Soft starters save in quarrying applications

Soft starters provide cost savings, and prevent failures of power transmission components in quarrying applications, says James Bowler of Fairford Electronics.

Soft starters provide cost savings, and prevent failures of power transmission components in quarrying applications, says James Bowler of Fairford Electronics.

Why use soft starters for starting electric motors in quarries when variable speed drives (VSD) appear to be so cheap?.

The simple answer to this question is that the majority of motor control applications in quarries are fixed speed and do not demand the complexity and, hence, the cost of VSD.

As a result, quarrying operators can save a lot of money through a combination of reduced equipment, setup and installation costs.

Soft starters are ideally suited to most equipment used in the quarrying process: conveyors, vibrating screens, crushers, stone saws and pumps.

They control high power ratings (15 to 250kW), so the savings against using AC drives are considerable.

To illustrate this, the cost of a Fairford QFE soft starter is approximately 33% of that of an equivalent variable speed drive at 22kW, and just 20% at 100kW.

Although cost is a major factor in any decision to use soft starters, quarry operators and owners are also looking for more reliability from their plant.

This can be achieved with soft starters providing optimal soft starting and stopping.

The result is less stress on motors, mechanical components and also on the electrical supply.

The electrical benefits are twofold.

First, by using soft starters the dips in mains voltages that occur due to current peaks inherent in direct-online (DOL) starting are avoided.

Secondly, by avoiding the considerable stresses on the motor windings, and the iron cores of the stator and rotor, which result in reduced motor life.

Although these benefits are considerable, the mechanical benefits can be greater still.

This is because the sudden impact at startup of uncontrolled motor starting, followed by the rapid acceleration to full speed, causes problems across a wider range of equipment types.

Sudden torque stresses can displace loads from conveyors, and cause excessive wear on belts, pulleys, gears, chains, couplings and bearings.

Experience shows that the reduced downtime from not having to replace these components so frequently ensures fast payback on any soft starter unit.

Complementing the savings that can be made on component costs is the additional benefit that energy costs can be controlled more effectively using soft starters.

Peak demand charges can often be reduced, especially when a number of motors are started at the same time.

Also, motors can be switched off and started repeatedly - if necessary - whereas, previously, they would have been left running due to concerns about the negative stress effects on mechanical components of frequent restarting.

In addition to protecting motors and the power transmission chains, Fairford soft starters have also helped one major quarry operator overcome a problem of mechanical shear pin breakage on his asphalt mixing plant.

The shear pin is required to protect mixer motors in the event of a process jam.

The previous mechanical shear pin arrangement provided this protection well enough, but the time taken to replace it, once it had sheared, often meant that the asphalt had solidified, resulting in an unpleasant and time consuming task to remove the material from the mixing vessel.

The electronic shear pin facility in Fairford QFE soft starters eliminates the need for a mechanical shear pin entirely.

The speed and extent of a sudden and rapid rise in motor torque (ie jamming) is immediately detected by the QFE which will then decide on a course of actions ranging from instantaneous shutdown to monitoring for recurrences if the blockage is released rapidly.

The electronic shear pin facility also overcomes the problems of replacing fuseable plugs that are employed to provide overload protection in the fluid couplings that are still used widely in quarries.

The plugs provide a safety measure in cases of conveyors or elevators jamming.

They are, however, difficult and time consuming to replace, unlike the electronic shear pin which can be reset immediately.

Underlining the benefits of soft starters in quarrying applications is a recent application of Fairford's new low cost SDF soft starters in the harsh environment of a Devonshire quarry.

The SDF units are being employed to provide soft starting and soft stopping of 37, 45 and 75kW motors that drive pumps to extract water from the quarry workings.

They have been installed as part of three complete, turnkey panel systems designed and installed by Portico Industrial Control Systems, an international provider of turnkey systems right across the spectrum from HV switching to control and plant interfaces.

"The SDF is so simple to wire, in contrast to units from many manufacturers which have lots of complex control wiring", said Mike Shoulder, Managing Director of Portico.

"This saves us time on installation and also reduces costs".

The first system installed by Portico controls a 37kW pump extracting runoff water from new quarry workings.

As a river runs through the site, it was necessary for the quarry to provide a man-made reservoir to collect the water runoff from the new workings.

When the reservoir level reaches a certain point, the pump is started to extract water and pump it up to a new level, where it is treated to remove all the heavy metals (etc) before being pumped into the river.

The control panel for this pump is fitted with a Fairford SDF-14 soft starter, which, in addition to its reliability and cost advantages, was chosen for its ease of installation.

The second and third systems supplied by Portico control two pumps.

The first, driven by a 45kW motor, takes surface water from the area where the operating machinery is located and pumps it into a settlement tank.

A second prime mover, driven by a 75kW motor, then pumps this water up to the treatment area, where it is treated before being discharged into the river.

The first pump is controlled by a SDF-22 soft starter, and the second by a SDF-24: both units being chosen for their ease of installation and also their competitive price.

"Like any contractor who has to guarantee their work, we want equipment that gives us trouble free operation, which means no costly returns to site", said Shoulder.

"We've used Fairford soft starters before, and they're a superb product, both in terms of performance and reliability - and also as regards Fairford's pre and after sales service".

Back to top