What is the working principle of a solenoid valve?

There are many types of solenoid valves, and different solenoid valves play roles at various positions within a control system. Commonly used ones include check valves, directional control valves, and flow-control valves.

　　So, let’s start by talking about the working principle of a solenoid valve.

　　The solenoid valve features a sealed chamber with through-holes located at different positions. Each hole is connected to a separate oil line. In the middle of the chamber is a piston, with two electromagnets on either side. When the coil of the electromagnet on one side is energized, the valve body is attracted toward that side. By controlling the movement of the valve body, different oil discharge ports can be opened or closed. Meanwhile, the oil inlet port remains normally open, allowing hydraulic oil to flow into the various discharge lines. The pressure of the oil then pushes the piston inside the cylinder, which in turn drives the piston rod. The piston rod, in turn, actuates the mechanical device. Thus, by controlling the on-off state of the electromagnets, we can precisely regulate the mechanical motion.

Solenoid valves can be classified into three major categories based on their operating principles.

　　1. Direct-acting solenoid valve

　　When power is applied, the electromagnetic coil generates an electromagnetic force that lifts the closure member off the valve seat, opening the valve. When power is cut off, the electromagnetic force disappears, and the spring presses the closure member against the valve seat, closing the valve.

　　2. Step-by-step direct-acting solenoid valve

　　It operates on a principle that combines direct-acting and pilot-operated mechanisms. When there is no pressure difference between the inlet and outlet, upon energization, the electromagnetic force directly lifts the pilot small valve and the main valve’s closure member upward in sequence, thereby opening the valve. Once the pressure difference between the inlet and outlet reaches the activation threshold, upon energization, the electromagnetic force actuates the pilot small valve, causing the pressure in the lower chamber of the main valve to rise while the pressure in the upper chamber drops. This pressure differential then pushes the main valve upward to open. When power is cut off, the pilot valve, aided by spring force or process medium pressure, moves its closure member downward, closing the valve.

　　3. Pilot-operated solenoid valve

　　When power is applied, the electromagnetic force opens the pilot port, causing the pressure in the upper chamber to drop rapidly. This creates a pressure differential—high pressure above and low pressure below—around the closing member. The fluid pressure then pushes the closing member upward, opening the valve. When power is cut off, the spring force closes the pilot port, and the inlet pressure quickly establishes a pressure differential—low pressure above and high pressure below—around the closing member via a bypass port. The fluid pressure subsequently pushes the closing member downward, closing the valve. There are many types of solenoid valves, each playing a role at different positions within the control system. Commonly used types include check valves, directional control valves, and flow-control valves.