I. Three-phase induction motor consists of stator, rotor and other parts.
(1) Stator - stationary part
1. Stator core
Purpose: part of the motor magnetic circuit in which the stator winding is located.
Construction: the core is usually made from stamped sheets of electrical steel 0.35–0.5 mm thick with surface insulation. On the inner circumference there are evenly spaced grooves for laying the stator winding.
The following stator slot shapes are used:
Semi-closed slots provide higher efficiency and power factor, but make winding installation and insulation more difficult. Typically used in small low voltage motors.
Semi-open slots allow the installation of finished coils and are commonly used in large and medium-sized low-voltage motors. The finished coil is pre-insulated and then placed in the groove.
Open slots are designed for finished coils, are convenient for insulation and are mainly used in high-voltage motors.
2. Stator winding
Purpose: the electrical part of the engine that creates a rotating magnetic field when three-phase alternating current is supplied.
Design: three identical windings, symmetrically arranged with a 120° electrical angle shift. Their coils are placed in the stator slots according to a certain pattern.
The main types of stator winding insulation ensure reliable separation of the conductive parts and the core, as well as parts of the winding itself:
(1) Frame insulation - between the entire stator winding and the stator core.
(2) Phase-to-phase insulation - between windings of different phases.
(3) Turn-to-turn insulation - between the turns of each phase winding.
Connections in the motor terminal box:
The box contains the terminal block. The six terminals of the three-phase winding are arranged in two rows. The top row from left to right is designated 1 (U1), 2 (V1), 3 (W1), the bottom row is 6 (W2), 4 (U2), 5 (V2). The windings are connected in star or triangle. This order must be followed during manufacture and repair.
3. Body
Purpose: Secures the stator core and the front and rear covers supporting the rotor, and also provides protection and heat dissipation.
Construction: the body is usually cast from cast iron; The housings of large asynchronous motors are welded from steel sheet, and micromotors are cast from aluminum. Fins are provided on the outside of the enclosed engine to increase the cooling surface. Protected end covers have ventilation holes that provide direct air convection between the internal cavity and the environment.
(2) Rotor - rotating part
1. Rotor core of a three-phase asynchronous motor
Purpose: part of the motor magnetic circuit with slots for the rotor winding.
Construction: like the stator, it is made of stamped sheets of electrical steel 0.5 mm thick. The holes for the winding are evenly spaced on the outer circumference. For the manufacture of the rotor core, internal cuttings of the stator sheets are usually used. In small engines, the core is pressed directly onto the shaft, and in medium and large engines, with a rotor diameter over 300–400 mm, it is attached to the shaft by means of a rotor support.
2. Rotor winding of a three-phase asynchronous motor
Purpose: when crossing the rotating magnetic field of the stator, EMF and current arise, creating an electromagnetic torque and rotation of the motor.
Design: squirrel cage or wound rotor.
(1) Squirrel-cage rotor: The winding consists of grooved bars and two closing rings. Without the core, it resembles a squirrel cage. In small motors, cast aluminum windings are used, and for powers above 100KW, a welded structure of copper rods and rings is used.
(2) Slip rotor: The winding is similar to the stator winding, it is a symmetrical three-phase system and is usually connected in star. The three leads are connected to the three slip rings of the shaft, and then through the brushes to the external circuit.
The design is more complex, so phase motors are less common than squirrel-cage motors. However, the inclusion of additional resistances and other elements in the rotor circuit through rings and brushes improves starting, braking and speed control. Such motors are used in devices that require a smooth change in speed within a certain range: cranes, elevators and air compressors.