CIM Bulletin, Vol. 3, No. 2, 2008
E. Pribonic and M. Thompson
There are no statistics on the number of persons transported on hoisting equipment, or the number of mine hoists in use worldwide. Permanent magnet (PM) linear eddy current brakes represent an exciting new technology for this very mature industry. Magnetic brakes are a simple and reliable alternative to destructive arresting systems — mechanical, pneumatic or electromagnetic — in mine hoists.
Magnetic arresting systems offer numerous benefits over all other existing systems. They are widely used in amusement park rides, such as roller coasters, and in vertical drop towers where riders free-fall from a height of 100 m to a safe landing. In addition, this is a technology that is infinitely re-usable, non-destructive, and requires no power or control system. Furthermore, the devices are essentially maintenance-free. Permanent magnet eddy current brake (ECB) technology traces its roots to Heinrich Lenz. In 1834, Russian physicist Heinrich Lenz discovered the directional relationships between induced magnetic fields, voltage, and current when a conductor is passed within the lines of force of a magnetic field. Lenz’s law states: “An induced electromotive force generates a current that induces a counter magnetic field that opposes the magnetic field generating the current.”
ECBs generate a resisting force in proportion to the linear velocity of the moving object. Faster motion creates a higher braking force. Therefore, as the arrestor reduces the speed of a skip, the resisting force reduces, thus providing a nearly linear deceleration. Amusement rides and mine hoists use rails or wire rope guides to support and/or guide them. The mine conveyance is guided at either end of its travel, and thus has reasonably constrained lateral motion. ECBs can be manufactured in the appropriate scale to arrest even the largest ore skip at very high speeds. Permanent magnet linear eddy current braking devices are currently utilized in vehicles, elevators and moving machinery of all sorts.
The use of a magnetic arresting system can be a huge improvement in mine hoisting safety and, in addition, provide major cost savings on installation and maintenance of arrestor systems.
The scaling of magnetic brake capabilities for controlling high-weight and high-speed ore skips will be demonstrated, and various applications and configurations of magnetic brake equipment will be presented to demonstrate the ability of the equipment to:
Provide a fail-proof controlled deceleration of skips at both the bottom of mine shafts and at the head frame.
Work without friction, and without contact parts.
Work without any power requirements or control systems.
Work safely in hazardous environments.
Be designed for any deceleration.
Work in any position from vertical, angled or horizontal.
Be adjusted quickly in the field during installation to provide perfect performance.
Require no maintenance.
Be utilized repeatedly without wear or damage.
Be configured to provide not only emergency stops, but to control descent and ascent throughout the lift.
Be accepted by safety agencies in the United States and Europe for years.
Have been certified for passenger operations in Europe and by TUV of Germany.
Limitations of ECAs
As with any device, permanent magnetic emergency arrestors cannot do everything. They operate with 100% reliability within a range of performance. The important criteria to understand are:
As speed increases, braking force increases, up to the maximum power capability of the arrestor.
A heavy conveyance will penetrate the arrestor set further than a lighter conveyance travelling at the same speed. Thus for operations, there will be a variation in deceleration depending upon vehicle weight.
An ECA cannot grasp and hold a conveyance since there are no moving parts and no contact between conductor fin and magnetic emergency arrestors. A vertically descending conveyance, or one at an incline, will force itself out of the ECA at a very slow rate if the conveyance is unrestrained at that point.
A shock absorber located at the exit end of the ECA will stop the conveyance and is a feasible addition to the system.