When designing its synchronous reluctance motor and drive (SynRM) packages for twin extruder applications, ABB had several objectives. The goal from the outset was creating 15% lower energy usage, lower motor maintenance costs and a drop in audible noise.
Radius Systems’ plant in Derbyshire, UK, houses 14 extrusion lines, each of which produces polyethylene pipes from 16 to 1,200 mm, together with an injection molding facility making fittings for utilities companies’ gas and water transfers.
Eight lines are powered by dc motors, many of which are more than 20 years old. In spite of Line 12’s high use rate, the 182-kW dc motor was regularly failing, leading to an increase in downtime and rising maintenance costs. The company estimated that the annual static and dynamic checks, brush changes and outsourced labor costs alone were about £2,000 per motor.
Before and after monitoring
Inverter Drive Systems (IDS), was tasked with carrying out “before” and “after” monitoring of the dc motor on Line 12. This would determine the energy savings while confirming that an investment in an ac motor and drive would meet Radius Systems’ three year capital payback policy.
“Monitoring of the application is critical to the success of any installation,” explained Phil Nightingale, sales engineer, IDS. “It allows us to determine real energy saving potential which means we can size the ac motor and drive correctly. We are often able to reduce the size of the motor and drive package required while exceeding production targets.”
IDS determined the speed range of the line’s dc motor which revealed that it was over-sized for the application, hence consuming more energy than necessary. Also, despite the motor having been regularly maintained, it was 20 years old and nowhere near the efficiency for which it was originally designed.
Energy saving bandwidth
It was estimated that potential energy savings from a SynRM package for Line 12 would be in the range from 8 to 15%, with a return on investment within two years. In addition, an excess of £2,000 cost avoidance was estimated with the removal of dc motor maintenance.
“To pick an exact production run that can give an accurate energy measurement is very difficult to quantify, hence the range from 8 to 15%,” explained Rob Betts, engineering manager at Radius Systems. “The actual energy savings is dependent on several process variables such as raw material type, different product ranges and profiles, the temperature at the die and the barrel and the general ambient environmental conditions. For instance, energy consumption is higher on a cold day as more energy is needed to warm up the process.”
The SynRM, a 200-kW, 1,500-rpm, IE4 component, together with the industrial drive ACS880, created a much higher efficiency than standard ac induction motors and drives. It’s also a quieter package than the existing dc motor. However, a challenge was presented by the physical footprint of the existing dc motor.
“Dc motors tend to be long and thin,” explained Betts, “whereas ac motors tend to be shorter but bigger in diameter.” The SynRM can be up to two frame sizes smaller than a conventional induction motor which is a benefit in the direct replacements of dc motors.
Challenges of constant torque
While SynRM is well tested on quadratic torque applications, like pumps, fans and compressors, there are fewer constant torque references, such as extruders. Despite the shortage of constant torque references, Betts was confident in the solution.
“At no time did we feel that we were taking a risk with this new technology,” he said. “It had been widely tested and we had every faith in the outcome. As such, we did not consider it to be a risk at all. We were able to meticulously plan the changeover of the dc drives to SynRM so that the entire installation and commissioning was carried out in just five days.”