The principle of energy conversion in electric motors

The energy conversion principle of an electric motor refers to its core mechanism of converting electrical energy into mechanical energy, which is based on the laws of electromagnetic induction and electromagnetic force (Ampere's Law)

Specifically, the motor achieves energy conversion through electromagnetic interaction between the stator and rotor: the stator winding generates a magnetic field after being energized, which interacts with the current in the rotor conductor to produce electromagnetic force (Lorentz force), thereby forming the torque that drives the rotor to rotate, and ultimately converting the input electrical energy into mechanical kinetic energy

 

The basic principle of energy conversion in electric motors

Electromagnetic induction and electromagnetic force: When current passes through the conductor of a motor (such as the stator winding), a magnetic field is generated around it; The magnetic field interacts with the current in the rotor, and according to Ampere's law of force, the conductor is subjected to a force that causes the rotor to rotate

Energy conversion path: After the electrical energy is input into the motor, it is converted into the rotational motion of the rotor (mechanical energy) through electromagnetic induction and electromagnetic force, which drives the external load to work

Key structure: The motor is mainly composed of a stator (fixed part, generating magnetic field) and a rotor (rotating part, carrying current). Some motors also include a commutator (DC motor) or a frequency converter (AC motor) to maintain unidirectional torque.

 

Classification and working characteristics of motors

Electric motors can be divided into DC motors and AC motors according to their power sources. Among them, AC motors are more widely used in power systems, including synchronous motors and asynchronous motors (asynchronous motors have rotor speeds that are not synchronized with stator magnetic field speeds)

6. The rotating magnetic field of an AC motor is generated by three-phase balanced currents passing through stator windings with a spatial difference of 120 °,

Among them, (omega=2 \ pi f) is the angular frequency, (f) is the power frequency, (p) is the logarithm of poles, and the synchronous speed (n0=60f/p)

7.The rotor speed of an asynchronous motor (n=(1- s) n0) always lags behind the synchronous speed, giving it a natural soft start capability due to its "asynchronous" characteristic

 

Historical background and application

The working principle of electric motors originated from the current magnetic effect discovered by Auster in 1820, and then Faraday invented the first electric motor device in 1821

6. Modern motors have been widely used in industries, transportation, and household appliances, and their energy conversion efficiency depends on the type, design, and usage conditions. For example, AC motors are usually more efficient than DC motors

1. With the development of materials science and control technology, motors are evolving towards higher power density and intelligence

The electric motor uses the principle of the force acting on an electrified conductor in a magnetic field (which is different from the magnetic effect of electric current, and the current People's Education Press ninth grade physics clearly separates the two). The discovery of this principle was made by Danish physicist Oster, born on August 14, 1777, in a pharmacist family in Rudjobin, Langlong Island. In 1794, he was admitted to the University of Copenhagen and obtained his doctoral degree in 1799. From 1801 to 1803, he visited countries such as Germany and France and met many physicists and chemists. From 1806, he served as a professor of physics at the University of Copenhagen, and from 1815, he became the Executive Secretary of the Royal Danish Society. In 1820, he was awarded the Copley Medal of the Royal Society of England for his outstanding discovery of the electric current magnetic effect.

Since 1829, he has served as the dean of the Copenhagen Institute of Technology. He passed away on March 9, 1851 in Copenhagen. He has conducted extensive research in physics, chemistry, and philosophy. Due to the influence of Kant's philosophy and Schelling's natural philosophy, I firmly believe that natural forces can be transformed into each other, and have long explored the connection between electricity and magnetism. In April 1820, the effect of electric current on magnetic needles, namely the magnetic effect of electric current, was finally discovered. On July 21 of the same year, he published his findings under the title "Experiment on the Electrical Conflict Effect on Magnetic Needles". This short paper caused a great shock in the European physics community, leading to the emergence of a large number of experimental results and thus opening up a new field of physics - electromagnetism.

Structural Classification

1, The structure of a three-phase asynchronous motor consists of a stator, rotor, and other accessories.

(1) Stator (stationary part)

1. Stator iron core

Function: As a part of the motor magnetic circuit, and on which the stator winding is placed.

Construction: The stator core is generally made by punching and laminating silicon steel sheets with insulation layers on the surface of 0.35~0.5 millimeters thick. There are evenly distributed slots punched in the inner circle of the core for embedding the stator winding.

There are several types of stator core slot types:

Half closed slot: The efficiency and power factor of the motor are high, but winding embedding and insulation are difficult. Generally used in small low-voltage motors.

Half open slot: capable of embedding molded windings, generally used for large and medium-sized low-voltage motors. The so-called formed winding refers to the winding that can be insulated before being placed into the slot.

Open slot: used for embedding molded windings, with convenient insulation method, mainly used in high-voltage motors.

 

2. Stator winding

Function: It is the circuit part of the electric motor, which is supplied with three-phase AC power to generate a rotating magnetic field.

Construction: Composed of three identical windings arranged symmetrically at an electrical angle of 120 ° in space, each coil of these windings is embedded in a certain pattern in each slot of the stator.

There are three main insulation items for stator windings: (ensuring reliable insulation between the conductive parts of the winding and the iron core, as well as reliable insulation between the windings themselves).

⑴ Ground insulation: The insulation between the entire stator winding and the stator core.

⑵ Inter phase insulation: The insulation between the stator windings of each phase.

⑶ Interturn insulation: Insulation between turns of each stator winding of each phase.

Wiring inside the motor junction box:

There is a terminal block inside the motor junction box, and the six wire ends of the three-phase winding are arranged in two rows, with the upper row of three terminal posts arranged from left to right numbered 1 (U1), 2 (V1), and 3 (W1), and the lower row of three terminal posts arranged from left to right numbered 6 (W2), 4 (U2), and 5 (V2). Connect the three-phase winding in a star or delta connection. All manufacturing and maintenance should be arranged according to this serial number.

 

3.Machine base

Function: Fix the stator core and front and rear end caps to support the rotor, and provide protection, heat dissipation, and other functions.

Construction: The base is usually made of cast iron. The base of large asynchronous motors is generally welded with steel plates, while the base of micro motors is made of cast aluminum. The enclosed motor has heat dissipation ribs on the outside of the base to increase the heat dissipation area, while the protective motor has ventilation holes on both ends of the base cover to allow direct convection of air inside and outside the motor, facilitating heat dissipation.