Basic knowledge of induction motors
The induction motor is a type of motor powered by 380V three-phase alternating current (phase difference 120°) at the same time. Three-phase AC induction motors are also called three-phase asynchronous motors.
(1) Structure
Although there are many types of three-phase asynchronous motors, the basic structure of all types of three-phase asynchronous motors is the same. They are all composed of two basic parts, the stator and the rotor, and there is a certain air gap between the stator and the rotor. In addition, there are end covers, bearings, fans, fan covers, junction boxes, lifting rings and other accessories, as shown in Figure 1.
① Stator. The stator is used to generate a rotating magnetic field. The stator of a three-phase asynchronous motor is composed of a shell, a stator core, and a stator winding.
a. Shell. The shell is an important part of the mechanical structure of the three-phase asynchronous motor. It consists of an end cover, a bearing, a junction box and a lifting ring. Usually, the outer surface of the shell is cast with heat sinks to expand the heat dissipation area, which is conducive to the heat dissipation of the motor, thereby reducing the insulation level and manufacturing cost. The bearing cover is made of cast iron or cast steel. Its function is to prevent the rotor from having excessive axial movement. In addition, it also serves to store grease and protect the bearing to prevent dust or dirt from entering the bearing. Accelerate the wear of the box bearing, thereby prolonging the service life of the motor. The junction box is generally made of cast iron, and its function is to protect and fix the lead-out terminal of the winding. The lifting ring is generally made of cast steel and is installed on the upper end of the machine base to lift and carry the motor.
b. The stator core. The stator core of a three-phase asynchronous motor is a part of the magnetic circuit of the motor. It is made of laminated silicon steel sheets coated with insulating paint on the surface of 0.35-0.5mm thick, as shown in Figure 2. Since the silicon steel sheet is thin and the sheet is insulated, the core eddy current loss caused by the passage of alternating magnetic flux is reduced. The inner circle of the iron core has evenly distributed notches, which are used to embed the stator windings.
c. Stator winding. The stator winding is the circuit part of a three-phase asynchronous motor. The three-phase asynchronous motor has a three-phase winding. When a three-phase symmetrical alternating current is applied, a rotating magnetic field will be generated.
The three-phase winding is composed of three independent windings, and each winding is connected by several coils. Each winding is called a phase, and the three windings are spaced apart by 120°. The coil is wound by insulated copper wire or insulated aluminum wire. Small and medium-sized three-phase asynchronous motors mostly use round enameled wire, while the stator coils of large and medium-sized three-phase asynchronous motors are wound with large-section insulated rectangular copper wire or aluminum wire, and then embedded in the stator core slot according to a certain rule.
The stator three-phase winding has two connection modes: star (also called Y-shaped) and delta, as shown in Figure 3.
The six outlet ends of the stator three-phase windings are all led to the junction box, the head ends are marked as U1, V1, W1, and the ends are marked as U1, V2, and W2. The arrangement of the six outlet terminals in the junction box is shown in Figure 4.
② Rotor Three-phase asynchronous electronic rotors are divided into two types: winding type and cage type. The corresponding motors are called winding asynchronous motors and cage asynchronous motors, (commonly known as squirrel cage motors).
a. Rotor of wound asynchronous motor The rotor of wound asynchronous motor is laminated with 0.5mm thick silicon steel sheet, which is sleeved on the shaft. On the one hand, it serves as a part of the motor magnetic circuit and on the other hand is used to place the rotor windings. . The wound asynchronous motor, like the stator winding, also has a three-wire winding, which is generally connected in a star shape with three-phase lead wires, respectively connected to the three collector rings on the rotating shaft insulated from the rotating shaft, and connected to the external circuit through a brush device , It is possible to connect resistors in series in the rotor circuit to improve the running performance of the motor. As shown in Figure 5.
b. Rotor of cage asynchronous motor. Insert a copper bar (called a guide bar) into each slot of the rotor core, and use a copper ring (called an end ring) at each end of the copper bar to connect the copper bars, which is called a copper row rotor, as shown in the figure As shown in 6; casting method can also be used to cast the rotor guide bar and end ring fan blades with aluminum liquid at one time, which is called a cast aluminum rotor, as shown in Figure 6, an asynchronous motor below 100kW Generally, cast aluminum rotors are used.
③Other parts. Other parts of the three-phase asynchronous motor include bearings, fans, etc. The fan is used to ventilate and cool the motor. The air gap between the stator and the rotor of a three-phase asynchronous motor is generally only 0.2~1.5mm, and the air gap cannot be too large. When the air gap is large, the air gap torque generated is small, which will reduce the power factor of the motor during operation; It should not be too small. If the air gap is too small, it will cause assembly difficulties. If there are foreign objects or the rotating shaft moves in the radial direction, it is easy to jam, the operation is unreliable, and the higher harmonic magnetic field increases, causing additional losses and poor starting performance.
(2) Working principle of three-phase asynchronous motor
Take the star connection of a pair of magnetic poles (P=1) of the stator three-phase winding as an example, as shown in Figure 7. The head ends U1, V1, and W1 of the three-phase winding are respectively connected to the phase lines A, B, and C of the three-phase alternating current. For the convenience of discussion, it is selected that the alternating current flows in from the head end of the winding during the positive half cycle and flows out from the end; on the contrary, during the negative half cycle, the current flows in the opposite direction. The stator windings synthesize a rotating magnetic field when the three-phase alternating current has different phases. When ωt=0°, the A-phase current is zero; the B-phase current is negative, and the current flows in from the V2 terminal and flows out from the V1 terminal; the C-phase current is positive, and the current flows in from the W1 terminal and flows out from the W2 terminal. According to the right-handed spiral law, the direction of the composite magnetic field generated by the stator three-phase winding current can be determined at this time. When ωt=90°, the current of phase A is positive, the current flows in from U1 and flows out from U2; the phase B current is negative, and the current flows in from V2 and flows out from V1; the current of phase C is negative. The current flows in from the W2 end and flows out from the W2 end. At this moment, the synthesized magnetic field has rotated 90° in space in the clockwise direction. In the same way, the synthetic magnetic field directions generated by the stator three-phase winding currents when ωt=180°, ωt=270°, and ωt=360° can be obtained respectively. Among them, the synthetic magnetic field direction when ωt=360° and ωt=0° same.
It can be seen that the current changes one cycle, and the resultant magnetic field also rotates one circle in space. The current continues to change, and the magnetic field continues to rotate. It can be seen from the above analysis that the composite magnetic field generated by the three-phase current passing through the stator windings is a magnetic field that rotates in space with the alternating current of the current. This rotating magnetic field plays the same role as the hoof magnet rotating in space.
In an AC asynchronous motor, a rotating magnetic field will be generated when the stator windings flow through three-phase alternating current with a phase angle of 120° in sequence. The rotating mechanical field generates an induced electromotive force in the rotor group. Because the winding is a closed loop, an induced current will be generated. The current winding conductor generates electromagnetic force in the rotating magnetic field, and forms an electromagnetic torque on the rotating shaft to drive the rotating shaft to rotate.
Since the rotor of a three-phase asynchronous motor generates induced electromotive force after the stator is energized, and is then dragged to rotate under the action of electromagnetic force, the speed of the rotor is always lower than that of the rotating magnetic field of the stator (the difference between the two speeds is called rotation The difference, expressed by the slip law, is usually within 10%), which is also the origin of the asynchronous motor.
The speed of the stator rotating magnetic field of the three-phase asynchronous electronic motor is called the synchronous speed, and the speed of the rotor is
n=(60f/P)(1－s)
Where: n—rotor speed, that is, motor speed, r/min;
f—Alternating current frequency, Hz;
P-the number of pole pairs;
S-slip rate, %.
In our country, the AC power frequency is 50Hz, so the synchronous speed of a three-pole motor is 3000r/min, and the synchronous speed of a four-pole motor is 1500r/min. Due to the slip rate, the actual speed of the three-phase asynchronous motor will be lower than the above-mentioned simultaneous rotation. For example, the synchronous speed of a large pole motor is 1000r/min, and its actual speed is generally 960r/min.
(3) Three-phase asynchronous motor model
The model (specification code) of the three-phase asynchronous motor is mainly composed of the center height, the length of the frame, the number of poles, and the power.
①The center height refers to the height from the motor shaft center to the bottom corner surface of the base. According to this, the motor can be divided into four types: large, medium, small and micro. Those with a center height of 45~71mm belong to micro motors; those with a center height of 80~315mm belong to small motors; those with a center height of 355~630mm belong to medium-sized motors; those above 630mm belong to large motors.
②The length of the base is indicated by capital English letters. S means short; M means medium; L means long. This letter in some motor models will have a subscript, such as 1, 2, etc., indicating the difference in power in the same series. The larger the number, the greater the power.
③The number of poles is expressed by Arabic numerals, for example, 2 means 2 magnetic poles.
④The power is expressed by Arabic numerals, for example, 2.3 means that the power is 2.3kW.
For example, the motor model Y100L2-4-3, the meaning of each code: Y stands for three-phase asynchronous motor; the number 100 means the center height of the motor is 100mm; L2 means the length of the base is long and the power is large; the number 4 stands for 4 Magnetic poles; the number 3 means that the rated power of the motor is 3kW.
(4) Features and applications of three-phase asynchronous motors
① Features.
a. Advantages. The rotor structure is simple and sturdy, easy to achieve high speed, small size and light weight; relatively high efficiency can be obtained, and the simultaneous use of field weakening control and maximum efficiency control can achieve the purpose of high efficiency; low price and good reliability.
b. Disadvantages. Because the excitation current is necessary, it will cause the deterioration of the power factor, especially in the low-speed area, the deterioration of the factor and efficiency is more serious; in addition, the torque control of this kind of motor is more difficult.
② Application. The three-phase asynchronous motor, as the driving motor of the automobile, is small and lightweight. The advantage of the asynchronous motor has been recognized, so it has applications in many electric vehicles. Electric vehicles such as Nissan March EV, Ford ETX-1, Toyota TownAce EV, and Mitsubishi RebelEV all use asynchronous motors as drive motors.
