New Variable Frequency Drive from Atlas Copco boosts raise boring productivity
Variable Frequency Drive gives the possibility to govern the rpm of the main motors and consequently the speed of the pilot bit and the reaming head by means of frequency conversion of motor current. This feature will be used for the first time on the two Robbins 73 RM-VF raise boring units to be delivered to the Norilsk Nickel poly-metallic mines in Northern Siberia early next year.
With 35 units delivered world wide since 1980, the Robbins 73 RM, a medium size unit, from Atlas Copco has become the most popular raise boring model today.
The 73 RM develops raises and shafts in the 1.8 to 3.1 m (6- 10 ft) diameter range.
In May 2001, Atlas Copco Rock Drills AB received a MUSD 5 order from Norilsk Nickel for two new state of the art Robbins 73 RM-VF. The order includes a complete package of spare parts, cutter heads, drill pipes and other drilling accessories. These units will increase the existing fleet of Robbins Raise Borers at the mines from 7 to 9 units and will be used mainly for development of ventilation shafts. Final testing and delivery will take place from the Örebro factory during the first quarter 2002
Extreme flexibility to geology
Variable speed control has so far been achieved by either a hydraulic or DC electrical motor drive. The advantages of the new AC drive, which is a fully so called torque vector controlled drive, is superior efficiency, reliability and reduced operational costs.
The possibility to vary the reamer head speed is particularly beneficial for operation under widely varying conditions, from hard to soft and from solid to fractured rock. Soft rock requires more torque than hard rock. Using conventional Raise Borers it is difficult to fully utilise the machine’s installed capacity in hard rock, unless the speed can be increased. Also the reaming head speed can readily be adapted to various reaming head diameters. In all, the final outcome is higher productivity and better cutter economy. Main features are:
the machine and the drill string is automatically protected from overtorque
using full torque at minimum current and low speed gives smooth start-up and stop proceedings at constant torque, reaming is performed in the 0 – 8 rpm range (0 – 50 Hz), and pilot hole drilling in the 0 – 30 rpm range.
At constant power, reaming is performed in the 8 – 16 rpm range (50 – 100 Hz), and drilling in the 30 – 60 rpm range, where the torque drops as the speed increases.
The enclosed graph illustrates the ratios between torque and speed during operation within the constant torque and the constant power zones
Photo no: 201828
The well-known Robbins 73 RM raise boring machine, from Atlas Copco, is now available with a Variable Frequency Drive for regulation of reaming head speed.
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