Over the last few years, many equipment
designers have moved away from low-efficiency motors, such as single-phase
induction motors or brushed dc motors. But often this transition results in a
higher cost for motion systems that comprise the motor, an inverter and a
controller, and require development of new control algorithms. But enterprises
must balance the higher initial cost of efficiency with the long-term overall
cost of ownership. Over its lifetime, a more efficient motor can save the
considÂerable cost of otherwise-wasted electricity.
"You can buy a one-horsepower 1,800-rpm
ac-induction motor for $40 to $50, but a comparable industrial-grade permaÂnent-magnet
brushless motor could cost as much as $400," says Scott Evans, an appliÂcation
engineer with Kollmorgen. "Large machine manufacturers have started to include
energy-efficiency criteria such as an upper current limit, power use and a high
power factor in their design specifications. And, of course, companies want
their customers to see them as âgreen.'"
"So rather than using less-expensive,
less-efficient motors to save on the initial purchase price of, say, a
million-dollar packaging line, they'll pay a fair price for highly efficient
motors that help save electrical costs long term," he continues.
"Just because you specify a
permanent-magnet brushless motor doesn't ensure a more-efficient system," Evans
stresses. "A permanent-magnet motor could use a ferrite magnet typical of those
in older brushed-dc motors. Those motors are a bit more efficient than a
comparable ac-induction motor but you don't get much for the increase in cost.
On the other hand, samarium-cobalt or neodymium magnets produce a much stronger
field, which manifests itself in higher thrust or torque. Specify one of these
rare-earth-type magnets in a brushless motor to improve efficiency."
"When you use BLDC motors or PMSMs
(permanent-magnet synchroÂnous motors), you must control them efficiently to
generate the highest torque," says Jorge Zambada, senior applications engineer
in the High Performance MicroÂcontroller Div. at Microchip Technology. "So you
use field-oriented control rather than the basic six-step control that just
spins a motor. In field-oriented control, or FOC, the controller measures
current in the stator coils and uses that information to precisely âposition'
the fields so you obÂtain the highest torque possible with the same current you
would use for a six-step controller." For more information about field-oriented
control. (Ref. 1)
"Engineers refer to the current meaÂsurement
technique as âsensorless,' but you can use sensor-based techniques instead,"
says Zambada. "In theory, a sensor, such as an encoder, could give you better
position information, but unless you have excellent motor coil-to-sensor
alignment, you won't get good feedback information. So in many cases, engineers
use sensorless techniques,
although they increase the
complexity of control algorithms."
"In a sensor-based design, the engiÂneers could use motors with
built-in Hall Effect sensors that detect magnetic fields," says Raju
Karingattil, MCU business development manager for moÂtor-control applications
at Texas InstruÂments. "But this approach adds wiring complexity and cost
because the motor requires more wires for the sensors and if a sensor burns
out, you must replace the motor. Sensorless designs simplify wiring and
mechanical challenges, but they inÂcrease the challenge of knowing the exact
position of the rotor."
"You can apply FOC to 3-phase inducÂtion motors, too," continues
Zambada. "Those motors traditionally have used what we call volts-per-hertz
control that employs a basic configuration of MOSÂFETs or IGBTs in an inverter
circuit to control the stator-coil currents. Engineers can use a basic 8-bit
microcontroller (MCU) to create a dedicated volts-per-hertz controller that
doesn't use feedback information. If they want to change to FOC, they must
measure field positions and choose a more-capable MCU that can run the FOC
algorithms. But they can use the same MOSFET- or IGBT-based inverter circuit to
drive the stator fields." In steady-state operation, for a motor under volts-per-hertz
control, the air-gap flux is approximately related to the ratio Vs/fs, where Vs
represents the amplitude of motor phase voltage and fs represents the
synchronous electrical frequency applied to the motor. Thus the volts-per-hertz
designation. (Ref. 2.)
"Engineers might say, âour
brushless application is 70-percent efficient now, what efficiency can we
expect from FOC?'" says Zambada. "It depends on their algorithm, the type of
motor, the type of load and other factors. Engineers can use a dynamometer to
analyze efficiency from electrical-energy input to mechanical-energy output."
"You can always look at
the pulse-width modulator (PWM) signals and the type of inverter circuit used
to control a motor for ways to save energy," says Zambada. "If you operate the
inverter at, say, 20 kilohertz you might get comparable results by dropping to
16 kilohertz. That represents a small change, but it eliminates some switching
loss at the MOSFETs or IGBTs. As a rule of thumb, use as low a PWM frequency as
you can to get the performance you need. The lower frequencies can increase
audible noise, but if you build the motor into a noisy compressor, for example,
a bit of added noise won't make much difÂference but you do save power."
"Put special emphasis on
inverter deÂsign," stresses Zambada. "The slower the MOSFETs turn on, the
higher the switchÂing losses due to the IR drop through the MOSFET. Aim for a
low gate resistance on the inverter MOSFETs so they turn on quickly. But if you
go too low, you can create switching noise that can adversely affect other
In a product such as a
washing machine, engineers can design a direct-drive or a belt-and-gear-drive
tub. The latter often puts restraints on spin speeds. "In that sitÂuation,
engineers can use a field-weakening algorithm to drive a motor at
higher-than-normal speeds, but at a loss of some efÂficiency," says Zambada.
"They can reduce the size of the motor and eliminate the gears and belts,
however." Motor speeds can exceed 1.5 times the rated speed. Up to that rated
speed, a motor operates in constant-torque mode. But above its rated speed, it
operates in constant-power mode. That means the increased speed comes at the
price of reduced torque. But in a washÂing machine, for example, the spinning
tub does not need a lot of torque to keep it going. (Ref. 3.)
"We have seen a four- or
five-fold increase above a motor's rated speed," says Zambada. "If a customer
has used an 8-bit MCU to control the motor they would need to change to a
higher-perÂformance MCU, perhaps a digital signal controller. The algorithm
requires a lot of feedback information and the MCU must continuously adjust the
fields to enÂsure the right commutation of the motor to raise it to those
realize these design techniques sound complicated, so they offer engineers
development kits, software libraries and reference designs to give them a head
start. "Motor control involves knowledge of the motor - an electromechanical
device - analog components, the MCUs, knowledge of
the software and power
electronics. So engineers have a lot to think about and a development kit or
reference design lets them start with from 50 to 75 percent of the electronics
and software already tested and ready to go," says Karingattil.
"Sometimes engineers need some help understanding the different
approaches they can take to control motors efficiently," he says. "They might
not understand how to compare field-oriented control with 6-step BLDC motor
control. For example, you can control a BLDC motor with either trapezoidal or
sinusoidal signals. But by usÂing a sinusoidal drive you get better control and
less vibration. Vibration means less efficient control of a motor."
understand, though, that the motor that comes with a reference design is
usually not the same one they will use in their equipment," cautions
Karingattil. "TI provides the parameters for the motors in its kits, so when
designers change to a different moÂtor, they must characterize it to obtain its
associated parameters and characteristics. Motor suppliers will give you a data
sheet, but you need the characteristics of the motor for your load conditions
and your operating environment so you can determine the control parameters for
your equipment. Those parameters lead to the values you use in the control softÂware
you create. So you must know what happens when the motor starts, stops, stalls
and so on. Don't take the data sheet as the âlast word' on a motor."
"When you develop a
control algorithm, the motor exists as a mathematical model," continues
Karingattil. "If you plug in inÂcorrect values, the motor will vibrate; have a
short life and waste power. So, characterÂize a motor as accurately as
With that mathematical
model in mind, engineers could look to modelÂing and simulation tools to lend a
big hand. "Our simulation tools let these engineers model more and more of the
motor down to nonlinearities," says Tony Lennon, industry marketing manager at
The MathWorks. "Modeling supports the capability to create algorithms that
provide better motor control and reduce the amount of power a motor uses."
Engineers may find,
though, that a vendor's prototype motor might have somewhat different
characteristics than the motors assembled later on a manuÂfacturing line. Also,
the characteristics of a production motor can change due to variations in the
wire used to wind the coils and magnetic materials used in rotors. So, those
small changes can cause significant variations in the overall system
have learned about these variations through a lot of lab testing and they
need to include those
variations in their simulation models," says Lennon. "Simulation tools can
incorporate these kinds of tolerances in the model and you can run parameter
sweeps to see how the system performance will change with the variations in the
motors that you receive. Being able to assess the motor variation in the
simulation lets you develop more robust motor control algorithms that can be
tested against operational profiles. It helps you take a systematic and repeatÂable
approach to making more informed design and cost trade-offs."
"If you put a motor on a
dynamomÂeter and you excite it, you can measure inputs and outputs for many
operating conditions," says Lennon. "Then you have data that represents the
dynamics of motor operation. Optimization routines can use this data to tune
parameters, such as rotor inertia and torque constant, in the motor model,
resulting in a more accurate model. "A more accurate model lets you make better
estimates of how the motor will perform with a variety of real operating loads,
which gives you better insight into the kind of controller needed to achieve
the desired efficiency of the motor-drive combination," says Lennon.
"Then in a simulation you drive the model with âreal' current
so you can deÂtermine how many kilowatts the motor will use. And you can put
different loads on the simulated motor to examine how it behaves."
In addition to a thorough analysis
of electronics and control algorithms, engineers must realize improvements in
efficiency don't stop at the end of a motor shaft. "People will spend a lot of
money on their motors and controller," says Kollmorgen's Evans. "Then they
purchase an inexpensive 50:1 worm-gear box, for example, instead of a more
expensive and high-efficiency helical-bevel, cycloidal or planetary gear box.
Instead of getting 0.95 horsepower from a 1 horsepower motor, they only get a
bit more than 0.5 horsepower after the worm gearbox. The wasted energy goes into
heat and audible noise, resulting in premature motor failure."
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