Parameters of the Solenoid Coil

Figure 1: A solenoid coil in operation.

The Solenoid coil is tightly wrapped up on a core to form a solenoid. The main parameter of the solenoid coil is the size of the inductance.
In addition, the wire wound around the solenoid coil generally always has a certain resistance, usually this resistance is very small and can be ignored. But when the current flowing in some circuits is very large, the small resistance of the coil cannot be ignored. This is because a large current will dissipate power on this coil, causing the coil to heat up or even burn out. Therefore, sometimes the electrical power that the coil can withstand is also considered.
This article will introduce several important parameters of the solenoid coil to help you to better learn about it.

Inductance

The inductance represents the inherent characteristics of the coil itself and has nothing to do with the magnitude of the current. Except for special solenoid coils, the inductance is generally not marked on the coil, but marked with a specific name. Inductance, also known as self-inductance coefficient, is a physical quantity that represents the ability of an inductor to generate self-induction.
The size of the inductance of the inductor mainly depends on the number of turns of the coil, the winding method, the presence or absence of a magnetic core and the material of the magnetic core, etc. Generally, the more coil turns and the denser the coils are wound, the greater the inductance. A coil with a magnetic core has a larger inductance than a coil without a magnetic core.
The basic unit of inductance is Henry, represented by the letter "H". Commonly used units are millihenry (mH) and microhenry (μH). The relationship between them is:
1H=1000mH
1mH=1000μH

Figure 2: Different types of solenoid assembly for hydraulic solenoid valve.

Inductive Reactance

The magnitude of the resistance of the solenoid coil to the alternating current is called the inductive reactance XL, the unit is ohm, the symbol Ω. Its relationship with inductance L and AC frequency f is XL=2πfL.

Quality Factor

The quality factor Q is a physical quantity that represents the quality of the coil, and Q is the ratio of the inductive reactance XL to its equivalent resistance, namely: Q=XL/R. It refers to the ratio of the inductive reactance presented by an inductor to its equivalent loss resistance when it operates under an AC voltage of a certain frequency. The higher the Q of the inductor, the lower its losses and the higher the efficiency.
The Q value of the coil is related to the DC resistance of the wire, the dielectric loss of the skeleton, the loss caused by the shield or the iron core, and the influence of the high-frequency skin effect. The Q value of the coil is usually tens to hundreds. The quality factor of the inductor is related to the DC resistance of the coil wire, the dielectric loss of the coil skeleton, and the loss caused by the iron core and shield.

Distributed Capacitance

For any solenoid coil, there is certain capacitance between turns and turns, between layers, between the coil and the reference ground, between the coil and the magnetic shield, etc. This capacitance is called the distributed capacitance of the solenoid.
If these distributed capacitances are combined together, it becomes an equivalent capacitance C in parallel with the solenoid coil. The existence of distributed capacitance reduces the Q value of the coil and deteriorates the stability, so the smaller the distributed capacitance of the coil, the better.

Rated Current

The rated current refers to the maximum current value that is allowed to pass through the inductor during normal operation. If the working current exceeds the rated current, the performance parameters of the inductor will change due to heat generation, and even burn out due to overcurrent.

Figure 3: Hydraulic valve coil-23-51-45mm solenoid coil.

Allowable Deviation

The allowable deviation refers to the allowable difference between the nominal inductance on the inductor and the actual inductance.
Inductors generally used in circuits such as oscillation or filtering require high accuracy, and the allowable deviation is ±0.2[%]~±0.5[%]; while the accuracy requirements for coils such as coupling and high-frequency blocking current are not high; allow The deviation is ±10[%]~15[%].

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