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Inductive coupling refers to the phenomenon that exists when a magnetic field created by an electrical current induces an effect on something else. When this happens, the two then become mutually reactive, or coupled, by the inductive effects of the magnetic field. For example, when an electrical current passes through a wire, the electromagnetic field created can induce an electrical current in another wire, causing the two to be inductively coupled. The principles and effects of inductive coupling find use in devices such as transformers and electric motors.
The effects of inductive coupling can be used in one of three primary ways. First, the inducing field can create a specifically desired electrical current, such as in transformers. Second, the inducing field can create a specifically desired mechanical effect, such as in electric motors. Finally, the inducing field can create a resonance, which itself can create specifically desired electrical currents, such as in radio transmission and reception and non-contact charging devices.
In transformers, an electrical current conducts through a wire wound around a core of some type, called the primary winding. This wire is intentionally placed near another wire wound around the same core, called the secondary winding. The electromagnetic field, created by passing current through the primary winding, then induces an electrical current in the secondary winding.
If the two windings have the same number of turns around the core, it allows the primary winding to pass an exact replica of its electrical current to the secondary winding. These types of transformers are typically called isolation transformers. Through induction, they allow two circuits to be electrically linked, or coupled, without actually coming into direct physical contact, which physically isolates the two circuits from one another.
When the primary and secondary windings are not of the same number of turns around the core, the inductive coupling causes a different effect. The electromagnetic field created by the primary winding will induce a current that is proportional in value to the difference between the two windings. For example, if the primary winding is 10 turns around the core, and the secondary winding is 20 turns around the core, the induced current in the secondary winding will be twice the voltage of the current passing through the primary winding.
An electric motor uses a different aspect of the electromagnetic field. In a simple motor, a wire is wound around a rotor forming the rotating shaft of the motor. When an electrical current is passed through the wire, it creates an electromagnetic field. This field then induces a mechanical force by pushing away from and pulling toward magnets mounted around the rotor, depending on the polarity of the magnetic fields.
Resonant devices work in a way similar to transformers, though, without the paired windings. In these devices, a standing electromagnetic field is created. When this field encounters an antenna, the effect of inductive coupling causes the antenna to resonate, which, in turn, induces an electrical current at its feed point. In the case of a radio, the induced current is amplified and heard over the radio. In a charging device, the induced current is directly applied to a battery’s terminals to recharge it.