Drummond Controls Conveyor Start to Boost Production Demands
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 explored 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 starters 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
Drummond's original drive system, built around a single drive and 300 hp wound rotor
motor, powered the existing slope belt with headpulley discharge and snub pulley. While
the wound rotor motor provided a moderate level of soft startup, 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 startup 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 startup. The wound rotor motor and drive 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 startup. "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.
Solution
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 gear drives connected
to 250 hp induction motors. The gear drives 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 startup, the motor comes up to full speed while the coupling
gradually engages the pulley. Startup 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 startup,"
notes Kosmus.
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 drives 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 gear drives.
"We used to have a floor fan blowing on the gear drive. 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. Startup 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 wound rotor motors, its cost
was up to three times as expensive as fluid couplings. Past experiences 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 base plate and belt
pulleys. Otherwise, the mine counted on its own ingenuity to complete the remainder of the
installation project 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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