Emergency Braking Systems For Mine Elevators
Emergency Braking Systems For Mine Elevators
Emergency Braking Systems For Mine Elevators
Thomas D. Barkand
U.S. Department of Labor
Mine Safety and Health Administration
Pittsburgh Safety and Health Technology Center
P.O. Box 18233
Pittsburgh, Pennsylvania 15236
ABSTRACT
CASE STUDY:
DYNAMIC BRAKING INSTALLATION AND TESTING
Performance testing of dynamic braking has been conducted on several
mine hoisting systems.[5] Two case studies of service elevators that had
dynamic braking installed, and were recently tested, will be presented for
illustrative purposes.
History
An elevator accident occurred on February 4, 1987 at a western
Pennsylvania coal mine due to a mechanical brake failure. The
counterweight fell to the bottom of the 400-ft shaft, causing the cage to
overspeed and crash into the headframe. The cage was unoccupied at the
time of the accident. The elevator was out of service for several months due
to the severity of the damage.
The governor tripped and attempted to set the safety catches. However, the
wedge design of the governor jaws and safeties rendered them ineffective
in the upward direction.
Dynamic braking was installed on the main elevator drive to prevent a
reoccurrence of this type of accident. Dynamic braking was also installed on
the auxiliary elevator to provide the same degree of safety.
Dynamic Braking Installation
A passive type of dynamic braking system was installed on both elevators
servicing the mine portal. The main elevator was a gearless design and the
auxiliary elevator was geared. The equipment needed for the modification
of each elevator included a three-pole loop contactor, a dynamic braking
resistor, a single-phase rectifier bridge, and a drive fault relay. A simplified
schematic diagram of the dynamic braking control circuit is shown in Figure
3.
When the mechanical brakes were called to set, the M contactor dropped
out and disconnected the armature from the power supply, and also applied
the dynamic braking (db) resistor across the motor armature. When the
field power supply was operative, the drive OK (DROK) relay was picked-up
and the field was supplied with normal standing field current.
The field current increased the strength of the magnetic field, which in turn
increased the generated armature current. This positive feedback
continued, causing the field current to build on the generated armature
current until a sufficient retarding torque was developed at 735 ft/min and
the car began to decelerate. The field and armature currents then began to
decrease as the car decelerated. The car slowed down to a steady-state
speed of 220 ft/min with an underdamped response.
The peak speed reached during the self-excitation process was primarily a
function of the time constant of the inductive motor field winding and the
acceleration rate of the cage. The inductance of the field winding was fixed;
however, the acceleration rate of the cage was a function of the load inertia
and the imbalance between the cage and counterweight. The maximum
acceleration rate for personnel load conditions occurs when one person is
transported. As more persons are added to the cage (up to the rated
personnel capacity) the load imbalance between the cage and
counterweight is reduced, thereby reducing the acceleration rate and the
peak speed.
Case Summary
These dynamic braking systems were designed to safely lower an
overhauling load, even under simultaneous failure of the mechanical brakes
and the main power supply. The simple dynamic braking system is an
economical method for providing ascending car overspeed protection.
ROPE BRAKE
A pneumatic rope brake has been developed by Bode Elevator
Components1 which grips the suspension ropes and stops the elevator
during emergency conditions.[6] A typical rope brake installation is shown in
Figure 6.
greatly increases the braking surface area. The increased surface area
dissipates the heat more effectively and therefore, reduces the peak
temperatures generated when the brake is applied. Lower brake lining and
suspension rope temperature increases the coefficient of friction and
consequently generates a greater braking effort.
Another factor which would increase the braking effort was the cleaning
effect the application of the rope brake would have on the suspension
ropes. The repeated application of the rope brake over the testing period
would have stripped the dirt and grease accumulations off a majority of the
suspension ropes. If the rope brake was applied on a cleaned portion of
suspension ropes, the braking effort would improve.
Low Air Pressure Tests: A series of tests were conducted with the air
compressor motor disconnected from the power source to determine the
number of times the rope brake could stop the elevator from the stored
pressurized air in the compressor tank. The tests were conducted with no
car load in the upward direction. The elevator was stopped by the rope
brake twelve times from rated speed with the air compressor power supply
disconnected as shown by the dashed line in Figure 8. Then the air pressure
fell to 52 psi, the pressure switch tripped and opened the elevator control
fault string and prevented operation of the elevator. The pressure switch
contact was temporarily bypassed to allow further testing. The rope brake
was activated eight additional times and the corresponding air pressure and
stopping distances are indicated by the solid line in Figure 8. The rope
brake was activated at speeds ranging from 640 to 680 ft/min. The
stopping distances were calculated from the actual deceleration rates based
on an initial speed of 600 ft/min. As expected, the stopping distance
increased as the available air pressure decreased. The rope brake was able
to effectively stop the elevator in 82 feet with as little as 30 psi in the air
compressor tank. After the rope brake set, only 22 psi was available in the
air compressor tank. The slight distortion in the curve may be attributed to
the varying condition of the suspension rope surface and initial speed
fluctuations.
[2]
W.J. Helfrich, "Island Creek Coal Company V.P.-5 Mine," MSHA, Mine
Electrical Systems Division Investigative Report No. C080978, August
1978.
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