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An MCE
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AC Inverter Drives
Electrical Noise & RFI
Motion Control
Engineering, Incorporated
Revised January 1996
©1996 Motion Control Engineering, Inc.
(Reproduced here with permission)
Purpose
This
Technical Publication discusses electrical noise and
Radio Frequency Interference (RFI) created by AC Inverter
drives and possible effects on other equipment.
Motion
Control Engineering, Inc.inverter drives
suggests that they can generate noise experience with AC
that may affect radio frequency sensitive equipment in
the building. This phenomenon needs to be understood and
considered prior to selection of an elevator drive
system.
Overview
It is
generally believed that AC inverter drives are the ideal
technology providing maximum power savings, reduced
motors cost and lower maintenance costs. AC inverter
drives have tradeoffs that need to recognized and
understood. These tradeoffs (potential drawbacks) include
greater harmonic distortion, radio frequency interference
and other idiosyncrasies that can make typically used AC
drives unfriendly.
In most
instances, new construction design can address these
issues; however, elevator modernization in existing
buildings requires thoughtful consideration. It is
important to have a basic understanding of the tradeoffs
that are determining factors in the drive selection
process.
Static
Drives
MCE Technical
Publications "Harmonic Analysis &
Comparison" and "Motor Generator vs SCR"
explored considerations for drive selection for a
particular elevator control application. Issues addressed
in these publications apply to all static drives,
including the typical AC inverter drive.
Radio
Frequency Interference "RFI"
AC inverter
drives can produce sufficient amounts of Radio Frequency
noise (RFI) that affect the operation of equipment
susceptible to Radio Frequency noise. This is
particularly true in older buildings when grounding is
lacking or otherwise inadequate.
One example
of a substantial RFI problem is a brick apartment
complex, built in the mid-20's, where the elevator
contractor was in the process of modernizing existing AC
elevator equipment. After the first cars were modernized
(new controllers included RFI filtering devices), the
building superintendent complained that he was unable to
listen to his favorite radio station because of
interference from the elevators. He stated that the
vintage AC elevator controls caused no problems; however,
the state-of-the-art static drives generated disruptive
RFI.
The building
manager, considering the complaint unfounded, suggested
that the superintendent select a different radio station.
The superintendent reported the incident to the FCC.
Subsequently, the contractor received an FCC notice to
immediately respond and resolve the problem.
At the
building the complaint was verified using an inexpensive
AC plug in radio and the superintendent's portable
battery operated radio equipped with all the latest
technology. In the elevator machine room the AC radio was
tuned to the AM band and, as expected, there was a
considerable amount of interference. At roof level the
battery operated radio, tuned to the same frequencies,
performed slightly better; however, a considerable amount
of interference was evidenced.
In an
apartment on the fourth floor, located in the middle of
the building, both radios demonstrated a similar level of
interference. Conditions were found to be the same in an
apartment on the first floor. Outside, in the courtyard
which is surrounded by many buildings, AM band station
signals were very strong and free of interference.
Simply
stepping back inside at the first floor entrance the
interference returned. Using the battery operated radio,
as the elevator ran one could hear interference during
both acceleration and deceleration.
The
conclusion, later confirmed by the drive manufacturer,
was that the building, without a solid earth ground, was
acting as an antenna. Grounding of the elevator drive
system and motor was occurring through water pipes and
whatever other steel may have been present in this brick
building.
The drive
manufacturer did additional research to identify some
probable causes. The contractor needed to provide a
proper earth ground, ground the controller and the motor
to this proper earth ground, and use insulated bushings
to isolate other devices from the controller and motor to
prevent grounding to or through the water piping system.
These recommendations are, generally, requirements of the
National Electrical Code, but they are sometimes
overlooked. An additional suggestion would have been to
try an isolation transformer. The drive manufacturer
subsequently confirmed the transformer may not have
helped in absence of a proper earth ground.
This is one
example of how RF noise pollution can unintentionally be
propagated throughout a building. Improper grounding
conditions make this possible. Nonetheless, grounding
alone may not be the cause of some RFI problems. Certain
incorrect installation and wiring practices can also
create serious RFI problems.
IGBT's
All modern AC
Inverter drives use power devices known as Insulated Gate
Bipolar Transistors (IGBTs). These devices make it
possible to minimize annoying audible noise by using
switching frequencies beyond the audible range.
Unfortunately, AC inverter drives using IGBTs, present a
high potential for generating RFI -- Radio Frequency
Interference.
Fast
switching in these devices generates sharp-edged
waveforms with high frequency components that generate
more RFI. The most likely complaint is interference with
AM band radios 500-1600 Khz. Nonetheless, sensitive
computers, medical equipment and other noise-sensitive
devices sharing the same power buss could experience
serious interference.
In extreme
cases, the AC inverter drive itself can experience
electrical noise interference. If elevator machine room
equipment is not correctly laid out and properly wired,
the electrical noise propagated by the elevator drive
system can interfere with the elevator controller.
An example is
the building lacking a solid grounding system where the
elevator system experienced multiple problems. A solid
earth ground was provided to eliminate many electrical
noise problems, yet the elevator controller itself was
being affected by undetermined sources of noise.
The routing
of the contractor's field wiring into the controller was
examined and several deficiencies were found and
corrected. It was subsequently determined that the step
down power/isolation transformer required by this
particular application was physically located too close
to the front of the controller. With the controller door
open, the transformer created interference that affected
the control microcomputers. The remedy was placement of a
shield between the transformer and the controller,
although other medthods may have also worked.
Reducing/Preventing
Electrical Noise
Electrical
noise whether it is conducted or radiated can create
unusual phenomena that are difficult to evaluate. To
avoid the effects of electrical noise pollution,
consider:
! Proper
grounding including correct ground conductor sizing
! Contractors routing of field wiring
! Controller and motor isolation to prevent indirect
grounds
! Controller design and layout
! RFI filters
! Isolation transformers
! Higher standards of care by the installing
contractor
Warnings
from Manufacturers
MCE
Motion Control Engineering warns, in job specific
manuals, "For proper operation of the AC inverter
drive unit in your controller, you must make sure that a
direct solid groud is provided in the machine room to
properly ground the controller and the motor.
Indirect
grounds such as building structure or water pipe may not
provide proper grounding and could act as an antenna to
radiate RFI noise, thus disturbing sensitive equipment in
the building.
Improper
grounding may also render any RFI filter and isolation
transformer ineffective."
SAFTRONICS
When experiencing RFI problems with AC inverter drives,
Saftronics has stated that the first step is to verify
the existence of a proper grounding system. All too
often, old commercial or residential construction relied
on "indirect" grounding methods in which the
building ground was accomplished via steel water pipes or
conduit instead of through solid, properly sized
conductors. This poor practice increases the likelihood
that common mode noise will be propagated throughout the
facility.
Conclusion
The
phenomenon of AC static drive noise generation can
adversely effect many devices including the controller
itself. Nonetheless, AC static drives offer technology
that, in numerous circumstances, can provide more
benefits than alternative drives. Awareness of the
circumstances that allow AC static drives to interfere
with other devices and proper design considerations will
greatly reduce the effects of these phenomena.
While this
publication addresses AC inverter drives, it is desirable
to continually explore issues relating to emerging AC
drive technology.
MCE's
Technical Publication series is intended to be an
informative catalyst for ongoing dialogue and sharing of
information between consultants, elevator contractors,
owners and other interested parties. Shortly, MCE
Technical Publications will be available via an
electronic library accessible through the MCE
On-Line! service.
Don
Alley, MCE Vice President, Research and
Development
MCE R&D Staff
January 1996
It is
MCE's philosophy to share information with interested
parties. To this end MCE grants unlimited reproduction
rights,with proper attribution, to NAVTP and/or NAEC to
further engineering and technical excellence within the
elevator industry.
Motion Control
Engineering, Incorporated
Copyright
(c) 1999 ElevatorPro.com All rights reserved.
Reproduction
in whole or in part in any form without express written
permission is prohibited.
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