Soft start system designed for coal mine conveyors

A Russian coal mine is set to install an explosion proof soft start for its vast conveyor system. Built in Britain, the explosion-proof soft start unit is rated at 6000V, 800kW, and effectively makes the conveyor fully automatic. It is designed to simultaneously start three separate motors, each rated at 400kW, and run the mine's main gate conveyor that carries coal from the working face to the mine shaft.

It was designed and built by Sheffield-based Baldwin & Francis, which specialises in electrical systems for extreme environments, from an OEM kit of parts supplied by Softstart UK. The kit consisted of thyristor power modules, a fibre optic EPT for firing the thyristors, and an electronic control module and user interface.

Baldwin & Francis took these components and built them onto a chassis, which was then enclosed in an Ex D housing. The whole system was custom-designed. For instance, it had to be compact, and this required that the thyristors be supplied with specially-made right-angled couplings rather than conventional in-line couplings. The use of fibre optics electrically isolates the high- and low-voltage parts of the soft start and removes the possibility of flashovers occurring.

Cooling of the soft start was a major design consideration, so that the electronic components could be used with confidence in such an aggressive environment. Several options were considered, with forced draught be chosen because it would be the most reliable over the expected long working life of the equipment.

Special requirements for mines

Mark Ramsden of Baldwin & Francis explains: "The worldwide mining industry is still fairly new to electronic solutions to do with starting motors underground and we are doing a lot of pioneering work in this respect. Electronics tend not to like the heat and gases typically found underground, and the thought that they may cause a spark is an anathema to mining engineers the world over. It is our job to show that electronics are robust enough and start sparking is not a problem."

Until four or five years ago, all mine-based conveyor starting would have been via a fluid coupling, but there is now a definite trend to more modern ideas. Baldwin & Francis is at the forefront of this, often transferring technologies developed in its other markets such as oil, process and marine.

Similarly, Softstart UK is riding a wave of interest in medium-voltage soft starts, particularly from the heavy and harsh-environment industries such as quarrying, groundwater pumping, offshore, waste management and recycling. The majority of its installations are bespoke and require considerable design input. Often this leads to specials parts being sourced or made for particular jobs.

Ramsden states: "Starting large motors direct on line underground is challenging. Often weak reticulation supplies preclude this from happening. Soft starting is essential in situations like this, and the expertise we can tap into at Softstart UK is invaluable."