What is a Switched Reluctance Motor?

Catalog

 I. Working Principle of Switched Reluctance Motor

The switched reluctance motor (SRM) used in the switched reluctance motor speed control system is the part that realizes the electromechanical energy conversion in the SRD, and it is also the main sign that SRD is different from other motors drive systems. SRM series double salient pole variable reluctance motor, the salient poles of the stator and rotor are laminated by ordinary silicon steel sheets. The rotor has neither windings nor permanent magnets. Concentrated windings are wound on the stator poles. The two diametrically opposed windings are connected together, which is called "one-phase". SR motors can be designed with a variety of different phase numbers. The stator and rotor There are many different combinations of poles. A large number of phases and small step angle is beneficial to reduce torque ripple, but the structure is complex, and the main switching devices are large, and the cost is high. Nowadays, four-phase (8/6) structure and six-phase (12/8) are more commonly used) structures. 

The figure shows the schematic diagram of a four-phase (8/6) structure SR motor. For the sake of simplicity, only the A-phase winding and its power supply circuit are shown in the figure. The operating principle of the SR motor follows the "principle of minimum reluctance"-'The magnetic flux always closes along the path of the least reluctance, and when the iron core with a certain shape moves to the minimum reluctance position, it must make its main axis and the magnetic field. The axes coincide. In the figure, when the stator D-D' poles are excited, 1-1' rotates to the position where the stator axis D-D' overlaps, and maximizes the inductance of the D-phase excitation winding. If the relative positions of the stator and the rotor in the figure are taken as the starting position, the D→A→B→C phase windings are energized in sequence, and the rotor will continue to rotate counterclockwise against the excitation sequence; on the contrary, if it is given in sequence When phase B→A→D→C is energized, the motor will rotate in a clockwise direction. It can be seen that the direction of the SR motor has nothing to do with the current direction of the phase windings, but only depends on the order in which the phase windings are energized. In addition, it can be seen from Figure 2 that when the main switching devices S1 and S2 are turned on, the A-phase winding absorbs electric energy from the DC power supply US, and when S1 and S2 are turned off, the winding current continues to flow through the freewheeling diodes VD1 and VD2. And feed back to the power source US. Therefore, the common feature of SR motor drive is that it has a regenerative effect and high system efficiency.

It can be seen that by controlling the amplitude and width of the current pulse added to the SR motor winding and its relative position with the rotor (ie conduction angle, turn-off angle), the magnitude and direction of the SR motor torque can be controlled. It is the basic principle of SR motor speed control.

II. Advantages and Disadvantages of Switched Reluctance Motor

Advantages

1. The structure is simple; the motor has a simple structure, low cost, and can be used for high-speed operation. The structure of SRM is simpler than that of squirrel cage induction motors. The rotor has extremely high mechanical strength and can be used for ultra-high-speed operation (such as tens of thousands of revolutions per minute). As for the stator, it has only a few concentrated windings, so it is easy to manufacture and simple in insulation structure.

2. The circuit is reliable; the power circuit is simple and reliable. Because the motor torque direction has nothing to do with the winding current direction, that is, only a single-phase winding current is needed, so the power circuit can achieve one power switch per phase. In contrast, the windings of asynchronous motors need to flow bidirectional currents, and the power circuit of the PWM inverter that supplies power to them requires two power devices for each phase. Therefore, the switched reluctance motor speed control system requires fewer power components than the PWM frequency converter power circuit, and the circuit structure is simple. In addition, two power switch tubes in each bridge arm of the PWM inverter power circuit are directly across the DC power supply side, which is prone to direct short-circuit and burn power devices. In the switched reluctance motor speed control system, each power switch device is directly connected in series with the motor winding, which fundamentally avoids the phenomenon of through short circuit. Therefore, the protection circuit of the power circuit in the speed control system of the switched reluctance motor can be simplified, which reduces the cost and has higher reliability.

3. System reliability; system reliability is high. From the perspective of the electromagnetic structure of the motor, each winding and magnetic circuit are independent of each other, and each generates electromagnetic torque within a certain axis angle range. Unlike in general motors, a rotating magnetic field must be generated under the combined action of each phase winding and magnetic circuit, so that the motor can operate normally. From the point of view of the control structure, each phase circuit supplies power to a phase winding and generally works independently of each other.

4. Starting advantages: large starting torque and low starting current. The controller outputs less current and the motor gets a larger starting torque is a major feature of this system. The data of a typical product is: when the starting current is 15% of the rated current, the starting torque is 100% rated torque; when the starting current is 30% of the rated current, the starting torque can reach 250% of its rated torque. Compared with the starting characteristics of other speed control systems, for example, the DC motor uses 100% current, and the squirrel cage induction motor uses 300% current to obtain 100% torque. The advantages of small starting current and large torque can also be extended to the low-speed operation section, so this system is very suitable for those machines that require heavy-load starting and long-time low-speed and heavy-load operation.

5. Frequent start and stop; suitable for frequent start and stop and forward and reverse conversion operation. This system has the characteristics of high starting torque and low starting current, so that the current impact is small during the starting process, and the motor heating is smaller than that during continuous rated operation. It is suitable for frequent start-stop and forward and reverse conversion operations.

6. Good performance; good speed regulation performance. There are at least four main operating parameters and common methods for controlling switched reluctance motors: phase conduction angle, related break angle, phase current amplitude, phase winding voltage, which is flexible and convenient. According to the operating requirements of the motor and the condition of the motor, different control methods and parameter values can be adopted to make it run in the best state (such as the largest output, the highest efficiency, etc.), and it can also achieve various functions. Specific curve. For example, the motor has exactly the same four-quadrant running capacity and has the highest starting torque and the load capacity curve of the series-excited motor. It can be used for speed closed-loop control, and it can be very convenient to form a non-static speed control system.

7. High efficiency; Switched reluctance motor system is an efficient speed regulation system. Take the 3kW SRM system as an example, the system efficiency can reach 87% in a wide range.

Disadvantages

Large torque ripple, noise, and vibration: From the working principle, the torque generated on the rotor of the switched reluctance motor is superimposed by a series of pulse torques, which affects the performance of the SRM.

III. Development of Switched Reluctance Motor

As a new type of speed-regulating drive system, switched reluctance motors are increasingly recognized and applied by people because of their simple structure, low cost, high efficiency, excellent speed regulation performance, and flexible controllability. It has been successfully used in many fields such as electric vehicle drive systems, household appliances, industrial applications, servo systems, high-speed drives, aerospace, etc., becoming AC motor speed control systems, DC motor speed control systems, and brushless DC motor speed control systems A strong competitor.

Nowadays, with the rapid development of power electronics technology, digital signal processing technology, and control technology, and with the continuous maturity of intelligent technology and the emergence of high-speed, high-efficiency, and low-cost digital signal processing chips (DSP), various complex algorithms are developed using high-performance DSP The indirect position detection technology without additional external hardware circuit greatly improves the reliability and applicability of switched reluctance motor detection, and will definitely show the superiority of SRD to a greater extent.

In the 1990s, it was further combined with computer-controlled flexible manufacturing systems, main warehouses, and robots for assembly. The computer controls the supply management of materials and components to achieve high-efficiency, high-quality, fully automated, and balanced production throughout the plant. The design and manufacturing levels continue Improved, dedicated control chips and integrated power devices have been continuously developed, and the performance and applicability of switched reluctance motors have been continuously enhanced, providing a huge potential market for switched reluctance motors.