Transformerless drives: Higher performance, lower cost
Reducing input harmonics using an input isolation transformer with multiple secondary windings to feed VFD bridges can compromise, and raise maintenance cost.
With electric motors providing the driving force behind most production output, selecting the proper ac variable frequency drive (VFD) will have a significant impact on a plant’s overall performance and operational efficiency. Application requirements dictate amperage ratings, optimal topology, options and accessories, and variable frequency drive (VFD) configurations are as numerous as the applications they control. Most topologies today generally utilize a pulse width modulated (PWM) inverter. However, rectifier designs in these VFDs vary significantly. Lower power designs often use a 6-pulse rectifier bridge.
As power levels rise, harmonic concerns also increase. A traditional method to reduce input harmonics was to provide an input isolation transformer with multiple secondary windings to feed multiple rectifier bridges in the VFD. This leads to 12-pulse and 18-pulse (usually the minimum required for IEEE 519-1992 compliance), or even higher pulse number configurations, depending on the number of windings/bridges. However, as system complexity increases, reliability may be compromised and maintenance costs increased.
Recent advances in medium-voltage VFD technology have resulted in significant cost and space saving advantages. A good example is direct-to-drive technology, where the VFD is connected directly to the electrical distribution system. These VFDs use PWM switching patterns to create an active front end (AFE) rectifier. This results in excellent harmonic reduction and allows the use of a simpler, more robust power structure that only requires a single rectifier bridge.
An integrated common mode choke is used to reduce common mode voltage so existing motors can be used. By addressing harmonics and common mode voltage, the isolation transformer becomes redundant. Connecting the drive directly, without an isolation transformer, results in a system that is ideal for retrofit, process improvement, or energy savings projects incorporating existing motors and control rooms, and where space is limited or at a premium.
— Frederick Jason , product marketing manager, medium voltage drives, Rockwell Automation
Edited by C.G. Masi , senior editor
Control Engineering Machine Control eNewsletter
Register here and scroll down to select your choice of eNewsletters free.
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.
Annual 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.