VSD Stands For Very Significant Dividend

Tailoring motor speed to the task offers a variety of important benefits.

By Seán Ottewell, Editor at Large

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In addition, after a shutdown the drive system returns to operating conditions within two minutes, compared with two hours for the steam-driven blowers. Another benefit is that the drive system can withstand micro power cuts at the refinery.

Projected savings for the first year should cover 33% of the project costs.

Preem Petroleum's facility in Gothenburg, Sweden, also has gained by switching to a VSD. It replaced a damper to control throughput from an exhaust fan that drives hot gases through a catalytic converter to reduce NOx levels to comply with strict government regulations.

The damper wasn't sufficiently accurate, occasionally leading to unacceptable NOx emission levels. These occurred when instabilities in the vacuum created by the fan led to inadequate control of heaters on the suction side of the fan that contribute to the processs, necessitating shutdown of the fan. When this happened, the refining process continued but the NOx-rich gases had to bypass the catalytic converter, venting directly into the atmosphere.

Motor control based on ABB's ACS 1000 drive delivers greater control of the process, cutting fan downtime and reducing emissions (Figure 4). The consistency of the vacuum on the suction side has eliminated the heater process stoppages and, with them, the need for regular catalytic converter shutdown.

Emissions now are consistently and predictably low and fully controllable. Other benefits include decreased downtime and improved product stability and thus consistency. Another advantage was that total system commissioning and startup took just three days, including complete electrical circuit redesign and motor reconnection.

Owens Corning, as part of a multi-million-dollar investment in energy efficiency at its Guelph, Ont., glass-fiber plant, targeted fans on the critical cooling section of the chop strand mat line. It wanted to reduce the speed of the 125-hp cooling fan and the three 40-hp recirculation fans on the oven, while not affecting the integrity of the product. Improper cooling can reduce the tensile strength of the web and cause it to break as it's wound into rolls.

The plant engineers turned to Rockwell Automation, Milwaukee, Wis., which installed a PowerFlex 700 AC drive to control the cooling fan, and three PowerFlex 70 AC drives for the oven recirculation fans.

This helped Owens Corning achieve a 57% energy saving on the chop-strand-mat-line fans. The lower fan speeds also bought additional advantages, including longer motor life, increased safety and reduced use of natural gas in the oven burners.

The four drives help provide total annual energy savings of 538 MW-hrs, or approximately $36,000/year — giving an investment payback of approximately ten months.

"The results were dramatic on the 125-hp fan," says Frank Peel, Owens Corning's electrical engineering specialist. "We were able to reduce the energy it was using from 88 kW down to 41 kW. The payback on that drive was under six months, and there was no negative effect on the product's tensile strength."

In August, Rockwell added the PowerFlex 755 AC drive to its range. That drive, aimed at motor control in heavy industries such as oil and gas, boasts expanded application flexibility, advanced diagnostics and easy-access design.

"Customer feedback about the desired attributes of a high-power drive gave us the information we needed to design our extended power range of PowerFlex 755 drives," notes Steve Perreault, drives product manager. "They told us they needed excellent reliability, ease of maintenance, and common control options to help reduce inventory and spare parts."

Siemens, Nuremberg, Germany, points out that while electric motors represent over 65% of total industrial power demand, approximately 70% don't use optimal motor control. So, VSDs can provide substantial further benefits at plants — cutting energy consumption in some cases up to 70%, and also helping to reduce production costs, improve product quality and ultimately lower CO>sub>2 emissions.

Using the company's SinaSave software tool can hugely aid in unlocking the real potential of VSDs, says Franz Ferdinand Friese of its Drive Technologies Division. The program can determine the potential savings and payback time for energy-efficient motors and frequency converters. The calculation also includes key figures for the volume flow, such as delivery rate, discharge head, operating times, as well as the delivery profile, which is crucial for the savings effect. "The user receives the result of the calculation, which shows him the potential savings over control methods without a frequency converter, and states the time within which the purchase of a frequency converter will pay for itself. This is usually just a few months," he explains.

"The program can be used by both machine and pump manufacturers, as well as by plant operators, to calculate how much of their energy costs they can save by using variable speed frequency converters and energy-efficient motors," notes Friese.

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