# Understanding Solenoid Design and Function

Figure 1: Solenoid coils

A solenoid is an electromechanical device that features electromagnetic linear actuation when subjected to an electric current. The device is a coil of wire that, when energized, generates a magnetic field within its core. The magnetic field moves an armature for several applications, including locking, industrial, automotive, and medical. This article describes the design of a solenoid and how it functions.

## Solenoid design

Figure 2: A diagram of a solenoid coil: coil (A), stationary core (B), shading ring (C), spring (D), and armature (E).

As seen in Figure 2, solenoid parts are the following:

• Coil (A): A copper wire tightly wound around the stationary core.
• Stationary core (B): A ferromagnetic cylinder
• Shading ring (C): A shading ring, or shading coil, is a single turn, or a few turns of an electrical conductor (copper or aluminum)
• Spring (D): A stainless steel spring that returns the armature to its normal position when the coil is de-energized.
• Armature (E): Part of the stationary core that moves when the coil is energized, also called the solenoid plunger.

## How does a solenoid work

A magnetic field forms when current in a solenoid flows through the coil. The strength of the field is directly proportional to the current, the number of windings, and the permeability of the stationary core's ferromagnetic material. The core works as a magnetic circuit for the magnetic field. In other words, the core acts as a closed path that confines the magnetic field.

The magnetic field induces a force on the armature that either pulls it up or pushes it down. The norm is to pull the armature up, but extending the side of the armature close to the stationary core with a pushrod results in the field pushing the armature down. In either scenario, a spring is compressed. The armature remains in the position as long as the field remains. When the field dissipates, the spring returns the armature to its original position.

Finally, the shading ring provides a low-impedance path for a high-voltage spike generated when the magnetic field dissipates. This diminishes the voltage spike's magnitude and duration, which protects the circuit.

### DC vs AC solenoid coils

There are advantages and disadvantages to working with DC or AC solenoids. A DC solenoid works as explained above. DC current flowing through a solenoid creates a magnetic force strong enough to overcome the spring force, lifting the armature into the coil. When the coil de-energizes, the spring pushes the armature back down.

AC coils are more complex because AC switches polarity. Twice per period, the current is zero, making the magnetic force zero. Because the spring force is constantly pushing the armature down, the armature vibrates and creates a humming sound and stresses the components. To solve this issue, a copper ring around the armature stores some of the magnetic energy, reducing the vibration.

When the magnetic force overcomes the spring force, and the armature is fully lifted, the current running through the coil can be reduced to save energy. DC responds slower than AC, and therefore AC coils are more energy efficient.

## Magnetic field of a solenoid formula

The magnetic field of a solenoid coil depends on the number of turns per unit length, the strength of the current through the coil, and the solenoid material’s permeability. The formula for measuring the strength of the field is:

Where:

• B: the magnetic flux density
• μ0: The permeability constant which is 12.57 x 10-7 Hm-1
• I: The current passing through the coil
• N: The number of turns
• L: The coil’s length

## Solenoid valve force equilibrium

Figure 3: To control flow, a solenoid valve uses a force balance between the coil's magnetic field (B) and the force of the spring (C) and the medium's pressure (D). The magnetic field generated by the shading ring (A) applies to AC coils.

The basic principle of a solenoid valve is a force balance of the magnetic force of the solenoid on one side, and the pressure of the medium and force of the spring on the other side. Calculate the required magnetic force for a direct operated solenoid valve with the following formula:

Where:

• Fs = solenoid force (N)
• p = pressure (Pa) (105 Pa = 1 bar)
• A = orifice (m2)
• Fspring = spring force (N)

### Example

A given solenoid provides a force of 15N. To use this solenoid to control a 10 bar pressure differential, the maximum orifice diameter can be calculated.

This formula cannot be used for indirect operated valves. Indirect operated solenoid valves have a smaller orifice and use the pressure of the medium to switch.

## Solenoid coil applications

• Locking applications: The magnetic field attracts the plunger within the solenoid, causing it to move and lock the mechanism in place. A spring pushes the plunger back when the current shuts off, releasing the lock. Solenoids can be used in various locking applications, including doors, vending machines, access barriers, and many other security devices.
• Automotive applications: Solenoids are used in various vehicle applications, including shifting the transmission, starting the engine, operating fuel injection systems, locking doors, and actuating valves.
• Medical applications: Solenoids are used in medical applications to control fluid flow, regulate valves in medical gas systems, operate pumps and dispensers, and control the movement of medical equipment.
• Railway applications: In the railroad industry, solenoids are used to operate switches and signals, control brakes, actuate doors and windows, and in diesel engines for fuel injection and exhaust gas recirculation.
• Industrial applications: Solenoids are used in industrial applications to control valves, operate pneumatic and hydraulic systems, actuate clutches and brakes, control the movement of equipment, and in automation, robotics, and manufacturing processes. An actuator of a solenoid is typically a solenoid with a ferromagnetic core.

Figure 4: A close-up of a solenoid valve in a tractor transmission system.

## FAQs

### What is a solenoid valve coil?

A solenoid coil is a tightly wound wire that produces a magnetic field when electricity flows through it and is used to move objects, create electricity or activate a solenoid actuator.

### Are solenoids AC or DC?

Solenoids convert AC or DC into linear motion.