Soft starters avoid by-passing and stay in-line

Edited by the Engineeringtalk editorial team Mar 30, 2005

Soft starters, as their name implies, allow electric motors to start up smoothly over a period of a second or two.

Soft starters, as their name implies, allow electric motors to start up smoothly over a period of a second or two.

That way, rather than coming on-line suddenly, they avoid causing physical damage through mechanical shock and cause voltage transients and power corruption in the mains supply especially on larger motors, believes drives and controls specialist Silverteam.

Motors tend to operate at about 80 per cent 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 and the actual cost of a motor only accounts for about three per cent over the lifetime of the equipment - the other 97 per cent is energy, according to Silverteam.

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 claimed to be 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 said the company.

In one example, a RVS-DN soft start has significantly reduced power consumption on a waste paper shredder being run by Mondi packaging in March, Cambridgeshire (UK), said Silverteam.

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

Site Engineer Peter Sweeney explained: "We manufacturer 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 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 fan is mounted in a housing and sucks the trim off the end of the conveyor and in to 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 in to the baler.

Silverteam and NET where called in to assess various options such as harmonic filters, variable speed drives and soft starts.

The companies 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.

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

Silverteam and NET suggested that an expert, Phil Windsor, from the Nottingham University School of Environmental Engineering was called in to assess the results in detail.

The university team 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 said Silverteam.

"The design of the soft start installation was quite complex, 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," commented Sweeney.