What Is a Stator?

A stator is the stationary component of a rotating electrical machine, such as a motor, generator, or alternator, that provides the magnetic field environment within which the rotor operates. The name derives from the Latin "stare" (to stand), distinguishing it from the rotor, which rotates. In a motor, the stator creates a rotating magnetic field that drives rotation of the rotor, converting electrical energy into mechanical energy. In a generator, the rotor's magnetic field sweeps past the stator windings, inducing voltage and delivering electrical energy to the external circuit.

As described in the Wevolver engineering reference on stator construction and function, the stator is typically the outer structural element of a radial-flux machine, though axial-flux designs invert this geometry. Its three principal sub-components are the stator frame, the stator core, and the stator windings, each designed to fulfill a distinct electromagnetic, mechanical, or thermal role.

Construction and Core Materials

The stator core is built from a stack of thin, electrically insulated laminations of silicon steel. Laminating the core in the direction perpendicular to the flux path restricts the paths available to eddy currents, which would otherwise circulate through the bulk iron and dissipate energy as heat. The thickness of each lamination, typically 0.3 to 0.65 mm, and the grade of electrical steel determine the balance between eddy-current loss and hysteresis loss at the operating frequency. Higher-grade grain-oriented silicon steel lowers core losses but carries a cost premium. For machines operating at high frequencies, such as those driven by variable-frequency drives or used in aerospace applications, amorphous and nanocrystalline alloys offer significantly lower core losses compared to conventional silicon steel, at the cost of greater mechanical fragility.

The core is assembled into the stator frame, a structural housing that transfers torque reactions to the machine's mounting surface and that often incorporates passages for cooling air or water. The frame material is commonly cast iron or fabricated steel in large machines.

Stator Windings

The stator windings are coils of insulated copper or aluminum conductors placed in slots cut into the inner surface of the stator core. In three-phase AC machines, three separate winding groups are arranged symmetrically around the stator bore, each displaced by 120 electrical degrees. When energized with balanced three-phase currents, these windings create a magnetic field that rotates at synchronous speed in the air gap, the narrow annular space between the stator bore and the rotor surface.

Each coil is insulated from the core and from adjacent coils by groundwall insulation, typically built from multiple layers of mica tape bonded with epoxy or polyester resin. The integrity of the groundwall insulation is critical to machine reliability; voltage stress, partial discharge, thermal cycling, and mechanical vibration all contribute to its degradation over the service life of the machine. IEEE Xplore papers on high voltage generator insulation document the failure modes and testing methods applied to stator winding insulation in utility-grade generators.

Stator in Motors and Generators

In motors, the stator windings carry the supply current and produce the rotating field that accelerates the rotor. In synchronous generators, the rotor carries the field winding and the stator carries the armature windings where output power is collected. In induction generators and doubly-fed machines, both the stator and the rotor carry AC currents, and power flows through both sets of windings. The IEEE paper on stator slot wedge testing for large generators illustrates how stator condition monitoring has become a structured discipline for machines where unexpected outages carry high economic consequences.

Applications

Stators have applications in a wide range of areas, including:

• Three-phase synchronous generators in power plants and wind turbines
• Induction motors in industrial drives and HVAC systems
• Permanent-magnet motors in electric vehicles and robotics
• Alternators in automotive charging systems
• Linear motors in magnetic levitation and precision positioning systems