Soft starters in quarrying applications

Why use soft starters for starting electric motors in quarries when variable-speed drives (VSDs) appear to be relatively 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, cost of a VSD. As a result, quarry operators who use soft starters can save a lot of money through a combination of reduced equipment costs, set‐up costs and installation costs.

Soft starters are well suited to most equipment used in the quarrying process, such as conveyors, vibrating screens, crushers and pumps. They control high kilowatt ratings (15kW to 250kW), so the savings compared with using AC drives are considerable. To illustrate this, the cost of a Fairford XFE soft starter is approximately 33% of that of an equivalent variable-speed drive at 22kW, and just 20% at 100kW.

While 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 as they provide optimal soft starting and stopping, resulting in less stress on motors and 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‐on‐line (DOL) starting are avoided. Secondly, the considerable stresses on the motor windings and the iron cores of the stator and rotor, which result in reduced motor life, are also avoided.

While these benefits are considerable, the mechanical benefits can be greater still. This is because, at start-up, the sudden impact of uncontrolled motor starting followed by the rapid acceleration to full speed can cause problems across a wide 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 due to frequent restarting.