2.
Controls (AFC control box and coal
volume detector):
n
n
Ex-proof AFC control box.
n
n
Ex-proof coal volume detector.
3.
Other system parts:
n
n
Main distributed control system
(DCS).
n
n
Signal system.
AFC drives
VFDs are the new drive solution for large
AFCs. Before this, when motor power
was higher than 855 kW, fluid couplings
were the only possible drive solution.
However, fluid couplings are far from
desirable in terms of low efficiency and
frequent maintenance.
Compared to fluid couplings and
traditional drive solutions, VFDs have
many advantages:
A wide speed range
Awide speed range is the foundation for
AFC speed adjustment and allows high
drive efficiency even when the motor is
working at a low speed.
In actual production, there are two
situations where theAFC is expected to
adjust speed. First, when two or more
working faces are producing coal
together, the control centre will give
different production orders to different
working faces for production
coordination. Some working faces need
to limit the production to a certain level,
80% for example. Therefore the shearer’s
cutting speed is limited and theAFC is
also required to lower its speed to reduce
wear and save energy. Second miners
often runAFCs at nominal speed to avoid
suspending production as a result of
changing coal volumes. However, the
nominal capability of anAFC is designed
for peak coal production, which can be
much higher than actual demand.
According toABB’s research, the average
working load of anAFC during a day is
between 50% and 67% of the designed
capacity. Thus, adjusting speed in line
with the actual coal volume will
significantly reduce wear and save
energy.
Up to 200% torque at low speed
Unlike traditional drive method, VFDs
can have full torque output at low speed.
With the DTC, torque output can reach
up to 200% at zero speed.
TheAFC is a machine that has
obvious load fluctuation; it needs high
torque output when starting-up. In
traditional drive solutions, torque
positively correlates with speed, which
means the torque is at a very low level at
start-up because of the low speed.
Sometimes, it needs several forward and
backward operations to start theAFC. In
contrast, VFDDTC technology can easily
start the machine, even when it is fully
loaded.
Smooth ramp-up
Starting time can be set, normally at
60 sec., which allows the speed control to
reach its highest performance and enable
a smooth start-up. By avoiding system
rush during starting, it effectively helps
reduce equipment wear.
Reduced and efficient maintenance
The mean time to failure (MTTF) for
high-quality VFDs is more than 10 000 hr;
little maintenance is needed. And as
electrical maintenance is easier than
mechanical, it reduces maintenance time
and frequency compared with fluid
couplings.
VFDs in underground mines
Despite all the advantages above, VFDs
must be specially re-designed to be
applicable to underground coal mining.
Practical limitations, such as the need to
be explosion proof, heat exchange,
limited space for installation, etc., must all
be removed before the new drive solution
is carried underground (Figure 2).
Anew cooling method called the
air-water heat exchange system (China
patent No. 201110324544.X) (Figure 3) is
now used in VFDs. The idea is to use
air-cooling inside the enclosure instead of
water-cooling. One air-water exchanger is
installed inside the enclosure for heat
exchanging.
The advantages of this method
include:
n
n
Air-water heat exchange system is a
heat exchange system (HEX) solution
applicable for all components.
In comparison, a traditional
water‑cooling system, is usually only
meant for high-heat equipment like
IGBTs.
n
n
No water deioniser equipment
is needed inside the enclosure as
pure cooling water is no longer
a requirement for this system,
which simplifies water supply
underground, as well as minimising
the size of the drive.
n
n
It is much safer to use air for cooling
electrical equipment than water.
AFC control
TheAFC is important in underground
coal mining. According to research in
Figure 1. AFC drive and control system.
64
|
World Coal
|
August 2015
