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Medium voltage drives add sensorless vector control

02 August, 2017

EXCLUSIVE  Rockwell Automation is expanding its family of medium-voltage variable-speed drives, adding sensorless vector control technology that will deliver improved performance compared to the previous V/Hz technology – including the ability to deliver 100% starting torque, compared to around 40% before.

The air-cooled PowerFlex 6000 family also has two new output voltages – 6.9kV targeting the North American market, and 11kV for EMEA and Australia – taking the number of voltage options to nine, covering the range 2.4–11kV.

The drives, aimed at heavy-duty uses such as compressors, pumps and fans, are suitable for variable- or constant-torque applications. They can handle motor currents of up to 680A for all voltage ratings, and offer a top power rating of 11MW at 11kV.

The drives’ power cells are now rated at 690V (up from 650V before), reducing the number of cells needed for the 6kV and 11kV drives, and resulting in smaller drives. Brad Bugiardini, Rockwell’s senior MV drives specialist, says that the reductions in footprint sizes will be around 10­–15%.

The addition of VDC feedback allows the drives to control the voltage on the DC bus, giving better control when ramping down high-inertia loads. It will also provide better control of fast-changing loads that typically need a quick response – such as reciprocating compressors.

An internal assumed motor model will improve torque accuracy, and help to provide functions such as automatic voltage control, slip compensation, bus voltage and current limits, and stability control. It will also provide a constant-torque capability, opening up new heavy-duty applications.

Rockwell will be offering a choice of rectifier configurations, ranging from 18-pulse (nine power modules) at 2.4/3.3kV, to 54-pulse (27 modules) at 11kV.  

The drives have eco-designed main cooling fans, helping users in Europe and other IEC-based markets to meet EC regulation 327 and ErP directive 2009, and avoiding the larger footprint of a liquid-cooled drive. The cooling fans are powered internally via a tertiary winding on the isolation transformer. This reduces the need for user-supplied control power, as well as cutting equipment and installation costs.

The medium-voltage drives with sensorless vector control are expected to open up new markets

An optional, automatic cell bypass rated at up to 680A, allows users to keep operations running at a reduced capacity if a power cell fails. This gives time to plan for a scheduled shut-down and reduces unplanned downtime. The solid-state bypass circuit is faster than a traditional DC contactor bypass, cutting interruption times.

An optional pre-charge circuit can reduce in-rush currents to the drives’ transformers and power cells for 2–6 seconds during start-up, reducing stresses on the power components.

An uninterruptible power supply provides back-up power for the drives’ control circuitry, helping to improve system reliability, minimise downtime and mitigate potential machine damage.

The drives are said to be easy to commission, use and maintain, with standard faceplates and EtherNet/IP connections.

Bugiardini believes that the new drives will open up new markets both geographically and in terms of applications. For example, he expects the new 11kV models to appeal to users in Europe and Australia, while the air-cooled capabilities will appeal to users in the oil and gas sector who have previously had to rely on liquid-cooled designs. “We expect a big impact, especially in the compressor market,” he says.

Bugiardini also expects the new models to increase Rockwell’s market share in North America, where the company already has its largest installed base of MV drives. This will be helped by a 5–10% cost reduction compared to the previous generation. The new drives will be “very cost-competitive,” Bugiardini promises.

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