Coil

A coil typically refers to a ring-shaped winding of conductive wire. Common applications include motors, inductors, transformers, and loop antennas.

In circuits, a coil refers to an inductor. It consists of conductive wires wound around a form, with each wire insulated from the others. The insulating tube may be hollow or contain an iron core or magnetic powder core, commonly abbreviated as an inductor.

Inductors operate based on the principle of electromagnetic induction. When current flows through a conductor, it generates an electromagnetic field around itself. This field induces a current in other conductors within its range.

The effect on the wire generating the electromagnetic field is called “self-inductance.” This means the changing current within the wire produces a changing magnetic field, which in turn affects the current in the wire. The effect on other wires within this electromagnetic field is called “mutual inductance.”

Inductors can be categorized into fixed inductors and variable inductors. Fixed inductor coils are commonly referred to as inductors or coils. They are denoted by L, with units including henry (H), millihenry (mH), and microhenry (μH). 1H = 10³ mH = 10⁶ μH.

Coils can be classified into single-layer coils and honeycomb coils.

In a wire excitation coil, a varying current flows through the coil. Magnetic flux lines pass through the coil's center. The greater the current, the more magnetic flux lines are generated, until saturation occurs. When the current is interrupted, the magnetic flux lines disappear.

A current-carrying coil used to induce magnetic flux in a magnetic circuit is called an excitation coil (or excitation winding). The current flowing through the excitation coil is called excitation current (or magnetizing current). If the excitation current is direct current (DC), the magnetic flux in the circuit remains constant and does not vary with time. Such a circuit is termed a DC magnetic circuit; the magnetic circuit of a DC motor belongs to this category. If the excitation current is alternating current (AC), the magnetic flux in the circuit varies periodically with time. Such a circuit is termed an AC magnetic circuit; the magnetic circuits of AC iron-core coils, transformers, and induction motors all belong to this category.