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.

07/24/2008


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
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