Electrical Circuit for Controlling a Lifting Electromagnet for Overhead Cranes with Top Running Trolley Hoist

When the need to conveniently pick up and lift heavy iron or steel objects for transferring from one place to another was seen as a necessity in heavy industrial facilities, the concept of using an electromagnet was implemented due to its ability to be turned ON and OFF which was then incorporated effectively to the hoist function of overhead cranes.

Overhead-travelling-crane magnets are electromagnetic device attached to the crane's hook to magnetically pick up heavy metallic loads for hoisting and transferring.  Cranes that are fitted with a lifting magnet are equipped with an electromagnet control circuit.

The control circuit of the electromagnet is designed with operator command switch that can power the magnet ON and OFF. The electrical system is controlled by an electronic circuit so constructed so that when the magnet is switched ON for lifting objects, it instantaneously energizes the magnet by applying electric current to produce magnetic field with enough force to pick up heavy objects by magnetic attraction to the metal surface of the object, and to also release the object after transferring it in place to a certain location by switching OFF the magnet.

Figure 1 below is an artist's rendition (that's me by the way) of a typical overhead crane with top running trolley hoist fitted with an electromagnet suspended from its hook.

Figure 1:  Typical overhead crane with top running trolley hoist fitted with an electromagnet suspended from its hook.

Although the electrical control circuit uses AC power converted to DC power which is responsible for supplying electric current to the coil of the electromagnet, the electrical system of the magnet should also be supported by an automatic transfer system that can immediately switch to back-up battery mode with a DC battery unit, this is for safety purpose to prevent any occurrence of accidental dropping of heavy metallic loads while it is suspended in midair halfway through the lifting process in case of sudden loss of AC mains power supply during electric power grid failures, and to also cope safely with instances of intermittent AC power fluctuations.

A battery charger should also be included in the control circuit of the electromagnet to ensure that the required battery power level is always maintained. The charger should be able to detect battery power depletion level and automatically charge to replenish it so that sufficient amount of electric power is efficiently available at all times to ensure that an uninterrupted supply can be expected from the battery for the electromagnet to work effectively.

The schematic diagram in Figure 2 below shows an example of a conceptualized electronic circuit intended for a typical electromagnet used for the hoist function of overhead cranes.

Figure 2: Electronic circuit schematic diagram intended to illustrate the electrical control system of a typical electromagnet for the hoist function of overhead cranes.

As mentioned earlier, the diagram above shows that the battery serves only as a backup unit and will remain idle for most of the time while the AC power supply is active in the circuit. Steady DC power comes from the rectifier's converted DC voltage. Turning the switch ON will activate the two transistors Q1 and Q2 which are connected together in parallel to withstand sufficient amount of electric current to flow across the collector to the emitter of these two transistors in order to supply adequate amount of power to the coil of the electromagnet.

The diode which is connected in series with the battery serves as a one way directional path that will block the flow of current in the opposite direction coming from the P terminal of the rectifier, since the voltage coming from the rectifier is more positive than the B+ terminal of the battery then the diode is reverse biased in such condition. When the supply voltage from the AC power source is removed, the positive side (B+ terminal) of the battery is free to flow through the forward direction of the diode, which is now forward biased to serve as substitute positive voltage to supply power to the coil of the electromagnet through transistors Q1 and Q2, in place of the missing positive voltage from the rectifier in the absence of the AC source voltage.

The B+ terminal between the diode and the positive plate of the battery is connected to the charger circuit located in the bottom part of the electronic diagram. The battery charger circuit consists of an OP Amp (operational amplifier) which amplifies the signal from the reference voltage set by the variable resistor (potentiometer) VR1. The zener diode connected across VR1 is intended as a clamp for the purpose of maintaining a fixed reference to protect the first stage OP Amp from unnecessary rise of voltage to enter the input of the first stage OP Amp.

A high input signal from VR1 is inverted to low output signal in the first stage OP Amp, which proceeds further as a low signal to the input of the second stage OP Amp where it is inverted once again as a high signal output going to the base of transistor Q3, which in turn activates transistor Q3 to supply high input signal to the base of transistor Q5. This switches ON transistor Q5 so that its positive collector voltage P, which comes from the P terminal of the rectifier, can flow down to the emitter of transistor Q5 to supply positive voltage to the battery, thus charging the battery.

Transistor Q4 serves as a comparator and also as a feedback that detects the voltage level from the battery which can be adjusted and set with a reference voltage from the variable resistor (potentiometer) VR2. When a preset feedback voltage level from VR2 is detected at the base of transistor Q4, it switches Q4 to ON state to pull down the high input signal supposedly for positive trigger input to the base of transistor Q3. Absence of the high input signal to the base of Q3 will shut OFF Q3 to remove the positive input signal to the base of transistor Q5, causing it to discontinue the flow of positive voltage P from the collector to the emitter of Q5, thus stopping the charging of the battery.

Another feedback connection is found between B+ and the input of the second stage OP Amp of the charger circuit. This is intended to check the voltage level from the B+ terminal of the battery so that when B+ is higher it will then take precedence over the low input signal of the second stage OP Amp, which will reduce the gain of this amplifier to turn OFF both transistors Q3 and Q5 to stop further charging of the battery.

The very stable DC reference voltages +15V and -15V for the charger circuit is supplied by two regulator ICs, the 7815 regulator IC outputs a steady +15V supply, while the 7915 regulator IC is responsible for supplying a very constant -15V volts.

To protect the entire electronic circuit from sudden rise of instantaneous reverse voltage from the coil of the magnet which will flow in the opposite direction to the original polarity of the supply voltage of the electromagnet after switching it OFF, a flywheel (or flyback) diode is connected across the coil of the electromagnet to serve as a shunt for back EMF (electromotive force) suppressor that catches the reversing spike voltage by shorting it out to the magnet's coil without causing damage to the electronic components of the circuit.

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