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.

Top tips for specifying VFDs - Joe Kimbrell - Variable frequency drives can reduce energy consumption, improve real-time control and lengthen motor life. Selecting the right one for your application requires asking the correct questions - from Process OnLine and Automation Direct.

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.

The Digital Drive - Jonas Berge explains how variable speed drives can become an integral part of the digital plant architecture. The final control element that first comes to mind for process applications is the control valve, but variable speed drives (VSD) also variously referred to as a variable frequency drives (VFD), adjustable frequency drives, or frequency converters are also used as final elements in closed loop flow or pressure control.  When it comes to set up, the sheer amount of configuration options available in a variable speed drive can seem overwhelming to technicians who have to commission them. A drive can have hundreds of parameters to customize AC motor controls for different applications. Most AC drives also have useful diagnostics about the motor and drive system and some even have predictive diagnostics. To help technicians more easily setup applications and diagnose problems, drive manufacturers now use electronic device description language (EDDL) to make drives easy to setup and diagnose by defining how the drives are displayed in the system. Bus technology and EDDL provide the ability to integrate instrumentation and controls with electrical and switchgear, enabling plants to freely select control system and electrical system independently, yet enjoy the ease of use as a result of tight integration - from Control Engineering Asia.

Clever pumping - All In One Drives Make the Water Industry’s Pumps Intelligent - Sanna-Kaisa Ehanto - The growing use of VSDs, particularly intelligent drives for pump control, represents a major departure from the standard operating practice of using control valves to vary fluid flow. With its low energy usage and low maintenance outlay, the total lifecycle cost of a VSD-controlled pumping system can be significantly less than traditional pump technology. Other VSD benefits include smoother start-ups and production changes, more precise control during continuous operation, and faster diagnosis of potential system problems before product quality or process operation is negatively affected. The emergence of intelligent pumps is a critical step forward in the evolution of process management. With embedded intelligence, VSDs can provide pump control, pump condition monitoring, protection and traditional energy saving benefits - from ABB Drives.

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 - from Belden.

Choosing the Right Cable for your Variable Frequency Drive (VFD) System - Brian Shuman - Like many engineering solutions, VFDs present not only benefits, but drawbacks as well. For example, the same fast switching rate of the transistors inside a pulse width modulated VFD that can accommodate an abrupt speed change in a motor (and offer precise control of processes) is also capable of generating unwanted noise in the drive system cable and in the drive itself. Electrical energy flowing in the cable contains frequencies as high as 30 megahertz. If this radio frequency energy is not contained within the cable, it can radiate out to interfere with the proper operation of nearby electronic equipment, less than robust, or commercial-grade Ethernet systems, and simple instrumentation wires even circuits that have absolutely nothing to do with the VFD system itself - from Belden.

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.

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.

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 to

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 - Do all Adjustable Speed Drives contributed current to a line short circuit? 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 CurrenSource Inverter (CSI) drives, but incorrect for PWM adjustable frequency drives.

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