Inverter topologies: Voltage-source or current-source

In very basic terms, a variable-frequency drive (VFD) consists of three sections, moving from the drive’s input to output. A rectifier (or converter) changes ac input to dc, followed by a dc link that serves as an energy storage circuit, and then an inverter switches dc back to variable frequency ac output.

08/11/2010


In very basic terms, a variable-frequency drive (VFD) consists of three sections, moving from the drive’s input to output. A rectifier (or converter) changes ac input to dc, followed by a dc link that serves as an energy storage circuit, and then an inverter switches dc back to variable frequency ac output. Among different ways to categorize VFDs, configuration of the inverter section is an important one—namely, current-source inverter (CSI) and voltage-source inverter (VSI).

One distinguishing characteristic is the energy storage section between converter and inverter. VSI drives use capacitive energy storage, while CSI drives use inductive energy storage in their respective dc links for voltage and current. Another topology of current-source drives is the load-commutated inverter (LCI), which also employs a dc link inductor, but relies on commutation by the connected motor (or load) via switching direct current to the motor windings. This contrasts with a standard CSI drive where a line-commuted rectifier and self-commutated inverter are typical.

VSI drives work with both induction and synchronous motors, some CSI drives also work with induction and synchronous motors, but LCI drives are limited to only synchronous motors.

According to TM GE Automation Systems (TMEIC GE), voltage-source inverter is the only choice for drives above a certain power rating, compared to older technology current-source inverter drives. “In addition, any drive load that requires high torques and high response, such as a steel mill drive, cannot use current-source because of its much slower response due to the inductive source,” says Tim Russell, senior system engineer. “CSI drives are best suited for pumps and fans.”

www.tmeicge.com

LCI drives are intended for very large power output, and in that sense are an exception to the overall power limit of current sourcing. LCI drives are advantageous for ratings up to 50,000 hp (37,500 kW) or even higher and for control of synchronous motors, explains Rick Hoadley—principal consulting applications engineer, MV drives—at ABB Inc. “LCI drives operate at a slightly leading power factor, which allows devices in their inverter section to be load commutated,” he says. “This eliminates induction motors, which can’t run with a leading power factor.” LCI drives are available from ABB and Siemens.

www.abb.us/drives

www.siemens.com

Power-switching devices

Power-switching devices constitute another difference between CSI and VSI drives. Whether a power device is current- or voltage-switched determines its applicability to the type of drive. These power semiconductors range from the venerable silicon-controlled rectifier (SCR) and gate turn-off (GTO) thyristor to newer symmetrical gate-commutated thyristor (SGCT) and injection-enhanced gate transistor (IEGT).

TM GE Automation Systems provides the following attributes and trade-offs among some of these devices:

  • Current- switched devices—SGCT and integrated gate-commutated thyristors (IGCT)—require many more parts in firing/gate control than voltage-switched devices, such as IEGT and insulated-gate bipolar transistors (IGBT), which are available in LV and MV versions.
  • Voltage-switched devices—IGBT and IEGT—have much lower switching losses than current-switched devices.
  • Conduction losses are nearly equal for equivalent voltage- and current-rated devices: SGCT, IGCT vs. IGBT, IEGT.
  • Voltage-switched devices allow higher switching rates and provide better output waveforms.

Also read:

Why Choose Medium-Voltage Drives?

What is medium voltage? 

Transformerless medium-voltage drives perspective

Frank J. Bartos, P.E., is Control Engineering consulting editor. Reach him at braunbart(at)sbcglobal.net.



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
The true cost of lubrication: Three keys to consider when evaluating oils; Plant Engineering Lubrication Guide; 11 ways to protect bearing assets; Is lubrication part of your KPIs?
Contract maintenance: 5 ways to keep things humming while keeping an eye on costs; Pneumatic systems; Energy monitoring; The sixth 'S' is safety
Transport your data: Supply chain information critical to operational excellence; High-voltage faults; Portable cooling; Safety automation isn't automatic
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Maintaining low data center PUE; Using eco mode in UPS systems; Commissioning electrical and power systems; Exploring dc power distribution alternatives
Synchronizing industrial Ethernet networks; Selecting protocol conversion gateways; Integrating HMIs with PLCs and PACs
Why manufacturers need to see energy in a different light: Current approaches to energy management yield quick savings, but leave plant managers searching for ways of improving on those early gains.

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.