What Is a Solenoid Switch?

Electromagnetic switch, as its name implies, is a switch controlled by an electromagnet, which is a combination of an electromagnet and a switch. When the electromagnet coil is energized, an electromagnetic suction force is generated, and the movable iron core pushes or pulls the switch contact to close, thereby connecting the controlled circuit. Electromagnetic switches have a wide range of applications in various industries, the most common being contactors in the industrial field. The electromagnetic switch is a control switch on the starter, and is one of the three major components of the starter (DC motor, transmission meshing mechanism, electromagnetic switch). Its working principle is that the electromagnetic force is generated after the coil is energized, which moves the movable iron core, thereby pulling The transmission meshing mechanism causes the starter pinion to move forward to mesh with the engine flywheel ring gear. On the other hand, it pushes the switch contact to make the DC motor energized and drives the engine to start.

The starter electromagnetic switch is a key component of the starter. The design quality of the electromagnetic switch directly affects the reliability of the starter.

The starter electromagnetic switch has two functions. One is to turn on the main circuit to rotate the starter. The other is to push the driving gear through the fork to mesh with the flywheel ring gear of the engine. Therefore, the electromagnetic switch is required to have corresponding suction and corresponding stroke. The contacts can reliably switch on and off large currents. In addition, it is required to reduce the volume and weight as much as possible.

Based on the above characteristics, the contact structure of the electromagnetic switch generally adopts a contact form, that is, the movable contact is made into a bridge piece shape or a disc shape, so that it has a large heat dissipation surface and heat capacity. The magnetic system mostly uses a solenoid-type electromagnet structure to meet the requirements of corresponding stroke, corresponding suction, small size and light weight.

The moving iron core of the electromagnetic switch can be smoothly moved in a brass shaft sleeve equipped with a solenoid coil. The moving iron core rod is connected by a fork unidirectional device. Under the action of a return spring, the moving iron core usually remains in the inactive initial state. Position, when starting, the starter drive gear should move forward about 12-20mm, so when the electromagnetic switch starts to attract, a large magnetic potential is needed to generate the necessary initial suction, and the air gap is reduced after the suction , The electromagnetic suction force increases sharply, greatly exceeding the force required to maintain the suction state. To this end, the electromagnetic switch uses two coils (attraction coil and holding coil). At the beginning, the attraction coil and the holding coil jointly generate an electromagnetic attraction force. When the switch contacts are attracted, the attraction coil is automatically released from the working state, and the holding coil is used alone To keep the switch attracted, keep the coil current relatively small, and use thinner wires to wind it. This can not only reduce the volume of the electromagnetic switch, but also achieve reasonable suction matching. In order to ensure the reliability of the starter, it is necessary to focus on the design quality of the electromagnetic switch ^[1]

The switch case of some electromagnetic switches is welded to the case and the bottom pole, so that the magnetic leakage is large. Therefore, in the design, if the shell and the bottom plate are integrated, and the bottom plate is thickened, there is no magnetic leakage, and the magnetic flux is also increased.

In order to reliably attract and disconnect the electromagnetic switch, it is necessary to control the additional stroke of the electromagnetic switch. The additional stroke is generally between 1.5-3.5mm. The additional stroke is too small, which may cause the unreliable attracting. The additional stroke is too large, causing contact. Not easy to disconnect, so the additional stroke of the electromagnetic switch is best controlled between 1.5-2.5mm ^[2]

The static contact material of the electromagnetic switch is generally pure copper. If the total amount of impurities is high, it is easy to cause the main contact to be difficult to open. Therefore, the static contact tends to use copper tungsten alloy and silver chromium key alloy. These alloy materials With high temperature resistance, high melting point, not easy to ablate, strong arc resistance, etc., can greatly improve the reliability and life of electromagnetic switches.

The material of the moving contact of the electromagnetic switch is also pure copper, and now the moving contact tends to use silver-copper alloy. Because silver copper alloy has good conductivity. The shape of the moving contact piece is bridge type and disc type, so that it can dissipate heat quickly when it is energized, and it is not easy to adhere to the static contact due to thermal ablation.

The return spring is generally a cylindrical spring, and tends to use a drum spring, so that the return spring will not be coiled at the end of the working compression, and it is not easy to skew.

Coil tubes have copper coils and brass tubes. Stainless steel tubes tend to be used because of the strength and durability of stainless steel tubes, and the moving iron core is also easy to pull in and return in the coil tube.

The air gap between the coil tube and the moving iron core is too large. Due to the large air gap, the moving iron core is abraded with the coil tube due to its own weight during the suction and release process, so it tends to be between the coil tube and the moving iron core. The gap is about 0.1mm ^[3] .