Abstract:
A magnet controller supplied by an AC source controls a lifting magnet. Two bridges allow DC current to flow in both directions in the lifting magnet. During “Lift”, relatively high voltage is applied to the lifting magnet until it reaches its cold current. Then voltage is lowered. After a desired interval, once the magnet has had time to build its electromagnetic field, voltage is further reduced to prevent the magnet from overheating. The magnet lifting forced is maintained due to the magnetic circuit hysteresis. During “Drop”, reverse voltage is applied briefly to demagnetize the lifting magnet. At the end of the “Lift” and the “Drop”, most of the lifting magnet energy is returned to the line source. A logic controller controls current and voltage of the magnet and calculates the magnet&#39;s temperature. In one embodiment, a “Sweep” switch is provided to allow reduction of the magnet power to prevent attraction to the bottom or walls of magnetic rail cars or containers.

Description:
REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from U.S. Provisional Application No. 61/066,121, filed Dec. 19, 2007, titled “METHOD FOR CONTROLLING A LIFTING MAGNET SUPPLIED WITH AN AC SOURCE,” the entire contents of which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for controlling a lifting magnet of a materials handling machine for which the source of electrical power is an AC power source. 
     2. Prior Art 
     Lifting magnets are commonly attached to hoists to load, unload, and otherwise move scrap steel and other ferrous metals. For many years, cranes were designed to be powered by DC sources, and therefore systems used to control lifting magnets were designed to be powered by DC as well. When using a hoist, due to the nature of the overhauling load, the torque and speed of the hoist motor need to be controlled. The traditional approach was to control the DC motor torque and speed by selecting resistors in series with the DC motor field and armature windings by means of contactors. In recent years, with the advance of electronic technology in the field of motor control, systems used to control lifting magnets, namely cranes, are now designed to be powered by AC sources. Cranes are now equipped with adjustable-frequency drives, commonly referred to as AC drives, which can accurately control the speed and torque of AC induction motors. The use of AC supplies removes the costs of installing and maintaining large AC-to-DC rectifiers, of replacing DC contactor tips, and of maintaining DC motor brushes and collectors. However, in order to use a lifting magnet on one of the new AC supplied cranes, a rectifier needs to be added to the crane. The rectifier that needs to be added to the crane is generally composed of a three-phase voltage step-down transformer connected to a six-diode bridge rectifier. The rectifier that is added to the crane is either mounted on the crane itself, where the rectifier becomes a weight constraint and an obstruction, or the rectifier is mounted elsewhere in the plant, in which case additional hot rails are required along the bridge and trolley in order for the DC electrical power to reach the DC-supplied magnet controller. 
     While lifting magnets have been in common use for many years, the systems used to control these lifting magnets remain relatively primitive. During the “Lift”, a DC current energizes the lifting magnet in order to attract and retain the magnetic materials to be displaced. When the materials need to be separated from the lifting magnet, most of the controllers automatically apply a reversed voltage across the lifting magnet for a short period of time to allow the consequently reversed current to reach a fraction of the “Lift” current. The phase during which there is a reversed voltage applied across the magnet is known as the “Drop” phase, during which a magnetic field in the lifting magnet of the same magnitude but in an opposite direction of the residual magnetic field is produced such that the two fields cancel each other. When the lifting magnet is free of residual magnetic field, the scrap metal detaches freely from the lifting magnet. This metal detachment is known as a “Clean Drop”. 
     Some control systems operate to selectively open and close contacts that, when closed, complete a “Lift” or “Drop” circuit between the DC generator and the lifting magnet. At the end of the “Lift”, which is called the “discharge” and at the end of the “Drop”, which is called the “secondary discharge”, these systems generally use either a resistor or a varistor to discharge the lifting magnet&#39;s energy. The higher the resistor&#39;s resistance value or varistor breakdown voltage, the faster the lifting magnet discharges, but also the higher the voltage spike across the lifting magnet. High voltage spikes cause arcing between the contacts. In addition, fast rising voltage spikes also eventually wear out the lifting magnet insulation, and the insulation of the cables connecting the lifting magnet to the controller. To withstand these voltage spikes, generally in the magnitude of 750 V DC with systems using DC magnets rated at 240 V DC, the lifting magnet, cables, and the control system contacts and other components need to be constructed of more expensive materials, and also need to be made larger in size. 
     Lifting magnets are rated by their cold current (current through the magnet under rated voltage, typically 250V DC, when the magnet temperature is 25° C.). These lifting magnets are designed for a 75% duty cycle (in a 10 minute period the magnet can have voltage applied at 250V DC for 7 minutes 30 seconds and the remaining 2 minutes 30 seconds the magnet must be off for cooling or the magnet will overheat). Today, magnet control systems are limited by the rectified DC voltage supplying the magnet control (typically 250-350V DC). These systems control the voltage to the magnet and as the magnet heats up, the resistance rises and the current drops. As a magnet heats up, the magnet loses 25-35% in lifting capacity because the resistance of the wire increases and the current through the lifting magnet decreases. 
     SUMMARY 
     These and other problems are solved by a new and improved method and apparatus for controlling a lifting magnet using an AC source, described here. 
     In one embodiment, the voltage and the current are controlled during the charging of the lifting magnet during the lift cycle. Charging involves the phase that begins the “Lift” mode during which the current in the lifting magnet increases. Voltage levels up to 500V DC or more are applied to the lifting magnet during the charge. When a current value related to the cold current rating of the lifting magnet is reached, then the current is limited to this value until the end of the “Lift” mode. The lifting magnet can overheat if the current is maintained at the cold current level or higher, so after a preset time, during which the material attaches to the lifting magnet, the voltage on the lifting magnet is reduced to a holding voltage which causes a relatively lower current than the current applied during the “Lift” of the lifting magnet. The period during which there is a holding voltage applied to the lifting magnet is the “Hold” mode and this “Hold” mode allows the lifting magnet to hold the material that the lifting magnet has already picked-up. 
     In one embodiment, the “Lift” mode is initiated by the operator. During the “Lift” mode, a first voltage is applied across the lifting magnet. Then, the operator can select a relatively higher voltage continue to be applied to the magnet in order to secure a load that has been picked up by the magnet. 
     In one embodiment, the voltage levels during “Lift” and “Hold” modes are user-selectable. 
     In one embodiment, the ratio of “Lift” to “Hold” voltages is user-selectable, based on the type of application sought. 
     In one embodiment, the magnetic field is maintained in the lifting magnet from the magnet&#39;s cold state to the magnet&#39;s hot state during the charging of the lifting magnet. Since the lifting magnet&#39;s field is primarily controlled by NI (where N=turns of wire and I=current), maintaining the same current for a cold or hot magnet maintains substantially the same magnetic field. 
     In one embodiment, most of the lifting magnet energy used during the “Lift” and the “Drop” phases is returned to the line source rather than being dissipated in resistors, varistors, or other lossy elements. 
     In one embodiment, if during “Lift” or “Drop”, the controller is accidentally disconnected from the line such that the current cannot keep flowing in the lifting magnet, the voltage across the lifting magnet sharply rises and consequently this fast voltage rise turns one or more voltage protection devices before their breakover voltage is attained. In addition, the lifting magnet controller circuitry can be protected by the use of circuit breakers, such as, for example, a high speed breaker. 
     In one embodiment, switching of current for the lifting magnet is provided by solid-state devices. 
     In one embodiment, the control system is configured to increase the useful life of the lifting magnet by reducing voltage spikes in the lifting magnet circuit. During operation, the instantaneous voltage across the magnet typically should not exceed the line voltage, i.e. for a system rated 460 V AC RMS, peak voltage is 460×√2=650 V, whereas voltages in prior art systems typically exceed 750 V. 
     In one embodiment, the control system is configured to increase the useful life of the lifting magnet, by providing a “Hold” mode that reduces magnet heating. 
     In one embodiment, the control system is configured to save energy by providing a “Hold” mode that reduces energy consumption. 
     In one embodiment, the control system is configured to reduce the “Lift” time. A shorter “Lift” time helps to increase production by reducing the lifting magnet cycle times. Using a higher AC voltage can provide relatively shorter “Lift” times. Some existing systems use a step-down voltage transformer which reduces the maximum voltage that can be applied to the magnet during “Lift”, and therefore these systems could not lift as quickly as systems with full line AC voltages. 
     In one embodiment, the control system is configured to reduce the “Drop” time. A shorter “Drop” time helps to increase production by reducing the lifting magnet cycle times. Some existing systems use a resistor, which causes voltage to decay with the current, leading to longer discharge times. Using a constant voltage source to discharge the lifting magnet energy allows a faster discharge. 
     In one embodiment, the control system is configured to monitor the lifting magnet resistance. Using the direct relationship between the magnet resistance and the magnet&#39;s winding temperature, resistance values corresponding to different meaningful temperature levels of the lifting magnet can be monitored. 
     In one embodiment, the control system is configured to indicate an alarm to the operator if the lifting magnet temperature rises above a threshold level. 
     In one embodiment, the control system is configured to protect and increase the useful life of the lifting magnet by providing a “Trip” mode, which, based on an indication of the lifting magnet&#39;s temperature, determines whether the system should directly enter “Drop” mode instead of “Lift” mode, to reduce magnet heating. 
     In one embodiment, the control system is configured to prevent the lifting magnet from sticking to the bottom and walls of magnetizable containers by providing a “Sweep” mode that reduces the voltage levels applied to the lifting magnet during the “Lift” and “Hold” modes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an overhead crane with lifting magnet. 
         FIG. 2A  shows an AC lifting magnet system. 
         FIG. 2B  shows an AC lifting magnet system with an optional DC Power Converter such as a DC Regulated Power Supply. 
         FIG. 3  illustrates an equivalent circuit for magnet resistance calculation. 
         FIG. 4A  shows voltage and current signals as the AC magnet controller is operated through “Lift”, “Hold” and “Drop” modes for handling scrap material, for example. 
         FIG. 4B  shows voltage and current signals as the AC magnet controller is operated through “Lift”, “Hold” and “Drop” modes for handling plates or slabs, for example. 
         FIG. 5  shows a general Sequential Function Chart (SFC). 
         FIG. 6  shows a flowchart for the Main SFC. 
         FIG. 7  shows a flowchart for the Ready SFC. 
         FIG. 8  shows a flowchart for the Lift SFC 
         FIG. 9  shows a flowchart for the Hold SFC. 
         FIG. 10  shows a flowchart for the Drop SFC. 
         FIG. 11  shows one embodiment of the DC Regulated Power Supply Voltage Selection. 
         FIG. 12  shows one embodiment of the DC Regulated Power Supply Current Selection. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an overhead crane with lifting magnet  113 . The lifting magnet  113  is attached by cables to the magnet controller which controls the lifting magnet  113  from the bridge of the overhead bridge crane. 
       FIG. 2  shows a lifting magnet controller circuit that includes a Logic Controller (LC)  100 . In one embodiment, the LC  100  can be a Programmable Logic Controller (PLC). The LC  100  receives input commands from an operator and provides alarm and trip relay outputs. Outputs from the logic controller  100  are provided to respective switches  101 - 112 . The switches  101 - 103  and  110 - 112  are configured in a positive bridge  250  to provide current to the lifting magnet  113  in a first direction, and switches  104 - 109  are configured in a negative bridge  251  to provide current to the lifting magnet  113  in a second direction. The switches  101 - 112  can be any type of mechanical or solid-state switch device so long as the devices are capable of switching at a desired speed and can withstand voltage spikes. For convenience, and not by way of limitation,  FIG. 2  shows the switches  101 - 112  as thyristors, each having an anode, a cathode and a gate. One of ordinary skill in the art will recognize that the switches  101 - 112  can be bipolar transistors, insulated gate bipolar transistors, field-effect transistors, MOSFETs, etc. One of ordinary skill in the art will also recognize that the number of switches used can be less or more than the twelve shown; using a greater number of switches reduces ripple. 
       FIG. 2A  shows the lifting magnet controller.  FIG. 2B  shows one embodiment of the lifting magnet controller where a DC Power Converter such as a DC Regulated Power Supply  400  is used. The DC Regulated Power Supply  400  is one embodiment of a DC Power Converter, and is used as an example and not by way of limitation. 
     In  FIGS. 2A and 2B , the thyristors  101 - 112  will initially conduct when the anode is positive with respect to the cathode and a positive gate current or gate pulse is present. The gate current can be removed once the thyristor has switched on. The thyristors  101 - 112  will continue to conduct as long as the respective anode remains sufficiently positive with respect to the respective cathode to allow sufficient holding current to flow. The thyristors  101 - 112  will switch off when the respective anode is no longer positive with respect to the respective cathode. The amount of rectified DC voltage can be controlled by timing the input to the respective gate. Applying current on the gate without delay to the natural commutation time will result in a higher average voltage applied to the lifting magnet  113  (where natural commutation time is understood in the art to be the time at which the SCRs would start conducting if they were replaced by diodes). Applying current on the gate later will result in a lower average voltage applied to the lifting magnet  113 . When the current in the magnet needs to be turned off, the application of the current on the gate can be further delayed to the point where voltage across the magnet  113  reverses, restoring the magnet energy to the AC supply. The period of time which precedes the “Drop” mode is called discharge. Six thyristors,  101 - 103  and  110 - 112 , are connected together to make a three-phase bridge rectifier  250 . The gating angle of the thyristors in relationship to the AC supply voltage determines how much rectified voltage is available. Converted DC voltage (V DC ) is equal to 1.35 times the RMS value of input voltage (V RMS ) times the cosine of the phase angle (cos α): V DC =1.35×V RMS ×cos α. The value of the DC voltage that can be obtained from a 460V AC input is thus −621V DC to +621V DC. The addition of the second, negative bridge  251  (i.e. connected in reverse with respect to the first positive bridge  250 ) in the circuit allows for four-quadrant operation. The positive bridge  250  charges the lifting magnet  113  during the “Lift” mode and returns energy from the lifting magnet  113  back to the AC input during discharge. This four-quadrant circuit can also be used to demagnetize the lifting magnet  113  by applying voltage in the opposite polarity by using the negative bridge  251  as the bridge used to bring voltage to the lifting magnet  113  and returning energy to the AC input (for example, at the end of “Drop”). The time during which the negative bridge  251  restores energy from the magnet back to the AC input is called the secondary discharge. Those skilled in the art will recognize that the polarity of the lifting magnet  113  is reversible, such that the positive bridge  250  can be used to demagnetize the lifting magnet  113  during the “Drop” mode and the negative bridge  251  can be used to magnetize the lifting magnet  113  during the “Lift” mode; the previous directions have been described for convenience. It will also be apparent to one skilled in the art that the use of three-phase power is not necessary for all cycles. 
     The thyristors  101 - 112  act as transient protection devices themselves, and prevent failures in the DC Regulated Power Supply  400  or in the AC input power from damaging components in the DC Regulated Power Supply  400  by conducting before the output voltage of the supply rises above the breakover voltage of the thyristors by freewheeling the magnet coil. The thyristors  101 - 112  are usually chosen so that their breakover voltage is higher than the greatest voltage expected to be experienced from the power source, so that they can be turned on by intentional voltage pulses applied to the gates. If other types of switches are used, those skilled in the art will recognize that transient protection devices can be added to protect against voltage spikes. 
       FIG. 3  shows the actual and equivalent circuits used for magnet resistance calculation. Overheating of the lifting magnet  113  can lead to melting or short-circuits, and a need to rewind the lifting magnet  113 . The internal temperature of the lifting magnet  113  can be measured by a thermistor or other temperature sensor, if such a device was embedded in the lifting magnet  113  during the process of magnet winding. In one embodiment, the temperature of the lifting magnet  113  is calculated by measuring the electrical resistance  301  of the magnet  113  because the resistance  301  of the lifting magnet  113  is substantially proportional to the temperature of the lifting magnet  113 . The magnet resistance  301  is calculated based on readings of voltage and current across the lifting magnet  113  or across the load side of the DC Regulated Power Supply  400  and by taking into account the resistance  302  of the cables. The resistance  302  of the cables can either be (1) calibrated out, (2) measured and subsequently subtracted from the total resistance reading, or (3) disregarded if the resistance  302  is assumed to be small in relation to the magnet resistance  301 . The cables are not expected to get hot because of the low value of their resistance  302  and their exposure to air. However, the lifting magnet  113  gets hot because of the relatively high density of windings in relation to the surface area available for cooling (typically, cooling is achieved by natural convection). Lifting magnets are generally designed for a resistance increase of about 50% when they get hot. The formula to calculate the magnet resistance  301  at a given temperature is: R H =R 0  (1+K Δθ), where R 0 =cold resistance of the lifting magnet  113 , in Ω, K=temperature coefficient of the magnet  113  (typically 0.004Ω/° C. for a copper- or aluminum-wound magnet), and Δθ=change in temperature, in ° C. 
     The lifting magnet&#39;s calculated resistance  301  is compared to two parameters: the “Alarm resistance” and the “Trip resistance”. The “Alarm resistance” is a threshold value which, if exceeded, triggers the system to provide an alarm to warn the operator to either turn off the lifting magnet  113  or to indicate that the system is picking up materials which are too hot, or that the cable is partially cut, or that a connection is loose. The “Trip resistance” is a threshold value which, if exceeded, triggers the system to protect the lifting magnet  113  from overheating. When the trip resistance is exceeded, the system activates a trip relay. If the trip relay is activated when the system is in “Hold” mode, the system will continue through the normal modes of operation of “Hold” and “Drop”. However, if the Trip relay is activate when the operator requests a “Lift”, the system will not enter into “Lift” mode and instead go directly to “Hold” mode. 
       FIG. 4A  shows voltage and current during the “Lift”, “Hold” and “Drop” modes for applications such as scrap material handling. The “Lift” mode is initiated by the operator. During the “Lift” mode, the positive bridge  250  applies a high voltage level across the lifting magnet  113  until the current reaches the limiting current for the lifting magnet  113  through the positive bridge  250 . The “Lift” mode lasts long enough to charge the lifting magnet  113  yet is short enough to prevent overheating of the lifting magnet  113 . The length of time for the “Lift” mode will vary based on the time constant of the lifting magnet  113 , the desired current for the lifting magnet  113  and the voltage applied to the lifting magnet  113 . During the charge, the first portion of the “Lift” mode, there is a relatively high average voltage applied to the lifting magnet  113  (typically adjusted around 500V for an AC supply of 460V AC) and the current rises relatively fast. Once the current has risen, then the current is limited and held at a plateau for a specified time to allow magnetic field to build up. 
     The “Hold” mode is initiated automatically after a specified time in “Lift” mode. During the “Hold” mode, the positive bridge  250  applies a different (lower) voltage level across the lifting magnet  113 , for as long as the operator needs in order to move the load. The “Hold” voltage is set below the lifting magnet  113  rated voltage, and the lifting magnet  113  is thus expected to cool down somewhat during the “Hold” mode. In other words, for safety reasons, an energized lifting magnet  113 , possibly carrying an overhead load, is not made to automatically shut down. Because of the reduced voltage level, in “Hold” mode, the current decreases to a second lower plateau. Under normal conditions, in the “Hold” mode, the load has already been attracted, air gaps are at a relatively low level, and therefore, less magnetic flux is required to keep the load attached. Therefore, the current and the magnetic field across the lifting magnet  113  can be reduced. At the end of the “Hold” mode, the firing angle of the thyristors phases back and energy from the lifting magnet  113  is returned to the AC input until current reaches zero. 
     The “Drop” mode is initiated by the operator and causes the “Lift” or “Hold” mode to terminate. During the “Drop” mode, the positive bridge  250  thyristors&#39; firing pulses get delayed to cause the polarity of voltage across the lifting magnet  113  to reverse. After the current from the “Drop” mode or the “Hold” mode reaches zero, the negative bridge  251  applies a voltage of reverse polarity across the lifting magnet  113 , i.e. reverses the sense of voltage signal until the current reaches the current limit for the lifting magnet  113  through the negative bridge  251 . The “Drop” mode expires after yet another specified time. During the “Drop” mode, the current value is specified such as to produce a magnetic field in the lifting magnet  113  that is of the same magnitude but in an opposite direction of the residual magnetic field across the lifting magnet  113 , such that the two fields cancel each other. When the lifting magnet  113  is free of residual magnetic field, the load detaches freely from the lifting magnet  113 . 
     In  FIG. 4A , during phase  0 , the lifting magnet  113  is idle. Phase  1  represents the “Lift” mode during voltage regulation, where the voltage can be adjusted to a relatively high value in order to magnetize the lifting magnet  113  relatively quickly. Phase  2  represents the “Lift” mode during current limiting, where the current limit can be adjusted close to the cold current rating for the lifting magnet  113 . Phase  3  represents the “Hold” mode, during which the current is adjusted to be a portion of the cold current such that the lifting magnet  113  does not warm up, while still holding the load; the magnitude of the current during the “Hold” mode can be adjusted such as to compensate for the amount of magnetic hysteresis. Phase  4  represents the “Drop” mode during transient, where the current is adjusted to compensate for the magnetic hysteresis. Phase  5  represents the “Drop” mode, where both current and voltage are held constant, in order to match the magnetic time constant of the lifting magnet  113 . 
       FIG. 4B  shows voltage and current during the “Lift”, “Hold” and “Drop” modes for applications such as handling of slab or plates material. The “Lift” mode is initiated by the operator. During the “Lift” mode, the positive bridge  250  applies a preset voltage level across the lifting magnet  113 . The length of time for the “Lift” mode will vary based on the time constant of the lifting magnet  113 . During the charge, the slab or plates attach to the lifting magnet  113 . After the charge, the operator starts to hoist the lifting magnet  113  for a few feet. If the operator wishes to hoist the load further, then the operator can apply a relatively higher voltage to the lifting magnet  113  during the “Hold” mode in order to maintain the load attached to the lifting magnet  113 . The “Drop” mode operates the same for this slab or plates&#39; material application as it does for the scrap materials handling application. 
     In  FIG. 4B , during phase  0 , the lifting magnet  113  is idle. Phase  1  represents the “Lift” mode where a preset voltage is applied to the lifting magnet  113 . Phase  2  represents the “Hold” mode, during which the operator selects a relatively higher voltage to apply across the lifting magnet  113 . Phase  4  represents the “Drop” mode during transient, where the current is adjusted to compensate for the magnetic hysteresis. Phase  5  represents the “Drop” mode, where both current and voltage are held relatively constant, in order to match the magnetic time constant of the lifting magnet  113 . 
     In addition to the above three modes, there is a “Sweep” mode, which is optionally activated by the operator. The “Sweep” mode is for applications where the rail car or container to be unloaded has its bottom or walls formed of magnetic material. When unloading is almost complete, to prevent the lifting magnet  113  from sticking to the bottom or walls of the rail car or container, a “Sweep” switch can be activated by the operator to reduce the “Lift” and “Hold” voltages. The reduced voltage across the lifting magnet  113  prevents the magnetized load from attaching to the bottom or walls of the rail car or container while the lifting magnet  113  is unloading. 
     In one embodiment, the “Lift”, “Hold”, “Drop” and “Sweep” modes of the magnet controller circuit described above, used to control the lifting magnet  113 , can be controlled through the use of the Logic Controller (LC)  100 . 
     The logical programming of the LC  100  is represented in sequential function charts (SFC). SFC is a graphical programming language used for logical controllers, defined in IEC 848. SFC can be used to program processes that can be split into steps. 
       FIG. 5  shows a general SFC. Main components of SFC are: steps with associated actions, transitions with an associated logic condition or associated logic conditions, and directed links between steps and transitions. Steps can be active or inactive. Actions are executed for active steps. A step can be active for one of two motives: (1) the step is an initial step as specified by the programmer, (2) the step was activated during a scan cycle and was not deactivated since. A step is activated when the steps above that step are active and the connecting transition&#39;s associated condition is true. When a transition is passed, the steps above the transition are deactivated at once and the steps below the transition are activated at once. 
     An SFC program has three parts: (1) preprocessing, which includes power returns, faults, changes of operating mode, pre-positioning of SFC steps, input logic; (2) sequential processing, which includes steps, actions associated with steps, transitions and transition conditions; and (3) post-processing, which includes commands from the sequential processing for controlling the outputs and safety interlocks specific to the outputs. 
       FIG. 6  shows a flowchart for the Main SFC. In  FIG. 6 , step “ 10  Main” has no associated actions and the transition to step “ 20  Ready” is true. Step “ 10  Main” can be accessed either if a “Drop” input is received by the operator while in step “ 20  Ready” or when the SFC is initialized. Step “ 20  Ready” is initiated either automatically after step “ 10  Main” or after a preset time TM 2  in step “ 50  Drop”. Step “ 20  Ready” starts the Ready SFC. From step “ 20  Ready”, a “Drop” command by the operator calls step  10 . Step “ 30  Lift” starts the Lift SFC. “Lift” is initiated by a lift command from steps “ 20  Ready” or “ 50  Drop”. Step “ 40  Hold” is initiated either automatically after a preset time TM 1  in step “ 30  Lift”, or immediately after a “Lift” input in step “ 20  Ready” if the magnet temperature trip relay is active. Step “ 40  Hold” initiates the Hold SFC. Step “ 50  Drop” is initiated by a “Drop” rising edge from either step “ 30  Lift” or “ 40  Hold”, and step “ 50  Drop” initiates the Drop SFC. 
       FIG. 7  shows a flow chart for the Ready SFC. Step “ 21  Ready” is the initialization step. Step “ 21  Ready” will be active when the Main SFC is not in step “ 20  Ready”. Step “ 21  Ready” is not associated with any actions. Step “ 20  Ready” getting active in the Main SFC causes transition X 20  to be true and to make step “ 22  Run Off” active. Once step “ 20  Ready” is active, unless step “ 20  Ready” stops to be active and causes  X 20    to be true and the SFC to return to step “ 21  Ready”, the SFC stays in step “ 22  Run Off”. While the SFC is in step “ 22  Run Off”, the LC  100  sends commands to the control circuitry to turn off the current in the magnet  113 . From step “ 22  Run Off”, the SFC transitions to step “ 23  Voltage Selection  1  Off” when the Send Command Done is true, and the SFC transitions from step “ 23  Voltage Selection  1  Off” to step “ 24  Negative Bridge Off” when the Send Command Done is true. From step “ 24  Negative Bridge Off”, the SFC transitions to step “ 27  Done” when the Send Command Done is true. 
       FIG. 8  shows a flowchart for the Lift SFC. The first step to be activated, “ 32  Run On”, is to reduce to a minimum the delay time between the activation of the “Lift” input by the operator and the response by the circuitry. Steps “ 35  Negative Bridge Off” and “ 36  Voltage Selection  1  Off” are used if the step before “ 30  Lift” was “ 50  Drop” in the Main SFC and the Send Command Done is true. “Sweep” is a switch that can be toggled by the operator. If “Sweep” is on, “Voltage Selection  2 ” and “Current Limit Selection  2 ” are on, and the system selects the second set of voltage references and the second current limit. If “Sweep” is off, “Voltage Selection  2 ” and “Current Limit Selection  2 ” are off, and the system selects the primary set of voltage references and the primary current limit. 
       FIG. 9  shows a flow chart for the Hold SFC. Step “ 41  Hold” is the initialization step. Step “ 40  Hold” getting active in the Main SFC causes transition X 40  to be true and to make step “ 42  Voltage Selection  1  On” active. Once the step “ 42  Voltage Selection  1  On” is active, unless step “ 40  Hold” stops to be active and causes  X 40    to be true and the SFC to return to step “ 41  Hold”, the SFC stays in step “ 42  Voltage Selection  1  On”. While the SFC is in step “ 42  Voltage Selection  1  On”, the LC  100  sends commands to control the lifting magnet circuitry. 
     The SFC transitions from step “ 42  Voltage Selection  1  On” to step “ 49  Run On” when Send Command Done is true. The SFC transitions from step “ 49  Run On” to step “ 90  Negative Bridge Off” when Send Command Done is true. The SFC transitions from step “ 90  Negative Bridge Off” to step “ 43  Ready” when Send Command Done is true. Once the SFC is in step “ 43  Ready”, after the timer TM 3  elapses, the voltage and current across the lifting magnet  113  are stabilized and the LC  100  gets updates from the system for readings of Volts across the lifting magnet  113  and Amps going across the lifting magnet  113 . Based on those readings, the LC  100  calculates the magnet resistance and determines whether or not the alarm resistance is exceeded, and whether or not the trip resistance is exceeded. Each of these updates is requested after the previous update is done. 
       FIG. 10  shows a flow chart for the Drop SFC. Step “ 50  Drop” getting active in the Main SFC causes transition X 50  to be true and to make step “ 52  Negative Bridge On” active. In step “ 52  Negative Bridge On”, the system selects the negative bridge  251 . The current limit for the negative bridge  251  is set at a fraction of the current limit for the positive bridge  250 . Then, in step “ 55  Voltage Selection  1  Off”, voltage selection is reset. The system remains in “Drop” mode until the Main SFC exits step “ 50  Drop” either after timer TM 2  expires or when a “Lift” command is requested by the operator. 
     In one embodiment, the circuitry used to control the lifting magnet  113  can be obtained by appropriately programming a DC Regulated Power Supply  400 , normally used to control motors. The LC  100  can be set up with access to the DC Regulated Power Supply  400  logic, allowing the setting of parameters to be changed to suit different operating conditions. 
     In one embodiment, the Mentor II DC Drive manufactured by Control Techniques of Minnesota, United States (Model M550R?). can be used as the DC Regulated Power Supply. 
     The thyristors in the DC Regulated Power Supply  400  are fired when the “Run ON” command is sent during step “ 32  Run On” of the Lift SFC. 
     During the “Lift” mode, the positive bridge  250  applies the voltage from the DC Regulated Power Supply  400 , usually set around 500V DC across a 240V DC rated lifting magnet  113  to boost the charge until the current gets limited by the limiting current for the lifting magnet  113 . In addition, the “Lift” time is controlled by the value in timer TM 1  of the LC  100 . 
     During the “Hold” mode, the positive bridge  250  applies a voltage of around 180 V DC across a 240 V DC rated magnet  113 . This holding voltage is adjustable and set in the LC  100 . In addition, after being in “Hold” mode for about 5 seconds, as preset in timer TM 3  of the LC  100 , and periodically at each period of time preset in timer TM 3 , the LC  100  reads the current and voltage across the DC Regulated Power Supply  400 . 
     During the “Drop” mode, the negative bridge  251  is turned on by changing the value in parameter “Bridge Selector”, shown in  FIG. 11 . During the “Drop” mode, the current can be limited by the parameter “Current Limit for Negative Bridge” shown in  FIG. 12 . In addition, the time for the “Drop” mode is preset by parameter TM 2 . 
     During the “Sweep” mode, depending on whether a “Sweep” command is received by the operator at the LC  100 , “Voltage Selection  2 ” is set to on or off in the DC Regulated Power Supply  400 . If “Sweep” is off, “Voltage Selection  2 ” is off, as shown in  FIG. 11 . Therefore, the reference voltages in “Voltage Reference  1 ” and “Voltage Reference  2 ” of the DC Regulated Power Supply  400  are respectively selected during “Lift” and “Drop”, depending on the value of “Voltage Selection  1 ”. On the other hand, if “Sweep” is on, “Voltage Selection  2 ” is enabled. By enabling “Voltage Selection  2 ”, the “Voltage Reference  3 ” and “Voltage Reference  4 ” of the DC Regulated Power Supply  400  are respectively selected during “Lift” and “Drop”, again, depending on the value of “Voltage Selection  1 ”. Furthermore, during the “Sweep” mode, the current is limited by parameter “Current Limit  2 ”, as shown in  FIG. 12 . 
     It will be apparent to those skilled in the art how the “Lift” and “Hold” modes described above function when the system is used in a slab or plates material handling application, and the voltage levels are adjusted accordingly. 
     The temperature protection for the lifting magnet  113  is controlled through the use of parameters “Alarm Resistance” and “Trip Resistance”. The resistance value at which the system activates an alarm relay during the “Hold” mode is set into parameter “Alarm Resistance”, based on the lifting magnet  113  manufacturer&#39;s rated hot current. The resistance value at which the system activates a trip relay is set into parameter “Trip Resistance”, based on the insulation class temperature of the lifting magnet  113 . When the resistance  301  of the lifting magnet  113  exceeds the value set in parameter “Trip Resistance”, the next cycle begins directly in “Hold” mode. When the lifting magnet  113  cools down and its resistance value  301  becomes less than the value set in parameter “Trip Resistance”, then the system enters “Lift” mode again. Cable ohmic resistance  302  of the wiring between the lifting magnet  113  and the LC  100  is set in parameter “Wiring Resistance”. To calculate the magnet resistance, the LC  100  divides the voltage by the current and then subtracts the value set in “Wiring resistance”. 
     In addition to the above parameter settings, some parameters in selected DC Regulated Power Supplies can be adjusted to accommodate for highly inductive loads like the lifting magnet  113 . Generally, voltage loop and current loop PID gain circuitries need to be optimized, current feedback resistors scaled to accommodate for the inductance of the magnet  113 , and a safety margin of 1 supply cycle added to the bridge changeover logic to prevent shorting the line by having a thyristor in one bridge firing while another thyristor in the other bridge were still conducting. 
     It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributed thereof, furthermore, various omissions, substitutions and changes may be made without departing from the spirit of the inventions. The foregoing description of the embodiments is, therefore, to be considered in all respects as illustrative and not restrictive, with the scope of the invention being delineated by the appended claims and their equivalents.