Experience in Repairing Valve Electromagnets

In hydraulic components, electromagnets are often used to generate suction force to push and pull the valve core, thereby controlling the direction, pressure and flow rate of the liquid flow. This type of electromagnet is generally called a valve electromagnet (hereinafter referred to as an electromagnet for short). In the control system, the electromagnet plays a pivotal role in connecting the upper and lower parts. It converts electrical energy into mechanical energy to drive the hydraulic valve to operate. Strictly speaking, an electromagnet consists of an electromagnetic coil and an armature action mechanism. In the market, the two are also supplied as a complete set. In the maintenance of construction machinery, the situation where electromagnetic coils are burned out is very common. Therefore, the electromagnet we refer to here mainly means the electromagnetic coil.

Electromagnets include two types: switch type and proportional type. Switch type electromagnets are often installed on directional control valves to control the valve's reversing, unloading and loading of the system, etc. Proportional electromagnets are installed on various proportional valves to control the direction, pressure and flow rate of the liquid flow. Many proportional pressure valves are installed on electrically controlled variable pumps and can control the pump's flow rate and direction.

Proportional electromagnets have a characteristic that the electromagnetic force remains largely unchanged throughout their stroke, at least during their working stroke, which distinguishes them from ordinary switch-type electromagnets. This suction characteristic is formed through the special shape of the working air gap and the guidance of the magnetic force of the magnetic conductor. The difference between switch-type and proportional electromagnets does not entirely depend on the electromagnets themselves. When a proportional electromagnet is subjected to the maximum current, it is equivalent to a switch-type electromagnet. When a switch-type electromagnet is subjected to different currents, it will also generate different thrust.

The performance indicators of an electromagnet mainly include applicable voltage (current), pushing (pulling) force and stroke, etc.

According to voltage, it can be classified into 12V DC, 24V DC, 110V AC, 220V AC, etc. The push-pull force of the electromagnet for the NG6 valve is generally 20 to 70N, and the stroke is between 3 and 7mm. When it is suspected that a certain solenoid valve is stuck or its coil is burned out, the solenoid valve can be turned over to see if its stroke is within the above-mentioned range.

The external characteristic of an electromagnet is mainly manifested as resistance. The resistance values of the most common 24V NG6 solenoid valve coils are generally between 16 and 26Ω, those of 24V cartridge valves are generally between 20 and 38Ω, and those of 24V proportional valves are generally between 21 and 26Ω. Theoretically, the electromagnetic suction force is proportional to the square of the current, so the resistance value of a 12V coil is generally about one quarter of that of a corresponding 24V coil.

Proportional solenoid valves generally require the current to reach a certain range. For instance, for REXROTH (Rexroth) series pumps, 24V proportional solenoid valves generally require a current of 200-600mA, while 12V ones require a current of 400-1200mA. The LINDE series of 24V proportional solenoid valves for pumps require a current of 220-405mA or 175-360mA. The 24V proportional solenoid valve for the SAUER series pumps requires a current of 13-85mA.

When the solenoid valve does not operate, you can use a multimeter to measure the resistance between the coil connection pins 1 and 2. If the resistance value is infinite, it indicates an internal open circuit. If the resistance value is very small, it indicates an internal short circuit and the coil needs to be replaced. Open circuits and short circuits are often caused by the heating of coils. To reduce the probability of valve jamming, hydraulic component manufacturers will lower the coil resistance value to increase the thrust. However, in this way, the coil will heat up very severely. After being continuously powered on for several minutes, the internal temperature may exceed 100°C. Therefore, in application, in addition to paying attention to the heat dissipation of the solenoid valve, it is also necessary to minimize the power-on time as much as possible. Solenoid valve coils with high on-off and power-off frequencies are more prone to burnout.

When there is no multimeter, you can use a wire or a small screwdriver to push the push rod and see if the working mechanism moves. You can also bring a wrench or other tools close to the electromagnet to see if there is sufficient suction. Of course, there are other electrical and hydraulic reasons for the solenoid valve not operating.

The wiring of electromagnets generally refers to the ISO4400/ DIN43650 standard. Pin 1 and Pin 2 are the two ends of the coil. No matter which end is the positive pole, the coil will exert a suction force on the armature, so the wiring is not divided into positive and negative. But conventionally, 1 is positive and 2 is negative.

A small number of on-off solenoid valves and some proportional valves use push-pull type electromagnets. Their wiring generally complies with the ISO4401 standard. Pins 1 and 2 are the positive poles, and 3 is the common negative pole.