Self Inductance

- basic information about self-inductance, how it occurs, the basic self-inductance formula and associated calculations.

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Self inductance is an effect that is noticed when a single coil experiences the effect of inductance.

Under the effects of self inductance and changes in current induce an EMF or electro-motive force in that same wire or coil, producing what is often termed a back-EMF.

As the effect is noticed in the same wire or coil that generated the magnetic field, the effect is known as self inductance.

Self-inductance definitions

There are various definitions associated with self inductance that are useful to mention.

  • Self-inductance:   Self inductance is defined as the phenomenon in which a change in electric current in a circuit produces an induced electro-motive-force in the same circuit.
  • Self inductance unit:   The self-inductance of a coil is said to be one henry if a current change of one ampere per second through a circuit produces an electro-motive force of one volt in the circuit.

Self-inductance basics

When current passes along a wire, and especially when it passes through a coil or inductor, a magnetic field is induced. This extends outwards from the wire or inductor and could couple with other circuits. However it also couples with the circuit from which it is set up.

The magnetic field can be envisaged as concentric loops of magnetic flux that surround the wire, and larger ones that join up with others from other loops of the coil enabling self-coupling within the coil.

When the current in the coil changes, this causes a voltage to be induced the different loops of the coil - the result of self-inductance.

Self induction effect
Self induction

In terms of quantifying the effect of the inductance, the basic formula below quantifies the effect.

V L = - N d ϕ d t

    VL = induced voltage in volts
    N = number of turns in the coil
    dφ/dt = rate of change of magnetic flux in webers / second

The induced voltage in an inductor may also be expressed in terms of the inductance (in henries) and the rate of change of current.

V L = - L d i d t

Self induction is the way in which single coils and chokes operate. A choke is used in radio frequency circuits because it opposes any change, i.e. the radio frequency signal, but allows any steady, i.e. DC current to flow.

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