Ampereturns

Ampereturns, or ampere-turns (unit: ampere-turns, abbreviated AT or A·T), are the unit of magnetomotive force (MMF) in electromagnetism. The MMF provided by a coil is the product of the current I (in amperes) and the number of turns N in the coil: MMF = NI. By convention, one ampere-turn is produced by a current of 1 ampere flowing through 1 turn of wire.

In a magnetic circuit, MMF is the line integral of the magnetic field H around a closed path: MMF = ∮ H · dl. For a simple, uniform magnetic core, MMF ≈ H l, where l is the magnetic path length. The magnetic flux density B relates to H by B = μH, with μ = μ0 μr the permeability of the material. The total magnetic flux φ through an area A is φ = B A. The relation to reluctance Rm of the core is MMF = φ Rm, with Rm = l/(μ A). Thus φ = NI / Rm = NI μ A / l.

Applications and significance: ampere-turns are used to specify the magnetomotive force required in electromagnets, transformers, inductors, and electric machines. Designers use NI as a primary figure when sizing windings to achieve a desired flux or torque, while accounting for core material saturation, fringing, copper losses, and leakage flux.

Limitations: the actual flux is limited by material saturation, geometry, and losses; nonuniform fields and fringing can complicate simple NI-based estimates, so detailed models are often needed for precise design.