### Back EMF and impedance

When passing an alternating current through a coil of wire, like a transformer or a solenoid valve, the expanding and collapsing magnetic field created by the alternating current induces a back-voltage (back EMF) in the coil which goes against the supply voltage. All coils of wire will have this.

If you have two sets of windings near one another, you can “induce” a voltage from one to the other. A transformer has two sets of windings. The primary and secondary windings. One coil (primary winding) is connected to alternating current, and the expanding and collapsing magnetic field from that coil will “induce” a voltage in the nearby coil (secondary winding). The voltage produced is in the opposite direction, opposing the original voltage.

Inductive reactance is a form of resistance that only exists when an alternating current is present due to frequency. Higher the frequency, lower the current due to higher resistance. It cannot be measured with an ohmmeter or Ohm’s Law. You’ll have to use the power factor multiplier for the piece of equipment.

The total resistance of a circuit containing a coil is called the impedance. It’s the sum of the measurable Ohm’s Law resistance you measure with no power applied, and the resistance due to the inductive reactance.