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Induction coil | Definition, Principle, & Design

Induction coil Definition Principle Design

The coil must be tightly coupled to the workpiece, but adequate mechanical and electrical clearances should be provided. The coil must usually be designed complete with operating devices and matching parts, such as capacitors, transformers, etc. The design must also take into account the effects of leakage flux heating, vibration, water and oil, so the coil should be manufactured after the electrical design is completed.

The conductor is usually copper with high thermal conductivity and is cooled by water. Most coils consist of tubes with segmented water supply. However, sometimes oil or a “sticky” quenching medium can be used to achieve the dual purpose of cooling the conductor and quenching the workpiece.

In some one-time spray quenching operations with short heating cycles, the conductor may not be cooled in segments because the spray quenching can perform the above two functions. Silver plating is often used to increase the thermal conductivity of the surface, especially the contact surface. A typical coil is shown in the figure. Most mobile coils are single-turn and carry high currents of up to 20 kA.

Since the reduced voltage eliminates the danger of voltage breakdown in the presence of the quenching medium, single-turn inductors powered by matching transformers (see Chapter 7) are satisfactory. The conductors should be strong and durable to ensure long life and dimensional stability.

induction coil

Another advantage of the reduced voltage is that it makes it possible to couple the induction coil closely to the workpiece, thereby improving heating efficiency and achieving a precise heating hardening zone. Because of the high frequency (3.10 or 450 kHz), the current is concentrated on the inner surface. All welds must be silver brazed.

High-temperature brass solders, such as copper-silver alloys, should not be used because the weld resistance is too high and they are brittle and tend to crack while thermally expanding. Because of the strong electromagnetic forces on the induction coil, it must be mechanically reinforced and firmly fixed. The water jet can be set inside the induction coil or made separately. Obviously, annular and cylindrical induction coils meet the requirements, but this is not the only shape. Sometimes other shapes are also suitable, such as horseshoes (straight or curved). The figure shows a curved horseshoe induction coil; it can be used where it is impossible to surround a cylindrical surface.