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Ask 6. What is an electromagnet?
An electromagnet is a component that converts electromagnetic energy into mechanical energy, which is divided into a direct current electromagnet and an alternating current electromagnet. (1) the structure principle of AC electromagnet
AC electromagnet can be regarded as a inductive load in series of resistance and inductance. Its structure is made up of coils, yokes (static iron cores) and armature (moving iron core), as shown in figure 16-5. In order to reduce the iron loss, the yoke and armature are all laminated by silicon steel sheet. The coil is wound on the skeleton with enameled wire, wrapped outside the insulation protection layer, and is made after being dipped and dried and fixed on the yoke. Figure 16-6 shows the working principle of the electromagnet. Alternating magnetic field is generated around the coil after alternating current. The magnetic yoke and armature are magnetized at the same time by alternating magnetic field, different polarity poles appear on the pole of the armature and the magnetic yoke, and the armature is sucked to the yoke according to the principle of same-sex repulsion and opposites attracting, until they match each other. Although the direction of the coil current is constantly changing, the direction of suction is always the same as the polarity of the armature and the yoke changes at the same time. Because the alternating current continuously assumes the sine change, when the current passes through the zero value, the suction of the electromagnet is zero, and then the armature will be released; the current is over zero, and the armature is sucked by the suction. In this way, with the constant change of alternating current, the armature will constantly be sucked and released, which is bound to produce violent vibration. In order to prevent this phenomenon, a copper ring is embedded on the polar surface of the magnetic yoke, which is called short circuit ring. The function of the short circuit ring is that when the alternating flux passes through the short circuit ring, the induced current is generated, which prevents the original magnetic field from vanishing when the alternating current passes through zero, a fixed suction force is maintained between the armature and the yoke, thus eliminating vibration at work. Because of the existence of remanence, the armature is sucked, and the method of leaving the demagnetization gap eliminates the phenomenon that the armature is sucked up after the power supply is cut off. (2). The structure principle of the DC electromagnet. A DC electromagnet consists of a coil, a yoke and a armature. As it is known, the yoke and armature are made of magnetically conductive soft steel because of the direct current and no eddy in the yoke and armature. After the DC electromagnet is connected to the power supply, the magnitude of the current (exciting current) in the coil is independent of the core condition, which is equal to the voltage divided by the resistance of the coil. The intensity of the magnetic field is affected by the size of the current and the number of turns. The larger the current, the stronger the magnetic field; the more the number of turns, the stronger the magnetic field. The DC electromagnet has stable suction force, no noise and small vibration after the suction, so the failure rate is small.
Figure 16-8 shows the DC electromagnet power supply circuit diagram. After the coil passes through the direct current, the magnetic field is generated, the magnetic yoke and the armature are magnetized at the same time, and the polarity of the pole face of the street iron and the magnetic yoke is different, so they attract each other, and the yoke and the armature attract people. An electromagnet coil is connected in series by a suction coil A and a holding coil B, and a pair of contacts ( conversion contacts ) controlled by the routing button are connected in parallel with the coil B, when the button is pressed and the yoke enters the yoke, the pressing contact disconnects the two contacts. When the electromagnet is not electrified and is energized, the two contacts are in the connection state, and the coil B is short connected with a large number of turns and a large resistance, only the suction coil A works, so the current is very large when it is energized, about 1.6A, can produce enough attractive force ( about 40N ) to suck the armature. When the armature is sucked in, the plate on the armature press the button into the yoke plate, so that the conversion contact is separated, so that the coil B is connected, the two coils are connected in series to work, and the resistance is about 31002 after series connection, so that the current of the coil of the electromagnet is reduced to about 0.06A. However, because the number of turns of the coil is large enough, the attraction can be kept at around 80N.

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The internal resistance of the electromagnet is the insurance resistance ( thermistor ). When the current is too large, the coil temperature is too high, the resistance increases dramatically, so that the circuit current becomes small, and the circuit is open. When the current becomes small and the temperature of the coil becomes low, the low resistance is restored, and the circuit is turned on. The resistance of the resistor is about 0.62 at room temperature. Some of the electromagnets do not have this resistance.

This article is from ALLICDATA ELECTRONICS　LIMITED.