Новости


Would you like to subscribe?

Download full pdf

Automation Today

Latest News

Feature story

Case studies

Medium voltage – high popularity


Jan Van Den Bossche, High-Power Drives Business Manager at Rockwell Automation, explains medium-voltage motors and drives – what they are, what are their benefits and what application opportunities they offer.

With potential energy savings of 30-35%, lower cost of ownership and greatly improved reliability, medium voltage (MV) AC drives are growing in popularity in motor applications in the 400kW to 2MW range.
Medium voltage drives between 2.4-6.6kV allow variable-speed operation in high power processes such as pulp and paper, steel, mining, petrochemical, food processing, water and waste water. Common applications include large pumps, fans, compressors, conveyors, mixers and extruders. Due to the sinusoidal motor supply, they are also ideal for applications where long cable runs and the resultant voltage drop could be a problem: in offshore, submersible or remote applications.

Considerable savings in energy costs and short pay back periods provide the most compelling reason to employ MV drives
Removing the need for inefficient step-up and step-down transformers improves control performance and use of space. MV drives also offer a high degree of motor control and can limit starting current without compromising nominal torque over the complete speed range. They are therefore ideal for starting large motors on ‘weak’ supplies, where high starting currents could result in serious voltage reductions.
The PowerFlex range of MV drives include high-voltage symmetrical gate commutated thyristors (SGCTs) in an active-front-end rectifier to reduce harmonics and transformerless common-mode voltage protection. The SGCTs have an integrated gate driver to provide more efficient and uniform gating. They are better suited to handling fluctuating voltage and current while switching on and off during gating. SGCTs are gate-commutated (fired) instead of load-commutated, making them ideal for use in pulse width modulated (PWM) current source inverters as they produce an inherently regenerative drive. Thanks to this, many other components can be reduced in size or eliminated entirely.

“Compared to early models, the modern MV drive has dramatically reduced in size.”
The PowerFlex 7000’s Direct-to-Drive active front-end (AFE) rectifier uses semiconductor switching to reduce line current harmonics to levels that comply with the world’s most accepted harmonic standards. The AFE rectifier, uses the SGCT to produce a PWM switching pattern that prevents the drive from producing high line current harmonics. The special DC-link provides common-mode voltage protection, reducing motor stress while avoiding the use of an isolation transformer.

On the load side, the inverter section produces high-quality voltage and current waveforms with very low harmonic content. At higher frequencies, PWM works in combination with a small integral capacitor to reduce the larger-order harmonics to deliver a near perfect sinusoidal waveform. Sensor-less flux-vector control gives better performance compared with volts-per-Hz drives, and also enables the motor torque to be changed quickly.

Upgrading to a MV transformerless solution is made even more straightforward thanks to PowerFlex’s sophisticated firmware, which can be programmed at the factory with application-specific parameters. A start-up wizard helps the commissioning process by prompting the user for the required information and warning if parameters are missed or entered incorrectly. These features, combined with the auto-tuning function, result in faster commissioning.