Problem Solution
A CASE STUDY FROM YOUR FALK SALES ENGINEER
Drummond Controls
Conveyor Start to Boost Production
Demands
GRAYSVILLE, AL-- When Drummond Coal added a second
continuous miner on each of the two sections at its Chetopa mine, essentially
doubling the mine's output potential, production forecasts heralded an initial
increase of as much as 1,000 tons per day. But, lack of horsepower on Drummond's
slope belt conveyor drive system threatened to stop these goals short of daily
projected output of
3,700
tons.
The company explores several ways of increasing
horsepower and gradually starting the conveyor to reduce the damaging effects of
belt slippage and over-torqued motors. They looked at soft-start options
including fluid couplings, soft start controls and wound rotor motors.
Eventually, Drummond settled on a system incorporating fluid couplings and dual
conveyor drives.
"Soft start is very important if you want to keep
your belt together and you want your motors to last," notes Chetopa's Chief
Electrician, Bill Kosmus.
Problem
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Drummond's original drive system, built around a single
reducer and 300 hp wound rotor motor, powered the existing slope belt with
head pulley discharge and snub pulley. While the wound rotor motor
provided a moderate level of soft start-up, its erratic, jerking motion
during start up created problems with other drive system
components.
For instance, the wound rotor motor caused the loaded
conveyor belt to slip during start-up as resistance, built up in the
motor, was released through the system in stages. The power surges
defeated the belt tension created by the snub pulley, causing damaging
slippage.
"The belt was slipping every day," says Kosmus. "The
slippage got to the point that sometimes you could see the belt
smoking."
The conveyor belt was not the only component to suffer
during start-up. The wound rotor motor and speed reducer worked overtime
to build up enough torque to start moving the loaded
conveyors.
"There were times when we would have to go down in
the mine and take coal off the belt just to get it started," says
Kosmus.
Despite these efforts, motors were frequently damaged
during start-up. "The wound rotor motor caused a lot of overloading since the
motor sees all of the load from the beginning," he says. "We botched a number of
motors just during start up and stopping."
As one of Drummond Coal's medium sized mines, Chetopa
depends on its 144 employees (120 miners) to bring high-grade clean coal from
the Mary Lee seam. The downtime began to haunt Drummond crews. "We could change
out a motor in four or five hours, but that's four or five hours of lost
production. Now, half a day of downtime could cost us up to 2,000 tons of coal,"
says Kosmus.
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Drummond-Chetopa installed two Falk fluid couplings and
double-reduction, parallel shaft speed reducers to achieve
more efficient system
start-up. | |
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Solution
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Chetopa Chief Electrician Bill Kosmus (left) and new
Mine Superintendent James A. Brown inspect the system that increased belt
speed on Drummond-Chetopa's slope belt conveyor to 650 fpm by utilizing a
dual conveyor drive system with fluid couplings.
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To solve the slippage problems and boost
conveyor speed, Sam Gilbert, Mine Superintendent, and Kosmus first
contacted Continental Conveyor & Equipment (Winfield, AL). Together
with Continental Conveyor they installed a dual pulley drive system with
remote discharge and fluid couplings for a true soft start.
The system incorporates two Falk
double-reduction, parallel shaft speed reducer drives connected to 250 hp
induction motors. The speed reducers feature a 17:10:1 ratio, which
produces 68.4 output rpm from an input of 1,170 rpm.
An extended delay fill chamber with adjustable
metering allows initial torque transferred by the coupling to be as low as
40 percent of the full load requirement of the conveyor. In a typical
start-up, the motor comes up to full speed while the coupling gradually
engages the pulley. Start-up time can be adjusted to more than a
minute.
"The fluid couplings are nice since you can
bring up your motors to full speed before the belt starts to work," Kosmus
adds.
The design also permits excellent motor
starting control. Both motors are started at the same time and the fluid
couplings allow both to come up to speed in the same time.
"The motors' amperage is within 2 or 3 amps of
each other during start-up," notes Kosmus.
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As
insurance against conveyor jams, a trip switch on the fluid couplings shuts down the electrical system when fluid coupling oil reaches a predetermined temperature. Without the safety measure, motors can burn out when a conveyor stalls.
To further enhance drive system life, Continental Conveyor boosted the thermal capacity of the reducers with shaft-mounted fans. The fans reduce the damaging effects of overheating like lubrication breakdown and premature bearing wear. They also allowed the use of smaller, less expensive speed reducers.
"We used to have a floor fan blowing on the speed reducer. It would still get hot enough to fry eggs," says Kosmus.
Perhaps most importantly, however, the configuration allowed Drummond to increase belt speed to 650 fpm while eliminating belt slippage.
"The wound rotor motor gave us soft start through resistance. It caused a lot of motor overloading," says Kosmus. "There is no comparison between fluid couplings and wound rotor motors. Start-up with fluid couplings is smooth whether or not you've got a load on the belt."
As another soft start option, Kosmus and Gilbert considered the soft start starters. While the electronic starter offered better control than would rotor motors, its cost was up to three times as expensive as fluid couplings. Past experience also influenced their decision.
"I had used soft start starters in the past," remembers Kosmus. "They either wouldn't come up to speed fast enough or they would come up too hard."
Surprisingly, installation of the fluid coupling/dual drive system resulted in no interruption in production.
Drummond-Chetopa relied on Continental Conveyor to build the baseplate and belt pulleys. Otherwise, the mine counted on its own ingenuity to complete the remainder of the installation product including the design and construction of a control box.
"We set ourselves up during the week for the installation of the system. The pulleys went in on one Saturday and the following Saturday we tied everything else in," says Kosmus. "We lost no production time."
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