①Working principle of AC induction motor
AC Induction Motor is also called asynchronous motor. Compared with DC motor, its structure is simple. From the technical level, induction motor drive system is an ideal choice for electric car motor drive system, especially when the power demand of the drive system is relatively high. Big large electric passenger car. At present, high-performance induction motor drive systems in various countries also mainly adopt two control methods, vector control and direct torque control.
The stator of an AC induction motor is used to generate a rotating magnetic field. It consists of a stator core, a stator winding, a shell outside the core, and a bearing that supports the rotor shaft. There are two types of rotor windings for an AC induction motor: cage and winding. Line type, in which the cage rotor winding structure is relatively simple, only composed of guide bars and end rings. Induction motors can also be divided into two categories according to the rotor winding structure: cage induction motors and wound induction motors, as shown in Figure 1.
The AC induction motor is made according to the principle of electromagnetic induction. When the U-shaped magnet rotates counterclockwise at a speed of n1, the wire in the coil will cut the lines of magnetic force, thereby generating induced electromotive force e.
e = Blν
In the formula, B is the magnetic induction intensity; l is the conductor length; ν is the cutting speed of the coil.
The direction of the induced electromotive force satisfies the right-hand rule, as shown by the arrow in Figure 2. Because the coil is a closed conductor, an induced current is generated. The direction of the current is shown by the arrow. The charged conductor in the magnetic field will be affected by the electromagnetic field F, and F=Bil, the direction meets the left-hand rule, the direction is shown in Figure 2. Under the action of electromagnetic force F, the coil will also rotate counterclockwise, and the magnetic field The direction of rotation is the same, the speed is n, and n<n1.
The rotating magnetic field of an AC induction motor is generated by a three-phase symmetrical alternating current that is passed into the stator winding. The equation of the current flowing into the three-phase stator winding is:
iA=Imsinωt
iB=Imsin(ωt﹣120°)
iC=Imsin(ωt＋120°)
The waveform is shown in Figure 3.
When ωt=0, there is no current in the AX coil, the current in the BY coil flows out from the B terminal and the Y terminal flows in; the current in the CZ coil flows out from the Z terminal and the C terminal flows in. According to the right-hand spiral rule, the position of the synthesized magnetic field, namely the position of the N pole and the S pole, is shown in Figure 4(a), and the positions of the synthesized magnetic field of ωt=60°, 120°, and 180° are shown in Fig. 4(b)~ (D) Shown.
A three-phase AC power supply is added to the stator winding to produce a clockwise rotating magnetic field. This magnetic field generates magnetic flux in the stator core. The rotor winding induces electromotive force due to the cutting of this magnetic field, which will induce induction in the closed rotor winding. The current flows. When the phase sequence is changed, the direction of the motor will be reversed. The speed of this rotating magnetic field is also called the synchronous speed, and its value is:
ns=60fs/Pn
In the formula, fs——the frequency of the stator current, Hz;
Pn——The number of pole pairs of the motor.
In AC induction motors, there is a very important physical quantity called slip, which is defined as the difference between the synchronous speed (ns) of the rotating magnetic field and the rotor speed (n), expressed by s, and its value is
s=[(ns﹣n)/ns]×100%
Rotor speed can be expressed as: n-(1-s)ns.
②Characteristics of AC induction motor drive system
Compared with DC motors, AC induction motors have many advantages such as high efficiency, simple structure, solid and reliable, maintenance-free, small size, light weight, easy cooling, and long life. The induction motor itself has a lower cost than a DC motor, but its inverter is more costly than a DC motor controller, but with the continuous advancement of power electronics technology, the cost gap between the two is getting closer and closer. From the current point of view, the total cost of an induction motor AC system is higher than that of a DC motor drive system. However, due to its light weight, high efficiency and effective regenerative braking, the operating cost of using an electric vehicle is higher than that of using a DC motor. The driving system is low in time, especially in high-power electric vehicles.
At present, the vector control theory is relatively complete and mature. The parameters of the motor can be accurately identified online, and the control performance is very superior. At present, due to the increasing computing power of microprocessors, the real-time performance of processing complex algorithms is also guaranteed. Many companies have introduced various types of vector control-based controllers to the market, and the control performance has basically met the power requirements of electric vehicles. .