What is the Inductance
Inductance is a physical quantity that measures the electromagnetic induction ability produced by the coil. Electrify a coil with current, the surrounding of the coil will then produce a magnetic field and there will be flux passing through the coil. The larger is the power that passes through the coil, the stronger the magnetic field will be and so is the magnetic flux passing through the coil. It has been found that magnetic flux passing through the coil is directly proportional to the passing current. Their ratio is called as self-inductance coefficient, and also called as inductance.
Classification of Inductance
Classification as per form of inductance: fixed inductance and variable inductance.
Classification as per magnetizer property: coil with air core, ferrite coil, iron-core coil, copper-core coil.
Classification as per job property: antenna coil, oscillator coil, choke coil, trap coil, deflection coil.
Classification as per winding structure: single layer coil, multi-layer coil, honeycombed coil.
Primary Characteristic Parameters of the Electric Induction Coil
1. Inductance quantities L
Inductance L denotes inherent characteristics of the coil itself，independent of current quantity. The inductance is normally not labeled specially on the coil, but labeled with a specific name, special electric induction coil (color code inductance) is an exception.
2. Inductive reactance XL
The scale of inhibition of the electric induction coil to alternating-current is called inductive reactance XL, the unit is Ohm. Its relation with inductance L and alternating current f is XL =2πfL
3. Quality factor Q
Quality factor Q denotes a physical quantity of the coil quality, Q is ratio of inductive reactance XL with its equivalent resistance, i.e: Q =XL/R. The higher is the Q value of the coil，the smaller the loss of the circuit will be. Coil's Q value is related to factors of wire DC resistance, skeletal dielectric loss, loss caused by shielded enclosure or iron core and impact of high frequency skin effect, etc. Coil's Q value is normally in tens to hundreds.
4. Distributed capacitance
The capacitance present between coil turns, coil and shielded enclosure, coil and platter is called as distributed capacitance. Presence of distributed capacitance will decrease coil's Q value and deteriorate the stability, therefore, the smaller is the distributed capacitance of the coil, the better.
III. Coils in Common Use
1. Single layer coil
Single layer coil is made by winding in turns insulated conductor around the paper drum or bakelite skeleton, like medium band antenna coil used for transistor radio.
2. Honeycombed coil
If the plane of the coil wound is not parallel to its rotation plane, but cross with a certain angle, this kind of coil is called as honeycombed coil, and times of its wire to and fro bending for its one round is often called as number of turning points. The advantage of honeycombed winding rests on compactness and small amount of distributed capacitance but large inductance. Honeycombed coil is wound by using honeycombed coil winding machine, the more are there turning points, the smaller the distributed capacitance will be.
3. Ferrite core and iron-dust core coil
The size of inductance of the coil is associated with presence of a magnetic core. If a ferrite core is inserted into a coil with air core, it can increase inductance and improve the quality factor of the coil.
4. Copper-core coil
Copper-core coil is mostly used in ultra-short band and it changes the inductance by turning the copper core position in the coil and this adjustment is relatively expedient and durable.
5. Color code inductor
Color code inductor is an inductor with fixed inductance, the inductance labeling method is the same as for resistance, and labeled with color circles.
6. Choke (choke coil)
The coil restricting alternating current to pass through the coil is called choke coil, which falls into high frequency choke coil and low frequency choke coil.
7. Deflection coil
Deflection coil is the load of output stage of TV set scanning circuit and the requirements for the deflection coil are high deflection sensitivity, uniform magnetic field, high Q value, compactness and low price.