be utilized to
eliminate the effects of electrically noisy environments. Differential line
drivers are designed to ensure uncorrupted position feedback from the
encoder to the control circuit.
Other options that can be selected and customized for particular
applications include connectors, custom cables, shaft modifications,
shaft-mounted pulleys and gears, special bearings and windings, and
electromechanical brakes. Each can deliver specific performance benefits
and ensure application requirements are satisfied.
Motor Selection Guided by Application
Despite the overall design and performance comparisons reviewed here
for slotless and slotted brushless DC motor types, one should remain
cautious in drawing any conclusion that one type is the ultimate choice
over the other. There are simply too many variables that must be evaluated, ranging from rotor size and windings to housing and special components. A given application and its requirements should (and will) be the
guiding factors in selecting a particular motor type and the customized
components to be incorporated.
Some encouraging news in those applications that would clearly benefit
from a slotless brushless motor is that costs are coming down to be more
in line with those for slotted motors. This is because of new streamlined manufacturing techniques and an increasingly available supply of
powerful magnets, which are both beginning to have a positive impact on
end-product costs.
Regardless of any cost differential, however, for many applications,
slotless brushless DC motors will be the preferred choice to resolve
specific requirement issues. While advances in electronics are beginning
to be applied that promise to reduce normal cogging in slotted products
as a step toward making these motors more smooth-running and quiet,
the industry is not there yet: slotless motors remain the best alternative
where cogging and life are defining performance issues.
This technical article was authored by the engineering team at Haydon
Kerk Pittman Motion Solutions, a leader in motion technologies. Complex
custom and ready-to-ship standard lead screw assemblies are made at
our facilities with a full range of onsite capabilities including designing,
engineering and manufacturing.
Shop Online. Products ship in 24-hours.
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