METHOD AND APPARATUS FOR CONTROLLING A LIFTING MAGNET OF A MATERIALS HANDLING MACHINE

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 is well as is 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). At least the drop cycle is controlled through use of a current transducer (200) that senses current flowing to the lifting magnet (12) so that the electronic controller is able to control the flow of current to the lifting magnet (12) based upon the sensed current in the magnet circuit.

Description: Description METHOD AND APPARATUS FOR CONTROLLING A LIFTING MAGNET OF A MATERIALS HANDLING MACHINE Technical Field The present invention relates to a method and apparatus for controlling a lifting magnet of a materials handling machine. It finds particular application in conjunction with lifting magnets used on cranes and other prime movers in the steel and scrap metal industries . Background Art Lifting magnets are commonly used in the materials handling industry to lift and move magnetic materials. For example, in the steel industry, lifting magnets are used to move intermediate products and finished goods. Also, in the scrap metal industry, lifting magnets are commonly attached to cranes and other prime movers and used to load, unload, and otherwise move scrap steel and other ferrous metals. While lifting magnets have been in common use for many years, the systems used to control these lifting magnets remain relatively primitive. Known control systems operate to selectively open and close contacts that, when closed, complete a circuit between a suitable source of DC electrical power and the lifting magnet. The source of DC power is generally at least 230 volts, and during certain lifting stages, the voltage can reach approximately 275 volts. Additionally, when the polarity of the voltage across the magnet is briefly reversed as is required to "push" a load of metal off of the magnet, voltages commonly reach 500 - 1000 volts. Thus, opening and closing the contacts during these conditions, to break or complete the magnet circuit, naturally results in arcing across the tips of the contacts and the creation of voltage spikes in the magnet control system. Arcing between the contacts of known magnet controllers causes burning and wear which eventually leads to the need to replace the contacts. The large variation in voltage also eventually wears out the generator (the typical source for the DC power) , the magnet and associated insulation, as well as the cables used to connect the magnet to the generator. To withstand the large voltages and voltage spikes, the magnet, cables, and the control system contacts and other components must be constructed of more expensive materials and must also be made larger in size . Also, with known magnet control systems, the control system must be matched to the particular magnet being used. For example, the contacts and associated circuitry in a known magnet controller for a 93 inch diameter, 40 kilowatt (kW) magnet must be able to pass approximately 175 Amperes of current and also withstand very large voltage spikes. Such a controller would not be effective when used in conjunction with a 30 inch diameter, 5kW magnet that draws only 20 Amperes of current. Of course, the components used in a controller for the smaller magnet would not be able to withstand the electrical current and voltage spikes associated with the larger magnet. Thus, with known systems, an operator of a scrap yard or other facility needs to restrict the use of different magnets on the various cranes and other prime movers or must switch the entire control system of the prime mover accordingly. For example, certain known magnet controllers are available in seven different capacities and each is unusable with magnets outside of its operational range. Therefore, a facility using different size magnets must also purchase and maintain a magnet controller suitable for use with each magnet. Known lifting magnet control systems are not "user-friendly." These control systems do not provide the operator of the magnet with sufficient information regarding the status of the magnet and the magnet control system. For example, known systems do not inform the operator if there exists an unwanted ground in the magnet circuit . Such a ground can damage the magnet or its controller and also adversely affect the operation both the magnet and controller, resulting in dropped loads or other malfunctions. A ground to the chassis of the prime mover can also damage the electronics of the prime mover which are preferably completely isolated from the magnet circuit but which are often grounded to the machine chassis. An unwanted ground in the magnet circuit is also potentially harmful to the generator supplying power to the circuit. Likewise, known magnet controllers do not monitor the "duty cycle" of the magnet. Duty cycle is the percentage of time that the magnet is energized or "turned on" relative to its total time in operation for a given period of time. Thus, to move a load of steel, an operator may have to energize the magnet 60% of the time, with the remainder of the time being accounted for by the time required to maneuver the magnet and its prime mover, as well as the time when the magnet is deenergized or "turned off" to drop a load. Modern magnets can withstand a 75% duty cycle. If this maximum duty cycle is exceeded, the magnet will be damaged. However, with known magnet control systems, operators are unable to effectively monitor duty cycle and known controllers do not inform the operator if the maximum duty cycle is being exceeded. Known systems also do not monitor the condition of the generator that supplies DC electrical power to the magnet circuit. If the magnet is being heavily used, it is possible for the generator to overheat. If an operator is unaware of a generator overheating problem, the generator will be damaged. Thus, it would be desirable to provide a magnet control system that continuously monitors the condition of the generator and informs the operator if the generator begins to overheat. Further, known system do not allow the operator to adjust the "drop time" - the amount of time a reverse voltage is applied to the magnet to reverse its polarity - without assistance or without leaving the operator's cab. Known systems require that this adjustment of drop time be made at the controller itself, which is usually accessible underneath or at the rear of the crane or other prime mover. This is dangerous and difficult, especially due to the fact that test lifts and drops must be made during the adjustment operation. Thus, either the operator of the prime mover machine must repeatedly exit the operator's cab and adjust the drop time or a second person must adjust the drop time in response to commands from the operator. This second person could easily be electrically shocked or otherwise injured should the operator unexpectedly activate the lifting magnet or the prime mover machine itself. Another drawback associated with known magnet control systems relates to the fact that the generator providing DC power to the magnet is generally driven through a belt -drive connection or using a hydraulic motor which is powered by a hydraulic pump connected to the main engine or an auxiliary engine of the prime mover. Thus, with known systems, an increase or decrease in revolutions per minute (rpm) in the engine driving the generator results in a corresponding increase or decrease in the rpm of the generator armature. This consequently results in an increase or decrease in the DC power output from the generator. While a certain amount of over-voltage from an increase in engine rpm is acceptable, a severe under-voltage, as might occur upon the driving engine becoming "bogged down" or otherwise slowed, can result in a severe drop in generator output to the magnet. If insufficient power is supplied to the lifting magnet, its load could be accidentally dropped. Attempts to utilize conventional voltage regulators to overcome these voltage variations have not been successful. Specifically, conventional voltage regulators cannot withstand the large voltage spikes associated with known magnet controllers. In order to drop a load from an electromagnetic lifting magnet, the forward voltage applied to the magnet must be turned off, and a reverse voltage must be applied to drive the current down to 0 and then momentarily reverse the current to correspondingly reverse the magnetic field of the magnet so that the load is repelled or "pushed" from the magnet. Merely interrupting the forward voltage to the magnet without applying a reverse voltage would cause at least part of the load to be held on the magnet due to the magnet's residual magnetism. Heretofore, lifting magnet controllers have relied solely on conventional mechanical or electronic timers or other timing devices such as relays and the like to control the onset and duration of magnet "reverse current" used to reverse the polarity of the lifting magnet to "push" the load off of the magnet during a drop cycle. However, use of these timing devices has been found to be inefficient, inconvenient, and often results in poor magnet drop characteristics. These deficiencies result from a variety of factors. For instance, these prior systems do not dynamically and automatically account for variations in lifting magnet size, lifting magnet temperature, load size, load type, and other such variations which alter the drop characteristics of the magnet. Therefore, for example, even if the magnet controller is manually adjusted so that the magnet properly picks and drops a particular type and size load, any variation in load size and type, and/or any large variation in magnet temperature, will cause the drop characteristics of the magnet to change. This, then, requires the operator or a co-worker to access the controller and manually vary the timer setting as needed. Obviously, such manual adjustment of the reverse current timer is inefficient and inconvenient. Furthermore, proper manual adjustment of the reverse current timer in prior controllers depends upon the skill of the operator in recognizing that an adjustment is needed and in actually adjusting the timer for optimal load-drop characteristics. Often, an operator either does not recognize an adjustment to the drop timer is needed, does not have the time or ability to adjust the timer, or improperly adjusts the timer mechanism. The foregoing deficiencies are especially critical given that even a 10 th of a second variation in the duration of the reverse current through the magnet can significantly affect the magnet drop performance . Because prior magnet controllers do not accurately control the onset and duration of the reverse current in a drop cycle, manufacturers of these prior controllers have designed the controllers to use only a portion of the forward or "lifting" voltage in the drop cycle. By applying only a reduced reverse voltage to the magnet to develop the reverse current, these prior controllers are attempting to accommodate the above-noted inaccuracies in the reverse current timer. That is to say, these prior controllers do not use a reverse or drop voltage that is equal to the forward or lift voltage because errors in the timing of the reverse current could cause the load to be initially repelled and then re-attracted to the magnet during the drop cycle. After termination of the reverse current, the load would merely dribble off the magnet. Use of a reduced drop voltage in prior controllers is intended to compensate for overly long drop cycles due to timer errors. However, the use of a reduced drop voltage also increases the time required to push a load from the magnet which leads to further inefficiencies. Disclosure of the Invention According to the present invention, a new and improved method and apparatus for controlling a lifting magnet, especially during a load-drop cycle, is provided. In accordance with first aspect of the present invention, a method of selectively energizing a lifting magnet of a materials handling machine includes connecting a magnet circuit including a lifting magnet to a voltage output of a separately excited generator that includes shunt field windings. The armature of the generator is rotated and the shunt field windings of the generator are selectively connected to an electrical power source to pass an electrical current through the shunt field windings, thereby exciting the generator, establishing a voltage at the output of the generator, and inducing current flow in the magnet circuit . Current flow in the magnet circuit is sensed and the flow of electrical current through the shunt field windings is controlled in accordance with the current sensed in the magnet circuit . In accordance with another aspect of the present invention, a controller apparatus for selectively energizing a lifting magnet includes an electrical power input for connection to a voltage output of a generator, and an electrical power output for selectively connecting the electrical power input to a lifting magnet. The controller also includes generator excitation means for selectively electrically connecting shunt fields of a generator connected to the controller to an electrical power source to thereby excite the shunt fields of the generator so that a voltage is established at the voltage output of the generator and so that electrical current flows through the lifting magnet. A means for detecting electrical current flow through the lifting magnet is connected to the generator excitation means so that the generator excitation means disconnects the electrical power source from the shunt fields of the generator when a select electrical current flow through the magnet is detected. In accordance with still another aspect of the present invention, a materials handling apparatus includes a prime mover and a separately excited generator including a rotatable armature, shunt field windings, and a voltage output for connection to a lifting magnet through a magnet circuit. A means for rotating the armature of the generator is provided. The apparatus also includes a controller for selectively connecting the shunt field windings of the generator to an excitation power source such that voltage is established at the generator voltage output and current flows through the magnet circuit . The apparatus further includes a current sensor for sensing current in the magnet circuit . One advantage of the present invention is the provision of a new and improved apparatus and method for controlling a lifting magnet. A second advantage of the present invention is the provision of a lower cost and more durable apparatus for controlling a lifting magnet. Another advantage of the present invention is the provision of an apparatus and method for controlling a lifting magnet that minimize voltage spikes in the magnet circuit . Still another advantage of the present invention is the provision of an apparatus and method for controlling a lifting magnet that eliminate arcing across the contacts in the magnet controller. Yet another advantage of the present invention is the provision of an apparatus and method for controlling a lifting magnet that increase the useful life of the magnet, the generator supplying power to the magnet, and the associated circuitry. A further advantage of the present invention is the provision of an apparatus for controlling a lifting magnet that is usable with a large range of different lifting magnets. A still further advantage of the present invention is the provision of an apparatus for controlling a lifting magnet that monitors for the existence of an unwanted ground in the magnet circuit and informs the magnet operator of any unwanted ground . A yet further advantage of the present invention is the provision of an apparatus for controlling a lifting magnet that monitors the duty cycle of the lifting magnet and informs the magnet operator if the maximum duty cycle is exceeded. Another advantage of the present invention is the provision of an apparatus for controlling a lifting magnet that provides a drop time control mechanism in the operator's cab of the prime mover carrying the lifting magnet. Still another advantage of the present invention is the provision of an apparatus for controlling a lifting magnet that monitors the temperature of the DC generator supplying electrical power to the magnet and informs the magnet operator if the generator temperature exceeds a select level . A further advantage of the present invention resides in the provision of an apparatus for controlling a lifting magnet that senses electrical current flow to the lifting magnet and controls the application of reverse polarity voltage to the magnet during a load-drop cycle in accordance with the sensed current . Another advantage of the present invention is found in the provision of an apparatus for controlling a lifting magnet wherein, during a load- drop cycle, a reverse voltage is applied to the magnet that is at least substantially equal in magnitude to the voltage applied to the magnet during a load-lift cycle to quickly and effectively drive the load off of the magnet . Still other benefits and advantages of the invention will become apparent to those skilled in the art upon reading and understanding the following detailed description. Brief Description of the Drawings The invention may take form in certain components and structures, preferred embodiments of which are illustrated in the accompanying drawings wherein: FIG. 1 is a side elevational view of a prime mover including a lifting magnet and a lifting magnet control system in accordance with the present invention; FIG. 2 schematically illustrates a lifting magnet control system in accordance with the present invention; FIG. 3 schematically illustrates a lifting magnet controller circuit and a generator circuit in accordance with the present invention; FIG. 4 is a flow chart showing a method for controlling a lifting magnet in accordance with the present invention; FIGS. 5 - 11 illustrate the various states of the circuit of FIG. 3 as the method shown in FIG. 4 is carried out; FIG. 12 graphically shows a voltage signal associated with a typical prior art lifting magnet controller as the lifting magnet is operated through a lift and drop cycle; FIG. 13 graphically shows a voltage signal associated with a lifting magnet controller of the present invention as the lifting magnet is operated through a lift and drop cycle; FIG. 14 is a perspective view of a hydraulic fluid manifold in accordance with the present invention; FIG. 15 is a schematic illustration of the manifold of FIG. 14 as it is connected between a hydraulic pump of the prime mover carrying the lifting magnet and a a hydraulic motor powering the a DC generator supplying electrical power to the lifting magnet ; FIG. 16 schematically illustrates a lifting magnet control system in accordance with another embodiment of the present invention; FIG. 17 is a more detailed schematic illustration of the magnet control system of FIG. 16; and, FIG. 18 is a flow chart illustrating a method of controlling a lifting magnet during a drop cycle in accordance with the present invention. Best Mode for Carrying Out the Invention Referring now to the drawings, wherein the showings are for purposes of illustrating preferred embodiments of the invention only and not for purposes of limiting the same, Fig. 1 shows a prime mover 10 carrying a electromagnetic lifting magnet 12. Although the prime mover 10 is shown herein as a crane, those skilled in the art will recognize that numerous other prime movers are suitable for use in carrying a lifting magnet 12. For example, overhead cranes, tractors and other wheeled vehicles, and excavators are examples of suitable prime movers 10. The present invention is suitable for use to control a lifting magnet 12 carried by any suitable prime mover 10 or a lifting magnet 12 not associated with a prime mover . The crane 10 includes an operator cab 14 from where an operator controls the crane 10 and the magnet 12. Typically, two hand control levers 16,18 are provided to maneuver the crane 10. The cab 14 includes a control panel 19 that displays information to the operator and also includes various control switches for operator control of the crane 10 and the magnet 12. The crane 10 is powered by an internal combustion engine 20, which may be fueled by gasoline, diesel, or any other suitable fuel. A direct current (DC) electrical generator 22 is driven by the engine 20 or by an auxiliary engine 24 that is optionally provided to power accessories of the prime mover 10. The generator 22 can be driven through a belt-drive or similar connection with an engine 20,24 but preferably is hydraulically driven as is described in detail below. With reference now also to FIG. 2, a lifting magnet control system 27 includes a magnet controller 26 in accordance with the present invention. The magnet controller 26 interconnects the lifting magnet 12 and the electrical output 23 of the DC generator 22 through cables 28,30 or another suitable electrical connection. The magnet controller 26 selectively energizes and deenergizes the lifting magnet 12 for lifting and dropping operations, respectively. When energized, the lifting magnet 12 attracts and retains ferrous metals and other magnetic substances. When the magnet 12 is deenergized, it is demagnetized. The magnet controller 26 is also connected to the generator 22 through cables 36,38 so that the controller 26 can control the operation of the generator 22 as is described in detail below. The operation of the magnet controller 26 as shown herein is preferably controlled by a programmable logic controller (PLC) 40 which is programmed to perform various operations as described below in response to the particular input thereto. One suitable PLC 40 is the Micrologix Model 1761 PLC available commercially from Allen-Bradley, Milwaukee, Wisconsin 53204, U.S.A. Other suitable PLC's, microcontrollers, or the like may be utilized without departing from the overall scope and intent of the present invention, and those of ordinary skill in the art will also recognize that the PLC 40 may be replaced by discrete components such as contactor relays and the like. With continuing reference to FIGS. 1 and 2, although the magnet controller 26 is generally located underneath or at the rear of the prime mover 10, it is connected to components in the cab 14 so that the operator can operate the magnet 12, adjust the controller 26, and receive information from the controller 26. Specifically, each control lever 16,18 in the cab 14 includes a push-button or similar switch S1,S2, respectively. One of the switches S1,S2 is manipulated by the operator to cause the magnet controller 26 to energize the magnet 12 for lifting a load. The other of the switches S1,S2 is manipulated by the operator to cause the controller 26 to deenergize the magnet 12 to drop a load. The magnet controller 26 is also connected to the control panel 19 in the operator' s cab 14 of the prime mover 10. The control panel 19 includes various gauges and other instruments that provide information to the operator about the operation of the prime mover 10 and the control system 27. For example, the control panel 19 includes a plurality of visual indicators such as indicator gauges or lights LI, L2 , L3 , L4 , L5 which are selectively illuminated by the PLC 40 in response to various system conditions to ensure operator awareness thereof. For example, the PLC 40 illuminates the light LI when the magnet is energized. The PLC 40 also monitors the output voltage received from the generator 22 to determine if the voltage is within an acceptable range of approximately 230 volts DC - approximately 275 volts DC, depending upon the operation being performed. An under-voltage condition can result in a dropped load and an over-voltage can damage the magnet and associated equipment. Therefore, if either an under- voltage condition or an over-voltage condition is present, the PLC 40 illuminates the light L2. To determine if there exits an unwanted ground in the control system 27, the PLC 40 continuously monitors the resistance between the control system 27 and an intentional ground connection 44 to ensure that the resistance to ground 44 is above a known threshold such as approximately 50,000 Ohms. A resistance to ground 44 less than this value indicates an unwanted ground in the magnet, the cables 32,34, or elsewhere in the system 27. An unwanted ground in the system 27 can result in dropped loads, insufficient reverse current (discussed below) during load drops, and other system malfunctions. Also, an unwanted ground can damage the generator 22 and presents safety concerns. Therefore, the operator is notified of this undesirable condition by the illumination of the light L3. The PLC 40 also maintains a measurement of the magnet duty cycle. This is accomplished by programming the PLC 40 to record and compare the amount of time the magnet 12 is energized relative to the total amount of time the system 27 is in operation. If the operator is exceeding the recommended duty cycle for the magnet 12, damage to the magnet 12 will result. Therefore, the PLC 40 illuminates the light L4 if the maximum duty cycle is exceeded. In addition to magnet damage, excessively heavy or prolonged use of the magnet 12 can overheat the generator 22 causing permanent damage. Therefore, a thermocouple 46 is positioned in the generator 22. The thermocouple 46 provides the PLC 40 with a temperature signal that represents the temperature of the generator 22. When the PLC 40 receives a signal indicating a generator temperature above an acceptable threshold, the PLC 40 illuminates the light L5 to notify the magnet operator. In FIG. 2, it can be seen that the lifting magnet control system 27, including the magnet controller 26 includes a power source 50, which is provided, for example, by one or more batteries supplying 24 volts DC. The power source 50 is selectively connected to the magnet controller 26 and to the switches S1,S2 in the cab 14 of the prime mover through one or more switches. Preferably, a single main switch 52 is operable to connect and disconnect both the magnet controller 26 and the switches S1,S2 from the power source 50. When the main switch 52 is closed, the source 50 is connected to the controller 26 through electrical connections 54,56 and to the switches S1,S2 through the electrical connection 58. When the switch 52 is opened, the controller 26 receives no electrical power, and the switches S1,S2 in the cab 14 are disconnected from the circuit. The switch 52 provides a main safety shut-off switch to the magnet control system 27. Preferably, opening the access panel of the controller 26 for maintenance requires the switch 52 to be opened as a safety measure so that the system 27 cannot be activated when the access panel to the controller 26 is open. It can be seen in FIG. 2, that when the switch SI is depressed and closed by the operator, a circuit is completed between a first input of the PLC 40 and the source 50, thereby causing the PLC 40 to execute the "lift" cycle of the controller 26. Likewise, depression of the switch S2 by the operator will complete a circuit between a second input of the PLC 40 and the source 50, thereby causing the PLC 40 to execute the "drop" cycle of the controller 26. With reference now also to FIG. 3, the magnet controller 26, the generator 22, and the relationship between the controller 26 and the generator 22 are shown in detail. The generator 22 can be any suitable DC generator that is separately excited - i.e., the generator 22 is the type that requires the shunt fields (also referred to as "shunt field windings" and "field windings") to draw current from a external voltage source in order "excite" the generator so that it produces DC electricity at its output 23. Although not required, the generator 22 is preferably a compound wound generator that supplies an essentially constant output voltage even as the load connected to the generator 22 varies. The generator 22 includes an armature 60 which is rotatably driven through a connection to the engine 20 or the optional auxiliary engine 24 of the prime mover 10. Preferably, as is described in detail below, the armature 60 of the generator 22 is connected to a hydraulic motor powered by a constant flow of hydraulic fluid from a hydraulic pump driven by an engine 20,24 of the prime mover 10. The preferred generator 22, as shown herein, includes a commutator field 62 and a series field 64 in series with the armature 60. The generator 60 also includes first and second shunt fields 66,68. As is known in the art of DC generators, when current is passed through the shunt fields 66,68, magnetic flux is established in the air gap between the armature 60 and the shunt fields 66,68. Rotation of the armature 60 through the magnetic flux induces a voltage in the armature 60 as a result of the relative motion between the armature 60 and the air gap flux. A commutator rectifies the induced voltage and carbon brushes connect the armature 60 to the generator output 23. However, if no current is passing through the shunt fields 66,68 of the generator 60, rotation of the armature 60 does not induce a voltage in the armature 60. Thus, when the shunt fields 66,68 are not energized, the generator produces no output voltage at the output 23. Furthermore, the direction and magnitude of the current in the shunt fields 66,68 controls the polarity and the magnitude of the voltage induced in the armature 60. In general, the magnet controller 26, in accordance with the present invention, selectively energizes the magnet 12 for lifting operations by selectively passing current through the shunt fields 66,68 of the generator 22. This eliminates the need to repeatedly open and close high voltage contacts between the magnet 12 and the output 23 of the generator 22 as is performed in prior art magnet controllers. Instead, as is shown in FIG. 3, the magnet controller 26 includes a plurality of contacts or "contactors" 70,72,74,76,78 (70-78) which are opened and closed by the PLC 40 to selectively connect the shunt fields 66,68 to the power source 50. The magnet controller 26 also includes a contactor 80 which selectively completes a circuit between the generator output 23 and the magnet 12. The contactor 80 is preferably normally closed. Contactor 80 may alternatively be replaced by a fuse or the like. As mentioned above, the power source 50 used to selectively energize or excite the shunt fields 66,68 of the generator 22 is relatively low voltage, preferably approximately 24 volts DC. Therefore, little or no arcing occurs when the contactors 70,72,74,76,78 of the controller 26 are opened and closed. The contactor 80 is preferably always closed before the shunt fields 66,68 are energized, and preferably never opened before the shunt fields 66,68 are deenergized. Thus, the contactor 80 is not opened and closed when the generator 22 is supplying power to the magnet 12. Various conventional contactors 70-80 may be used in the magnet controller 26. Suitable contactors include 200 Ampere normal current carrying contactors with a maximum resistance load break rating of 200 Amperes at 50 volts D.C. Each contactor 70-80 is electrically connected to and selectively opened and closed by the PLC 40 in accordance with the method of the present invention. FIG. 4 shows a method of controlling a lifting magnet 12 in accordance with the present invention. FIGS. 5-11 show the opening and closing of the various contactors 70-80 of the controller 26 along with the opening and closing of the main switch 52 as the method shown in FIG. 4 is carried out. The outline of the generator 22 has been omitted from FIGS. 5-11, but those skilled in the art will recognize that the armature 60, the commutator field 62, the series field 64, and the shunt fields 66,68 are contained within the generator 22 as shown in FIG. 3. Of course, before the system 27 can be operated as described below, the prime mover 10 is preferably turned on and the armature 60 of the generator 22 is rotatably driven. With reference to FIGS. 4 and 5, a step 100 turns the system 27 on using the main switch 52. This step is typically performed manually. As is seen in Fig. 5, the step 100 results in the closure of the switch 52 such that the power source 50 is connected to the magnet controller 26 through the electrical connections 54,56. Closure of the main switch 52 also connects the switches S1,S2 (Fig. 2) on the levers 16,18 to the power source 50 through the electrical connection 58. Until the step 100 is carried out to turn the system on, the magnet control system 27 is inoperable . Although the magnet contactor 80 that selectively connects the output 23 of the generator 22 to the magnet 12 is a normally closed contactor, a step or means 102 verifies that the magnet contactor 80 is closed. If the contactor 80 is open, the step or means 102 closes it. This step is performed by the PLC 40. It is important to ensure that the contactor 80 is closed at this point to eliminate the need to close the contactor 80 when there exists a large voltage across the tips thereof which would result in voltage spikes and arcing. Using the method of the present invention, it is also possible to eliminate the magnet contactor 80 altogether, although such is not preferred. When a lifting magnet 12 is used to lift a load, it is generally preferable to supply the magnet 12 with an initial boost of high power for a brief period of time and to thereafter reduce the power to the magnet 12 to maintain the load on the magnet . For example, a boost voltage of 275 volts DC can be applied to the magnet 12 for a period of approximately three seconds. Thereafter, the power to the magnet 12 can be reduced to 230 volts DC to hold the load. Of course, the actual boost time can vary for the particular magnet 12 and the particular material being handled thereby. Therefore, when the operator initiates a lift by pressing the switch SI on the lever 16, a step or means 104 transmits a boost level excitation current through the generator shunt fields 66,68. This boost level current passing through the shunt fields 66,68 causes a corresponding boost in the output of the generator 22. This boost step 104 is preferably carried out as shown in FIG. 6 wherein the contactors 70,72,74 are closed by the PLC 40 in response to the operator depression of the switch SI. The closure of the contactors 70,72,74 completes a circuit from the power source 50 through the shunt fields 66,68. Closing the contactor 72 partially bypasses a resistor Rl to lower the total resistance in the circuit and thus increase the level of current passing through the fields 66,68. At this stage, current passes through the magnet 12 in a first direction as indicated by the arrows I to establish a first magnetic polarity (indicated with conventional (+) and (-) symbols) in the magnet 12. As mentioned, the boost stage is relatively short in duration. Thus, a step or means 106 preferably reduces the current passing through the shunt fields 66,68, thereby causing a corresponding decrease in the output of the generator 22 and the power transmitted to the magnet 12. The step or means 106 is preferably carried out as shown in FIG. 7. The PLC 40 is programmed with the desired boost time. After the passage of this select boost time, the PLC 40 opens the contactor 72 such that the resistor Rl is no longer partially bypassed. This increases the resistance in the circuit and decreases the current flowing through the fields 66,68 and thus decreases the output of the generator 22. When it is time to drop the load, a step or means 108 interrupts the voltage to the generator shunt fields 66,68 to cut the flow of current therethrough. This preferably occurs in response to operator depression of the switch S2 on the lever 18. As is shown in FIG. 8, closure of the switch S2 causes the PLC 40 to open the contactors 70,74 to disconnect the shunt fields 66,68 from the voltage source 50. With a separately excited generator 22, there is no voltage is present at the output 23 unless current is passing through the shunt fields 66,68. Therefore, power to the magnet 12 is interrupted by the step 108 without having to open the magnet contactor 80. When power to the magnet 12 is cut, the residual magnetism in the magnet 12 induces a current through the magnet 12, as indicated by the arrows I, which is dissipated through the armature 60 and other components of the generator 22 which are in series with the magnet 12. Although the load carried by the magnet 12 should drop under the force of gravity upon the power to the magnet 12 being interrupted at step 108, it is preferably to immediately reverse the polarity of the magnet 12 for a brief time - known as the "drop time", to "push" the load off of the magnet 12. The reversal of polarity in the magnet 12 must be brief or else the load will be attracted once again to the magnet 12. Also, the drop varies depending upon the particular magnet 12 and upon the particular load being lifted thereby. Therefore, a step or means 110 transmits a reverse current through the shunt fields 66,68 of the generator 22. This results in a reversal of the polarity of the voltage at the output 23 of the generator 22 and a reversal of direction in the current flowing through the magnet 12. As is shown in FIG. 9, the PLC 40 closes the contactors 76,78 to complete a circuit between the shunt fields 66,68 and the power source 50 wherein the orientation of the source 50 in the circuit is reversed compared to the orientation shown in FIGS. 6 and 7. This causes a reverse current to flow through the shunt fields 66,68 which consequently reverses the polarity of voltage output by the generator 22. The reversal of polarity of the generator output voltage causes a reverse current I' to flow through the magnet 12. This reverses the polarity of the magnet and pushes the load from the magnet 12. Industrial Applicability With reference to FIG. 2, the drop time of the magnet controller 26 is controlled by the operator using a drop time control 81 positioned on the control panel 19. Using the control 81, the operator can select a drop time in the PLC 40. The drop time can be easily adjusted by the operator without assistance and without leaving the cab 14 of the prime mover 10. The drop time generally needs to be adjusted when the magnet 12 is first connected to the prime mover 10 or when the type of load being moved varies. After the passage of the selected drop time, a step or means 112 interrupts the reverse current flowing through the generator shunt fields 66,68. This is preferably carried out as shown in FIG. 10, wherein it can be seen that the PLC 40 opens the contactors 76,78 to break the circuit and stop the flow of current through the shunt fields 66,68. Residual magnetism in the magnet 12 induces a current to flow through the magnet as indicated by the arrows I' . This residual current dissipates over a brief time, and as is shown in FIG. 11, the magnet is once again demagnetized without the magnet contactor 80 having been opened. As is indicated in FIG. 4 at 114, while additional lifting is to be performed, the process begins again with step 102. Otherwise, step 116 turns the system off by opening the main switch 52 to remove the voltage source 50 from the circuit (FIG. 3) . FIG. 12 graphically illustrates the undesirable voltage spikes that occur in typical prior art magnet controllers and control systems in a lift and drop cycle. The boost level voltage is omitted for clarity. The magnet is energized at point PI with 230 volts. Once the 230 volts is present at point P2 , the voltage level to the magnet remains constant. At point P3 , the polarity of the voltage to the magnet is briefly reversed to push the load from the magnet . However, with known controllers, this reversal of magnet polarity causes a large reverse voltage spike P4. Often, as shown in FIG. 12, the voltage spike P4 is approximately -1000 volts. Furthermore, it can be seen in FIG. 12 that, before returning to 0 volts, the voltage climbs back to 230 volts at point P5. In contrast, FIG. 13 graphically illustrates the voltage levels associated with the magnet control system 27 and method of the present invention in a typical lift and drop cycle. Again, the boost level voltage is omitted for clarity. The shunt fields 66,68 are energized at point PI' and the voltage output to the magnet climbs to 230 volts at point P2 ' . The voltage remains constant until point P3 ' where the current to the generator shunt fields 66,68 is interrupted. Immediately thereafter, a reverse current is passed through the shunt fields 66,68 to reverse the polarity of the voltage output from the generator 22. This causes the voltage level to drop and reverse to point P4' which is approximately -250 volts. Once the reverse current through the shunt fields 66,68 is interrupted, the voltage output by the generator goes to 0 volts without first returning to 230 volts. It can be seen from FIGS. 12 and 13 that the apparatus and method of the present invention eliminate wide voltage fluctuations and spikes associated with known magnet controllers. The elimination of voltage spikes and arcing in the magnet controller 26 allows the contactors 70- 80 to be made smaller in size. Furthermore, only the contactor 80 directly passes current to the magnet 80. Therefore, for example, the magnet controller 26 of the present invention can be safely utilized with magnets that vary from a small magnet such as a 5kW, 30 inch, 20 Ampere magnet to a large magnet such as a 40kW, 93 inch, 175 Ampere magnet. Preferably, the generator 22 is powered by a hydraulic motor. With known systems, the hydraulic motor receives a flow of hydraulic fluid directly from a hydraulic pump provided to drive a generator or other accessories of the prime mover 10. With known systems, variations in the speed of the engine driving the hydraulic pump of the prime mover 10 results in corresponding variations in the flow of hydraulic fluid from the hydraulic pump to the generator powering the lifting magnet. This consequently causes fluctuations in generator speed and the voltage transmitted to the lifting magnet. Therefore, another aspect of the present invention is illustrated in FIGS. 14 and 15. The generator 22 of the present system 27 is preferably driven by a hydraulic motor 120. The hydraulic motor is connected to a hydraulic pump 122 of the prime mover 10 through a hydraulic circuit 124. The pump is driven by an engine 20,24 of the prime mover 10. The hydraulic circuit 124 is preferably defined in a manifold 130 which may be aluminum or any other suitable material. The circuit 124 regulates the flow of hydraulic fluid to the hydraulic motor 120 and thus ensures that the armature 60 of the generator 22 rotates at an essentially constant speed, independent of the speed of the pump 122 to thus regulate the voltage output of the generator 22. Specifically, the pump 122 of the prime mover generally produces excess flow of hydraulic fluid. The pump 122 pumps fluid from a reservoir R to a manifold inlet 132. The manifold includes a relief valve assembly 134 that can either act as a conventional relief valve to limit the maximum hydraulic pressure in the circuit 124 or can be set to divert all of the fluid flow from the pump 122 directly to the outlet 136 of the manifold 130. The relief valve assembly 134 thus includes an adjustable vented relief valve 138 and a solenoid valve 140. When the solenoid 140 is energized, the vent of the relief valve is closed and the relief valve 136 acts as a conventional pressure relief valve which opens only when the upstream pressure reaches a set threshold. When the solenoid 140 is deenergized, the relief valve 136 is vented and opens at "zero" pressure and thus diverts all fluid from the pump 122 immediately back to the reservoir R to cut the flow of fluid to the hydraulic motor 120. When the relief valve assembly 134 is set to act as a conventional relief valve, fluid that is not diverted by the relief valve assembly 134 reaches a pressure compensated flow control valve assembly 140. The assembly 140 includes an adjustable flow control valve 142 that regulates the flow of hydraulic fluid to the motor 120. The assembly 140 also includes a pressure compensator 144 that ensures a select pressure differential across the flow control valve 142 such that the flow through the valve 142 remains at least essentially constant. For example, a pressure differential of approximately 135 to approximately 165 pounds per square inch (p.s.i.) can be maintained across the flow control valve 142. This constant flow through the pressure compensated flow control valve assembly 140 ensures an essentially constant rotational speed of the motor 120 and thus, the armature 60 of the generator 22. This is so, even if the output of the pump 122 increases. The hydraulic motor 120 is connected to a motor outlet 150 of the manifold 130. Fluid passes through and drives the motor 120 and returns into the manifold 130 at a motor return port 152. Fluid from the motor inlet flows back to the reservoir R. When the flow of fluid to the motor 120 is interrupted, the motor will continue to rotate for a time. To ensure that the motor does not pump itself dry or pump large volumes of air into the circuit 124, the circuit preferably includes an anti-cavitation valve 160 that allows the motor 120 to recirculate fluid to itself when the pump 122 is stopped or when the relief valve assembly 134 is opened to divert fluid to the reservoir R. The adjustable flow control valve 142 is set such that a predetermined flow of hydraulic fluid is delivered to the hydraulic motor 120. However, it has been found that in certain instances, for example when a light flow of hydraulic fluid is needed at the motor 120 (as is required when a smaller magnet 12 is being used) , the pressure compensator 144 has difficulty in accurately regulating the pressure upstream and downstream of the flow control valve 142. Therefore, the manifold 130 optionally includes an adjustable cross-over flow control valve 170 that diverts or "bleeds" a small amount of hydraulic fluid from the circuit 124 between the flow control valve 142 and the motor 120. This prevents surges in the circuit 124 and helps the pressure compensator 144 to regulate the pressure at the flow control valve 142. Finally, as is known in the art of hydraulics, the motor 120 includes a case drain line 172 to prevent the build-up of excessive hydraulic pressure in the motor housing. With reference now to FIGS. 16 and 17. an alternative magnet control system in accordance with the present invention is shown generally at 27'. For convenience and ease of consideration, like components relative to the control system 27 are identified with like reference numerals including a primed (') suffix and new components are identified with new reference numerals beginning with 200. The lifting magnet control system 27' is similar in all respects to the system 27 described above, except as shown and described herein. In particular, the control system 27' includes a magnet controller 26' including means for sensing electrical current flow through the magnet circuit . The magnet circuit is defined by the magnet 12', itself, as well as the cables 28' ,30' and other elements connecting the magnet 12' to the generator output 23'. Preferably, the means for sensing current in the magnet circuit is a current transducer or current sensor such as a current transformer or the like. Use of a current transformer allows the current in the magnet circuit to be sensed without the introduction of a load into the high voltage - high current magnet circuit. Of course, other suitable components exist and are known for measuring directly and/or indirectly deriving current flow in a circuit, and the same may be used herein as the means for sensing current in the magnet circuit without departing from the overall scope and intent of the present invention. As illustrated in FIG. 16, the current transducer 200 outputs a select electrical signal to the electronic controller such as PLC 40' which varies depending upon the magnitude and direction of current flow in the magnet circuit. As is described in detail below, the electronic controller 40' controls the drop cycle of the control system 27' depending upon the signal received directly or indirectly from the current transducer 200. With particular reference also to FIG. 17, the current transducer 200 interfaces with the electronic controller 40' through a converter 220 which translates the output signal of the current transducer 200 into a suitable format for input to the controller 40'. For example, in one preferred arrangement, the current transducer 200 is a current transformer that outputs a voltage signal of 0 - 5 volts DC depending upon the current flow in the magnet circuit. In such case, the converter 220 may be a voltage-to-frequency converter that converts the voltage signals from the current transducer 200 into corresponding frequency signals as are suitable for input to a controller 40' such as a PLC. Of course, other suitable arrangements may be employed to sense the current in the magnet circuit during the drop cycle of the control system 27'. The operator control panel 19' located in the cab 14' includes a drop cycle control means 281 by which an operator is able to adjust the cut-off point of the reverse current during the drop cycle. Preferably, the drop cycle control means 281 is used to select a magnitude and/or direction of current in the magnet circuit at which the reverse voltage to the generator shunt fields 66 ',68' is cut to consequently cut the output of the generator 22' and the flow of reverse current in the magnet circuit. Preferably, the control means 281 includes a control knob with a plurality of preset locations, each of which locations define a different input to the electronic controller 40' so that the controller 40' terminates the reverse current in the magnet circuit when a select current is sensed in the magnet circuit. In one arrangement, for example, the drop cycle control means 281 comprises a network of resistors which are connected to the controller 40' in different select patterns depending upon the particular adjustment made by the operator. In certain instances, depending upon the load being lifted, the particular magnet being used, and other like variables, the reverse voltage to the generator shunt fields 66', 68' will be cut when the current in the magnet circuit is sensed at 0 Amperes - i.e., that is, when the reverse current has counteracted the remaining forward current flowing in the magnet circuit due to the residual magnetism in the lifting magnet 12'. In other instances the control system 27' will be set through the drop cycle control means 281 so that the reverse voltage to the shunt fields 66', 68' is cut either before or after the residual forward current in the magnet circuit is overcome or "reversed." The particular current level in the magnet circuit at which the reverse voltage to the generator shunt field windings 66', 68' is cut is controlled by the operator, within a certain range, depending upon the setting of the drop cycle control means 281' . Because the current in the magnet circuit is able to be determined at all times during the drop cycle, it is not necessary to reduce the voltage to the magnet circuit during the drop cycle as is done with prior lifting magnet controllers. Instead, the drop voltage applied to the magnet 12' is at least substantially equal to the voltage applied to the magnet 12' during the lift cycle. With reference now to FIG. 18, a method of using the control system 27' in accordance with the present invention is illustrated. The drop cycle is initiated at 250 by the operator by depression of the drop control switch S2'. Step or means 250 reverses the current in the generator shunt fields 66', 68' by application of a reverse voltage thereto. Step or means 270 senses the current in the magnet circuit. Step or means 280 determines if the sensed current in the magnet circuit is at the select level as is controlled by the drop cycle control means 281. If the current in not at the select level, step or means 260 continues the flow of reverse current through the magnet circuit and steps or means 270 and 280 continue to sense the current in the magnet circuit and determine if it has reached the select level . On the other hand, if the step or means 280 determines that the current in the magnet circuit has reached the select level as is desired, a step or means 290 cuts the voltage to the generator shunt fields 66 ',68' so that the drop cycle is terminated. Although the use of a current sensor 200 finds particular application for use to control the drop cycle of the magnet control system 27', those skilled in the art will recognize that the output of the current sensor 200 can also or alternatively be used during the lift cycle of the control system to monitor and/or control the flow of current to the lifting magnet 12' . Furthermore, the invention has been described with reference to preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding specification. It is intended that the invention be construed as is including all such modifications and alterations insofar as is they come within the scope of the appended claims or the equivalents thereof.

Claims: Claims 1. A method of selectively energizing a lifting magnet (12) of a materials handling machine (10), said method comprising: (a) connecting a magnet circuit including said lifting magnet (12) to a voltage output of a separately excited generator (22), said generator including shunt field windings (66,68); (b) rotating an armature (60) of said generator (22) ; (c) selectively passing an electrical current through said shunt field windings (66,68) to excite said generator (22), thereby establishing a voltage at said output (23) of said generator (22) and inducing current flow in said magnet circuit; (d) sensing the current flow in the magnet circuit; and, (e) controlling the flow of electrical current through the shunt field windings (66,68) based on the current sensed in the magnet circuit . 2. The method as is set forth in claim 1, wherein step (c) comprises the substeps : (cl) connecting said shunt field windings (66,68) of said generator (22) to an electrical power source in a first orientation to pass an electrical current through said shunt field windings (66,68) in a first direction; and thereafter (c2) connecting said shunt field windings (66,68) of said generator (22) to said power source in a second orientation to pass an electrical current through said shunt field windings (66,68) in a second direction opposite said first direction, wherein steps (d) and (e) are carried out in connection with substep (c2) to terminate the flow of electrical current in the shunt field windings (66,68) in the second direction upon the sensed current in the magnet circuit reaching a select level . 3. The method as is set forth in claim 2, wherein substep (cl) comprises: connecting said shunt field windings (66,68) of said generator (22) to said electrical power source to pass a first electrical current of a first magnitude through said shunt field windings (66,68); and thereafter, connecting said shunt field windings (66,68) of said generator (22) to said electrical power source to pass a second electrical current of a second magnitude through said shunt field windings (66,68), said first current having a magnitude greater than said second current . 4. The method as is set forth in claim 1, further comprising: (f) continuously monitoring the magnet circuit interconnecting said lifting magnet (12) and said generator output (23) for the existence of a ground; and (g) notifying an operator of said lifting magnet (12) upon sensing a ground in said circuit interconnecting said lifting magnet 912) and said generator output (23) . 5. The method as is set forth in claim 1, further comprising: (f) continuously monitoring the electrical voltage level at said generator voltage output; and, (g) notifying an operator of said lifting magnet (12) if said voltage level is not within a predetermined range. 6. The method as is set forth in claim 1, further comprising: (f) maintaining a record of the amount of time said lifting magnet (12) is energized by said generator voltage output (23) relative to the total time said magnet (12) is in use; and (g) notifying an operator of said lifting magnet (12) if said lifting magnet (12) is energized more than a predetermined percentage of the total time said magnet (12) is in use. 7. The method as is set forth in claim 1, further comprising: (f) monitoring the temperature of said generator (22); and, (g) notifying an operator of said lifting magnet (12) if said temperature of said generator (22) exceeds a predetermined value . 8. The method as is set forth in claim 1, wherein step (b) comprises the substeps : (bl) connecting said armature (60) to a hydraulic motor (120); and, (b2) providing an essentially constant flow of hydraulic fluid to said hydraulic motor (120) to rotate said armature (60) at an essentially constant speed. 9. The method as is set forth in claim 8, wherein said step (b2) includes: passing hydraulic fluid through a valve (142) to restrict the flow of hydraulic fluid; and, maintaining an essentially constant hydraulic pressure drop from an upstream side of said valve (142) to a downstream side of said valve (142) . 10. A control system apparatus for selectively energizing a lifting magnet (12), said apparatus comprising: an electrical power input for connection to a voltage output of a generator (22) ; an electrical power output for selectively connecting the generator voltage output to a lifting magnet (12) ; generator excitation means for selectively electrically connecting shunt fields (66,68) of the generator (22) to an electrical power source to thereby excite said shunt fields (66,68) so that a voltage is established at the voltage output of the generator (22) and so that electrical current flows through said lifting magnet (12); and, means, connected to said generator excitation means, for detecting electrical current flow through said lifting magnet (12) , said generator excitation means disconnecting the electrical power source from the shunt fields (66,68) of the generator (22) when a select electrical current flow through the magnet (12) is detected. 11. An apparatus as is set forth in claim 10, wherein said generator excitation means for selectively electrically connecting shunt fields (66,68) of the generator (22) to an electrical power source comprises: a plurality of contactors (70-78) for selectively completing a circuit between the shunt fields (66,68) of the generator (22) and the electrical power source; and, an electronic controller (40') for opening and closing said plurality of contacts (70-78) . 12. An apparatus as is set forth in claim 11, wherein said electronic controller (40') comprises a programmable logic controller. 13. An apparatus as is set forth in claim 11, wherein said plurality of contactors (70-78) includes: a first set of contactors (70-74) which, when closed, complete a first circuit between the shunt fields (66,68) of the generator (22) and the electrical power source to pass electrical current through said shunt fields (66,68) in a first direction; and, a second set of contactors (76,78) which, when closed, complete a second circuit between the shunt fields (66,68) of the generator (22) and the electrical power source to pass electrical current through said shunt fields (66,68) in a second direction. 14. An apparatus as is set forth in claim 13, wherein said plurality of contactors (70-78) includes at least one contactor (72) which, when closed, alters the resistance in said first circuit to thereby alter the magnitude of electrical current flowing in said first circuit. 15. A materials handling apparatus comprising : a prime mover (10) ; a separately excited generator (22) including a rotatable armature (60) , shunt field windings (66,68), and a voltage output (23) for connection to a lifting magnet (12) through a magnet circuit; means for rotating said armature (60) of said generator (22) ; a control system (27) for selectively connecting said shunt field windings (66,68) of said generator (22) to an excitation power source such that voltage is established at said generator voltage output (23) and current flows through the magnet circuit; and, a current sensor (200) for sensing current in the magnet circuit. 16. The materials handling apparatus as is set forth in claim 15, wherein said means for rotating said armature (60) of said generator (22) comprises: a hydraulic pump (122) driven by an engine of said prime mover (10) ; and, a hydraulic motor in fluid communication with said hydraulic pump (122) and drivingly connected to said armature (60) . 17. The materials handling apparatus as is set forth in claim 16, further comprising: a hydraulic manifold (130) connecting said hydraulic pump (122) in fluid communication with said hydraulic motor (120) , said hydraulic manifold (130) defining a hydraulic circuit (124) including means for delivering an essentially constant flow of hydraulic fluid from said hydraulic pump (122) to said hydraulic motor (120) independent of the speed of said hydraulic pump (122) . 18. The materials handling apparatus as is set forth in claim 17, wherein said manifold (130) comprises a pressure compensated flow control valve assembly (140) for delivering a select flow of hydraulic fluid from said hydraulic pump (122) to said hydraulic motor (120) . 19. The materials handling apparatus as is set forth in claim 15, wherein said control system (27) comprises: a plurality of contactors (70-78) for selectively completing an electrical circuit between said excitation power source and said shunt field windings (66,68) . 20. The materials handling apparatus as is set forth in claim 19, wherein said control system (27) further comprises: an electronic controller (40') operatively connected to said plurality of contactors (70-78) and to said current sensor (200) for selectively opening and closing said plurality of contactors (70-78) according to current sensed by said current sensor (200) . 21. The materials handling apparatus as is set forth in claim 20, further comprising: a drop cycle duration adjustment control positioned in an operator's cab (14) of said prime mover (10) and electrically connected to said electronic controller (40') for operator selection of one of a plurality of select levels of current flow in the magnet circuit at which the electronic controller (40') terminates excitation of the shunt fields (66,68) of the generator (22). 22. The materials handling apparatus as is set forth in claim 20, further comprising: a plurality of visual indicators (L1-L5) positioned in an operator's cab of said prime mover (10) and electrically connected to said electronic controller (40'), said indicators (L1-L5) indicating to an operator of said prime mover (10) at least one of: a magnet power-on condition wherein the lifting magnet (12) carried by said prime mover (10) is energized; an over-voltage condition of said generator (22) ; an under-voltage condition of said generator (22) ; a ground between said generator output and the lifting magnet (12) carried by said prime mover (40') ; an excessive magnet duty cycle; and, a generator overheating condition.

Inventors: CHLAD KENNETH J, CLUTTER RONALD W, COOPER TIMOTHY L, ROBINSON CHARLES A, CHLAD, KENNETH, J., CLUTTER, RONALD, W., COOPER, TIMOTHY, L., ROBINSON, CHARLES, A.
Applicants: CATERPILLAR INC, CATERPILLAR INC.
Application year: 1999
Main classification: B66C1
IPC codes: B66C1