With the development of modern high-power semiconductor switching devices and modern control technology, the switch reluctance motor (SR motor) drive technology with high efficiency, high reliability and a new type of mechatronics, with soft The appearance of speed-regulating transmission technology with start-up characteristics has appeared, and currently it has more applications in the field of electric vehicle driving.
1. Working principle of switched reluctance motor
The SR motor has a simple structure. It is a double salient pole structure in which the stator and rotor are laminated by ordinary silicon steel sheets. There is no winding in the rotor, and the stator is equipped with a simple concentrated winding. Generally, two diametrically opposed windings are connected in series to form a phase. SR motors can be designed in a variety of different phase structures, and there are many different combinations of stator and rotor poles, such as three-phase 6/4 structure, four-phase 8/6 structure, etc. The air gap magnetic field of SR motor has three types: radial magnetic field, axial magnetic field and mixed magnetic field.
The switched reluctance motor works according to the principle of magnetic field force. The four-phase (8/6) SR motor shown in Figure 1 is composed of an 8-pole stator with windings and a 6-pole rotor without windings. Each stator pole is energized with direct current to form a magnetic field. The magnetic field causes the corresponding pair of rotor magnetic poles to rotate and coincide with the stator magnetic pole centerline. The operating principle of the SR motor follows the “principle of minimum reluctance”. When the core moves to the minimum reluctance position, its main axis must coincide with the axis of the magnetic field. When the stator D-D’ poles are excited, the generated magnetic force causes the rotor to rotate to the rotor axis 1-1, which coincides with the stator pole axis D-D’, and maximizes the inductance of the D-phase excitation group. →A→B→C phase winding is energized, the rotor will continue to rotate in the counterclockwise direction according to the excitation sequence; on the contrary, if B→A→D→C phase is energized in sequence, the motor will rotate in the clockwise direction. By controlling the amplitude and width of the current pulse added to the windings of the SRD motor and its relative position to the rotor, the magnitude and direction of the torque of the SR motor can be controlled.
2. Mathematical model of switched reluctance motor
Unlike traditional AC motors, SR motors use a salient pole core structure, and only install each phase field winding on the stator. The non-sinusoidal winding current and the high saturation of the magnetic core flux density are two characteristics of SR motor operation. According to the basic law of the circuit, write the voltage balance equation of the main electrical circuit including each phase circuit. Each phase of the motor requires an equation. The voltage balance equation of the k-th phase of the motor is:
Uk=Rkik+(dψk/dt)
In the formula, Uk——the voltage applied to the k-phase winding;
Rk-the resistance of the k-phase winding;
ik——-k-phase winding current;
ψk — the flux linkage of the k-phase winding.
ψk is a function of winding current ik and rotor displacement angle θ, namely
Ψk=ψk(ik,θ)
The flux linkage of the motor can be represented by the product of inductance and current, namely
Ψk=ψk(θk,ik)ik
Set dynamic inductance Lk (θ, ik) = αψk/αik
Figure 2 is a simplified one-phase model of a switched reluctance motor. The torque of an SR motor is generated by the tendency of the magnetic circuit to select the smallest reluctance structure. Appropriate saturation helps to improve the overall performance of the SR. Therefore, the saturation of the magnetic circuit of the motor is Another important feature of SR motors. Due to the non-linearity of the magnetic circuit of the motor, usually the torque of the SR motor should be calculated based on the magnetic co-energy, namely
T(θ,i)=(1/2)i²(αL/αθ)=(1/2)i²(dL/dθ)
The direction of the torque of the motor is not affected by the direction of the current, but only depends on the change of the inductance with the angle of rotation. If dL/dθ>0, current flows through the phase windings, producing electric torque; dL/dθ<0, when the direction of current flowing does not change, braking torque will be produced. Therefore, the magnitude and direction of the motor torque can be controlled by controlling the amplitude, width and relative position of the current pulse added to the motor windings and the rotor. When the torque direction is unchanged, the average torque can be adjusted by adjusting the current. There are usually two methods for current regulation: PWM chopping control during the winding conduction period; and changing the conduction angle of the winding.
3. Electric vehicle SR motor control system
Figure 3 is a common power converter main circuit of a four-phase SR motor. In the figure, A, B, C, and D are the motor phase windings, Sa~Sd are the main switching devices of each corresponding phase, and VDa~VDd are corresponding Freewheeling diode.
The switch reluctance motor speed control system is mainly composed of SR motor, power converter, controller, and position sensor, as shown in Figure 4.
The power converter is to provide the energy required for the operation of the SR motor. It is powered by the DC power obtained after the rectification of the battery or the AC power. The winding current of the SR motor is unidirectional. The controller comprehensively processes the feedback signals of instructions, speed, current and position sensors, controls the working state of the power device, and realizes the state control of the SR.
The operating characteristics of SR motors can be divided into three areas: constant torque area, constant power area, and natural characteristics area. In the constant torque area, because the motor speed is low and the motor back electromotive force is small, the current needs to be chopped and limited, which is called the current chopping control method. In the constant power zone, the constant power characteristic is obtained by adjusting the turn-on and turn-off angles of the main switch tube, which is called the angle position control method. In the natural characteristic area, the power supply voltage, turn-on angle and turn-off angle are all fixed. Since the natural characteristic is similar to that of a series-excited DC motor, it is also called the series-excited characteristic area, as shown in Figure 5.
4. Features of Switched Reluctance Motor Drive System
(1) Advantages of switched reluctance motor
①The motor rotor has no windings, low cost, no obvious heat generation, extended bearing life, low inertia of the salient pole rotor, and easy speed regulation and control;
②The integral coil of the stator is easy and firm to embed, and most of the heat consumption is concentrated in the stator, which is easy to cool. The rotor has no permanent magnets and can have a higher maximum allowable temperature rise;
③The advantages of small starting current and large starting torque are also obvious in the low-speed operation section, which is very suitable for machinery that needs frequent starting and long-time low-speed and heavy-load operation.
(2) Disadvantages of switched reluctance motor
①The energy conversion density is lower than that of electromagnetic motors;
②The torque ripple is large, the noise caused by the torque ripple and the resonance problem at a specific frequency are also more prominent;
③The more the number of phases, the more the number of main wiring, and the more complicated the main circuit