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Variable Speed Drives
AC
and DC Variable Speed
Drives - Application Considerations - Knowing what
controller/motor package to use in a given situation is just one of the subjects
covered in this overview of AC and DC drive application considerations - From
Reliance Electric
Silence
of the Drives - Along with energy savings and other benefits,
adjustable-speed drives have a downside—a tendency to generate disruptive
electrical noise that must be minimized in the drive’s design and installation
- Frank J. Bartos - They’re great for saving energy and optimizing automation
systems, but adjustable-speed drives (ASDs) inherently produce high-frequency
electrical noise. Fast switching transistors producing large voltage changes in
the drive’s power section are the prime source of electrical noise, also known
as electromagnetic interference (EMI). From Control Engineering.
Perplexing
Variable Frequency Drive Vibration Problems - Brian Howes -
Several unusual vibration problems have been seen recently that involve variable
frequency drives and different types of driven equipment. The drives and motors
are from different manufacturers and vary widely in size. The vibrations seem to
be consistent in that there is a vibration frequency that remains constant as
the shaft speed of the motor changes. Sometimes the vibrations are seen in
accelerometer readings, but more consistently the vibrations are torsional.
Generally, the solutions have involved changes to the software in the VFD
control system - From Beta Machinery Analysis Ltd.
Variable
Frequency Drives Optimize Performance and Protection of Offshore Oil Electric
Submersible Pumps - Electric submersible pumps (ESP’s) offer the
highest yield of most deep-well artificial lift systems, but suffer the highest
frequency of expense and repair. This profile outlines how a careful selection
of technologies, specifically the use of electric submersible pumps with
variable frequency drives, can optimize the performance in well extraction on
offshore oil production platforms - From Allen Bradley
Variable
Speed Drive Cuts Compressed-Air Costs and Helps Ensure Consistent Quality
- Wayne Perry - Any evaluation about air-compressor selection will be based on a
study of available technologies and existing equipment. In some instances, such
as when there is continuous demand at full load, a fixed-speed, rotary-screw
compressor may be the best solution. When the base load varies with an
additional load, it might be best to consider supporting the base load with
fixed-flow compressors and adding a unit with a variable speed drive (VSD) as a
"trim" device to carry the variation in the load - From Impo.
4.10
Building
a Reliable VFD System - Brian Shuman - A Variable Frequency Drive (VFD)
regulates the speed of a 3-phase AC electric motor by controlling the frequency
and voltage of the power it delivers to the motor. VFDs offer many benefits;
principle among them the ability to save a substantial amount of energy during
motor operation. In that sense, these devices represent both an attractive, “green”
engineering solution, and an economical choice. Other benefits worth mentioning
include the following: they can maintain torque at levels to match the needs of
the load, improve process control, reduce mechanical stress on 3-phase induction
motors by providing a “soft start,” and improve an electrical system’s
power factor. What’s more, legacy systems that now use throttling devices to
regulate motor speed can be retrofitted with VFDs to make speed regulation much
more efficient and precise. Special consideration must be given to the proper
installation and operation of the overall system that comprises the VFD, the
motor it controls, and the cable that connects them. This paper is primarily
concerned with the motor-supply cable in the VFD/motor system. It looks at some
fundamental cable design considerations, and presents suggestions for
installation. However, to give the reader some context, it makes sense to first
describe VFDs, their benefits and potential problems, and their relationship to
the motors they control.
1.11
Picking
the Right VFD Cable - Reduce Problems and Increase Reliability
-Variable-frequency drives (VFD) allow sophisticated control by AC motors by
allowing their speed and torque to be precisely adjusted. Control is achieved by
varying the frequency to the motor. It does not, however, send a pure sine-wave
frequency to the motor. Rather it sends a series of pulses, whose width varies,
to the motor. This technique—pulse-width modulation— supplies the drive with
the same power profile as a sine-wave frequency. PWM also allows the effective
voltage to the motor to be varied. While PWM provides an excellent way to
control a motor’s speed and torque, it creates several phenomena that can
affect the power, create noise, reduce the life of the cable, and disrupt
operation of the drive system. Understanding these phenomena explains why
selecting the proper VFD cable is important to maintaining reliable, long-term
operation - from Alpha Wire.
4.10
Cable
Alternatives for PWM AC Drive Applications - This paper
describes an alternative solution for cables used with Insulated Gate Bipolar
Transistor (IGBT) Variable Frequency Drives (VFDs). New IGBT technology has
introduced voltage stresses on motors and cables that leads to unpredictable
system performance and reliability. This paper includes a performance and cost
comparison between a continuously welded armored option, the option of lead wire
in conduit, and a proposed shielded tray cable. Unique physical characteristics
of the cables are discussed. A proposed cable with increased insulation
thickness is discussed that insures long-term cable service life under VFD
operation, while the shielded coaxial braid contains VFD EMI emissions. Other
applications, options, and termination considerations with respect to the
petro-chem industry is discussed. Cable performance is documented with
theoretical and experimental support. From Belden.
The Following Technical Papers are from Rockwell
Automation.
8.10
The
Drive for Harmonic Balance - Peter Tomazic - The presence of harmonics
can drastically alter the quality of the power provided to electrical systems
and may affect equipment on that supply. this paper explains how harmonics are
generated, why they are a problem and discusses various engineered solutions
that can mitigate their effects - Thanks also to www.processonline.com.au
EMI
Emissions of Modern PWM AC Drives - Gary L. Skibinski, Russel J. Kerkman,
and Dave Schlegel - This article provides a common understanding of the EMI
issues and provides simple pre-installation and post installation guidelines
useful for all interested parties involved in industry application of adjustable
speed PWM ac drives. From Rockwell Automation.
Interaction
of Drive Modulation & Cable Parameters on AC Motor Transients - R.
Kerkman - This paper investigates over-voltage transients on ac induction motors
when connected through a cable of arbitrary length to a Variable Frequency Drive
(VFD) consisting of a Pulse Width Modulation (PWM) inverter with Insulated Gate
Bipolar Transistor (IGBT) power devices. From Rockwell Automation.
Straight
Talk About PWM AC Drive Harmonic Problems and Solutions - Though much
has been written about harmonics and related issues with respect to AC drives,
many drives users still seek clear answers to some basic questions. The purpose
of this paper is to provide the interested reader with some basic information
regarding AC drives and harmonics with a simplified explanation of harmonics and
power factor, showing how both can affect a distribution system. It is the
intention of the author to dispel some of the myths as well as point out
legitimate concerns, show some viable solutions and their pros and cons.
Considerations
for the Use of AC Induction Motors on Variable Frequency Controllers in High
Performance Applications - Until recently the majority of AC
variable speed drives have been applied to variable torque, pump and fan
applications. Advances in drive technology have led to the use of induction
motors in high performance applications that exceed the capability of motors
designed for operation on sine wave power. These applications, which have
traditionally been served by DC systems, have created the need for definite
purpose AC induction motors designed specifically for operation on adjustable
frequency controllers. This paper will discuss many of the considerations for a
successful application and will highlight the limitations of standard motor
designs.
AC
Drive Ride-Through Techniques - While most AC drive applications do not
require the drive to ride through a power interruption, many of those that do
are crucial to a process. In those applications where ride through is required,
it is often very important that the ride through provided meets the demand of
the application to the extent that the process is not adversely affected. The
shear number of variables between applications, and variations in drive features
of different drives and drive manufacturers can make handling ride through a
real challenge.
Line
Reactors and AC Drives - Quite often, line and load reactors are
installed on AC drives without a solid understanding of why or what the positive
and negative consequences are for adding this piece of hardware. The purpose of
this document is to provoke some thought on the part of the person(s)
responsible for the successful installation of the drive, and to provide some
guidelines as to if, where and when a reactor is needed and what size reactor to
use.
Harmonic
Calculators - Tool for determining how much voltage and current
distortion might exist on your distribution system when operating non linear
loads. This program is not intended to determine harmonic distortion with single
phase loads. The analysis with single phase load is quite different from
analyzing a 3 phase ac motor circuit. For single phase circuits, a distorted
current wave form can place more stress on the supply system. For a 3 phase
motor circuit converted to variable speed, the distorted current wave form does
not place more stress on the supply system if the reactive component of current
is removed.
Eliminating
Voltage Notching on the Distribution System - When Silicon Controlled
Rectifiers (SCRs) are used in electrical controls, it is possible to experience
line voltage distortion in the form of "notches" in the waveform. The
types of equipment that utilize SCRs in converters or rectifiers, to change the
ac line voltage to a dc voltage, and thus experience notching include DC motor
speed controls and induction heating equipment.
Installation
Considerations - In the last few years, Adjustable Speed AC Drive
(ASD) manufacturers have migrated from Bipolar Junction Transistor (BJT)
semiconductors to Insulated Gate Bipolar Transistors (IGBTs) as the preferred
output switching device. The advantage of IGBTs over BJTs is that device rise
and fall time switching capability is 5 - 10 times faster, resulting in lower
device switching loss and a more efficient drive. However, for a similar motor
cable length as the BJT drive, the faster output voltage risetime of the IGBT
drive may increase the dielectric voltage stress on the motor and cable due to a
phenomenon called reflected wave. Faster output dv/dt transitions of IGBT drives
also increases the possibility for phenomenon such as increased Common Mode (CM)
electrical noise, Electromagnetic Interference (EMI) problems and increased
capacitive cable charging current problems. This paper is discusses these issues
and give solutions for each phenomenon.
Drive
and Motor Basics - An adjustable speed drive is a device that
controls speed, and direction of an AC or DC motor. Some high performance drives
are able to run in torque regulation mode.
Pulse
Width Modulated (PWM) - A short technical description
Adjustable
Speed Drives and Short Circuit Currents - Per
IEEE standard 141-1993 (Redbook), section 4.2.5, "adjustable speed drives
can contribute current from the motors to a short circuit..." This
statement has led to confusion regarding adjustable speed drives and short
circuit currents. The statement is true for some adjustable speed drives such as
dc drives and Current Source Inverter (CSI) drives, but incorrect for PWM
adjustable frequency drives.