Inductance in AC

In DC you learned that current through a coil creates a magnetic field. In AC that field constantly grows, shrinks, and reverses because current reverses every half-cycle. Every change in the field induces a voltage in the coil itself.

That means every AC circuit has some inductance — even a straight wire has a tiny amount. In practice we treat inductance as significant when the load contains a coil: motors, transformers, fluorescent ballasts, reactor chokes, contactor coils, solenoids, and filter inductors.

Loads with coils behave very differently from resistive heaters or incandescent lamps. Current does not simply follow Ohm's law with a fixed R value. The coil fights the change, and that fight shows up as inductive reactance (next lesson) and phase shift.

Four facts from basic inductance theory carry directly into AC work:

1. When magnetic flux cuts through a coil, a voltage is induced in that coil.
2. The induced voltage is always opposite in polarity to the applied voltage — counter-electromotive force (CEMF).
3. The induced voltage is proportional to the rate of change of current (how fast the field changes).
4. An inductor opposes any change in current — it does not oppose steady DC once the field is built, but in AC the current is always changing.

Picture a 120 V AC line connected to a large coil. Most of the applied voltage may be "used up" fighting the induced voltage, leaving only a few volts to push current through the wire resistance inside the coil. A clamp meter might read 0.8 A while an ohmmeter on the de-energized coil reads 6 Ω. Only about 4.8 V is needed for I × R through the copper; the rest is absorbed by CEMF.

That is why you cannot judge an energized motor winding by resistance alone.

On AC, applied voltage and induced voltage are 180° out of phase. They oppose each other. The net voltage that actually drives current through the coil's resistance is small compared with the line voltage.

This is not "lost" energy in the sense of billing watts — it is the mechanism that limits current without burning heat (in an ideal pure inductor). Real coils still have I²R losses in the copper, plus core losses in iron-core devices.

The inductor symbol is a coil of wire (series of loops or semicircles):

• Air-core inductor: loops only, no core lines.
• Iron-core (or ferromagnetic core): loops with two parallel lines beside them — the core.

Chokes and ballasts often use laminated iron cores. High-frequency filter inductors may be air-core or powdered-iron core. Read the schematic before you assume "just a wire."

Labeling convention:

Real parts also carry current rating, voltage class, and sometimes kVAR rating — not just henrys.

At no load, a motor looks mostly inductive — current is limited by XL. As mechanical load increases, the motor converts electrical energy to mechanical work and the circuit looks more resistive (power factor rises toward unity under load, though rarely to 1.0).

In a pure resistive AC circuit, voltage and current rise and fall together — 0° phase difference.

In a pure inductive circuit, current lags applied voltage by 90°. Voltage peaks first; current peaks one quarter-cycle later.

Why? Induced voltage is proportional to the rate of change of current:

• When current is at its peak, it is momentarily not changing → induced voltage is zero.
• When current passes through zero and changes fastest → induced voltage is maximum.

Current leads induced voltage by 90°. Induced voltage is 180° from applied voltage. Therefore current lags applied voltage by 90°.

Memory aid for the trade:

ELI the ICE man — in an L (inductive) circuit, E (voltage) Leads I (current).

(You will meet ICE — current leads voltage — when you study capacitors.)

In a pure inductive circuit, voltage and current are 90° apart. Half the time they have the same polarity (energy goes into building the magnetic field); half the time opposite polarity (energy returns as the field collapses). Net true power (watts) over a full cycle is zero for an ideal inductor.

Real coils still consume watts because of wire resistance (I²R), eddy currents, and hysteresis in iron cores. Those are losses, not "useful" magnetic storage.

Situation. A maintenance tech replaces fluorescent ballasts in a plant wired for 60 Hz. New ballasts shipped from Europe are marked 50 Hz. Same lamp type, same voltage label.

What happens. At 60 Hz the magnetic field changes 20% faster than the ballast was designed for. Inductive reactance XL = 2πfL rises with frequency. Current through the choke can exceed design value → overheating, nuisance tripping, shortened life.

Applied lesson. Inductance is not just "how many turns" — frequency matters. Always match Hz rating on magnetic components. When nameplates are unreadable, measure operating current on a known-good identical unit before ordering.