The mine, in the western coalfields of NSW, had recently moved from the southern area to the new northern area. At this time a new, high capacity maingate conveyor system was required. The longwall blocks were in excess of 3 kilometre in length and the conveyor capacity was planned at 3500 tph.
The first northern block, LW19 used a maingate conveyor powered by a twin 500kW Dodge CST transmission drive-head using 3.3kV motors. This drive-head used proven technology and was successful apart from the fact that a planned tripper drive was not install, limiting belt capacity at the beginning of the wall.
A second longwall maingate conveyor was required to operate the next longwall, LW20. A tender document was prepared for a drive identical to the existing CST main drive, and a twin 500kW CST tripper drive.
All of the main suppliers quoted on the conveyor and several non complying tenders were also submitted, one from Nepean Mining offering a Variable Frequency (VF) Drive in place of a CST. The offer was for a 415V ABB Variable frequency drive unit and motors, directly coupled to the drive through a David Brown gearbox. The complete drive module would be mounted on a swing arm.
The Nepean tender primarily attracted the attention of the accounts department because the quote for the VF drive and gearbox was about 40% of the price of the CST. Senior management, though not keen on the idea, asked the question of the company and mine engineers, if we go with the VF drive would it work? This would be the first time a VF drive was to be used in an Australian coal mine in a maingate drive installation and would have be integrated with a CST driven tripper drive.
Nobody at the group engineering level wanted much to do with the project and all the other conveyor companies came up with reasons why it wouldn’t work. Everybody hated the idea of using low voltage drives.
The site electrical and mechanical engineers along with Nepean Mining looked at each of the engineering issues and concluded it should work very well. Senior management, after receiving the feedback from the mine engineers, and taking into consideration the fact that the mine was just about sold and they would be back home in the USA before anyone turned the conveyor on, gave the go ahead for the VF option.
VF technology using 415V systems are now commonplace in outside industry, although multiple 500kW units would be rare. At the time of tender much of the design of the VF drive was conceptual. The nuts and bolts were worked out by a consortium of the drive manufacture ABB, the panel manufacture MI Power, the tripper and belt signaling system manufacture Ringway Control, the primary contractor Nepean Mining and Baal Bone’s engineers.
Many engineering hurdles had to be overcome; the combined transformer starter is positioned on the surface in its first installation but needed to be able to move around a mine with a minimum height of 2.2 meters. One of the most important decisions was to cool the starter using a closed loop air-conditioning system. This provided a dust free environment at a controlled temperature for the VF drives. Earth leakage protection on the drive output was also an issue that was addressed. The decision was made to use the drive’s own ramp control. The starter PLC sends only sends a digital signal to start and stop the drive.
The use of 415V drives was a financial decision and was always a concern to engineering. As it turned out, using low voltage units had as many advantages as drawbacks.
This was proven when one of the ABB electric motors was accidentally dropped and destroyed just prior to installation, and a new 415v 500kW motor was purchased off the shelf as a replacement. The new transformer was basically an off the shelf unit also. Two cables were run to each motor to make physical handling of large cables easy. The distance between the starter and drive was kept to a minimum and VF drives don’t have high inrush currents to worry about. During the belt acceleration ramp, motor currents never exceed normal motor full load current.
The longwall panel had been developed using a small temporary conveyor drive. The main drive and loop take-up had to be installed during the longwall move. The site engineers felt some degree of trepidation when the final go ahead to proceed with the VF drive option was approved. Engineering development and extended commissioning time on a new longwall belt wasn’t going to make anyone happy. It had to go first time and keep going, a tough ask.
Did it work? At the time of writing, longwall 20 has produced more than 1 million tons. From start up the belt has had an availability of 98% and no downtime related to the VF drive has been recorded. We turned it on set it up and it went. The belt uses a simple linear speed control; 100-second start ramp up, a 15-second ramp down on a normal stop. The belt uses a 55kW eddy current loop with 330 m of storage which is working well, with such a controlled start and stop, the reaction time of the loop is not as critical as with harsher starting methods.
The VF drive has many additional advantages that the mine hasn’t utilized yet, like reduced belt speeds for developing longwall panels. The VF drive module is scheduled to be used as a tripper drive for the LW21 panel, in this application the VF drive will be used in a torque control mode rather than a speed control mode. Integration with a 500kW CST tripper was very successful; the tripper used local tension sensing to control drive output and the only communication to the main drive was a simple handshake signal through the signal line. Alignment of the swing arm drive module to the drive is quick and easy.
Baal Bone will definitely be considering VF drives for our future belt starting requirements. Many people have already been to site to have a look at our VF drive belt and we believe they have all been impressed. The mine is also using VF drives on the main ventilation fans, controlling underground water pressure and operating the showers in the bathroom so every body can have a great shower at the end of a hard day’s work.