In the past, fuses, overcurrent relays, thermal relays, thermostats, etc. were used to protect motors, but each of these methods has its advantages and disadvantages. There is no absolutely reliable protection method for electric devices, which are load changing equipment. Therefore, various combinations must be adopted, which can be summarized into two types: one is to judge the increase or decrease of motor input current; The second is to judge the heating condition of the motor itself. In both cases, the time allowance given by the motor thermal capacity shall be considered.

Pneumatic ball valve The two-piece pneumatic ball valve is used in the pipeline to cut off faster, distribute and change the flow direction of the medium. Pneumatic ball valve is a new type of gate valve widely used in recent years, which has the following advantages:
1. The hydrodynamic friction resistance of pneumatic two-piece ball valve is small, and its friction resistance index is the same as that of pipes of the same length.
2. Simple structure, light weight, very light.
3. It is inseparable. At present, plastic is widely used as the raw material for the sealing surface of valves, with good sealing performance, and it has also been widely used in ultrafiltration devices.
4. The pneumatic two-piece ball valve is easy to operate and can be opened and closed quickly. If it rotates 90 ° from full opening to full closing, it is conducive to long-distance operation.
5. The maintenance is convenient, the pneumatic ball valve is simple in structure, the sealing ring is generally subject to theme activities, and the disassembly and replacement are relatively convenient.
6. When the valve is fully opened or closed, the sealing surface of the ball and high-pressure gate valve is isolated from the medium. When the medium is used, it is not easy to cause corrosion of the sealing surface of the gate valve.
7. Pneumatic two-piece ball valve can be used in a wide range of applications, ranging from nominal diameter of mm to the size of meters, and from high vacuum pump to working pressure.