There is provided an electromagnet control apparatus that continuously attracts crushed pieces of metals with the core of a magnetized electromagnet without immediately releasing the attracted pieces in a composite operation of a construction machine. The electromagnet control apparatus includes a control unit that controls a demagnetized state, a magnetized state, or a counter-magnetized state of an electromagnet. Until the end of a first threshold time t.sub.th1 from a transition to the demagnetized state from the magnetized state, the control unit ignores a turn-on voltage for turning on first transistors (a first transistor, a fourth transistor) and turns on the first transistors after a lapse of a second threshold time that is shorter than the first threshold time. The demagnetized state and the magnetized state are repeated until the end of the first threshold time t.sub.th1 from the transition to the demagnetized state.

There is provided an electromagnet control apparatus that continuously attracts crushed pieces of metals with the core of a magnetized electromagnet without immediately releasing the attracted pieces in a composite operation of a construction machine. The electromagnet control apparatus includes a control unit that controls a demagnetized state, a magnetized state, or a counter-magnetized state of an electromagnet. Until the end of a first threshold time t.sub.th1 from a transition to the demagnetized state from the magnetized state, the control unit ignores a turn-on voltage for turning on first transistors (a first transistor, a fourth transistor) and turns on the first transistors after a lapse of a second threshold time that is shorter than the first threshold time. The demagnetized state and the magnetized state are repeated until the end of the first threshold time t.sub.th1 from the transition to the demagnetized state.

Simplification of a device configuration and control is realized, which enables attraction and release by a magnet without using a battery. A magnet work machine includes a generator motor (8) driven by an engine (7); and an inverter (10) which controls a magnet (6) and the generator motor (8) such that the magnet (6) is energized by electric power from the generator motor (8) to attract metal scraps or the like, and during demagnetization for releasing metal scraps or the like, electric power applied to the magnet (6) is transmitted to the generator motor (8) as regenerative power so as to be consumed. The inverter (10) determines a torque of the generator motor (8) as a motor according to a voltage change of a main circuit based on a detected main circuit voltage so as to suppress the voltage change, and gives the torque as a command.

Simplification of a device configuration and control is realized, which enables attraction and release by a magnet without using a battery. A magnet work machine includes a generator motor (8) driven by an engine (7); and an inverter (10) which controls a magnet (6) and the generator motor (8) such that the magnet (6) is energized by electric power from the generator motor (8) to attract metal scraps or the like, and during demagnetization for releasing metal scraps or the like, electric power applied to the magnet (6) is transmitted to the generator motor (8) as regenerative power so as to be consumed. The inverter (10) determines a torque of the generator motor (8) as a motor according to a voltage change of a main circuit based on a detected main circuit voltage so as to suppress the voltage change, and gives the torque as a command.

A notification system includes: a notifier which notifies an operator of a state of a lifting magnet; a sensor which detects at least one of a voltage, an electric current, and an electric power supplied for exciting the lifting magnet; a determination part which determines, based on detection data from the sensor, whether an attraction quantity of the lifting magnet reaches a maximum attraction quantity after excitation of the lifting magnet is started; and a notification control part which causes the notifier to notify that the attraction quantity reaches the maximum attraction quantity when the determination part determines that the attraction quantity reaches the maximum attraction quantity.

A notification system includes: a notifier which notifies an operator of a state of a lifting magnet; a sensor which detects at least one of a voltage, an electric current, and an electric power supplied for exciting the lifting magnet; a determination part which determines, based on detection data from the sensor, whether an attraction quantity of the lifting magnet reaches a maximum attraction quantity after excitation of the lifting magnet is started; and a notification control part which causes the notifier to notify that the attraction quantity reaches the maximum attraction quantity when the determination part determines that the attraction quantity reaches the maximum attraction quantity.

There is provided an electromagnet control apparatus that continuously attracts crushed pieces of metals with the core of a magnetized electromagnet without immediately releasing the attracted pieces in a composite operation of a construction machine. The electromagnet control apparatus includes a control unit that controls a demagnetized state, a magnetized state, or a counter-magnetized state of an electromagnet. Until the end of a first threshold time t.sub.th1 from a transition to the demagnetized state from the magnetized state, the control unit ignores a turn-on voltage for turning on first transistors (a first transistor, a fourth transistor) and turns on the first transistors after a lapse of a second threshold time that is shorter than the first threshold time. The demagnetized state and the magnetized state are repeated until the end of the first threshold time t.sub.th1 from the transition to the demagnetized state.

There is provided an electromagnet control apparatus that continuously attracts crushed pieces of metals with the core of a magnetized electromagnet without immediately releasing the attracted pieces in a composite operation of a construction machine. The electromagnet control apparatus includes a control unit that controls a demagnetized state, a magnetized state, or a counter-magnetized state of an electromagnet. Until the end of a first threshold time t.sub.th1 from a transition to the demagnetized state from the magnetized state, the control unit ignores a turn-on voltage for turning on first transistors (a first transistor, a fourth transistor) and turns on the first transistors after a lapse of a second threshold time that is shorter than the first threshold time. The demagnetized state and the magnetized state are repeated until the end of the first threshold time t.sub.th1 from the transition to the demagnetized state.

A method for using a lifting magnet and a lifting magnet. The lifting magnet includes a plurality of electromagnet coils that are each independently ON/OFF-controllable and voltage-controllable, and a magnetic pole that is excited by application of a voltage to the electromagnet coils. An electromagnet coil to be used for lifting steel plates is determined based on a total thickness of the steel plates to be lifted. An amount of passing magnetic flux .sub.r in the magnetic pole in a case where magnetic flux passes through only the steel plates to be lifted when the electromagnet coil is used is calculated. An application voltage to be applied to the electromagnet coil used for lifting the steel plates is determined based on the amount of passing magnetic flux .sub.r. The application voltage is applied to the electromagnet coil.

A method for using a lifting magnet and a lifting magnet. The lifting magnet includes a plurality of electromagnet coils that are each independently ON/OFF-controllable and voltage-controllable, and a magnetic pole that is excited by application of a voltage to the electromagnet coils. An electromagnet coil to be used for lifting steel plates is determined based on a total thickness of the steel plates to be lifted. An amount of passing magnetic flux .sub.r in the magnetic pole in a case where magnetic flux passes through only the steel plates to be lifted when the electromagnet coil is used is calculated. An application voltage to be applied to the electromagnet coil used for lifting the steel plates is determined based on the amount of passing magnetic flux .sub.r. The application voltage is applied to the electromagnet coil.