China, it is the actual capacity of theAFC
that mostly affects the final production.
While the maximum capacity of theAFC
has reached 4500 tph, the actual efficiency
is, on average, only 17%. Even in large
coal mines, an efficiency of only 35% is
hardly satisfying. ABB found that high
maintenance rates and unstable loads are
the main reasons for low efficiency.
New AFC starting method
As mentioned before, VFDs are capable
of large starting torque. This reduces the
risk of unsuccessful starts and saves
man-hours for clearance of overload coal
on the AFC when starting.
AFCs normally have one motor for
the head drum and one for the tail
drum. Before starting, spare chains are
stacked under the head drum and this
creates the potential risk that they will
block the return way underneath. Using
the tail drum to tension the return chain
first and then starting the two drums
together will prevent this risk, while
accomplishing an efficient start.
Compared to the current practice of
running the tail drum for 1 – 7 sec. in
advance, this control idea will detect the
pre-running time automatically and run
the two drums together. It can make sure
that the two drums both contribute via
the tensioned return chain during the
starting.
New balance method to reduce the
wear of tail area
Optimised power distribution can be
easily achieved based on VFD with
DTC technology. The head drum
controls the speed of the whole AFC
and the tail drum tensions the return
chain first and contributes in heavy
load. To optimise performance, a
dynamic adjustment between head
drum and tail drum, also known as the
‘new balance method’, is completed
during operation.
Comparing to 1:1 power
distribution, the new balance method
has advantages both in light and heavy
load situations.
In light loads, the tail drum can get
more power for return chain tension to
avoid stack risk. Tensioning of the return
chain also means that the spare chain
loosens the chain on the top in the tail
area, which will lower the pressure of
the chain against the upper armor plate
and thus reduces wearing (Figure 4).
In heavy loads, the tail drum should
be precisely controlled to make a
contribution to the load, while avoiding
too much tension power frommaking the
release of the chain from the head drum
difficult.
Dynamic chain tension adjustment
system
Deformation of the chain is happening
all the time and a loose chain will cause
unstable running of the AFC and
increase the stack risk. In traditional coal
mines, there is a tension pump for the
chain at the tail area. An experienced
worker will tension the chain and
manually fix extension length
occasionally with a pin.
Dynamic chain tension adjustment
systems can automatically adjust the
chain according to a set pressure all the
time when working. It leaves out the
risk of human error and saves man
power underground.
Figure 3. Air-water heat exchange system.
Figure 2. 3300V Ex-proof drive with air-water HEX method.
Type: BPJV-xxxx/3.3(xxxx: 1000 KW~2000 KW).
Nominal Voltage: 3300 V.
Overload capability: 150% 1 min./10 min.
Length of motor cable: about 2000 m.
Topology: 2-Q 12-pulse converter, voltage source 5-level inverter.
Control method: DTC.
HEX solution: Air-water HEX system.
Ex-proof type: Exd[ib] I Mb.
August 2015
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World Coal
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65
