Basic Structure of a Three-Phase Asynchronous Motor

A three-phase asynchronous motor consists of two fundamental parts: a stationary stator and a rotating rotor. The rotor is housed within the inner cavity of the stator and is supported by bearings mounted on the two end shields. To ensure that the rotor can rotate freely within the stator, a gap-known as the air gap-must exist between the stator and the rotor. The motor's air gap is a critical parameter; its magnitude, symmetry, and other characteristics exert a significant influence on the motor's performance. Figure 2 illustrates the constituent components of a three-phase squirrel-cage asynchronous motor.

Stator
The stator comprises the three-phase stator windings, the stator core, and the frame.
The three-phase stator windings constitute the electrical circuit of the asynchronous motor; they play a pivotal role in the motor's operation and serve as the key component responsible for converting electrical energy into mechanical energy. The structure of the three-phase stator windings is symmetrical, typically featuring six terminal leads-U1, U2, V1, V2, W1, and W2-which are housed within a terminal box located on the exterior of the frame. Depending on operational requirements, these leads can be connected in either a star (Y) or a delta (△) configuration.
The stator core forms part of the asynchronous motor's magnetic circuit. Since the main magnetic field rotates relative to the stator at synchronous speed, the core is constructed from a stack of 0.5 mm-thick, high-permeability silicon steel laminations to minimize losses induced within the core material. Both sides of these silicon steel laminations are coated with an insulating varnish to reduce eddy current losses within the core.
The frame (also referred to as the housing) serves primarily to support the stator core; concurrently, it withstands the reaction forces generated during the motor's operation under load. Furthermore, heat generated by internal losses during operation is dissipated externally through the frame. The frames of small and medium-sized motors are typically cast from iron, whereas the frames of large motors-due to their substantial size and the associated difficulties in casting-are commonly fabricated by welding steel plates.

Rotor
The rotor of an asynchronous motor consists of the rotor core, the rotor windings, and the shaft.
The rotor core also constitutes a part of the motor's magnetic circuit and is likewise constructed from a stack of silicon steel laminations. Unlike stator core laminations, rotor core laminations feature slots cut into their outer circumference. When stacked together, these laminations form a rotor core with numerous uniformly distributed, identically shaped slots on its outer cylindrical surface, designed to house the rotor windings.
The rotor winding constitutes the other part of the induction motor's electrical circuit. Its function is to cut through the stator's magnetic field, thereby generating an induced electromotive force (EMF) and current; subsequently, under the influence of the magnetic field, it experiences a force that causes the rotor to rotate. Structurally, rotor windings can be classified into two types: squirrel-cage windings and wound-rotor windings. The primary characteristics of these two rotor types are as follows: the squirrel-cage rotor features a simple structure, is easy to manufacture, and is both economical and durable; the wound rotor, conversely, has a more complex structure and is more expensive, but it allows for the introduction of external resistors into the rotor circuit to enhance starting and speed-control performance.
A squirrel-cage rotor winding consists of conductive bars placed within the rotor slots, connected at both ends by end rings. To conserve steel and boost manufacturing efficiency, the conductive bars and end rings in small-to-medium power induction motors are typically formed in a single operation by die-casting molten aluminum. For high-power motors-where ensuring the quality of cast aluminum can be challenging-copper bars are commonly inserted into the rotor core slots, and end rings are subsequently welded to both ends. The squirrel-cage rotor winding forms a self-contained, closed circuit that requires no external power supply; its physical appearance resembles a cage, hence the name "squirrel-cage rotor."

Air Gap
The air gap in an induction motor is extremely small; for small to medium-sized motors, it typically ranges from 0.2 to 2 mm. The larger the air gap, the greater the magnetic reluctance; consequently, generating a magnetic field of a specific magnitude requires a correspondingly larger excitation current. Due to the presence of this air gap, the magnetic circuit reluctance of an induction motor is significantly higher than that of a transformer; as a result, the excitation current required by an induction motor is substantially larger than that of a transformer. While a transformer's excitation current typically amounts to approximately 3% of its rated current, an induction motor's excitation current is roughly 30% of its rated current. Since the excitation current is a reactive current, a larger excitation current implies...