Hits:Updated:2020-05-14 09:05:58【Print】
The electromagnet is to use the magnetic circuit to convert electrical energy into magnetic energy, and then convert the magnetic energy into mechanical energy that causes the moving iron core to move linearly. The electromagnet is usually composed of three main parts: excitation coil, static iron core and moving iron core.
According to the current classification through the exciting coil, there are DC electromagnet and AC electromagnet.
Since there is no hysteresis eddy current loss in the DC electromagnet, the iron core can be made of a whole piece of ferromagnetic material. The moving iron core is often made into a cylindrical shape, called I-shaped electromagnet or solenoid electromagnet. Compared with T-shaped electromagnet, it has a compact structure, small volume, short stroke, light moving parts, small impact force and full air gap. In the coil of the solenoid, the suction force is greater. But the DC electromagnet should prevent the residual magnetism from being too large and affect the normal work.
When an alternating current is passed through an alternating current, there is a hysteresis eddy current loss in the core. On small electromagnets and electromagnetic pilot valves, integral I-shaped electromagnets can be used. On the direct-acting solenoid valves that require a large suction force, in order to reduce the iron consumption and heat, T-shaped electromagnets are often used, and the iron core is made of silicon steel sheets to reduce the iron consumption and heat. The movable parts of the T-shaped electromagnet are heavy, and the impact force is large during the action, but the suction force is large and the stroke is large.
(2) Buzzer
For 50HZ alternating current, there is zero suction 100 times per second. The moving iron core began to return to its original position due to the loss of suction. But within a short period of time, the suction force increased again, and the moving iron core was sucked in again, forming a moving iron core vibration and making a buzzing sound. The precautionary measure is to install a magnetic ring on the suction surface of the electromagnet. The magnetic flux in the part of the magnetic pole surrounded by the sub-magnetic ring and the magnetic flux in the part of the magnetic pole at the end are sometimes different, and the corresponding suction force is also different, so the total suction force of the moving iron core at any instant will not be zero, which can eliminate vibration. The smaller the resistance of the sub-magnetic ring, the better (such as brass, copper), but too small will cause the current flowing through the sub-magnetic ring to be too large, and the loss will be large. Setting the sub-magnetic ring naturally reduces the mechanical strength. Because the sub-magnetic ring is often broken, the life is reduced.
There is no beeping sound from the DC solenoid.
(3) The relationship between suction and stroke
The relationship between electromagnetic suction force and stroke (ie air gap) is called suction characteristic. The stroke and relationship between the sum of spring force and gravity is called the reaction force characteristic. It is hoped that in any stroke of the electromagnet, the suction force is slightly greater than the reaction force, which can ensure that the moving iron core is attracted, and avoid the impact and rebound caused by the action speed too fast. This can avoid contact welding and burning, to improve life.
The suction force of the DC electromagnet is inversely proportional to the square of the stroke. When the stroke is large, the suction force is very small, that is, the starting power is small.
The suction characteristic curve of the AC electromagnet is relatively flat, that is, it has a large suction force even when the stroke is large.
(3) Starting current and holding current
When the voltage of the AC electromagnet is constant, although the magnitude of the exciting current is related to the resistance of the coil, it is mainly affected by the stroke. The stroke is large, the magnetic resistance is large, and the exciting current is also large. The current at the maximum stroke is called the starting current, and the current at the time of suction is called the holding current, also called the excitation current. In large electromagnets, the starting current can reach more than 10 times the holding current. The starting current of small electromagnets can also reach 2 to 4 times the holding current. If the moving iron core is stuck and the starting current continues to flow, the coil heats up violently and is even burned. The solenoid pilot valve with a small stroke is not easy to burn. The AC electromagnet should not be switched on and off frequently. The service life is not as long as the DC electromagnet.
When the voltage is constant, the coil current of the DC electromagnet only depends on the coil resistance, regardless of the stroke size. Therefore, the coil will not be burned when the moving iron core is stuck. The DC electromagnet can be switched on and off frequently, and the work is safe and reliable. However, when the high voltage is misconnected and the current is too large, the coil will still be burned.
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