emi filter design and optimization for
three-phase motor drive systems
lei xing
an abstract of a thesis submitted to the graduate
faculty of rensselaer polytechnic institute
in partial fulfillment of the
requirements for the degree of
doctor of philosophy
major subject: electric power engineering
the original of the complete thesis is on file
in the rensselaer polytechnic institute library
examining committee:
jian sun, thesis adviser
joe h. chow, member
kenneth a. connor, member
sheppard j. salon, member
david a. torrey, member
rensselaer polytechnic institute
troy, new york
march, year
(for graduation may, 2011)
electromagnetic interference (emi) is a common problem in power electronics
systems. pulse-width modulation (pwm) control of semiconductor devices in a power
converter circuit creates discontinuity in voltage and current with rich harmonics over a
broad frequency range, creating both conducted and radiated noise. the increase in
switching speed enabled by new power semiconductor devices helps to reduce converter
size and reduce switching losses, but further exacerbates the emi problem. complying
with regulatory emi emission limits requires the use of emi filters in almost all power
converter designs, and emi filters are often the dominant elements for system volume,
weight, and cost. this is particularly true for motor drives used in an aircraft
environment, where filtering of both input and output conducted emi may be necessary.
the traditional approach to emi filter design requires measurements of unattenuated
emi emission of a converter, which cannot start until a complete system prototype has
been built and tested. this “emi-last” approach results in sub-optimal solutions at best
and often results time-consuming and costly redesign of part or the entire system. to
solve this problem and to enable a “concurrent emi design” approach, emi modeling of
motor drive systems is studied first in this work. piece-wise linear behavioral models are
used to model different components of a typical motor drive system, such as semicon-
ductor devices, passive components, interconnects and cables, and motors. such behav-
ioral models can be established in parallel with motor drive circuit and control design,
thereby providing a means to evaluate different design options in terms of system emi
performance and to optimize emi filter design without requiring prototype measurement
the second part of this work deals with new methods to reduce emi emission from
motor drive systems. one technique proposed is to reduce system common-mode (cm)
emi emission by inserting passive components around the cm voltage source to create a
balanced wheatstone impedance bridge where the cm current generated through one
path is cancelled by that in another path, significantly reducing cm current emission at
the input or output terminals that needs to be attenuated. the inserted passive components
carry only the cm current, hence can be much smaller than traditional cm filter compo-
nents which have to carry the full input or output current. the second technique, which is
effective in reducing both cm and differential-mode (dm) emi, is by phase-shifting the
pwm signals of parallel converter modules. in particular, asymmetric interleaving, in
which the phase-shift angle is varied from module to module is applied to optimize the
cancellation of harmonics that otherwise drive the emi filter size.
the third part of this work deals with damping of emi filters. it is known that
peaking in the output impedance of an emi filter due to parallel resonance of the filter
inductor and capacitor may create an intersection point with the input impedance of the
converter, causing degradation in converter control performance or even instability of the
filter-converter system. in this work, we demonstrate that dipping in the input impedance
of an emi filter due to series resonance of filter inductor and capacitor can also cause
intersection with the source output impedance and lead to source-filter system instability.
optimal damping of emi filter input impedance to avoid interactions with the source
output impedance is studied. closed-form analytical results are developed for optimal
design of different damping circuits. the optimization method is further extended to
multi-stage lc filters.