What is an electric motor?
An electric motor (English: Electric machinery, commonly known as a "motor") is an electromagnetic device that converts or transmits electrical energy based on the law of electromagnetic induction.
In circuits, an electric motor is represented by the letter M (D in the old standard). Its main function is to generate driving torque and serve as a power source for electrical appliances or various machines. A generator is represented by the letter G in circuits. Its main function is to convert electrical energy into mechanical energy.
Motor Classification and Classification
1. According to the type of power supply: motors can be divided into DC motors and AC motors.
1) DC motors can be classified into brushless DC motors and brushed DC motors according to their structure and working principle.
Brushed DC motors can be classified into two types: permanent magnet DC motors and electromagnetic DC motors.
Electromagnetic DC motors can be classified into: series-wound DC motors, shunt-wound DC motors, separately excited DC motors, and compound-wound DC motors.
Permanent magnet DC motors can be classified into three types: rare earth permanent magnet DC motors, ferrite permanent magnet DC motors, and AlNiCo permanent magnet DC motors.
2) AC motors can be further divided into single-phase motors and three-phase motors.
2. According to structure and working principle, motors can be divided into DC motors, asynchronous motors, and synchronous motors.
1) Synchronous motors can be classified as: permanent magnet synchronous motors, reluctance synchronous motors, and hysteresis synchronous motors.
2) Asynchronous motors can be divided into: induction motors and AC commutator motors.
Induction motors can be classified as: three-phase asynchronous motors, single-phase asynchronous motors, and shaded-pole asynchronous motors, etc.
AC commutator motors can be classified as: single-phase series motors, AC/DC universal motors, and repulsion motors.
3. According to the starting and running methods, single-phase asynchronous motors can be divided into: capacitor-start single-phase asynchronous motors, capacitor-run single-phase asynchronous motors, capacitor-start-run single-phase asynchronous motors, and split-phase single-phase asynchronous motors.
4. According to their purpose, electric motors can be divided into drive motors and control motors.
1) Electric motors for driving can be divided into: electric motors for power tools (including tools for drilling, polishing, grinding, grooving, cutting, and reaming), electric motors for household appliances (including washing machines, electric fans, refrigerators, air conditioners, tape recorders, video recorders, DVD players, vacuum cleaners, cameras, hair dryers, electric shavers, etc.), and electric motors for other general-purpose small mechanical equipment (including various small machine tools, small machinery, medical devices, electronic instruments, etc.).
2) Control motors are further divided into stepper motors and servo motors, etc.
5. According to the rotor structure, induction motors can be divided into: squirrel-cage induction motors (formerly known as squirrel-cage asynchronous motors) and wound-rotor induction motors (formerly known as wound-rotor asynchronous motors).
6. According to operating speed, motors can be classified as: high-speed motors, low-speed motors, constant-speed motors, and variable-speed motors. Low-speed motors are further divided into geared motors, electromagnetic geared motors, torque motors, and claw-pole synchronous motors, etc.
In addition to being classified into stepped constant speed motors, stepless constant speed motors, stepped variable speed motors, and stepless variable speed motors, speed-regulating motors can also be classified into electromagnetic speed-regulating motors, DC speed-regulating motors, PWM frequency conversion speed-regulating motors, and switched reluctance speed-regulating motors.
The rotor speed of an asynchronous motor is always slightly lower than the synchronous speed of the rotating magnetic field.
The rotor speed of a synchronous motor remains constant regardless of the load.
What is an electric motor?
An electric motor is a device that converts electrical energy into mechanical energy. It utilizes a rotating magnetic field generated by a current-carrying coil (stator winding) that acts on a rotor (such as a squirrel-cage closed aluminum frame) to create magnetoelectric torque. Electric motors are classified into DC motors and AC motors based on the power source they use. Most motors in power systems are AC motors, which can be synchronous or asynchronous (the stator magnetic field speed and rotor rotation speed are not synchronized). An electric motor mainly consists of a stator and a rotor. The direction of the force on a current-carrying conductor in a magnetic field depends on the direction of the current and the direction of the magnetic field lines. The working principle of an electric motor is that the magnetic field exerts a force on the current, causing the motor to rotate.
Basic structure of electric motor
I. The structure of a three-phase asynchronous motor consists of a stator, a rotor, and other accessories.
(a) Stator (stationary part)
1. Stator core construction
Used as part of the motor's magnetic circuit, on which the stator windings are placed.
Structure: The stator core is generally made of silicon steel sheets with an insulating layer on the surface, which are stamped and stacked. The inner circle of the core has evenly distributed slots for embedding the stator winding.
The stator core slot types include the following:
Semi-closed slots: These motors have higher efficiency and power factor, but winding and insulation are more difficult. They are generally used in small, low-voltage motors.
Semi-open slots: These slots can accommodate pre-formed windings and are generally used in large and medium-sized low-voltage motors. Pre-formed windings refer to windings that have been pre-insulated before being placed into the slot.
Open slots: used to embed shaped windings, providing convenient insulation methods, and are mainly used in high-voltage motors.
2. Stator winding
Function: It is the circuit part of the electric motor. When three-phase alternating current is applied, it generates a rotating magnetic field.
Construction: It consists of three identical windings arranged symmetrically in space, spaced 120° apart electrically. The coils of these windings are respectively embedded in the slots of the stator according to a certain pattern.
The main insulation items of the stator winding are as follows: (ensuring reliable insulation between each conductive part of the winding and the iron core, as well as reliable insulation between the windings themselves).
1. Ground insulation: Insulation between the stator winding as a whole and the stator core.
2. Phase-to-phase insulation: Insulation between stator windings of each phase.
3. Inter-phase insulation: Insulation between turns of each phase stator winding.
Wiring inside the motor junction box:
Every motor junction box contains a terminal block. The six wire ends of the three-phase windings are arranged in two rows, top and bottom. The top three terminals are numbered 1 (U1), 2 (V1), and 3 (W1) from left to right, and the bottom three terminals are numbered 6 (W2), 4 (U2), and 5 (V2) from left to right. The three-phase windings are connected in either a star or delta configuration. This numbering should be followed during manufacturing and maintenance.
3. Base
Function: To fix the stator core and the front and rear end covers to support the rotor, and to provide protection and heat dissipation.
Construction: The frame is usually made of cast iron. The frame of large asynchronous motors is generally welded from steel plates, while the frame of micro motors is made of cast aluminum. The frame of enclosed motors has heat dissipation fins on the outside to increase the heat dissipation area, and the frame of protective motors has ventilation holes at both ends of the end cover to allow direct air convection between the inside and outside of the motor, which is conducive to heat dissipation.
(ii) Rotor (rotating part)
1. Rotor core of a three-phase asynchronous motor:
Function: As part of the motor's magnetic circuit and to house the rotor windings within the iron core slots.
Construction: The materials used are the same as the stator, made of 0.5 mm thick silicon steel sheets, punched and stacked. The outer circumference of the silicon steel sheets has evenly distributed holes punched to house the rotor windings. The rotor core is usually made by punching the inner circumference of the silicon steel sheets after the stator core has been punched. Generally, the rotor core of small asynchronous motors is directly pressed onto the shaft, while the rotor core of large and medium-sized asynchronous motors (rotor diameter of 300-400 mm or more) is pressed onto the shaft with the help of a rotor support.
2. Rotor windings of a three-phase asynchronous motor
Function: It cuts the rotating magnetic field of the stator to generate induced electromotive force and current, and forms electromagnetic torque to make the motor rotate.
Structure: It is divided into squirrel cage rotor and wound rotor.
1. Squirrel-cage rotor: The rotor winding consists of multiple conductor bars inserted into the rotor slots and two annular end rings. If the rotor core is removed, the entire winding resembles a squirrel cage, hence the name squirrel-cage winding. Small squirrel-cage motors use cast aluminum rotor windings, while motors above 100kW use copper bars and copper end rings welded together.
2. Wound rotor: The winding of the wound rotor is similar to that of the stator winding. It is also a symmetrical three-phase winding, usually connected in a star configuration. The three leads are connected to the three current collectors on the shaft and then connected to the external circuit through brushes.
Features: Due to its more complex structure, wound-rotor motors are not as widely used as squirrel-cage motors. However, by using slip rings and brushes to connect additional resistors and other components in series in the rotor winding circuit, the starting, braking, and speed regulation performance of asynchronous motors can be improved. Therefore, they are used in equipment requiring smooth speed regulation within a certain range, such as cranes, elevators, and air compressors.
(iii) Other accessories for three-phase asynchronous motors
1. End cap: provides support.
2. Bearing: Connects the rotating part to the stationary part.
3. Bearing end cap: Protects the bearing.
4. Fan: Cools the electric motor.
II. The DC motor adopts an octagonal fully laminated structure with series winding, suitable for automatic control technologies requiring forward and reverse rotation. It can also be manufactured with series winding according to user needs. Motors with a center height of 100–280 mm have no compensating winding, but motors with a center height of 250 mm and 280 mm can be manufactured with compensating winding depending on specific circumstances and needs. Motors with a center height of 315–450 mm have compensating winding. The external dimensions and technical requirements of motors with a center height of 500–710 mm conform to IEC international standards, and the mechanical dimensional tolerances of the motor conform to ISO international standards.
Cooling method
1) Cooling: When a motor converts energy, a small portion of it is lost and converted into heat. This heat must be dissipated through the motor casing and the surrounding medium. This process of dissipating heat is called cooling.
2) Cooling medium: a gaseous or liquid medium that transfers heat.
3) Primary cooling medium: a gaseous or liquid medium with a temperature lower than that of a certain component of the motor. It comes into contact with that component of the motor and carries away the heat it releases.
4) Secondary cooling medium: a gaseous or liquid medium with a temperature lower than that of the primary cooling medium, which carries away the heat released by the primary cooling medium through the outer surface of the motor or the cooler.
5) Final cooling medium: Heat is transferred to the final cooling medium.
6) Ambient cooling medium: The gaseous or liquid medium in the environment surrounding the motor.
7) Remote medium: A medium that is far from the motor, which draws in heat from the motor and discharges cooling medium to a distant location through inlet and outlet pipes or channels.
8) Cooler: A device that transfers heat from one cooling medium to another while keeping the two cooling media separate.
Is there a difference between an electric motor and a motor?
Electric motors include both electric motors and generators. "Electric motor" is a general term encompassing both, but the two are conceptually distinct.
An electric motor is just one way a motor operates. When a motor operates in electric mode, it converts electrical energy into other forms of energy. Another way a motor operates is as a generator, in which it operates in generator mode, converting other forms of energy into electrical energy.
However, some motors, such as synchronous motors, are often used as generators, but can also be used directly as electric motors. Asynchronous motors are more often used as electric motors, but with the addition of simple external components, they can also be used as generators.
