For many years, designers have been making a conscious
tradeoff between the cost of the motor and the cost of motor control. But that tradeoff has presented a moving
target. But for recent sharp increases
in the cost of copper and other components, the cost of the motor has tended to
be relatively stable. Meanwhile, the cost of control technology has tended to
decrease over time thanks to the continued effects of Moore's Law, which accurately models the
declining cost and increasing capability of semiconductor products.
This sharp dichotomy now opens the door to achieving more
efficient and cost-effective variable speed drives for applications such as
HVAC that were traditionally limited to at most one or a few speed options.
With the cost of control becoming ever more affordable - flash-based digital
signal controllers (DSCs) often cost less than $3.00 in low quantity and
substantially lower at larger volumes -
even low volume motor control applications are now candidates for new
thinking.
Ways
designers can reduce cost
To take advantage of this new price-performance environment,
designers should consider several steps.
1. Design in a lower priced controller from the start (digital signal
processor integrated circuit (dsPIC)
digital signal control (DSC) now cost less than many 8-bit MCus with dedicated
motor control peripherals).
2. Combine functions - Add power factor correction (PFC) computation
to the same chip that performs motor control (DSCs can do it) Designers can
even add user interface or other upstream functions normally handled by a
separate MCU with the leftover resource on the dsPIC DSC).
3. Eliminate expensive position sensors - sensorless strategies use
resistors to reliably sense motor currents and thereby accurately infer rotor
position.
4. Eliminate op-amp arrays for sensorless applications (required by
some 8-bit systems), the dsPIC analog-to-digital conversion (ADC) has features
optimized for motor control such as multiple sample and holds for simultaneous
sampling.
5. For applications with extensive filtering requirements, a digital
filter can reduce component count and product cost.
6. Use the on-chip hardware pulse width modulation (PWM) with dead
time insertion to reduce the cost of external power-drive hardware that
performs simple digital functions like dead time insertion and generating
complementary motor drive signals. Power semiconductors are not cost-optimized
for digital logic.
7. Use libraries and application notes. Higher performing algorithms
can make an inexpensive motor perform like a more expensive one. What's more,
using sophisticated algorithms can allow stretching the motor rated specifications.
In this way designers can use a less expensive motor to fulfill its design
challenges and requirements.
8. Save on the cost of materials by including the motor controller in
the motor casing - (this requires small size, and in fact, 6x6mm DSCs are now
available).
Finally, it is worthwhile
considering the design
of a motor control platform, where
the same design, layout and bill of materials (BOM) is used for several
projects, requiring only a software update for a particular end product. For
example, in an air conditioning application, companies may have different
compressor sizes, or even different motor types (permanent magnet synchronous
motor - PMSM, and alternating current induction motor --ACIM) for their lines
of products. Instead, they can design a single board with the same hardware,
and then apply a different version of the software to control a particular
motor. This helps achieve higher volume for their BOM, which minimizes direct
and costs as well as simplifying inventory and service procedures.
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