High Voltage 
Continuous Miners 

By: John Willison
Joy Mining Machinery

    Underground coal mining managers, engineers and equipment specialists recognized long ago the relationship between input voltage and mining rate. Starting in the late 1970s mining machines progressed steadily from 440 volts to 950 volts with a corresponding increase in productivity, and by 1990 virtually all continuous miners were powered with 950 volts.
    JOY longwall shearer development made a very successful leap into high voltage during the late 80s. It was natural to extend the same philosophy into the continuous miner fleet as technology permitted. The latest JOY continuous miner high voltage series is the product of a development effort to offer the most cost-effective designs practical. The “27” series includes the 2,300-volt JOY 12CM27 and 14CM27 in various configurations. The new models are also produced in 3,300-volt, 50-hz versions for applications outside the USA. 
    The decision to develop and market higher voltage continuous miners was based on a number of key factors: performance potential, safety, maintenance and repair cost, regulatory issues, capital cost and haulage capacity.
    Primarily, increased performance potential is due to reduced voltage drop (for a given current) occurring in a trailing cable when using higher voltage. Since motor torque varies with the square of the voltage, any decrease in machine voltage has a drastic effect on motor performance. If voltage drops to 90 percent of nominal rating (a generally accepted lower limit), torque output will be reduced to 81 percent. For a given cable size, the 10 percent voltage drop that would occur when operating at 950 volts will be less than 2 percent on a 2,300-volt machine. In addition to the increased torque and horsepower levels attained, this improved voltage regulation leads to reduced heating of cables, motors and electrical components, positively impacting maintenance and repair costs.
    Safety features of the machine and its cable prevent high voltage from being more hazardous than their lower voltage counterparts. For example, high voltage cables feature shielding around each conductor. Medium voltage cables are required to have shielding only around the circumference of the cable. Also, high voltage cables include a double jacket with different colors used for each jacket to aid in visibility and damage detection. 
    While current Federal law makes no provision for the use of high voltage equipment on a continuous miner face, obtaining a waiver to use such equipment has been accepted practice since the first high voltage miner went into service in 1997. These waivers, which do place some additional requirements on high voltage operations, are becoming less restrictive as high voltage use increases.
    Of course, the increased capital expense inherent in producing high voltage miners must also be considered. More complex motors, switchgear and cable as well as increased machine mass and higher capacity gear trains increase cost. Despite these increased costs, the high voltage machines operating to date have achieved a lower life cycle cost than the lower voltage units they replaced.
    Finally, the decision to build higher capacity continuous miners can be dependent on haulage systems, as any haulage bottleneck could negate miner productivity gains. Joy Mining Machinery addresses that issue by offering products like the flexible conveyor train (FCT) that are matched to the high-voltage production rates. A 42-inch-wide conveyor operating at 700 feet per minute complements increased hopper capacity to handle high miner output. Other FCT developments have reduced maintenance time and costs. Another related development, the Dynamic Move Up (DMU) system, provides an extensible conveyor at the end of the panel belt to minimize FCT length, reducing system complexity and parts count. JOY battery-powered articulated haulers also have been designed to complement the high voltage miners. The 20AH27 offers an increased payload, decreased cycle time and increased reliability.

Two high voltage models
The initial thrust into 2,300-volt continuous miner designs focused on high seam applications owing to their higher production potential. Introduced in 1997, the JOY 12CM27 features 335-horsepower cutter motors and a machine mass of more than 80 tons to increase sumping and shearing forces. The 12CM27 has achieved record-breaking productivity numbers in some applications, and its extended rebuild cycle has proven its durability.
    Joy engineers later designed the 14CM27 high voltage continuous miner for mid-seam applications. The first 14CM27 was introduced in 2000, and the model is now being proven in the field. Like it’s bigger cousin, the 14CM27 offers increased mass (75 ton) and cutting power (twin 295 Hp motors) to maximize production potential. The 14CM27 also features a new, 1000-volt, 130-hp variable frequency AC traction system. The drive offers improved maneuverability through smooth operation and increased tram speeds. The JNA II (Joy Network Architecture) machine control system directs all operating, informational and diagnostic functions. The system allows adjustment of many machine functions via an onboard color display with a pictorial interface. The JNA II platform is adaptable and will accommodate increased automation functions.

    The use of high voltage continuous miners is still in the early stages, but the benefits have proven to be significant. Joy believes that the decision to convert to high voltage is economically justified in many instances once all factors are considered.      cl

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