Electronics Principles V11

Clive W. Humphris

INDUCTANCE: Storing Energy in an Inductor.  

Connect a battery across an inductor and a current will be made to flow through the coil winding. As the current changes (builds-up) energy is stored in the coil as a magnetic flux the density of which is dependent upon the amount of current flowing, determined by R1. A magnetic field exists only whilst a current is flowing in an inductor. When the current ceases to flow, the field collapses.

If S1 is opened and S2 closed, the energy produced by the collapsing field will be dissipated in R2. The rate of collapse can cause a very large voltage spike to occur which other circuits need to be protected from. This safety measure is usually in the form of a reverse biased (commutating) diode connected across the coil which conducts on the reverse voltage.

The rate of current change will determine the back-EMF (reverse voltage) developed across L. This can reach extremely high levels (thousands of volts) and should be controlled. The faster switch S1 is opened and S2 is closed and the lower the value of R2 then the higher the back-EMF.

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