Variable speed drives (VSDs) can provide a host of advantages, claim vendors. They cite substantially improved energy efficiency, better performance and reliability, reduced maintenance costs, enhanced operating flexibility and lower emissions — to name but a few benefits.
But are these claims backed up in reality?
Yes, for the most part, says Kurt Bieniek of the technical materials management group of BASF, Ludwigshafen, Germany. "In many applications the above statements are true, for example, with extruders, large reciprocating or displacement pumps, large agitators, and high-inertia systems."
While BASF doesn't have specific figures on the benefits of using VSDs, they form an important part of an overall strategy aimed at improving the energy efficiency of its production processes by 25% by 2020.
The company finds the greatest benefits when VSD use is oriented toward a specific service or mode of operation (Figure 1).
"For some applications VSDs are predetermined solutions, such as for extruders and for larger reciprocating pumps (»1 kW). However, in other applications the use of VSDs is considered and analyzed. If the machines are frequently operated in partial load the savings potential rises. In individual cases the application of a frequency converter can reduce the energy costs by 40%," Bieniek adds.
BASF is working closely with original equipment manufacturers to develop solutions that will improve both its own device-testing capabilities and extend VSD applicability around the plant.
"In most cases, reliability and plant availability slightly deteriorate and maintenance costs slightly increase when using VSDs. But these effects have to be assessed in connection with the improvements caused by VSDs, especially in energy consumption and in production performance," notes Bieniek.
Reduced energy consumption and improved production performance, of course, appeal to all processors. However, plastics manufacturing is one sector in which such gains are especially crucial because companies are really struggling to maintain plant profitability in the face of rising energy costs, says Scott Barlow, vice president of Integrated Control Technologies, Carrollton, Tex., a distributor and authorized service provider for Yaskawa Industrial Drives, Waukegan, Ill.
"For most plastic extrusion manufacturing facilities, approximately 30% of the energy consumed can be attributed to extruder motors. If the line is more than five years old, it is more than likely that a direct current (DC) motor is being used as the extruder motor. Today, the majority of extruder machinery manufacturers are installing alternating current (AC) vector motors and drives on their extruders instead of DC systems. There are multiple reasons that they are making this change, but the biggest reasons are lower costs and better performance of the AC alternative," he explains.
Barlow cites the results achieved at one U.S. plastic sheet manufacturer that decided to replace a DC motor and drive system used on its primary extruder with a 500-hp AC vector motor and drive. The line has three extruders with its own service from the power company, so a power quality meter and analysis of the utility's bill enabled easy verification of the retrofit's impact.
"The retrofit was performed between billing cycles. The billing cycle prior to the retrofit showed an overall power factor of 0.49 with a peak power of 371 kW. After the retrofit, the bill showed a power factor improvement to 0.86 and the peak power was reduced to 360 kW. It is clear that the retrofit had an impact on the peak kW, which demonstrates an energy efficiency improvement, as well as a power factor improvement."
"The energy saving in this application was estimated at $2,500 a month, equating to $30,000/year. Combined with the maintenance costs associated with the DC motor, the estimated return on investment for this application was 1.4 years," he notes.
For its part, Yaskawa has just added the A1000 range of VSDs to its portfolio. The company claims the A1000 is robust, flexible and user-friendly, and provides twice the calculated design life of previous generations (Figure 2).
Meanwhile, ABB, Zurich, Switzerland — which recently expanded its drive (and motor) portfolio with the purchase of Baldor, Ft. Smith, Ark. — says six main drivers spur developments in its VSD technology: making devices more user friendly, with common user interfaces across the whole range; simplifying maintenance and installation; reducing the footprint; integrating certified safety features; providing improved lifecycle services; and creating industry- and application-specific features.
"In the chemical, oil and gas (COG) industry there is reluctance to use the newest technology. So we are working closely with our customers to see what would be the best solution for them and we are giving them training to use the products in the best possible way. Also, we are working to ensure that the drives are extremely reliable," explains Markus Eklund, market manager COG.
ABB's experience at Repsol's refinery in La Plata, Argentina, highlights the benefits that VSDs can provide.
The 750-acre site contains 35 different processing unit. Originally, steam turbines handled a large number of drive applications. This posed a number of disadvantages, including the higher cost of steam versus electricity, the greater maintenance costs of gas turbines relative to electric drives, and high operating costs due to the large amount of cooling water circulating through the surface condensers.
So, Repsol replaced one of the three steam-turbine-powered blowers on fluid catalytic cracking (FCC) unit A with an electric system consisting of an ABB KTMP 4400 double secondary transformer, an ACS 1000 medium-voltage AC drive and an AMB 560 L2L motor. Benefits realized include more efficient use of power because of better speed control, far smoother torque application throughout the required power range, and improved power index (Figure 3).
"The energy intensity index of the FCC unit improved by 10.5% within one year by replacing one steam turbine with an electric drive," notes Marcelo Ruiz, manager of the FCC units at the refinery.
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.
ANOTHER FAN CHALLENGE
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.