Modern Mining June 2023

POWER SUPPLY AND ENERGY EFFICIENCY

VSD technology for energy efficient mining, and more

As South African mines work towards improved efficiency in their energy usage, they are also dealing with the various impacts of load shedding; the use of variable speed drives (VSDs) holds many advantages on both counts, according to Zest WEG Gqeberha branch manager Pieter de Villiers.

“ T he benefits of VSDs in making mining operations more efficient are well known, but there are plenty of other advantages,” says de Villiers, who was previously the VSD Service Manager at Zest WEG. “Many users will be pleasantly surprised by the ways that VSDs can assist in reducing the disruption caused by load shedding.” He highlights that a core advantage of VSDs is to avoid the high start-up current that an electric motor would normally draw in direct-on-line starting mode. This start-up current is typically six times higher than the motor’s rated current and contributes to an oper ation’s maximum power demand. “VSDs reduce a motor’s start-up current to just the current required to drive the load,” he explains. If VSDs were more widely applied across industries and households, he argues, the peak demand on the country’s power system could in fact be consider ably reduced – perhaps even making an important impact on load shedding levels. Power factor Another important contribution that VSDs make is in improving the power factor of electric motors. Where the current and the voltage in a motor are

not in phase with each other, the phase-shift power factor is not optimal. “The application of a VSD can improve the sys tem power factor to typically 0,97 – which is quite a drastic improvement and saves on considerable wastage,” he says. “By correcting the power factor, the motor uses less current and thereby contributes directly to efficiency.” VSDs also help reduce energy wastage in com mon applications of motors such as pumping. Traditionally, water pumps are expected to run at a constant speed, with the flow being controlled – or throttled – by valves as required. De Villiers points out that the more efficient approach is to control the actual speed of the motor, so it pumps only the vol umes required. The direct benefit is reduced energy consumption resulting in a reduction in energy costs. Load shedding disruption When it comes to the disruption caused by load shedding, many of the difficulties faced by mines and mineral processing plants relate to the re-starting of systems and workflows when the power supply is resumed. In many cases, there is a sequence that must be followed when machines are re-activated – for operational or safety reasons.

Pieter de Villiers, Gqeberha branch manager for Zest WEG.

A typical mine plant where WEG motors and VSD solutions can be applied.

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32  MODERN MINING  June 2023