FORGE CRANE AND CONTROL METHOD THEREOF

BACKGROUND OF THE INVENTION

Field of the Invention

[0001]

The present invention relates to a forge crane, which suspends an object to be forged during a forging operation, and a control method thereof. In particular, it relates to a forge crane in which an electric motor serving as a power source thereof is inverter-controlled, and a control method thereof.

Priority is claimed on Japanese Patent Application No. 2010-073172, filed March 26,2010, the content of which is incorporated herein by reference.

Description of Related Art

[0002]

In general, a forge crane used at a forge plant is a crane that conveys an object to be forged, and is also used to continue holding the object to be forged during forging by a forging apparatus. A forging process is performed on the object to be forged through the operation of the forge crane, which operates in connection with such a forging apparatus, such that the object to be forged is processed into a predetermined shape.

[0003]

To be specific, the construction of the forge crane comprises, for example, a trolley which travels on rails, a drum which is supported on the trolley and has a cable wound there around, an electric motor which rotates the drum, and a brake which brakes the drum (for example, refer to Patent Document 1). Here, in such a forge crane, when a forging operation is executed, not only the weight of the object to be forged, which is suspended load, but also a load which occurs due to pressing of the forging apparatus sometimes acts on the cable. In order to deal with such a load from the forging apparatus, the forge crane is characterized in that it is designed with a safety factor greater than that of a normal crane, which only performs simple load suspension, and the weight of each member thereof is heavy. Moreover, in order to mitigate the impact which occurs due to the load from pressing of the forging apparatus, a spring-operated or hydraulic-operated buffer device is provided between the drum and the trolley in some cases. Consequently, in the forge crane, the weight of the entire apparatus becomes greater than that of the normal crane, and this also contributes to an increase in the size of a building facility where the forge crane is installed. Therefore, there is a demand for a reduction in the weight of the entire apparatus.

[0004]

From this type of point of view, there has been proposed a forge crane which is equipped with an inverter and an alternating current type electric motor, and which drives the electric motor with inverter control so as to wind up and down the cable, By employing the inverter control type, the electric motor can be made smaller than a direct current type electric motor, and there is no need for facilities such as a secondary resistor. As a result, it is possible reduce the weight of the entire forging apparatus. Moreover, with the inverter control, the speed and torque of winding up can be freely changed to perform a suspending operation.

Prior Art Documents

Patent Documents

[0005]

[Patent Document 1] Japanese Examined Patent Application, Second Publication

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

[0006]

Incidentally, in a forge crane, there is a possibility of a heavy load acting suddenly when performing a forging operation even if it is provided with the above buffer device, and in this type of case, in order to avoid overload, the drum is reverse-rotated to temporarily wind down the cable in some cases. However, as pointed out in Patent Document 1, even if the motor is excited when a sudden heavy load acts, heretofore, there are limitations for rapidly accelerating the cable to be unreeled from the drum with inertia moment of the electric motor and the drum. In particular, in a case where inverter control is employed with use of an alternating current type electric motor, initial response is slower compared to the case of secondary resistance control with use of a direct current type electric motor. As a result, it is difficult, with rotational drive of die electric motor, to promptly wind down the drum according to a suddenly acting load.

[0007]

The present invention takes into consideration the above circumstances, with an object of providing a forge crane in which reduction in the weight of the entire apparatus can be achieved by rotation-driving an electric motor with inverter control while a winding down operation can be promptly executed in order to avoid overload, even in a case where a sudden heavy load acts, and a control method of the forge crane.

Means for Solving the Problem

[0008]

A forge crane of the present invention comprises a drum having a cable which suspends an object to be forged wound there around, an electric motor which drives the drum to rotate, an inverter which is connected to a power supply and controls alternating current output to the electric motor, a brake section which brakes rotation of the drum, a mode switching section which can set a mode to either one of two types of mode, namely a forge mode in which the object to be forged is suspended and a forging operation is executed, and a normal mode in which the object to be forged is suspended without executing a forging operation, and a main control section which controls the inverter. In a case where the mode switching section sets the mode to the forge mode, the main control section controls rotation of the drum with the brake section, and outputs a pre-excitation command to the inverter, causing the inverter to apply a preliminarily set pre-voltage to the electric motor

[0009]

According to the forge crane of the present invention, in a case where the mode
switching section sets the mode to the forge mode, the main control section causes the brake section to brake rotation of the drum, and outputs a pre-excitation command to the inverter, causing the inverter to apply a preliminarily set pre-voltage to the electric motor. As a result, the electric motor can promptly respond to an alternating current output from the inverter to the electric motor which performs a cable-winding down operation, and the drum can be rotation-driven to wind down the cable. Consequently, it is possible to promptly execute a winding down operation in order to avoid overload, even in a case where a sudden heavy load acts on the cable from the object to be forged at the time of a forging operation.

[0010]

The forge crane of the present invention further comprises an overload detecting
section which outputs an overload detection signal to the main control section in a case here a load acting on the cable exceeds a preliminarily set overload value. In a case here the overload detecting section outputs the overload detection signal, the main control section may output a winding down command to the inverter, causing the inverter o apply a preliminarily set initial voltage, which is greater than a normal-rated voltage, o the electric motor, so that braking of the drum performed by the brake section is released, in a case where a torque current output to the electric motor becomes greater than predetermined current.

[0011]
According to the forge crane of the present invention, in a case where a load
acting on the cable exceeds the overload value and the overload detecting section outputs an overload detection signal, the main control section outputs a winding down command to the inverter. As a result, the inverter applies an initial voltage, which is greater than the normal-rated voltage, to the electric motor, based on the winding down command. Consequently, it is possible to more promptly output a torque current to the electric motor until a current value at which winding down can be performed has been met, and in a case where the torque current has become greater than or equal to a predetermined current value corresponding to a state where winding down is possible, the main control section causes braking of the drum performed by the brake section to be released and the cable is wound down from the drum, and thereby overload can be prevented. Methods of the overload detecting section include spring flexure detection, hydraulic pressure detection, and suspended load detection.

[0012]

The forge crane of the present invention may further comprises an emergency load detecting section which outputs an emergency load detection signal in a case where a load acting on the cable exceeds a preliminarily set emergency load, which is greater than the overload value, and a brake release section which forcefully releases braking of the drum performed by the brake section in a case where the emergency load detecting section outputs the emergency load detection signal.

[0013]

According to the forge crane of the present invention, in a case where the overload detecting section detects a load exceeding the overload value; overload can be prevented by performing a winding down operation of the cable based on the control of the main control section. Furthermore, even in the worst case where an emergency load which exceeds the overload value acts for any reason, the emergency load detecting section detects the emergency load and outputs an emergency load signal, and consequently the brake release section releases braking of the drum performed by the brake section. As a result, overload can be prevented more reliably.

[0014]

Moreover a control method of a forge crane of the present invention is a method of controlling a forge crane having a drum having a cable which suspends an object to be forged wound there around, an electric motor which drives the drum to rotate, an inverter which is connected to a power supply and controls alternating current output to the electric motor, and a brake section which brakes rotation of the drum. In this method there are provided a mode setting step which sets a mode to either one of two types of mode, namely a forge mode in which the object to be forged is suspended and a forging operation is executed, and a normal mode in which the object to be forged is suspended without executing a forging operation, and a pre-excitation step which causes a main control section to make the brake section brake rotation of the drum, and outputs a pre-excitation command to the inverter, thereby causing the inverter to apply a preliminarily set pre-voltage to the electric motor, in a case where the mode is set to the forge mode in the mode setting step.

[0015]

According to the control method of a forge crane of the present invention, in a case where the mode is set to the forge mode in the mode setting step, the main control section causes the brake section to brake rotation of the drum, and outputs a pre-excitation command to the inverter, thereby causing the inverter to apply a preliminarily set pre-voltage to the electric motor in the pre-excitation step. As a result, the electric motor can promptly respond to an alternating current output from the inverter to the electric motor which performs a cable-winding down operation, and the drum can be rotation-driven to wind down the cable. Consequently, it is possible to promptly execute a winding down operation in order to avoid overload, even in a case where a sudden heavy load acts on the cable from the object to be forged at the time of a forging operation.

[0016]

Moreover, the control method of a forge crane of the present invention may further comprises an overload monitoring step which monitors whether or not a load acting on the cable exceeds a preliminarily set overload value, in a state where the pre-voltage is applied to the electric motor, an initial voltage applying step which causes the main control section to make the inverter apply a preliminarily set initial voltage, which is greater than a normal-rated voltage, to the electric motor, in a case where the load acting on the cable exceeds the overload value in the overload monitoring step, a torque current monitoring step which monitors a torque current output to the electric motor, to which the initial voltage is applied, and a brake releasing step which causes the main control section to release braking of the drum performed by the brake section, in a case where the torque current is greater than or equal to a predetermined current.

[0017]

According to the control method of a forge crane of the present invention, in a case where the load acting on the cable exceeds the overload value, in the overload monitoring step, in the initial voltage applying step, the inverter applies the initial voltage, which is greater than the normal-rated voltage, to the electric motor, based on control of the main control section. As a result, it is possible to more promptly output the torque current to the electric motor until a current value at which winding down can be performed has been met. Moreover, in a case where the torque current is greater than or equal to a predetermined current value which corresponds to a state where an object to be forged can be held and lowered in the torque current monitoring step, in the brake releasing step, the main control section releases braking of the drum performed by the brake section, to thereby wind down the cable from the drum, and it is consequently possible to prevent overload.
Effects of the Invention

[0018]

According to the present invention, the electric motor is rotation-driven with inverter control. As a result, reduction in the weight of the entire apparatus can be achieved, and the winding down operation can be promptly executed in order to avoid overload, even in a case where a sudden heavy load acts.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]

FIG. 1 is a schematic configuration diagram showing a forge crane according to the present invention.

FIG 2 is a block diagram showing the forge crane of the present invention.

FIG. 3 is a block diagram showing in detail an inverter in the forge crane of the present invention.

FIG. 4 is a flow chart showing in detail a control method of the forge crane of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020]

An embodiment of the present invention is described, with reference to FIG. 1 to FIG 4. FIG 1 and FIG 2 show a forge crane 1 of the present embodiment. As shown in FIG. 1, the forge crane 1 suspends a work piece W, which is an object to be forged, and works in cooperation with a forging apparatus 100 to perform a forging process on the work piece W.

[0021]

The forging apparatus 100 includes; a supporting base 101 on the top surface of which the work piece W is mounted during forging, a press 102 which is located above the supporting base 101 and applies pressure to the work piece W placed between itself and the supporting base 101, and cylinders 103 which raise and lower the press 102. The forging apparatus 100 rotates the work piece W with a turning apparatus 110 suspended by the forge crane 1, when performing a forging operation of the work piece W. Here the turning apparatus 110 comprises a belt 111 for holding a shaft W1 of the work piece W, and a rotating apparatus 112 for rotating the work piece W together with the shaft W1by rotating the belt 111.

[0022]

Next is a description the forge crane 1. The forge crane 1 includes: a supporting section 2 capable of traveling on rails L; suspending equipment 3 which is supported on the supporting section 2 and suspends the work piece W, which is a suspended load, via the turning apparatus 110, by a cable 31; a brake 4 serving as a brake section which brakes unreeling of the cable 31 performed by the suspending equipment 3; and a PLC (programmable logic controller) 5 serving as a main control section that performs control of respective sections of the forge crane.

[0023]

The supporting section 2 has a pair of traveling girders 21 which travel on the rails L using wheels 20, a trolley 22 capable of moving on the respective traveling girders 2 in a depth direction on the paper surface orthogonal to the extending direction of the rails L, and a traveling motor (not shown in the drawing) which rotation-drives the wheels 20 and serves as a traveling driving section. The traveling motor is controlled by a traveling inverter (not shown in the drawing), and this traveling inverter drives the traveling motor based on control of the PLC 5.

[0024]

The suspending equipment 3 has: a suspending device 30 which supports the turning apparatus 110; the cable 31 which suspends the suspending device 3 0; a plurality of pulleys 32 and 33 around which the cable 31 is wound; a drum 34 on which one end 31 a of the cable 31 that passes round the pulleys 32 and 33 is wound; a main hoist motor 35 serving as an electric motor which rotates the drum 34 (refer to FIG. 2); and an inverter 36 which controls the main hoist motor 35 (refer to FIG. 2). The pulley 32 is supported on the trolley 22 via a supporting spring 32a. Moreover, the pulleys 33 are provided as a pair, and support the suspending device 30 via a supporting spring 33a. One end 3 la of the cable 31 is wound on the drum 34 and wound sequentially on one of the pulleys 33, the pulley 32, and the other one of the pulleys 33, and the other end 31b is fixed on the trolley 22 of the supporting section 2. Moreover, the turning apparatus 110 engages with hooks 30a of the suspending device 30.

[0025]

Furthermore, as shown in FIG. 2, the forge crane 1 includes a winding operation section 6 which operates winding up and down of the cable 31, and a mode switching section 7 which can set a mode to either one of two types of mode, namely a forge mode and a normal mode. The winding operation section 6 can switch between winding up and winding down of the cable 31, and can also receive an input of each winding speed of the winding up and winding down. Moreover, it outputs a winding speed signal to the PLC 5 based on this input. A winding speed signal is, for example, output as a positive value when performing a winding up operation, and it is output as a negative value when performing a winding down operation. Moreover, the forge mode, which can be set by the mode switching section 7, refers to a mode in which the work piece W is suspended and a forging operation is executed, and the normal mode refers to a mode in which the work piece W is suspended without executing a forging operation. When the mode switching section 7 sets the mode to one of the modes, a corresponding mode setting signal is output to the PLC 5, and the PLC 5 executes a different control according to this mode setting signal. The detail of the control is described later.

[0026]

An alternating current power supply is connected to the main hoist motor 35 via the inverter 36, and the alternating current power supply is also connected to the brake 4. Between the inverter 36 and the alternating current power supply, there is provided a main hoist motor switch 10, and connection and disconnection can be switched based on control of the PLC 5. Moreover, between the brake 4 and the alternating current power supply, there are provided a first brake switch 11 and a second brake switch 12. The first brake switch 11 can switch between connection and disconnection based on the PLC 5, Furthermore, the second brake switch 12 is connected to a brake release circuit 13, and can switch between connection and disconnection based on an output from the brake release circuit 13.

[0027]

Moreover, the brake 4 brakes rotation of the drum 34 to thereby restrict unreeling of the cable 31, and for this, an electromagnetic brake is employed in the present embodiment for example. Normally, when the second brake switch 12 is in a connection state and the first brake switch 11 is in a disconnection state, the brake 4 maintains the braking state of the drum 34, and when the first brake switch 11 is brought to a connection state, electric current flows to the brake 4 and braking of the drum 34 by the brake 4 is released, consequently bringing the drum 34 to a rotatable state. Moreover, when the second brake switch 12 is brought to a disconnection state, the brake 4 can brake the drum 34 regardless of whether the first brake switch 11 is in a connection state or in a disconnection state.

[0028]

Moreover, in the present embodiment, the forge crane 1 includes; a brake limit switch 14 serving as a braking detecting section which detects a release state caused by the brake 4, an overload limit switch 15 serving as an overload detecting section which respectively detects a load acting on the cable 301 and outputs an overload detection signal in a case where a preliminarily set overload value is exceeded, and an emergency load limit switch 16 serving as an emergency load detecting section which outputs an emergency load detection signal in a case where a preliminarily set emergency load value is exceeded. The emergency load value set for the emergency load limit switch 16 is a value greater than the overload value. Furthermore, the overload detection signal output from the overload limit 6witch 15 is input to the PLC 5. Meanwhile, the emergency load detection signal output from the emergency load limit switch 16 is output respectively to the PLC 5 and the brake release circuit 13, and the brake release circuit 13 outputs an electric signal to the second brake switch 12 according to the emergency load detection signal. As a result, the second brake switch 12 can be brought to a connection state.

[0029]

As shown in FIG. 3, the inverter 36 includes: an inverter body 36a which, after converting an alternating current input from the alternating current power supply into direct current, outputs it as alternating current having a corresponding frequency and amplitude, based on a speed command ω0 from the PLC 5; a differentiator 36b which obtains angle information 6 from a pulse generator 17 fitted to the main hoist motor 35, and calculates a current rotation speeds a subtracted 36c which calculates a difference ∆ ω found by subtracting the rotation speed ω output from the differentiator 36b from the speed command ω 0 from the PLC 5, and outputs it to the inverter body 36a; and a comparator 36d which compares the current rotation speed with a threshold value ω r to determine which value is greater. The threshold value or set for the comparator 36d is a rotation speed corresponding to 1% of a normal-rated speed of winding down of the cable 31 for example. The comparator 36d can output a detection signal to the PLC 5 when the received rotation speed ω is smaller than the threshold value ωr.

[0030]

Next, the effect of the forge crane 1 of the present embodiment, and the detail of control of the forge crane 1 performed by the PLC S and the inverter 36, are described, based on the flow chart shown in FIG 4.

When the mode switching section 7 sets the mode to either the forge mode or normal mode, the PLC 5 receives an output of set-mode information, which indicates either the forge mode or normal mode, from this mode switching section 7 Having received the set-mode information output from when the mode switching section 7 sets the mode to either one of the modes, the PLC 5 determines whether or not the set mode is the forge mode, based on the received set-mode information (mode setting step: step S1). If it is determined as not being the forge mode, that is, it is determined as being the normal mode (NO), a main winding notch manual operation by the winding operation section 6 is prioritized, and the PLC 5 outputs a speed command ω 0, which corresponds to the output signal from the winding operation section 6, to the inverter 36. Consequently, with the alternating current supplied from the inverter 36, the main hoist motor 35 rotates the drum 34 at a rotation speed according to the operation amount input in the winding operation section 6, to perform winding up or winding down of the cable 31 (stepS2).

[0031]

On the other hand, in step S1, if it is determined as being in the forge mode (YES), the PLC 5 outputs a brake closing command to the first brake switch 11, and outputs a pre-excitation command to the inverter 36. Consequently, in a state where the first brake switch 11 is open and the brake 4 has braked the drum 34, the inverter 36 applies voltage to the main hoist motor 35 only by a preliminarily set pre-voltage (pre-excitation step: step S3). Therefore, the forging operation, which is performed in cooperation with the forging apparatus 100 is performed in a state where the pre-voltage is being applied to the main hoist motor 35.

[0032]

Furthermore, when the forging operation is being performed, the PLC 5
monitors whether or not an output from the overload limit switch 15 is present (overload monitoring step: step S4). In a state where no output is made from the overload limit switch 15, that is, in a state where the load acting on the cable 31 has not reached the overload value and there is no overload (NO), a main winding notch manual operation is prioritized by the winding operation section 6, and the PLC 5 outputs a speed command ω 0, which corresponds to the output signal from the winding operation section 6, to the inverter 36. Consequently, with the alternating current supplied from the inverter 36, the main hoist motor 35 rotates the drum 34 at a speed according to the operation amount input in the winding operation section 6, to perform winding up or winding down of the cable 31 (step S5). On the other hand, in a case where the overload limit switch 15 outputs an overload detection signal to the PLC 5, that is, the load acting on the cable 31 has reached the overload value (YES), it is considered that there is a possibility of an overload state, and the process proceeds to the next step.

[0033]

That is to say, the PLC 5 outputs an initial voltage applying command to the inverter 36. Consequently, the inverter 36 applies a preliminarily set initial voltage. Here, the initial voltage is a voltage greater than a normal-rated voltage, and it is, for example, a voltage approximately 300% of the normal-rated voltage (initial voltage applying step: step S6). Next, after step S6 has been performed and a predetermined amount of time has elapsed, the PLC 5 monitors whether or not a torque current value output from the inverter 36 has exceeded a preliminarily set threshold value (torque current monitoring step: step S7). The predetermined amount time here refers to an extremely short period of time by which a predetermined torque current can flow by applying the initial voltage, and it is approximately 0.2 seconds for example, Moreover, the threshold value corresponding to the torque current value is a torque current value required when performing winding down, and the winding down operation can be performed if the threshold value is exceeded. In a case where the torque current value does not exceed the threshold value (NO), the process repeats from step S6 again.

[0034]

On the other hand, in a case where the torque current value exceeds the threshold value (YES), the PLC 5 outputs a normal-rated voltage applying command to the inverter 36. As a result, the inverter 36 applies a preliminarily set normal-rated voltage (step S8). Subsequently, the PLC 5 outputs a brake releasing command to the first brake switch 11. As a result, the first brake switch 11 is connected, and electric power is supplied to the brake 4 from the alternating current power supply, so that the brake 4 is released and braking of the drum 34 is released (brake releasing step S9), and the brake limit switch 14 outputs a brake release detection signal (step S10). As a result, in a state where a constant torque is being given to the drum 34 from the main hoist motor 35, the drum 34 reverse-rotates and the cable 31 is wound down (step SI 1). At this time, in the inverter 36, a feedback of angle information 6 has been made from the pulse generator 17 and the current rotation speed ω has been calculated. As a result it is possible to accelerate the rotation speed to a normal-rated speed (for example, 4.S m/min) in a preliminarily set acceleration time (for example, 0.9 seconds), to execute a winding down operation at this normal-rated speed. Moreover, the PLC 5 uses a timer (not shown in the drawing) to monitor the time elapsed since the brake release detection signal was output.

[0035]

Here, at the point in time when the winding down operation is commenced, the load exceeding the overload value is still acting on the cable 31, and the PLC 5 is monitoring the overload detection signal output from the overload limit switch 15 (stepS12). If the overload detection signal is no longer output (NO), the PLC 5 outputs a winding-down stop command to the inverter 36. As a result, the inverter 36 changes the frequency of the alternating current output to the main hoist motor 35 so as to decelerate the speed of winding down the cable 31 in a preliminarily set decelerating time (for example, 2,0 seconds) (step S1 3).

[0036]

While the speed of winding down the cable 31 is being decelerated, the PLC 5 is monitoring the output from the comparator 36d of the inverter 36 (step S14). In a case where no detection signal is output from the comparator 36d of the inverter 36, that is, the winding down speed has not reached 1 % of the normal-rated speed for example (NO), step S13 is continued. On the other hand, in a case where a detection signal is output from the comparator 36d of the inverter 36, that is, the winding down speed has reached 1% of the normal-rated speed for example (YES), the PLC 5 outputs a brake closing command to the first brake switch 11. As a result, the first brake switch 11 is opened and the supply of electric power to the brake 4 from the alternating current power supply is no longer made, so that the brake 4 is closed, and the drum 34 is braked (step S15). As a result, the output of the brake release detection signal from the brake limit switch 14 to the PLC 5 stops (step S16), and the forced winding down operation ends.

[0037]

On the other hand, in step S12, in a case where the output of the overload detection signal continues (NO), the PLC 5 determines whether or not the time elapsed since the output of the brake release detection signal was made, which is measured by the timer (not shown in the drawing), has reached a preliminarily set limit time (for example, 5 seconds) (step S17). If the limit time has been reached (YES), that is, if the load acting on the cable 31 is not reduced even if the forced-winding down operation is performed, then a malfunction caused by another reason is considered. Accordingly an instruction is issued using a siren or lamp to stop the forging operation so that all operations are stopped and the operation of the forging apparatus 100 is also stopped (step SI 8).

[0038]

On the other hand, if the limit time has not been reached (NO), the PLC 5 determines whether or not the emergency load limit switch 16 has output an emergency load detection signal (step S19). If the emergency load detection signal has not been output (NO), the process returns to step S1 1 and the winding down operation is continued. On the other hand, if the emergency load detection signal has been output (YES), it is considered that the load acting on the cable 31 has reached an emergency load value, which is greater than the overload value, although the winding down operation is being performed, and there is a risk of an overload state. Accordingly, the PLC 5 issues an instruction, using a siren or lamp to stop the operation so that all operations are stopped and the operation of the forging apparatus is also stopped. Moreover, the detection signal from the emergency load limit switch 16 is input to the brake release circuit 13, and the second brake switch 12 is forcefully opened, so that rotation of the drum 34 is forcefully braked by the brake 4 (step S20).

[0039]

As described above, according to the forge crane 1 and the control method of the forge crane 1 of the present embodiment, in a case where the mode switching section 7 sets the mode to the forge mode, the PLC 5 causes the brake 4 to brake rotation of the drum 34, and outputs a pre-excitation command to the inverter 36, to thereby cause the inverter 36 to apply a preliminarily set pre-voltage to the main hoist motor 35. As a result, the main hoist motor 35 can promptly respond to an alternating current output from the inverter 36 to the main hoist motor 35 for performing a winding down operation of the cable 31, and the drum 34 can be rotation-driven to wind down the cable 31. As a result, in a case where it is determined in step S4 that a sudden heavy load acts on the cable 31 from the work piece W when performing a forging operation, and an overload is acting, the PLC 5 outputs a winding down command to the inverter 36, so that the main hoist motor 25 can be driven based on control of the inverter 36, and the winding down operation can be executed promptly so as to avoid overload.

[0040]

Moreover, in the forge crane 1 of the present embodiment, the inverter 36 applies an initial voltage, which is greater than the normal-rated voltage, to the main hoist motor 35, based on the winding down command. Consequently, it is possible to more promptly output a torque current to the main hoist motor 35 until a current value at which winding down can be performed has been met, and in a case where the torque current has become greater than or equal to a predetermined current value corresponding to a state where winding down is possible, the PLC 5 releases braking of the drum 34 by the brake 4, so that the cable 31 is wound down from the drum 34, and thereby overload can be prevented.

[0041]

Furthermore, in the forge crane 1 of the present embodiment, in addition to the overload limit switch 15, there are provided the emergency load limit switch 16, and the brake release section 13 which forcefully releases braking of the drum 34 by the brake 4, based on the output of the emergency load limit switch 16. Consequently, even in the worst case where an emergency load which exceeds the overload value acts for any reason, the emergency load limit switch 16 detects the emergency load and outputs an emergency load signal. As a result, the brake release section 13 forcefully releases braking of the drum 34 by the brake 4, without use of the PLC 5, so that overload can be more reliably prevented.

[0042]

The embodiment of the present invention has been described in detail, with reference to the drawings. However, the specific configuration is not limited to this embodiment, and design modifications may be made without departing from the scope of the invention.

What is claimed is:

1. A forge crane comprising:

a drum having a cable which suspends an object to be forged wound therearound;

an electric motor which drives the drum to rotate;

an inverter which is connected to a power supply and controls alternating current output to the electric motor;

a brake section which brakes rotation of the drum;

a mode switching section which can set a mode to either one of two types of mode, namely a forge mode in which the object to be forged is suspended and a forging operation is executed, and a normal mode in which the object to be forged is suspended without executing a forging operation; and

a main control section which controls the inverter,

wherein, in a case where the mode switching section sets the mode to the forge mode, the main control section controls rotation of the drum with the brake section, and outputs a pre-excitation command to the inverter, causing the inverter to apply a preliminarily set pre-voltage to the electric motor.

2. A forge crane according to claim 1, further comprising an overload detecting
section which outputs an overload detection signal to the main control section in a case where a load acting on the cable exceeds a preliminarily set overload value,

wherein in a case where the overload detecting section outputs the overload detection signal, the main control section outputs a winding down command to the inverter, causing the inverter to apply a preliminarily set initial voltage, which is greater
than a normal-rated voltage, to the electric motor, so that braking of the drum performed by the brake section is released, in a case where a torque current output to the electric motor becomes greater than a predetermined current.

3. A forge crane according to claim 2, further comprising:

an emergency load detecting section which outputs an emergency load detection signal in a case where a load acting on the cable exceeds a preliminarily set emergency load, which is greater than the overload value; and

a brake release section which forcefully releases braking of the drum performed by the brake section in a case where the emergency load detecting section outputs the emergency load detection signal.

4. A control method of a forge crane for controlling a forge crane having: a drum
having a cable which suspends an object to be forged wound there around; an electric
motor which drives the drum to rotate; an inverter which is connected to a power supply and controls alternating current output to the electric motor; and a brake section which brakes rotation of the drum, the control method comprising:

a mode setting step which sets a mode to either one of two types of mode, namely a forge mode in which the object to be forged is suspended and a forging operation is executed, and a normal mode in which the object to be forged is suspended without executing a forging operation; and

a pre-excitation step which causes a main control section to make due brake section brake rotation of the drum, and outputs a pre-excitation command to the inverter, thereby causing the inverter to apply a preliminarily set pre-voltage to the electric motor, in a case where the mode is set to the forge mode in the mode setting step.

5. A control method of a forge crane according to claim 4, further comprising:

an overload monitoring step which monitors whether or not a load acting on the cable exceeds a preliminarily set overload value, in a state where the pre-voltage is applied to the electric motor;

an initial voltage applying step which causes the main control section to make the inverter apply a preliminarily set initial voltage, which is greater than a normal-rated voltage, to the electric motor, in a case where the load acting on the cable exceeds the overload value in the overload monitoring step;

a torque current monitoring step which monitors a torque current output to the electric motor, to which the initial voltage is applied; and

a brake releasing step which causes the main control section to release braking of the drum performed by the brake section, in a case where the torque current is greater than or equal to a predetermined current.