Importance

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Abstract

An apparatus and method for controlling a lifting magnet (12) of a materials handling machine (10) to eliminate arcing between contacts (70-80) within the magnet controller (26) as well as large voltage spikes. The controller (26) selectively excites the shunt field windings (66,68) of a direct current generator (22). The magnitude and direction of the current passing through the shunt field windings (66,68) is varied by the magnet controller (26) to control the magnitude and polarity of the voltage at the generator output (23). The armature (60) of the generator (22) is rotatably driven by a hydraulic motor at an essentially constant speed to minimize voltage variations at the output (23) of the generator (22). The magnet controller (26) includes a programmable logic controller (40) which selectively opens and closes the contactors (70-80) within the controller (26). The programmable logic controller 40 is also connected to indicator lamps (L1-L5) within the operator's cab (14) of the materials handling machine (10), and selectively illuminates the lamps (L1-L5) to provide an operator of the apparatus with information regarding the status of the magnet control system (27) including the controller (26). A drop time adjustment control (81) is also positioned in the operator's cab (14) and operable to vary the drop time of the magnet controller (26).

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First Claim