primary
A directly driven inductive coil, as in a transformer or induction motor that is magnetically coupled to a secondary.
primary: the input coil that starts the magnetic action
In a transformer or induction motor, the primary is the coil connected to the power source. It receives electrical current and generates a magnetic field that couples to a secondary coil through the iron core. The primary winding is where energy enters the system; the secondary is where it emerges, either transformed in voltage or converted to mechanical motion.
The primary winding's physical characteristics determine how efficiently it transfers energy. Wire gauge, number of turns, and core material all affect the primary's resistance and its ability to establish a strong magnetic field. In a step-down transformer stepping from 480V to 120V, the primary has more turns than the secondary. In a step-up transformer, it has fewer. The turns ratio between primary and secondary directly governs the voltage transformation.
Primary failure and common problems
Primary coils fail most often through insulation breakdown. Moisture, thermal cycling, and voltage surges degrade the enamel coating on wire or the paper insulation between layers. A shorted primary, even a partial short across a few turns, causes excessive current draw, core heating, and potential fire risk. Testing a suspected primary involves measuring resistance (should be relatively low but finite) and insulation resistance to ground (should be very high, typically megohms).
The primary and secondary are magnetically coupled but electrically isolated in a transformer. This isolation is critical for safety in many applications: a transformer powering a low-voltage control circuit from a 480V line keeps dangerous voltage away from operators. In an induction motor, the primary (stator) creates a rotating magnetic field that induces current in the secondary (rotor), which then experiences a force and spins.
The term primary comes from Latin primus, meaning first. It is the first coil in the energy path, the one that initiates the electromagnetic process. Secondary follows naturally as the second coil in the chain. This naming is universal across transformers, motors, and coupled inductor circuits throughout electrical engineering and power distribution.