Basic knowledge of switched reluctance motor

Switched reluctance motor (SRM) is a typical mechatronics motor, also known as “switched reluctance motor drive system (SRD)”. Converter), rotor position sensor and controller are composed of four parts, as shown in Figure 1.
According to the excitation mode, switched reluctance motors are divided into two types: excitation type and permanent magnet type.

(1) Excitation type switched reluctance motor
① Structure.
a. Motor body. The motor body adopts the stator and rotor double salient pole structure, unilateral excitation, that is, only the stator salient pole adopts concentrated winding excitation, and the rotor salient pole has neither winding nor permanent magnet; the stator and rotor are both laminated by silicon steel sheets; The diametrically opposed poles of the stator winding are connected in series to form a phase. Its structure principle is shown as in Figure 2.

The structure of the rotor and stator of the switched reluctance motor is shown in Figure 3. The number of phases of the stator and rotor of the switched reluctance motor is different, and there are many combinations. The most common ones are three-phase 6/4-pole structure, three-phase 6/8-pole structure and three-phase 12/8-pole structure, as shown in Figure 4. .

The three-phase 6/4-pole structure shows that the motor stator has 6 salient poles and the rotor has 4 salient poles. The concentrated windings on the two symmetrical salient poles of the stator are connected in series to form a phase. There are no windings on the rotor, and 6 salient poles on the stator are called three-phase switched reluctance motors, and those with 8 salient poles on the stator are called four-phase switched reluctance motors. The more the number of phases, the smaller the step angle, the more stable the operation, the more conducive to reducing torque fluctuations, but the more complex the control, resulting in increased main switching devices and increased costs. The calculation method of the step angle is
Step angle (a)=360°×2/(number of stator poles? rotor pole number)
For example, a three-phase 6/8-pole motor has a step angle (a)=360°×2/(6×8)= 15°. Generally, a switched reluctance motor lower than a three-phase has no self-starting ability. At present, three-phase, four-phase and five-phase structures are mostly used.

Figure 5 (a), Figure 5 (b) and Figure 5 (c) are three-phase 6/4-pole structure, three-phase 12/8-pole (dual winding) structure and four-phase 8/6-pole structure of the switched reluctance A schematic cross-sectional view of the stator and rotor structure of the motor.

b. Rotor position sensor. The rotor position sensor has many kinds of Hall type, electromagnetic type, photoelectric type and magnetic sensitive type, and it is permanently installed at the non-output end of the motor, as shown in Figure 6.

The photoelectric position detector is composed of a gear plate and a photoelectric sensor. The cross-sectional shape of the toothed disk is the same as the cross-sectional shape of the rotor, which is installed on the rotor, and the photoelectric sensor is installed on the stator. When the gear plate rotates with the rotor, the photoelectric sensor detects the position signal of the rotor teeth.
The detection principle of the rotor position is shown in Figure 7. Among them, Figure 7 (a) is a four-phase 8/6 pole motor position detector, it only sets up S_p and S_Q two sensors, they are spaced apart by 15°, and there are six teeth spaced by 30° on the gear plate. Slot, the basic signal detected is shown in Figure 7(b).

The introduction of position sensors increases the complexity of the structure of the switched reluctance motor and affects its reliability. Therefore, people are working on sensorless solutions to obtain rotor position information by detecting phase inductance. This has been recognized as a very meaningful research. direction.
c. Power converter. The power converter of the switched reluctance motor provides electrical energy for the operation of the switched reluctance motor. In the cost of the entire switched reluctance motor system, the power converter occupies a large proportion. The reasonable selection and design of the power converter is to improve the switched reluctance motor. One of the keys to the performance-price ratio of the motor. The selection of the main circuit form of the power converter also has a direct impact on the design of the switched reluctance motor. The best combination plan should be comprehensively considered according to the specific performance and use place. There are many types of power converter main circuits commonly used in switched reluctance motors, and there are three most commonly used, as shown in Figure 8.

The main circuit shown in Figure 8(a) is a single power supply mode, with two main switching devices per phase, and the working principle is simple. There are three conduction modes: two main switches are turned on at the same time; one main switching device is turned on and the other switching device is turned off; two main switches are turned off at the same time. The rated power supply voltage that the main switch bears in this main circuit is U_d. It can be used in any number of phases, any power level, and has obvious advantages in high-voltage and high-power situations.
The characteristic of the main circuit shown in Figure 8(b) is that each phase must have two windings, one of which is connected in series with the switch tube, the other is connected in series with the freewheeling diode, and the two windings are fully coupled (usually double-stranded parallel winding). When working, the power supply supplies power to the windings through the switch tube. After the switch tube is turned off, the magnetic field energy storage is fed back to the power supply through the freewheeling diode. The maximum working voltage of the switch tube is 2U_d.
The main circuit shown in Figure 8(c) is a split-phase circuit, powered by a symmetrical power supply. There is only one main switch for each phase. The upper coil absorbs energy from the upper power supply and feeds the remaining energy back to the lower power supply; the lower coil absorbs energy from the lower power supply and feeds the remaining energy back to the upper power supply. Therefore, in order to ensure the balance of the upper and lower bridge arm voltages, this main circuit can only be used in even-numbered phase motors. The maximum reverse voltage of the main switch during normal operation is U_d. When each phase winding is turned on, the voltage across the winding is only U_d/2.
②Working principle. It can be seen from Figure 2 that when the A-phase winding current control switches K₁ and K₂ are closed, the A-phase is energized and excited, and the generated magnetic field tries to make the rotor rotate to the position where the rotor pole axis aa and the stator pole axis AA’ overlap, thereby generating reluctance Nature of electromagnetic torque. If the A, B, C, and D phase windings are energized in sequence, the rotor will continuously rotate in the counterclockwise direction; if the B, A, D, and C phase windings are energized in sequence, the rotor will rotate in the clockwise direction. In the actual operation of a multi-phase motor, two or more phases of windings are often turned on at the same time. When the stator winding wheel in a certain phase is energized once, the rotor rotates through a rotor pole pitch.

(2) Permanent magnet switched reluctance motor
High-performance neodymium iron boron permanent magnets are symmetrically embedded in the stator yoke of the excited switched reluctance motor. The magnetic field of the permanent magnet and the magnetic field of each phase winding form a new type of motor magnetic field, forming a permanent magnet switched reluctance motor (PMSRM). Its basic structure, magnetic flux and back electromotive force are shown in Figure 9.

The permanent magnet switched reluctance motor is also called a double salient permanent magnet motor, which can adopt cylindrical radial magnetic field structure, disc axial magnetic field structure and annular transverse magnetic field structure. The motor superimposes the permanent magnet torque on the basis of the reluctance torque. The existence of the permanent magnet torque helps to increase the power density of the motor and reduce the torque ripple, so as to facilitate its application in the electric vehicle drive system. It can accelerate the speed of winding commutation, reduce fluctuations, and improve energy utilization.
The control part of the permanent magnet switched reluctance motor is basically the same as the excited switched reluctance motor. Its working principle is: the magnetic flux generated by the permanent magnet is the largest when the salient poles are aligned, and the smallest when one salient pole is opposite to one pole slot. The magnetic flux generated by the stator winding is the same as the original one. After the two magnetic fluxes are superimposed, they act together on the rotor to drive the motor rotor to rotate. If the magnetic field effect of the permanent magnet is not considered when the stator winding is energized separately, it will bring greater reluctance to the winding magnetic flux loop and reduce the stator winding inductance, which also provides favorable conditions for rapid commutation between motor phases. Therefore, the improved permanent magnet switched reluctance motor has a smaller volume, higher efficiency and better stability. Currently, the double salient permanent magnet switched reluctance motor is a hot research topic, and it is usually made into an outer rotor type hub drive motor.

(3) Features of switched reluctance motor
① Advantages.
a. The system has a wide range of speed regulation, which can be operated at low speed or high speed (the maximum speed can reach 15000r/min).
b. The structure is simple, the rotor moment of inertia is small, the cost is low, and the dynamic response is fast.
c. The operating efficiency and reliability are better than induction motors and synchronous motors.
d. Low heat emission and strong chemical resistance. It can operate in an environment with poor heat dissipation and chemical pollution.
e. Low price, suitable for mass production.
② Disadvantages.
a. When the magnetic energy changes little, the efficiency will decrease and the noise will increase.
b. Compared with other types of motors, the inverter structure is more complex.

(4) Application of switched reluctance motor
Switched reluctance motors are suitable for use in low-cost, low-speed and small-sized electric vehicles. At present, due to many shortcomings, they are rarely used in electric vehicles. But after the permanent magnet switched reluctance motor is made into an outer rotor motor, it is often used in the wheel hub drive system of electric vehicles.