How much do you know about induction motors?

Induction motor is also known as "asynchronous motor", that is, the rotor is placed in a rotating magnetic field, and under the action of the rotating magnetic field, a rotational torque is obtained, so the rotor rotates. The rotor is a rotatable conductor, usually in the shape of a squirrel cage. The stator is the non-rotating part of the motor and its main task is to generate a rotating magnetic field. The rotating magnetic field is not achieved mechanically. Instead, alternating current is passed through several pairs of electromagnets, so that the properties of the magnetic poles are cyclically changed, so it is equivalent to a rotating magnetic field. This kind of motor does not have brushes or collector rings like DC motors. There are single-phase motors and three-phase motors according to the type of AC power used. Single-phase motors are used in washing machines, electric fans, etc.; three-phase motors are used as power for factories. equipment.

Nikola Tesla (July 10, 1856 - January 7, 1943) was a Serbian-American inventor, mechanical engineer, and electrical engineer. He is considered one of the key drivers of the commercialization of electricity, and is best known for presiding over the design of the modern alternating current system. Based on the electromagnetic field theory discovered by Michael Faraday, Tesla has a number of revolutionary inventions in the field of electromagnetic fields. Invented the induction motor in 1887, his many related patents and theoretical research work on electromagnetism is the cornerstone of modern wireless communication and radio.

Through the relative movement of the rotating magnetic field generated by the stator (the speed of which is the synchronous speed n1) and the rotor winding, the rotor winding cuts the magnetic field line to generate an induced electromotive force, thereby generating an induced current in the rotor winding. The induced current in the rotor winding acts with the magnetic field to generate electromagnetic torque, which makes the rotor rotate. Because when the rotor speed gradually approaches the synchronous speed, the induced current gradually decreases, and the generated electromagnetic torque also decreases accordingly. When the asynchronous motor works in the motor state, the rotor speed is less than the synchronous speed. In order to describe the difference between the rotor speed n and the synchronous speed n1, a slip is introduced.

Basic structure of single-phase asynchronous motor

A single-phase asynchronous motor is a motor that only needs a single-phase AC power supply. Single-phase asynchronous motor consists of stator, rotor, bearing, casing, end cover, etc. The stator consists of a frame and an iron core with windings. The iron core is formed by punching and laminating silicon steel sheets, and two sets of main windings (also called running windings) and auxiliary windings (also called starting windings to form secondary windings) are embedded in the slots with an electrical angle of 90° from each other. The main winding is connected to the AC power supply, and the auxiliary winding is connected in series with the centrifugal switch S or the starting capacitor, the running capacitor, etc., and then connected to the power supply. The rotor is a squirrel-cage cast aluminum rotor, which is formed by laminating the iron core and casting aluminum into the slot of the iron core, and casting end rings together to make the rotor bars short-circuit into a squirrel-cage type.

Single-phase asynchronous motors are further divided into single-phase resistance starting asynchronous motors, single-phase capacitor starting asynchronous motors, single-phase capacitor running asynchronous motors and single-phase dual-value capacitor asynchronous motors.

Basic structure of three-phase asynchronous motor

Three-phase asynchronous motor is mainly composed of stator, rotor and bearing. The stator is mainly composed of iron core, three-phase winding, frame and end cover.

The stator core is generally punched and laminated by 0.35~0.5mm thick silicon steel sheets with insulating layers on the surface, and evenly distributed slots are punched in the inner circle of the core to embed the stator windings.

The three-phase winding is connected by three windings with the same structure and arranged in a 120° electrical angle in space. The coils of these windings are respectively embedded in each slot of the stator according to a certain rule. Its function is to pass in three-phase alternating current to generate a rotating magnetic field.

The frame is usually cast iron, the frame of a large asynchronous motor is generally welded with steel plates, and the frame of a micro motor is made of cast aluminum.

There are cooling ribs on the outside of the frame of the enclosed motor to increase the heat dissipation area, and the end caps at both ends of the frame of the protective motor are provided with ventilation holes, so that the air inside and outside the motor can be directly convected to facilitate heat dissipation. The end cover mainly plays the role of fixing the rotor, supporting and protecting.

The rotor is mainly composed of iron core and windings. The material of the rotor core is the same as that of the stator, which is punched and laminated from 0.5 mm thick silicon steel sheets. The outer circumference of the silicon steel sheet is punched with evenly distributed holes for placing the rotor windings. Usually, the inner circle of the silicon steel sheet after the stator core is punched is used to punch the rotor core. Generally, the rotor core of small asynchronous motors is directly press-fitted on the rotating shaft, and the rotor cores of large and medium-sized asynchronous motors (with a rotor diameter of 300-400 mm or more) are pressed on the rotating shaft with the help of the rotor bracket.

Rotor windings are divided into squirrel-cage rotors and wound rotors.

(1) Squirrel-cage rotor: The rotor winding consists of a plurality of bars inserted into the rotor slots and two annular end rings. If the rotor core is removed, the entire winding looks like a squirrel cage, so it is called a cage winding. Small cage motors use cast aluminum rotor windings, and for motors above 100KW, copper bars and copper end rings are welded. Squirrel cage rotors are divided into: impedance rotors, single squirrel cage rotors, double squirrel cage rotors, and deep groove rotors, with different starting torque and other characteristics.

(2) Wound rotor: The wound rotor winding is similar to the stator winding, and it is also a symmetrical three-phase winding, generally connected in a star shape, and the three outlet heads are connected to the three collector rings of the rotating shaft, and then pass the brush connected with external circuits.