In this article, we want to describe the function of the solenoid valve coil and explain the difference between AC or DC solenoid coil in detail.
Principles of Solenoid Operation
Solenoids are the most important components used in solenoid valves to control the flow of liquids and gases. Solenoids are electromechanical devices that convert AC or DC electrical energy into linear motion. They usually consist of a helical coil centered around a moving cylinder, called an armature, made of a ferromagnetic material such as iron or steel. Most solenoid valves have an interchangeable coil and can be used with coils of different voltages.
As current flows through the coil, it creates a magnetic field inside the coil that, using the same basic principles as conventional electric magnets, attracts the armature to the center of the solenoid valve . Because the armature is pulled toward the center of the solenoid regardless of the polarity of the current, an opposite force is required to return the armature to the starting position if the coil is not energized. This is achieved by using a spring mechanism. Ideally, to activate the solenoid valve, the force generated by the solenoid valve must be greater than the combined forces of the spring and hydraulic pressure as well as the friction.
Lifting the armature opens a small port in the valve that allows media to flow. The flow can be controlled by passing the valve through the excitation of the coil. While there are different types of solenoid valves that differ in mechanical construction, the basic idea of an electric actuator operating on a control surface remains the same in all types of solenoid valves. The pole of the electrical contacts with the AC and DC solenoid valves does not matter. With AC solenoids, this may be obvious because the current changes the pole twice anyway. With DC solenoid valves, the reason is that the current passing through the coil creates an electric magnet that creates an absorption force on the armature. When current passes through the coil, regardless of the contact and polarity of the current, the armature is always pulled towards the coil.
The difference between AC and DC solenoid coils
At the most basic level, the operation of DC solenoids is relatively simple – the solenoid may be activated, allowing the magnetic force generated by the solenoid to overcome spring resistance and moving the armature toward the center of the coil, or allowing Release the spring force to push the armature back to the starting position.
Using an AC solenoid valve, performance theory is a bit more complicated. AC current can be approximated using a sine waveform. As a result, the passage current becomes zero twice in each period, meaning that the current passing through the coil at that time is zero.
Because the magnetic force generated by the solenoid valve is directly proportional to the current flowing through the coil, the spring force will overcome the force generated by the solenoid valve twice in a short time. This is a problem that manifests as vibration of the armature, which produces a whispering sound and can put pressure on the solenoid valve components. To prevent this, a simple conductive ring, called a shading ring, is installed near the coil around the armature. The shading ring is usually made of copper. The function of the shadow ring is to store the energy of the magnetic field and release it with a phase difference of 90 degrees.
The effect of a shading ring is that while the magnetic field produced by the primary coil decreases to zero, the magnetic field produced by the shading ring peaks, effectively closing the gap in the amplitude of the magnetic field as it passes. Fills from zero, and eliminates vibrations. Most solenoid valves that can be used with different coil voltages have an internal shading ring. If dust accumulates around the reinforcement, the effect of the shading ring may be limited and another solution may be needed. Another example is the use of an electronic circuit that filters the electric current so that there is no zero passage. These circuits can be installed in the solenoid coil itself or can be made from the outside. It is usually performed in a full-wave rectifier topology using rectifier diodes and a filter capacitor.
Use AC coil with DC current and vice versa
In some cases, AC current coils with DC power supply and vice versa can be used. However, there are some limitations that you need to keep in mind. It is possible to use a coil for AC current with a DC power supply, but the voltage (and current) must be limited otherwise the solenoid valve may burn out. This is because in the AC structure, the coils have an inductive reactivity that increases with the electrical resistance of the coil. As a result, the winding impedance in the AC structure is several times higher than in the DC organization. For example, using a 24-volt solenoid with a 24-volt DC power supply is more likely to damage the solenoid because the effective current flowing through the DC-voltage solenoid will be much higher. Unfortunately, there is no fixed factor for reducing the voltage of the power supply. The effective current must be measured in the AC structure and that current must also be designated as the target for the DC regime. Some ways to achieve this are to reduce the supply voltage or to use a current limiting resistor. Using a DC current coil with an AC power supply poses a risk of vibration because DC solenoid valves may not contain a shading ring or rectifier circuit. These vibrations may damage the solenoid over time by squeezing the components, and they can contribute to the sound level in the room. This can be done by using a full-wave external rectifier circuit with a capacitive filter. Another problem is that the effective current in this case will be reduced several times and the magnetic force generated by the coil may not be large enough to move the armature from its resting position. One solution is to use a larger voltage so that the effective current corresponds to the rated current of the solenoid valve. Using a DC current coil with an AC power supply poses a risk of vibration because DC solenoid valves may not contain a shading ring or rectifier circuit. These vibrations may damage the solenoid over time by squeezing the components, and they can contribute to the sound level in the room. This can be done by using a full-wave external rectifier circuit with a capacitive filter. Another problem is that the effective current in this case will be reduced several times and the magnetic force generated by the coil may not be large enough to move the armature from its resting position. One solution is to use a larger voltage so that the effective current corresponds to the rated current of the solenoid valve. Using a DC current coil with an AC power supply poses a risk of vibration because DC solenoid valves may not contain a shading ring or rectifier circuit. These vibrations may damage the solenoid over time by squeezing the components, and they can contribute to the sound level in the room. This can be done by using a full-wave external rectifier circuit with a capacitive filter. Another problem is that the effective current in this case will be reduced several times and the magnetic force generated by the coil may not be large enough to move the armature from its resting position. One solution is to use a larger voltage so that the effective current corresponds to the rated current of the solenoid valve. These vibrations may damage the solenoid over time by squeezing the components, and they can contribute to the sound level in the room. This can be done by using a full-wave external rectifier circuit with a capacitive filter. Another problem is that the effective current in this case will be reduced several times and the magnetic force generated by the coil may not be large enough to move the armature from its resting position. One solution is to use a larger voltage so that the effective current corresponds to the rated current of the solenoid valve. These vibrations may damage the solenoid over time by squeezing the components, and they can contribute to the sound level in the room. This can be done by using a full-wave external rectifier circuit with a capacitive filter. Another problem is that the effective current in this case will be reduced several times and the magnetic force generated by the coil may not be large enough to move the armature from its resting position. One solution is to use a larger voltage so that the effective current corresponds to the rated current of the solenoid valve.
Solenoid design considerations in AC with DC
Ideally, when the solenoid valve goes from OFF to ON, the solenoid valve should first generate more force to pass through the spring tension along with the hydraulic pressure acting on the valve. Once the current is generated, the hydraulic forces that affect the valve mechanism are reduced, and electricity can reduce the power generated to reduce power consumption and heating.
The AC solenoid follows this ideal behavior more than the DC solenoid. In DC solenoids, when the solenoid is turned on, depending on the resistance of the coil, the current increases alternately to a certain value. This translates to less initial flow (and less initial force, which leads to slower valve opening). Once the valve is opened, the flow tension remains at a constant value greater than that required to keep the valve open. As a result, DC solenoids waste a significant amount of open energy without any external circuitry.
For AC circuits, the impedance of the coil is calculated using the following formula:
Where Z is the impedance, R is the electrical resistance of the coil, j is a constant equal to the square root of -1, which in this equation has a phase change effect of 90 degrees, f is the frequency and L is induced from the coil. As a result, the inductance of the coil is small, leading to smaller impedance and more current through the solenoid valve. The larger the current, the higher the magnetic force on the armature.
As the valve opens, the air gap decreases and the coil impedance increases rapidly and the current through the coil decreases. Reducing the current through the coil reduces power consumption and heat loss. Because of this, AC solenoids create an initial current jump, which allows the valve to open faster and more powerfully. As soon as the valve is opened, the flow decreases, which reduces power consumption. Although AC solenoids are inherently more energy efficient, they do have some potential drawbacks. One of them is the loss of power due to eddy currents due to electromagnetic induction in the armature. Another drawback is the risk of vibration, which can be reduced by using fully engineered solenoid valves that use proper shading rings. In addition, modern control systems tend to have a simpler interface with DC outputs, so using an AC solenoid valve with these systems can be more cumbersome and requires the use of additional relays. DC solenoids can be made more efficient by using external circuits that can shape the winding current so that there is an initial current spike to open the valve. Once the valve is opened, the flow can be reduced to a maintenance flow level, which is enough to hold the valve securely by pulling the armature against the spring tension.
These external circuits can be simply connected in series with the resistor and capacitor in parallel. In such a circuit, the capacitor is charged through the coil providing the primary spike of the coil. After charging the capacitor, the current limiting resistor passes all the current. The disadvantage of such a simple approach is that some energy is wasted to heat the current-limiting resistor.
Much more sophisticated approaches involve switching power supplies that provide programmable coil flow. These power supplies may work with AC and DC solenoid valves and power supplies. They ensure that the valve opens well and reduces power consumption when the valve is open, resulting in better energy efficiency, less heating and longer solenoid life.