“Where others may have built machines with greater power, Joy has concentrated on machines that run consistently with all the power required to mine the coal, and then some,” according to Joe Heinzer, electrical engineer in Joy’s longwall shearer development group.
“Where other designers may concentrate on questionable ever-greater power, we concentrate on reliability and innovation and more than sufficient power to mine the coal as economically and efficiently as possible.”
So, just as Joy was not the first manufacturer in the industry with a longwall shearer, neither were they among the first to offer AC haulage.
“In both instances, being first was not as important as being the best,” Heinzer said. “When we did introduce our variable frequency/variable voltage AC haulage, it had to be the 7LS shearer’s single most attractive innovation. Today, Joy’s AC haulage system is the industry standard that never has been equaled.
“The introduction of this system with the 7LS shearer in 1997 was ‘THE’ breakthrough in longwall haulage, finally achieving the highly desirable but elusive variable frequency drive that gives the operator absolute speed control while providing greater speed for cutting and flitting, traveling without cutting,” Heinzer said. “In the Pittsburgh Seam of the eastern United States Appalachian coal field, the 7LS shearer has achieved cutting speeds in excess of 20% higher than that of the previous 4LS shearer.
Variable frequency drive (VFD) is accomplished by varying the frequency of the voltage going to the drive. In the US, all AC motors operate on 60 hertz, the electrical potential rising and falling 60 times a second and to attain variable speed drive you vary that frequency. The 7LS AC drive frequency ranges from zero (stopped) to 125 hertz (top speed). This is accomplished utilizing a semiconductor switch, called the Insulated Gate Bipolar Transistor, a key component of the variable frequency drive. (While the drive is variable, the cutter motors are wired direct and operate at a constant 60 hertz.)
“This was—and is—quite an innovation,” according to electrical engineer Peter Dalton, Joy’s VFD expert. “In addition to the higher speeds and infinite speed control now provided the operator, the AC drive also eliminated the maintenance and repair problems associated with DC motors, primarily the upkeep and replacement of DC motor brushes.
“Another benefit of the 7LS shearer I would have to say,” said Niederriter, “is the improved ranging arm cylinders. Through the acquisition of Longwall International in 1995, we gained access to a new level of technology allowing us to build hydraulic cylinders for the ranging arms with higher capacities than hitherto had been possible.”
Niederriter said he had to believe there were several elements beyond the obvious mechanical and electrical advantages that have made Joy the world’s preeminent manufacturer of longwall shearers and mining systems.
“Joy is a company that pays attention to details, a demonstration of which is the arrangement of component parts on a machine. The components in the controller are arranged in a manner to facilitate maintenance and maximize uptime,” he said.
“The location on the machine of each component is determined after carefully considering machine vibration and its effect on electrical systems and electronics, as well as considering the environment in which the machine will be operating. We are meticulous when it comes to component location.
Niederriter said unlike other manufacturers that use off-the-shelf industrial drives, the Joy-designed AC VFD was developed specifically for the mine environment and for mining applications.
“Also, the sheer numbers of Joy machines out there works to the customer’s advantage. Unlike some companies that must maintain two and three times the parts in inventory per machine they have out there, with so many Joy machines in operation around the world, we have the critical mass that can maintain the flow of parts.”
Like Joy’s in-house designed drives, the JNA control system is proprietary; developed in-house for mining applications in a mining environment.
“The JNAI system is used on all 7LS shearers. The JNAII system is used on continuous miners and Joy’s continuous haulage system, the Flexible Conveyor Train. The JNAII system will be introduced on shearers toward the end of this year,” Dalton said.
The JNA system regulates the cutting speed of the shearer to limit the mechanical, electrical and thermal stresses on the haulage and cutter motors, and on other machine components.
The JNA system also provides extensive diagnostic capabilities, as well as trouble shooting aids and continuous monitoring functions to assist the operator in fault-finding. The system displays temperature of motors, current being drawn and hydraulic pressure, and has alarms to signal malfunctions or stress to the system. The JNA systems, which also maintain event logs for up to three months, have considerable input/output capabilities and are able to transfer data from the shearer outby and to the surface, as well as having the potential to network.
Existing shearers with JNAI software can be retrofitted with JNAII but retrofits would be best suited to the 7LS series.
A major advantage is the system’s ability to provide extreme overload capability to the AC drive of up to 400% for 15 seconds, important in traction applications. While a shearer may have a nameplate rating of 100kW rating for continuous operation, the operator may need 200 or 300kW for short periods of time in traction, if a shearer encounters difficult conditions.
The shearer’s onboard electronics are also the means by which Shearer Initiated Roof Support Advance is achieved.
“The resultant minimum manpower, maximum production and Joy’s increased availability translates into higher productivity and lower operating costs for the mine operator,” Niederriter said. “That’s what the operator requires in today’s market; that’s what Joy’s 7LS shearer and variable frequency drive delivers.”
