Abstract:
A method for operating an electric crane, comprises the steps of activating a magnet controller to cause a current to flow through a magnet for creating a magnetic field about the magnet for securing a load to the magnet, receiving a feedback input value at a logic controller from a device associated with the electric crane, in response to the received feedback input value at the logic controller, receiving a command value at the magnet controller from the logic controller, and in response to the received command value at the magnet controller, modifying the current flow from the magnet controller to the magnet to change the magnetic field about the magnet. A system is also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/889,664 filed on Feb. 13, 2007. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates in general to material handling machines and more particularly relates to the control of electro-magnets manipulated by cranes. 
       BACKGROUND 
       [0003]    Electro-magnetic lifting magnets are commonly associated with cranes. Cranes with lifting magnets are utilized for manipulating relatively heavy magnetic materials, such as, for example, scrap steel, ferrous material, and the like. 
         [0004]    In operation, if electric current is delivered, without interruption, to the lifting magnet, the lifting magnet generates heat which detracts from its magnetic strength. To compensate for this loss of magnetic strength, the operator often increases current flow to the magnet. The increased current flow may solve the immediate problem by re-establishing the magnet&#39;s strength; however, it exacerbates the heating of the magnet due to I 2 R losses generated in the windings of the lifting magnet. If this current escalation is carried out to an extreme, it can lead to destruction/failure of the lifting magnet. 
         [0005]    An experienced crane operator may, however, manipulate the electromagnet controls in other ways in an effort to manually establish an efficient operation of a crane lifting magnet combination. For example, an efficient operation of a crane can be manually controlled by the operator by manipulating the timing of an energize-to-de-energized duty cycle period (i.e., a rest period) of a lifting magnet during each load-unload-reload cycle (hereinafter lift cycle). The “load” portion of the lift cycle may be, for example, thirty seconds long and the “unload” period (i.e. the period between unloading and reloading) may be, for example, three seconds long. As such, an operator may be able to regain a certain efficiency by manually reducing the current to the magnet during the unload period. Of course, the relationship between duty cycle and loss of efficiency is generally not linear. 
         [0006]    If a crane operator falls behind schedule, the crane operator may not appropriately time or otherwise provide the lifting magnet with a rest period, thereby causing the lifting magnet to overheat due to a constant, high current that passes through the lifting magnet when it is energized. If the electro-magnet is utilized for a long period of time during a daily shift (without appropriately apportioning the rest period in each lift cycle), an over-heating condition may result in a temporary failure of the lifting magnet. Even further, if this operation is practiced in a similar manner over a protracted period, the repetitive over-heating condition may result in permanent damage to the lifting magnet. 
         [0007]    In addition, several drawbacks including, for example, voltage spiking of a hoist motor and whipping of the crane derrick may occur should a crane operator improperly de-energize a lifting magnet during a condition when a crane&#39;s hoist motor is generating high torque during a lifting operation. 
         [0008]    Accordingly, there is a need in the art for a method and apparatus for improving the control of a crane magnet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The disclosure will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0010]      FIGS. 1A-1D  each illustrated an environmental view of a lifting magnet and a crane in accordance with an exemplary embodiment of the invention; 
           [0011]      FIG. 2  is a flow chart illustrating a method for providing efficient operation of the electric crane in accordance with an exemplary embodiment of the invention; 
           [0012]      FIG. 3  is a timing diagram associated with the method of  FIG. 2  in accordance with an exemplary embodiment of the invention; 
           [0013]      FIG. 4  is a flow chart illustrating a method for providing efficient operation of the electric crane in accordance with an exemplary embodiment of the invention; 
           [0014]      FIG. 5  is a flow chart illustrating a method for providing efficient operation of the electric crane in accordance with an exemplary embodiment of the invention; and 
           [0015]      FIG. 6  is a timing diagram associated with the method of  FIG. 5  in accordance with an exemplary embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The Figures illustrate an exemplary embodiment of a method and apparatus for controlling a lifting magnet of a crane in accordance with an embodiment of the invention. Based on the foregoing, it is to be generally understood that the nomenclature used herein is simply for convenience and the terms used to describe the invention should be given the broadest meaning by one of ordinary skill in the art. 
         [0017]    Referring to  FIGS. 1A-1D , a system for moving magnetic material is shown generally at  10   a - 10   d , respectively, according to an embodiment. The system  10   a - 10   d  is generally defined by a crane  12  and an electro-magnet referred to herein as a lifting magnet  14 . The crane  12  is generally defined to include an operator cabin  16  and a derrick  18 . The crane  12  also includes a lift cable  20  that is reeled from a hoist assembly including a hoist motor  22 . 
         [0018]    The lift cable  20  is supported by a pulley  24  and serves as a bearing surface for spatially supporting the lifting magnet  14  above ground, G, by way of the lift cable  20 . According to an embodiment, the lift cable  20  may provide a dual function in that the lift cable  20  structurally supports the load of the magnet  14  while also serving as a support structure for supporting an electric conductor (not shown) used to deliver electrical current to lift magnet  14  from magnet controller  26 . 
         [0019]    According to an embodiment, although not required, the magnet controller  26  is shown generally disposed within the operator cabin  16 . According to an embodiment, the magnet controller  26  may provide a flow of current to the lifting magnet  14  in order to create a magnetic field about the magnet  14  for lifting magnetic material, such as, for example, a small load, L S , a medium-sized load, L M , or a larger load, L L . 
         [0020]    According to an embodiment, although not required, a controller  28 , such as, for example, a programmable logic controller (PLC) is shown generally disposed within the operator cabin  16 . As illustrated, the PLC  28  may receive information from operator inputs  30 , which may include, for example, joy sticks, levers, dials, switches, or the like. In addition, the operator inputs  30  may be provided directly to the hoist motor  22  by way of the magnet controller  26 . In an embodiment, the operator inputs  30  may include levers, dials, and/or switches for initiating the energizing and de-energizing of the magnet  14  that, respectively, activates or deactivates a magnetic field about the magnet  14  for respectively retaining, moving, and releasing the load L S , L M , L L  therefrom. 
         [0021]    The inclusion of the PLC  28  in the system  10  provides for an efficient operation of the crane  12 . Although operational information may be provided to the PLC  28  from the hoist motor  22  and/or operator inputs  30 , the PLC  28  may also receive operational information from a device  32   a - 32   c . The device  32   a  ( FIG. 1B ) may include, for example, a load cell. The device  32   b  ( FIG. 1C ) may include, for example, an imaging camera. The device  32   c  ( FIG. 1D ) may include, for example, a magnet temperature sensor. Accordingly, with the inclusion of a device  32   a - 32   c , the PLC  28  may provide a closed-loop feedback system that effects control over numerous output devices including, for example, the magnet controller  26 . 
         [0022]    Operation Mode 1—Power Adjust Mode 
         [0023]    According to an embodiment, the PLC  28  may receive information from one of more of the hoist motor  22 , load cell  32   a , camera  32   b , and/or temperature sensor  32   c  to provide a signal to the magnet controller  26  that references an amount of current, I 1 -I 3  ( FIG. 3 ), provided to the lifting magnet  14 . In addition, the information received at the PLC  28  from the hoist motor  22  and/or devices  32   a - 32   c  may also be supplemented with or effected by information from operator inputs  30 . The information provided to the PLC  28  may be conducted in any desirable fashion, such as, for example, a hardwired communication (see, e.g., feedback  102   a  from hoist motor  22 /signal  108  from operator inputs  30 ), or, alternatively, wireless communication (see, e.g., feedback  102   b  from devices  32   a - 32   c ). Although the signal from devices  32   a - 32   c  is illustrated to be wireless, it will be appreciated that the feedback from devices  32   a - 32   c  may be hardwired as well. 
         [0024]    As seen in  FIG. 2 , a method  100  including steps S. 101 -S. 108  for providing efficient operation of the lift magnet  14  is shown according to an embodiment. In general, the method  100  operates on the principle of providing an input  102   a ,  102   b ,  108  ( FIGS. 1A-1D ) to the PLC  28 , which may be provided, for example, from the hoist motor  22 , operator inputs  30 , or devices  32   a - 32   c . In correlation with the input  102   a ,  102   b ,  108 , efficient operation of the lift magnet  14  is enabled by providing a command  104  ( FIGS. 1A-1D ) to the magnet controller  26  from the PLC  28  that results in a controlled, output  106  ( FIGS. 1A-1D ) of current from the magnet controller  26  to the lifting magnet  14 . 
         [0025]    Prior to operating the system  10   a - 10   d  according to the method  100 , the PLC  28  may be pre-programmed at step S. 101  to associate the input  102   a ,  102   b ,  108  of  22 ,  30 ,  32   a - 32   c  with an amount of weight that is to be lifted by the magnet  14 . In the following description, according to an embodiment, the amount of weight is defined to include either the weight of the small load, L S , which is less than the weight of the medium load, L M , which is less than the weight of a large load, L L . Additionally, according to an embodiment, it may be assumed that the type and density of material defining the load identified at L S , L M , and L L  may be similar; the only difference, for example, between the three loads identified at L S , L M , and L L  may be the relative mass of each load L S , L M , and L L . 
         [0026]    According to an embodiment, at step, S. 101 , the PLC  28  may be pre-programmed with, for example, a data map or a look-up table by associating the input  102   a ,  102   b ,  108  in relation to a weight range defined by each load L S , L M , L L . Referring first to  FIG. 1A , for example, the data map or look-up table may be constructed by associating a weight range of the load (i.e. L S , L M , L L ) with a respective input  102   a  to be provided by the hoist motor  22 . In an embodiment, the input  102   a  provided by the hoist motor  22  may be an amperage utilized by the hoist motor  22 . As such, if the amperage  102   a  utilized by the hoist motor  22  is relatively low, the PLC  28 , by referring to the data map or lookup table, may be able to determine that the load is relatively light (i.e., a small load, L S ), and therefore, the PLC  28  may instruct the magnet controller  26  to reduce the current  106  provided to the magnet  14 . 
         [0027]    Referring to  FIG. 1B , for example, the data map or look-up table may be constructed by associating a weight of the load (i.e. L S , L M , L L ) with a respective input  102   b  to be provided by the load cell  32   a . In an embodiment, the input  102   b  provided by the load cell  32   a  may be a gauge factor. As such, if the gauge factor  102   b  is relatively low, the PLC  28 , by referring to the data map or lookup table, may be able to determine that the load is relatively light (i.e., a small load, L S ), and therefore, the PLC  28  may instruct the magnet controller  26  to reduce the current  106  provided to the magnet  14 . 
         [0028]    Referring to  FIG. 1C , for example, the data map or look-up table may be constructed by associating a weight of the load (i.e. L S , L M , L L ) with a respective visual attribute  102   b  to be provided by the camera  32   b . In an embodiment, the input  102   b  provided by the camera  32   b  may be a captured image of the load L S , L M , L L . As such, once the captured image  102   b  is scrutinized by, for example, the PLC  28 , the PLC  28  may determine that the image of the load evidence that it is comprised of a class of materials that are relatively easy to pick up (perhaps because of the geometry or topography of the materials, or some other correlating visual feature), and therefore, the PLC  28  may instruct the magnet controller  26  to reduce the current  106  provided to the magnet  14 . 
         [0029]    Referring first to  FIG. 1D , for example, the data map or look-up table may be constructed by associating a weight of the load (i.e. L S , L M , L L ) with a respective input  102   b  to be provided by the magnet temperature sensor  32   c . As such, if the temperature of the magnet  14  is relatively high, and the load is relatively light, and therefore, the PLC  28  may instruct the magnet controller  26  to incrementally reduce the current  106  provided to the magnet  14  to a threshold that permits retention of the load to the magnet while also reducing the temperature of the magnet  14 . 
         [0030]    Although a data map or look-up table may be programmed to function in a closed-loop feedback system described above, it will be appreciated that the invention is not limited as such. If desired, inputs  108  from the operator controls  30  may be provided to the PLC  28  (see, e.g., step, S. 106   b , below). For example, the input  108  provided by way of the operator controls  30  may include, for example, a signal from a rheostat that reduces the current flow to the magnet  14 . Thus, the automatic, closed-loop nature of the invention, as described in relation to the inputs  102   a ,  102   b , may also be supplemented with manual inputs  108  originating from the crane operator positioned within the operator cabin  16 . In addition, it will be appreciated that other feedback parameters may be provided by any device that is/are directly or indirectly useful in determining the minimum current needed by the lift magnet  14  to pick up the weight of the load L S , L M , L L . 
         [0031]    Referring now to step S. 102 , the crane  12  may be operated by spatially positioning the magnet  14  proximate a load L S , L M , L L  that is to be lifted. Then, at step S. 103 , the magnet  14  is energized and the load L S , L M , L L  is drawn and secured to the magnet  14  by way of a magnetic field. 
         [0032]    At step S. 104 , the hoist motor  22  or device  32   a - 32   c  is activated to determine the weight of the load L S , L M , L L  according to the pre-programmed mapped data of step S. 101 . If, for example, the hoist motor  22  is utilized at step S. 104 , the data map may be programmed at step, S. 101 , such that the data map may know that the hoist motor  22  may range in operation between a low end of 250 amperes, which is associated with an amperage needed to lift small class of material defined by load, L S , and a high end of 600 amperes, which is associated with an amperage needed to lift a large class of material defined by load, L L . 
         [0033]    Then, at step S. 105   a , once the PLC  28  has been provided with a feedback input  102   a ,  102   b  that is associated with a weight of the load L S , L M , L L , the PLC  28  selects a current from the data map for operating the magnet  14  and sends a the current command signal to the magnet controller  26 , which is shown generally at  104  in  FIGS. 1A-1D . In effect, the current command  104  provides an instruction to the magnet controller  26  that sets the magnitude of current  106  to be provided to the magnet  14  at step, S. 106   a . According to one aspect of the method  100 , the current that is selected from the data map may be a minimum amount of current needed to create a magnetic field that will lift a corresponding weight of the class of material L S , Lm, L L . As such, a smaller/medium class of material, L S , L M , may result in the magnet  14  needing a lower current than that of a “per unit load”/larger class of material, L L . Thus, when a smaller/medium class of material, L S , L M , is lifted by the magnet  14 , the magnet  14  may be operated at a lower current level, thereby increasing the efficiency of the system  10  by operating the magnet  14  at a lower temperature. Classification of material can be directed to one or more physical features (except for weight). For example, topography, geometry, chemical make up, volume characteristics, etc. 
         [0034]    As described above, if, for example, the operator provides a manual input  108 , the PLC  28 , at step, S. 105   b  may monitor for such a condition. If no manual input  108  by the operator is provided, the method  100  is advanced to step  105   a . However, if a manual input is provided at step, S. 105   b , the current command  104  is provided to the magnet controller  26  and is then altered according to the manual input  108  provided by the operator at step S. 106   b.    
         [0035]    In operation, the current provided at either step S. 106   a  or S. 106   b  is associated with electrical power provided by the magnet controller  26 . The current provided by the magnet controller  26  may be less than a maximum potential current provided by the magnet controller  26  in view of the different classification of material L S , L M , L L  to be lifted by the magnet  14  according to the pre-programmed data map or look-up table of step S. 101 . Thus, because a limited current may be provided to operate the magnet  14 , the magnet  14  may produce less heat, H ( FIGS. 1A-1D ), and therefore, is less susceptible to failure or damage. In addition, because there is a smaller amount of heat, H, produced by the magnet  14 , the system  10  may operate with a reduced rest period in a lift cycle, thereby increasing efficiency of the system  10 . 
         [0036]    Referring to  FIG. 3 , an exemplary embodiment of the operation of the system  10  is shown. If, for example, the hoist motor  22  is activated at time, T 1  (i.e. steps, S. 103 , S. 104 ), and, for example, operates with a high end current of 600 or more amperes, the PLC  28 , according to the data map, may determine that the weight of the load is that of a large load, L L ; as such, the PLC  28  may provide an instruction  104  to the magnet controller  26  at step S. 105  to limit a current, I 3  (i.e., the signal  106 ), provided to the magnet  14  at step S. 106   a . Thus, for a large load, L L , the current, I 3 , flowing through the magnet  14  may be, for example, approximately 77 amperes, which is adequate to create a magnetic field that retains the large load L L  to the magnet  14 . 
         [0037]    At step, S. 107 , the operator of the crane  12  may move and position the large load L L  to a desired location. Then, at time, T 2  (i.e., step S. 108 ), the magnet  14  may be de-energized such that the large load, L L , is released from the magnet  14  at step, S. 108 . Then, a rest period may occur from time, T 2 , until time, T 3 . Later, at time, T 3 , the method may be returned to steps S. 102  and S. 103  where the magnet  14  is positioned and energized so that the hoist motor  22  is activated again at step S. 104 . 
         [0038]    At time, T 3 , the hoist motor  22  may operate with a low end current of approximately 250 amperes, which causes the PLC  28 , according to the data map, to determine, at step S. 104 , that the weight of the magnetic load is that of a small load, L S ; as such, the PLC  28  may provide an instruction  104  to the magnet controller  26  at step S. 105  to limit a current, I 1 , provided to the magnet  14 . Thus, the current, I 1 , flowing through the magnet  14  may be, for example, approximately 50 amperes, which is adequate to provide a magnetic field that retains the small load, L S , without unnecessarily overheating the magnet  14  by otherwise operating the magnet  14  with a current (e.g., I 3 ) higher than 50 amperes. 
         [0039]    The magnet  14  is then de-energized at time, T 4 , and a rest period occurs between time, T 4 , and time, T 5 . Then, from time, T 5  to T 6 , a similar operation as that described above is provided for a medium load, L M , which may result in a current, I 2 , flowing through the magnet  14  that is approximately equal to 65 amperes. Thus, the because the current, I 2 , flowing through the magnet  14  is approximately 65 amperes, the current, I 2 , is adequate to provide a magnetic field to retain the medium load, L M , thereto without unnecessarily overheating the magnet  14  by otherwise operating the magnet  14  with a current higher (e.g., I 3 ) than 65 amperes. 
         [0040]    Accordingly, it will be appreciated that the limited supply of current (e.g., I 1  or I 2 ) to the magnet  14  provides a cooler magnet  14  due to less operational heat, H, that is related to conventional higher operating currents of conventional systems. Because conventional systems do not consider the weight of the load, conventional systems must operate a magnet  14  at a higher current in order to adequately cover the upper load. 
         [0041]    Because the PLC  28  may recognize that the magnet  14  is lifting, for example, a lighter load (i.e., a smaller load, L S ), the power consumed from a current draw, I 1 , of 50 amperes may be only 8537 BTUs (i.e., 50 2 ×3.4149) whereas a heavier load (e.g., the larger load L L ) consuming a current draw, I 3 , of 77 amperes may be approximately equal to 20,246 BTUs (i.e., 77 2 ×3.4149). As such, the PLC  28  also may provide a cost savings for the host company of the crane operator with respect to a smaller amount of consumed electricity, which results from a more efficient operation of the crane  12 . 
         [0042]    Although the method  100  is based upon a data map or look-up table that considers a weight of the load, L S , L M , L L , it will be appreciated that the invention is not limited to a data map or look-up table utilizing a weight characteristic of the load L S , L M , L L  to determine a current provided to the magnet  14 . For example, referring to  FIG. 4 , a method  200  is related, in general, to any visual characteristic of the load, L S , L M , L L , or, alternatively, an operational characteristic of the system  10   a - 10   d  rather than a weight of the load, L S , L M , L L . 
         [0043]    Referring to  FIG. 4 , the method  200  may be related to, for example, a material class of the load, L S , L M , L L , including, for example, a geometric size of the constant particles that make up the load, topography, or constituent elements having visual manifestations, of the load, L S , L M , L L , determined by the camera  32   b  at step S. 204 . Upon learning the geometric size, material class, or material constituent of the load, L S , L M , L L , the PLC  28  may send a control signal  104  at step S. 206   a  to adjust the current  106  provided to the magnet  14 . 
         [0044]    Accordingly, if, for example, the camera  32   b  detects a large object (e.g., L L  of classification “x”, at step, S. 204 ) the PLC  28  may automatically tell the magnet controller  26  at  104  to set a current  106  at step S. 206   a  to a highest possible setting, whereas, alternatively, if, the camera  32   b  detects a large object (e.g., L L , of classification “y” where “x” and “y” are classifications of the topography of the constituent pieces that make up load L L  at step, S. 204 ) the PLC  28  may automatically command the magnet controller  26  at  104  to set a current  106  at step S. 206   a  to a lower setting. 
         [0045]    If, for example, the current  106  is over- or under-compensated by the PLC  28  according to the input  102   b  provided by the camera  32   b , an operator input  108  may be provided at step, S. 206   b , to provide the needed current compensation in order to arrive at the desired behavior of the magnet  14 . The desired behavior of the magnet  14  may be, for example, a decrease in current to reduce the magnetic field about the magnet  14 , or, alternatively, an increase in the magnetic field about the magnet  14 . According to an embodiment, over time, the PLC  28  may include intelligence that permits the PLC  28  to be “trained” by monitoring the operator&#39;s actions in conjunction with characteristics of images captured by the camera  32   b  temperature of the magnet, and weight of load L compensate for current delivered to the magnet  14 . 
         [0046]    According to an embodiment, the method  200  may be related to an input factor or characteristic of the system  10  including, for example, a temperature of the magnet  14  determined by the temperature sensor  32   c  at step S. 204 . Upon learning the temperature of the magnet  14 , the PLC  28  may send a control signal  104  at step S. 206   a  to adjust the current  106  provided to the magnet  14 . 
         [0047]    Accordingly, if, for example, the temperature sensor  32   c  detects a high operating temperature of the magnet  14 , which may, for example, be associated with the lifting of a large object (e.g., L L ), the PLC  28  may automatically command the magnet controller  26  at  104  to set a current  106  to a reduced setting to reduce the operating temperature of the magnet  14 . If, for example, the current  106  is over- or under-compensated by the PLC  28  according to the input  102   b  provided by the temperature sensor  32   c , an operator input  108  may be provided at step, S. 206   b , to provide the needed current compensation in order to arrive at the desired behavior of the magnet  14 . 
         [0048]    One skilled in the art will readily recognize that an “N” dimensional map can be created (using empirical testing) to map multiple inputs against magnet current. For example, magnet temperature, load weight, load classification, can all be used as map inputs to generate a unique magnet current output. 
         [0049]    Operation Mode 2—Auto-Drop Mode 
         [0050]    As seen in  FIGS. 5 and 6 , a method  300  including steps S. 301 -S. 307  for providing an improved operation of the crane  12  is shown according to an embodiment. In general, the method  300  operates on the principle of providing feedback  102   a  ( FIGS. 1 and 6 ) to the PLC  28 , which may be provided, for example, from the hoist motor  22 . In correlation with the feedback  102   a , less derrick whip and reduced voltage spiking of the hoist motor  22  is enabled by providing a regulated, control input  104  ( FIGS. 1A-1D ) to the magnet controller  26  that originates from the PLC  28 . 
         [0051]    Prior to operating the system  10   a - 10   d  according to the method  300 , the PLC  28  may be pre-programmed at step S. 301  to associate a torque output  102   a  from a hoist motor  22  with a drop release signal  104  to be sent to the magnet controller  26  by way of the PLC  28 . In operation, at step S. 302 , the crane  12  spatially positions the magnet  14  proximate a load L S , L M , L L  that is to be lifted. Then, at step S. 303 , the magnet  14  is energized and the load L S , L M , L L  is drawn and secured to the magnet. Although not required, step, S. 303 , may simultaneously occur with an activation of the hoist motor  22  at step, S. 304 , which is illustrated in  FIG. 6 . 
         [0052]    Referring to  FIG. 6 , at time, T 1  (i.e., steps S. 303 , S. 304 ), the hoist motor  22  is activated to lift the load L S , L M , L L  above the ground, G, such that the reeling-in of the lift cable  20  sharply increases the torque on the hoist motor  22  until the torque reaches a torque load value, T load . The torque load value, T load , may be substantially constant from time, T 2 , to a time, T 3 , as the crane operator moves the suspended load L S , L M , L L  generally horizontally above the ground, G. 
         [0053]    Then, at time, T 3 , the crane operator may decide to suddenly drop the load L S , L M , L L  to the ground, G. The PLC  28 , as such, at step S. 305  prevents an abrupt cessation of the current flow in the magnet  14  as would otherwise be associated with a conventional “auto-drop” operation of the crane  12 , but rather, at step, S. 305 , the PLC  28  commands the magnet controller  26  with a command signal  104  that instructs the magnet controller  26  to reduce the torque on the hoist motor  22  to a value less than the torque load value, T load , prior to de-energizing the magnet  14 . 
         [0054]    At step, S. 306 , the PLC  28  monitors the value of the reduced torque  102   a  after time, T 3 , until the torque  102   a  on the hoist motor  22  is associated with a hoist motor torque output  102   a  that is correlated with the drop release signal  104  associated in step S. 301 . Once the torque  102   a  of the torque motor  22  is reduced below a predetermined threshold T dropthres. , at step, S. 307 , the PLC  28  provides the signal  104  to the magnet controller  26  at time, T 4   a , to cease a current flow to the magnet  14 , which is seen at  106 , thereby dropping the load L S , L M , L L . 
         [0055]    Thus, because there is a reduced amount of torque  102   a  (i.e., a torque equal to T drop-thres. ) seen by the hoist motor  22 , there is a less likelihood for undesirable derrick  18  ‘whip’ or voltage spiking across the hoist motor  22  to occur during the operation of the crane  12 . Once the load L S , L M , L L  has been dropped as described above, at step, S. 307 , the method may be returned to steps S. 302  and S. 303  where the magnet  14  is positioned and energized so that the hoist motor  22  is activated again at step S. 304 . 
         [0056]    Although three distinct methods  100 ,  200 ,  300  have been described as related to the PLC  28 , it will be appreciated that one or more of the methods  100 ,  200 ,  300  may be conducted sequentially or simultaneously. For example, if, for example, the auto-drop mode  300  is conducted and the magnet  14  is operating relatively hot, the power adjust mode  200  may be activated during the operation of the auto-drop mode  300  to reduce the temperature of the magnet  14 . Alternatively, for example, if the auto-drop mode  300  has been completed, the power adjust mode  100  may be conducted subsequently to operate the system  10   a - 10   d  at a reduced power and therefore, at a potentially reduced operating temperature of the magnet  14 . 
         [0057]    The present invention has been described with reference to certain exemplary embodiments thereof. However, it will be readily apparent to those skilled in the art that it is possible to embody the invention in specific forms other than those of the exemplary embodiments described above. This may be done without departing from the spirit of the invention. The exemplary embodiments are merely illustrative and should not be considered restrictive in any way. The scope of the invention is defined by the appended claims and their equivalents, rather than by the preceding description.