High Voltage Continuous Miners
John Willison, Joy Mining Machinery of Franklin, Pennsylvania presented a paper High Voltage Continuous Miner Applications with Complimentary Haulage Solutions to the 2003 SME Annual Meeting.
A leading advancement over the past 30-40 year period has been the introduction of higher voltages to operate mining equipment. This advance enables higher power transmission for a given cable size, operation at lower amperages resulting in less voltage fluctuation, and reduced heat of the cable and electrical components. The industry progressed from 440-volt to 550-volt and upward to 950-volt steadily in the late 1970’s and 80’s with a corresponding increase in tons produced per man-hour, until virtually all continuous miners were being manufactured with 950-volt input by 1990.
Advancement in mine productivity and efficiency continued throughout the 1990’s as the continuous miner was modified to improve on its ability to turn machine horsepower to raw tonnage. These modifications included increases in the individual motor horsepower, increases in mass and center gravity optimization, machine reconfigurations for maximizing coal flow into the conveyor and maneuverability improvements.
In the mid-90’s, it was realized that voltage regulation had become a limitation to machine performance. Joy longwall shearer development had made a very successful leap into high voltage during the late 80’s progressing from 950-volt to 2300-volt to 4160-volt in the span of a few years. It was therefore natural to extend this philosophy into the continuous miner fleet, as technology permitted, and the latest Joy continuous miner series was introduced, referred to as the “27” including the 2300-volt 12CM27 and 14CM27 models.
The decision to develop higher voltage equipment includes performance potential, safety, maintenance costs, regulatory issues, capital costs, and haulage capacity. The improved performance potential is due to the decrease in percent voltage drop for a given current occurring in a trailing cable when higher voltage is induced on that cable. Since motor torque varies with the square of the voltage, any decrease in machine voltage has a drastic effect on motor performance.
A safe working environment for the miner, operator, and maintenance crew is of the highest priority and equipment designed for high voltage must be, at minimum, no less safe than their 950-volt counter parts. Cables include shielding around each phase. High voltage enclosures are required to include cover interlocks that deenergize the machine once the cover is removed. Ground fault protection is set at a low level and the high voltage power centers are required to have a ground fault test circuit that energizes the individual ground fault system for operational verification. For a given power requirement, the potential exists for use of cable with a smaller conductor size when compared to a medium voltage machine. The smaller and lighter cable resulting, will provide the benefit of fewer injuries due to cable handling. There is less heat in the cable, motors, and electrical components in general and this results in reduced maintenance costs and less lost time.
Regulatory issues include the necessity for the mine operator to submit a petition that requests granting a waiver for its use. Joy has developed a standard petition to simplify the process. Physical handling of energized trailing cable is approved only with the use of hot sticks or high voltage rated gloves. The means of protecting slack cable from damage and limiting human contact has been a major detail in the petition rulings thus far. The entire trailing cable length must be walked on a per shift basis to inspect for damage. Higher voltage continuous miners are more expensive to produce because of the added expense in motors, cable, and switch gear. The capital expense relating to conversion of the power center must also be factored into the equation. According to Willison, despite the increased capital costs, the high voltage installations operating to date have yielded a lower life cycle cost than the 950-volt unit they replaced.
Willison reviewed the Flexible Conveyor Train (FCT) which has been an ongoing development with Joy for over 25 years and it is now particularly suitable for use behind high voltage equipment. The FCT with 42” conveyor width operating at 700 fpm and increased hopper capacity matches the loading rate capabilities of the high voltage miner designs. New features, such as variable frequency traction and ladder chain, address past issues of belt and chain life coupled with the major reengineering effort focused on reducing maintenance time and cost.
A new product from Joy called the dynamic move up (DMU) has been designed for use with the FCT. The system consists of two conveying components, one movable and one fixed at the end of panel belt with the movable conveyor overlapping the fixed unit, being progressively extended on skids as the FCT advances behind the miner tail. A major advantage with this system is that belt moves can be scheduled to coincide with maintenance shift timing, thus minimizing production shift delays.
The Joy range of battery powered articulated haulage equipment has been designed to compliment the high voltage miner products. The equipment operates at 240-volt compared to the typical 128-volt and offers a number of features geared toward higher productivity including increased payload, decreased transit time, and increased reliability. cl
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