Soft start saves 30% power on paper shredder

A packaging company in Wisbech, Cambridgeshire has cut power consumption on its waste paper shredding machine by 30% after installing a soft start controller that is acting like an energy controller. The company, Mondi, recently re-engineered its waste processing plant with the help of its local distributor Nottingham Electrical Transmissions (NET). Soft starts, as most design engineers will know, enable electric motors to start up steadily over a period of a few seconds, rather than coming on line suddenly, which can cause physical damage and wear in the motor, as well as voltage spikes and power corruption in the mains supply. Peter Sweeney, site engineering manager at Mondi commented: "We manufacture corrugated paper and card using a continuous web process. This is trimmed to width and we shred and bale up the trimming for recycling. "Originally, we had the shredder running 24 hours a day, but a site-wide energy audit made us realise that it [the shredder] is only in use about 75 per cent of the time. Stopping it rather than having it idling would obviously save energy, but we were worried that the stopping and starting would cause problems with the mains supply." The basic design of the shredder includes an in-feed conveyor, a shredding fan and a baler. The fan is a solid metal disc with six cutter blades mounted on its front face. The 22kW fan is mounted in a housing and sucks the trim off the end of the conveyor and into the blades. As the trim is shredded to the required size, the centrifugal force moves it to the outer edge of the disc from where the through draught draws it over the disc and into the baler. NET considered using harmonic filters and variable speed drives, but since the average load factor was 9.3kW (less than half of its capacity) a soft start solution from Silverteam (the UK distributor for Hitachi) was recommended. An environmental engineering expert from the University of Nottingham was called in to assess the results of the soft start in detail, who confirmed that the power consumption had fallen from 9.3kW to 6.7kW. "The design of the soft start installation was quite complex," said Sweeney "because we had to allow for intermittent running, varying load, varying trim size and fan speed." When the load is less than 90% of the motor rating, the soft controller reduces the current from the power supply, reducing the voltage and therefore saving energy even though the motor remains running at its set speed.