Motor soft starters provide for energy saving

Soft starters allow smooth start up over a period of a second or two: plus motors normally operate very inefficiently when unloaded - this enabled significant energy savngs at Mondi Packaging

Soft starters, as their name implies, allow electric motors to start up smoothly over a period of a second or two, rather than coming on-line suddenly which can cause physical damage through mechanical shock and cause voltage transients and power corruption in the mains supply especially on larger motors.

Motors tend to operate at about 80% efficiency when under full load and far less when unloaded, inefficient motors generate energy waste in the form of heat, vibration and noise increasing maintenance costs and reducing the life cycle.

Motors are actually sized and designed for load conditions that rarely occur thus contributing to energy waste, Motors alone do not have soft start technology and without a soft starter, the energy demand alone can dramatically increase energy costs: the actual cost of a motor only accounts for about 3% over the lifetime of the equipment, the other 97% is energy! Soft starters are commonly used to reduce the current when starting AC Motors and then they are usually by-passed, however the RVS-DN series from Silverteam is different, fully rated it does not need to be by-passed and as a result can stay in line and act as an energy controller.

In a recent example an RVS-DN soft start has significantly reduced power consumption on a waste paper shredder being run by Mondi packaging in March, Cambridgeshire.

The soft starter was recommended and supplied by drives and controls specialists Silverteam to their distributor Nottingham Electrical Transmissions (NET), which was helping Mondi re-engineer its waste materials processing plant.

Site Engineer Peter Sweeney explains.

"We manufacturer corrugated paper/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 is only in use about 75 percent 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 in fact a solid metal disc with six cutter blades mounted on its front face.

The 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 trough draft draws it over the disc and into the baler.

Experts from Silverteam and NET where called in to assess various options such as harmonic filters, variable speed drives and soft starts, who noted that the nominal rating of the fan was 22kW while the average load factor was 9.3kW, or less than half of its capacity.

This lead to the suggestion of a soft start solution, with the tantalising promise of significant energy savings as well as protection of the mains quality.

In fact Silverteam/NET were so confident that it suggested that an expert Mr Phil Windsor from Nottingham University school of Environmental Engineering were called in to assess the results in detail.

They monitored the performance of the fan before and after installation of the soft start and confirmed in an official report that the power consumption fell and the soft start was acting as an energy controller.

Energy controllers allow motors to run at constant (optimum) speed.

When the load is less than 90 per cent of the motor rating the controller reduces the current from the power supply, reducing the voltage and hence saving energy even though the motor remains running at its set speed.

"The design of the soft start installation was quite complex," says Sweeney, "because we had to allow for intermittent running, varying load, varying trim size and varying fan speed.

Thanks to Silverteam, the fan drive is saving energy, as are many other parts of the plant thanks to the re-engineering, and our mains supply is probably better than previously too.".

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