AC Motor Construction

Three-phaseAC induction motors are commonly used in industrial applications. This type ofmotor has three main parts, rotor, stator, and enclosure. The stator and rotordo the work, and the enclosure protects the stator and rotor.

StatorCore

Thestator is the stationary part of the motor’s electromagnetic circuit. Thestator core is made up of many thin metal sheets, called laminations.Laminations are used to reduce energy loses that would result if a solid corewere used.

StatorWindings Stator laminations are stacked together forming a hollow cylinder.Coils of insulated wire are inserted into slots of the stator core.

Whenthe assembled motor is in operation, the stator windings are connected directlyto the power source. Each grouping of coils, together with the steel core itsurrounds, becomes an electromagnet when current is applied. Electromagnetismis the basic principle behind motor operation.

RotorConstruction

Therotor is the rotating part of the motor’s electromagnetic circuit. The mostcommon type of rotor used in a three-phase induction motor is a squirrel cagerotor. Other types of rotor construction is discussed later in the course. Thesquirrel cage rotor is so called because its construction is reminiscent of therotating exercise wheels found in some pet cages.

Asquirrel cage rotor core is made by stacking thin steel laminations to form acylinder.

 Ratherthan using coils of wire as conductors, conductor bars are die cast into theslots evenly spaced around the cylinder. Most squirrel cage rotors are made bydie casting aluminum to form the conductor bars. Siemens also makes motors withdie cast copper rotor conductors. These motor exceed NEMA Premium efficiencystandards.

Afterdie casting, rotor conductor bars are mechanically and electrically connectedwith end rings. The rotor is then pressed onto a steel shaft to form a rotorassembly.

 

Enclosure

Theenclosure consists of a frame (or yoke) and two end brackets (or bearinghousings). The stator is mounted inside the frame. The rotor fits inside thestator with a slight air gap separating it from the stator. There is no directphysical connection between the rotor and the stator.

Theenclosure protects the internal parts of the motor from water and otherenvironmental elements. The degree of protection depends upon the type ofenclosure. Enclosure types are discussed later in this course.

 

Bearings,mounted on the shaft, support the rotor and allow it to turn. Some motors, likethe one shown in the following illustration, use a fan, also mounted on therotor shaft, to cool the motor when the shaft is rotating.