By Randy Dube
Principle Mechanical Engineer
Otis Elevator Company
Farmington, Connecticut
The electronics packaging challenges facing the designers
of elevator control equipment have significantly increased
over the past decade. The market demands for reduced space
utilization within the building by the elevator system has
driven the design of smaller sized controls, making it possible
to place them in the hoistway rather than a separate machine
room. Eliminating the machine room benefits building owners
by increasing rentable space, but making the controls smaller
has substantially increased power density, which makes thermal
management more difficult than in the past.
At the same time, electromagnetic compatibility (EMC) design
for elevator control equipment has become more challenging
than in the past, largely due to stricter requirements that
have been issued by the European Community. In Europe, elevator
controls must meet the requirements of EN 12015 (emissions)
and EN 12016 (susceptibility). Otis products are used around
the world so they are generally designed to meet the toughest
global standards. Finally, electromagnetic interference (EMI)
has become a more important consideration than ever before
because emissions generated by the control equipment can
interfere with the increasing number of wireless communication
systems distributed within buildings. In addition, better
EMC design improves immunity of the elevator controls from
emissions of licensed transmitters such as cell phone services,
commercial broadcasters, and emergency communications.
Designing new motor drive
 All of these issues came to the fore in the design of a
new regenerative variable frequency drive for AC motors used
to drive Otis elevators. Regenerative drives are attractive
to building owners and managers because of their improved
energy efficiency and consistency with “green” building
iniatives. The advantage of regenerative drives stems from
the fact that instead of the motor doing the work to move
the elevator car, under certain load conditions such as empty
car up or full car down, the weight of the car or counterweight
drives the motor, generating electricity that is put back
into the grid, reducing energy costs. In addition, regenerative
drives offer greater line voltage flexibility, low harmonic
distortion, and high power factor.
The new drive has three major sections. The input section
includes EMC filtering, inductive line coupling, precharge
circuitry and AC-to-DC conversion. The power section includes
the power semiconductors used to drive the motor, control
circuitry for these semiconductors, and the capacitors used
for DC bus energy storage. The control section uses digital
signal processors to perform the conversion from DC-to-variable
frequency AC and contains the proprietary algorithms to efficiently
control the torque and speed of the motor by dictating appropriate
vectors.
Otis engineers recognized from the start that a well-executed
electronics packaging design of the new motor drive would
present a formidable challenge. Thermal management, EMC,
and acoustic noise requirements had all increased over previous
generations of controls. Just as important, all three of
these factors interact with each other to further increase
the difficulty of simultaneously meeting all the requirements.
For example, increasing the openings in the enclosure to
improve thermal performance will at the same time have a
negative impact on EMC and acoustic noise.
Move to integrated thermal/EMC analysis
 Otis has used Flomerics’ Flotherm thermal simulation
software since 1994 and their FLO/EMC simulation software
since 2000 in order to identify problems early in the design
process when they can be inexpensively corrected. Simulation
can significantly reduce time to market by making it possible
to specify critical custom components such as large heatsinks
that are required in motor control drives much earlier than
was possible in the past. In their traditional design methodology,
Otis engineers separately simulate the design from a thermal
and an EMC standpoint. In the design of this drive, it was
important early in the project to achieve greater collaboration
between what are traditionally discrete engineering specialties
to reduce risk and avoid substantial delays from iterative
design changes. A major concern was that thermal design often
conflicts with EMC design so fixes that are implemented to
address thermal concerns often exacerbate or create EMC problems.
The most obvious example is that thermal performance often
is improved by using large openings to enable adequate airflow
while EMC design requires small openings to attenuate high
frequency emissions. Another example is that heatsinks are
often added to dissipate heat away from hot components. Switching
currents can then couple to the finned surfaces causing them
to radiate like an antenna.
Otis worked on thermal and EMC design in parallel so these
types of tradeoffs could be optimized simultaneously. Flomerics
has introduced an integrated analysis environment that makes
it practical to address thermal and EMC design in a collaborative
manner from the earliest design stages.
Starting with hand calculations and rough models
The first step in the process was to use elevator system
simulation results to develop a spreadsheet to track power
dissipations and estimate airflow requirements. Hand calculations
and experience with previous designs indicated that it should
be possible to cool the motor drive with a combination of
natural convection for the input and control sections, but
forced air convection would be required for the power section.
Then engineers created a series of very basic thermal models
of the design using Flotherm software in conjunction with
their PTC Pro/Engineer® CAD solids modeling tool for
creating geometry. These coarse models were used primarily
for preliminary power section heatsink design, placement
of high-dissipation components in the power section, selection
of air movers, and optimization of air mover location.
At the same time the thermal design was being addressed,
other members of the engineering team were looking at EMC
issues. A subset of the geometry used to create the thermal
model was also used to create an EMC model in FLO/EMC simulation
software. Engineers considered the effect of the location,
number, size, and type of openings on radiated emissions.
They addressed conductive emissions by simulating the major
sources and evaluating changing their position.
Otis engineers considered several major aspects of EMC design
as submodels in order to efficiently use FLO/EMC to evaluate
emissions from major sources. Examples are how to mate the
cover with the enclosure to avoid gaps, the best way to integrate
drip proof ventilation openings, and the analysis of cable
routing effects in more detail. As they considered various
design alternatives from an EMC standpoint, Otis engineers
also looked at impact on thermal management, ensuring that
each decision made at this early stage took effects on both
thermal and EMC into account.
Refining thermal and EMC models
 The next stage of the design process involved refining both
the thermal and EMC models in order in order to incorporate
PCB layout changes and implement design changes requested
by field and manufacturing groups obtained through periodic
design reviews. The simulation models were also expanded
beyond the power section to incorporate more parts and to
include appropriate component physical attributes deemed
important to the simulation accuracy. Component selections
were stable, but other aspects of the design such as cable
routing and improving overall performance were refined. The
new motor drive uses an enclosure 650 mm high by 420 mm wide
by 210 mm deep, which results in a power density considerably
greater than previous designs.
At this stage, ideas for improvement were rapidly analyzed
from both a thermal and EMC standpoint. For example, small
changes were made to the location of openings in the enclosure
to simultaneously determine their impact on thermal and EMC.
The refinements that were added to the simulation made it
possible to start looking at the effect of physical layout
on radiated emissions. At this point, engineers also began
to consider the effects of barriers required in the control
system to prevent contact with dangerous high-voltage components
by field personnel. These barriers impact airflow so Otis
engineers simulated their effect on thermal performance.
The more detailed system-level model helped engineers quickly
assess problems areas such as a heatsinks that acted as antennas
and how the structure of the box resonated and contributed
to electromagnetic interference. At this point, engineers
also made several modifications to the enclosure, internal
bracketry, and cover to improve the design for manufacturability
and ease of assembly again considering the impact on both
thermal and EMC. Robust design of the interface between the
cover and the enclosure was needed to prevent radiated emission
leaks. Integrating this functionality into the sheet metal
itself reduced the cost of the design as opposed to the alternative
approach, used in the past, of adding components such a wire
mesh or wire impregnated elastomer gaskets to provide these
capabilities.
First prototype worked as predicted
 The effectiveness of Otis’ improved engineering methodology
is illustrated by the fact that when components finally became
available to build the first prototype of the design, temperatures
and EMC performance were as predicted within the expectations
requested of the simulation tools. All thermal and nearly
all EMC requirements were met from the very first iteration.
Minimal modifications were required to the enclosure and
the cooling system from the first prototype through to the
final release.
All in all, this project demonstrates that the integrated
approach to thermal and EMC design is ideally suited to meeting
increasing electronics packaging design challenges that are
caused by higher power density and more stringent EMC regulations.
The ability to evaluate design alternatives from both a thermal
and EMC standpoint prior to the prototyping stage made it
possible to order prototype components and build the first
prototype earlier than would have otherwise been possible.
Integrated thermal/EMC simulation also helped to ensure that
the first prototype worked as predicted from a thermal and
EMC standpoint, which reduced prototyping expenses and helped
to get the product to market earlier.
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