Solenoids, or solenoid actuators, are the actuator devices that attract the ferromagnetic materials (such as armature) to move by the magnetic field that generated by the energized solenoid coil. Solenoids have been widely applied to relay actuators, contactors, and other automatic devices.

A common solenoid is consist of coils, magnetic field, armature or plunger, which has been used as the power element of solenoid valves, such as hydraulic valves, pneumatic valves. During the working process, the metallic core is magnetized, so the plunger is moving under the influence of the electromagnetic force, and the plunger can trigger the solenoid valves to realize all types of control actions. When the power goes out, it kills the magnetic fields, too, so the plunger has been released under the force of the elastic force.

Solenoid is a basic component of many different devices, such as cars, mining machines, hydraulic systems, environmental sanitary vehicles. Therefore, there are many different ways to classify solenoids, based on solenoids applications, the working method, such as hydraulic solenoids, pneumatic solenoids, rotary solenoids, etc. One of the basic ways to classify solenoids is based on different power systems (the electric current pass through the solenoid coils), solenoids can be divided into two types, ac solenoids, and dc solenoids.

a picture of different types of solenoids

About DC Solenoids

DC solenoids refer to the solenoids for dc excitation current.

DC Solenoid Attractive Force

The exciting current for dc solenoid is direct current, therefore, the attractive force for dc solenoid plungers is F=4B02S ╳ 105.

Elements of the formula of dc solenoid attractive force:

  • B0 refers to the magnetic induction intensity of air gap (Wb/m2);
  • S refers to the cross section area of the air gap magnetic field;
  • F refers to the attractive force of the solenoid.

The Features of DC Solenoids

DC solenoids adopt the dc excitation, the magnetic flux of the iron core is constant. There is no induced electromotive force has been generated. So the excitation current of dc solenoid coil is determined by the electric voltage and the internal resistance of solenoid coils. Therefore, if the electric voltage and the solenoid coil internal resistance are invariable, the excitation current will not change, and the magnetic motive force (MMF) is constant, too.

Therefore, dc solenoid holds the following features:

  1. Direct excitation current of solenoid coil is totally determined by the voltage and solenoid coil internal resistance, so the direct current is constant.
  2. During the pull-in process of dc solenoid armature, the air gap is progressively smaller, and the magnetic resistance of the magnetic circuit keeps smaller, too.
  3. According to Ohm’s law for magnetic circuit, if the field current is invariable, the magnetic flux is in inverse proportion to the magnetic resistance. During the solenoid armature pull-in process, the magnetic flux grows larger, which means the attractive force of dc solenoid is related to the position of the solenoid armature. The solenoid force when the solenoid armature starts to close is much lighter than the solenoid force of a working solenoid.

a picture of dc 12v and 24v solenoids

About AC Solenoids

AC solenoids refer to the solenoids for ac excitation current.

When the ac solenoid coil has been energized by sinusoidal alternating current, the alternating magnetic flux is generated, and the relation between solenoid coil voltage and the magnetic flux is U = 4.44FNφm

Elements of the ac solenoid formula:

  • F refers to the frequency of excitation alternating current (Hz);
  • N refers to the number of windings of ac solenoid coils;
  • φm refers to the maximum value of ac solenoid magnetic flux (Wb);
  • U refers to the virtual value of ac current voltage.

According to the formula, as the power frequency and solenoid coil winding numbers are invariable, the maximum value of ac solenoid magnetic flux is in direct proportion to the virtual value of ac solenoid current voltage. Also, when the current voltage is changeless, the maximum value of ac solenoid magnetic flux is constant, it has nothing to do with the magnetic circuit.

AC Solenoid Attractive Force

The exciting current for ac solenoid is alternating current. The magnetic induction intensity of air gap is changing over time, so the ac solenoid force is also changing over time. AC solenoid force formula is: Fav = 2Bm2S ╳ 105 .

Elements of the formula of ac solenoid attractive force:

  • Bm refers to the maximum value of air gap magnetic induction intensity;
  • S refers to the cross section area of the air gap magnetic field;
  • Fav refers to the average force of ac solenoid.

Short Circuit Ring

From the ac solenoid force formula we can find out, ac solenoid force is changing between zero and the maximum value over time, and the vibrating armature will cause noise.

At the end of the iron core, there is a short circuit ring (also known as divide magnetic ring). It is a closed copper ring which can subdue the noise.

The Features of AC Solenoids

AC solenoid holds the following features:

  1. During the closing process of ac solenoid armature, ac solenoid force maintains the same. Therefore, in the ac solenoid magnetic circuit, the magnetic flux has no relationship with the materials used in the magnetic circuit and the air gap size. AC solenoid magnetic flux is determined by ac voltage and its frequency.
  2. There is a big difference of the ac solenoid excitation current between the before and after of the solenoid armature closing process.
  3. The air gap before and after ac solenoid closing process is different, therefore, the magnetic resistance is going to be changed before and after ac solenoid closing process. The current when ac solenoid starts is much stronger than the current when ac solenoid is working.
  4. The experiment results show that, for a U-shape ac solenoid, its excitation current when ac solenoid starts is 10-15 times of the excitation current when ac solenoid is working. The solenoid coil rated current is designed based on the current value after the solenoid closing process. Therefore, if the solenoid coil has been energized but the solenoid armature cannot be closed, or this action has been operated continually, the ac solenoid coil may overheat and burn. This is one of the main reasons that make the ac solenoids more easier to be burnt than dc solenoids.
  5. The alternating magnetic flux of the iron core of ac solenoid will generate eddy current and magnetic hysteresis loss. In order to minimize the iron core loss, the iron core of ac solenoid is made by silicon steel sheet.

a picture of ac 110v and 220v solenoids