What Is a Switched Reluctance Motor?

Switched Reluctance Motor (Switched Reluctance Motor: SRM) is a type of speed-regulating motor developed after DC motors and brushless DC motors (BLDC). The research on switched reluctance motors in the United Kingdom and the United States started early and has achieved significant results. The power levels of the products range from several w to hundreds of kw. They are widely used in household appliances, aviation, aerospace, electronics, machinery and electric vehicles field.

Switched Reluctance Motor

Switched Reluctance Motor (Switched Reluctance Motor: SRM) is a type of speed-regulating motor developed after DC motors and brushless DC motors (BLDC). The research on switched reluctance motors in the United Kingdom and the United States started early and has achieved significant results. The power levels of the products range from several w to hundreds of kw. They are widely used in household appliances, aviation, aerospace, electronics, machinery and electric vehicles field.

Chinese name

Switched Reluctance Motor

Foreign name

Switched Reluctance Motor

Types of

Speed motor

component

Concentrated winding stator, salient pole rotor, etc.

Short name

SRM

Switched reluctance motor is a new type of speed-regulating motor. It is the latest generation of speed-regulating system following frequency conversion speed-regulating system and brushless DC motor speed-regulating system. Its structure is simple and sturdy, the speed range is wide, and the system has high reliability. The complete system is mainly composed of motor entity, power converter, controller and position detector. The controller contains a power converter and a control circuit, while a rotor position detector is installed at one end of the motor.

Today, the application and development of switched reluctance motors have made significant progress, and have been successfully used in various fields such as electric vehicle drives, general industry, household appliances and textile machinery, with power ranging from 10W to 5MW (lack of reliable information), The maximum speed is up to 100,000 r / min.

China s research and trial production of the switched reluctance motor speed control system started in the late 1980s and early 1990s, and has achieved results and progress in various aspects from basic theory to design and manufacturing technology, but the industrialization and application research work has lagged behind. .

Switched reluctance motor speed control system is mainly composed of four parts: switched reluctance motor (SRM), power converter, controller, and rotor position detector. The system block diagram is shown in Figure 1. The controller contains the control circuit and power converter, and the rotor position detector is installed at one end of the motor. The motor has the same power, same frame size and shape as the domestic Y series induction motor.

Figure 1 SRM system block diagram

Switched reluctance motor ^[1]

Main features of the switched reluctance motor speed control system:

The structure is simple; the motor has a simple structure, low cost, and can be used for high-speed operation. The structure of the SRM is simpler than that of a squirrel-cage induction motor. Its mechanical strength of the rotor is extremely high, which can be used for ultra-high-speed operation (such as 10,000 revolutions per minute). On the stator side, it has only a few concentrated windings, so it is simple to manufacture and the insulation structure is simple.

The circuit is reliable; the power circuit is simple and reliable. Because the direction of the motor torque is not related to the direction of the winding current, that is, only a single-phase winding current is required, so the power circuit can achieve a power switch for each phase. In contrast to the asynchronous motor windings, which need to flow bidirectional current, the power circuit of the PWM inverter powering it requires two power devices per phase. Therefore, the switching reluctance motor speed regulation system requires fewer power components than the PWM inverter power circuit, and the circuit structure is simple. In addition, in the power circuit of the PWM frequency converter, two power switches of each bridge arm directly cross the DC power supply side, and it is easy to cause a direct short circuit to burn out the power device. In the switched reluctance motor speed control system, each power switching device is directly connected in series with the motor winding, which fundamentally avoids the phenomenon of shoot-through short circuit. Therefore, the protection circuit of the power circuit in the speed regulation system of the switched reluctance speed control motor can be simplified, that is, the cost is reduced, and the reliability is high.

High system reliability; high system reliability. From the perspective of the electromagnetic structure of the motor, the various windings and magnetic circuits are independent of each other, and each of them generates electromagnetic torque within a certain range of shaft angles. Unlike in ordinary motors, a rotating magnetic field must be generated under the combined action of the windings and magnetic circuits of each phase for the motor to operate normally. From the control structure point of view, each phase circuit supplies power to one phase winding, and generally works independently.

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 typical products are: when the starting current is 15% of the rated current, the starting torque is 100% of the rated torque; when the starting current is 30% of the rated current, the starting torque can reach 250% of its rated torque . The starting characteristics of other speed-regulating systems are compared. For example, the DC motor has a current of 100%, and the squirrel cage induction motor has a current of 300%, and obtains 100% of the torque. The advantages of small starting current and large torque can also be extended to the low-speed running section, so this system is very suitable for those machines that require heavy-load starting and long-time low-speed and heavy-load operation.

Frequent start and stop; suitable for frequent start and stop and forward and reverse switching 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 heating of the motor is smaller than that during continuous rated operation. It is suitable for frequent starting and stopping and forward and reverse switching operation.

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

High efficiency; the switched reluctance motor system is an efficient speed regulation system. Taking a 3kW SRM system as an example, its system efficiency can reach 87% over a wide range.

The main advantages of the switched reluctance motor drive system are as follows:

1. The structure of the motor is simple, there is no winding of any kind on the rotor; there is only a simple concentrated winding on the stator, the end is short, and there is no interphase jumper wire. Therefore, it has the characteristics of fewer manufacturing processes, low cost, reliable work, and small maintenance.

2. The torque of the switched reluctance motor has nothing to do with the polarity of the current. Only one-way current excitation is required. In theory, each phase in the commutation conversion circuit can use only one switching element, and it is connected in series with the motor winding. There is a danger that the two switching elements will pass through like the power supply of the inverter. Therefore, the switched reluctance motor drive system has a simple circuit, high reliability, and lower cost than an AC speed regulation system.

3. The structure of the switched reluctance motor rotor has a small restriction on the speed, which can be made into a high-speed motor, and the rotor's moment of inertia is small. The magnitude and direction of the phase turn torque can be changed each time the current is commutated. Have good dynamic response.

4. Because SRM uses a unique structure and control method, it has advantages in some aspects compared with induction motors. The efficiency and power density of SRM systems can be maintained at high levels over a wide range of speeds and loads.

The main disadvantages of the switched reluctance motor drive system SRM system are:

1. Large torque ripple and large noise and vibration: From the working principle, it can be known that the torque generated on the rotor of a switched reluctance motor is a series of pulse torques superimposed, which affects the performance of SRM.

At present, there are two main types of washing machines that are widely used in the world and accepted by the majority of users: one is a pulsator type automatic washing machine; the other is a drum type full automatic washing machine.

The two types of washing machines have common performance requirements for the electric motor: the motor should rotate at low speed during washing, and it can frequently rotate forwards and backwards; the motor must be rotated at high speed during dehydration.

For a long time, these two types of washing machines have basically used a pole-changing two-speed single-phase induction motor to barely meet the requirements, but the disadvantages are obvious:

Poor speed regulation performance, only one speed during washing is difficult to adapt to the requirements of various fabrics on the speed of washing, and the so-called "strong washing", "weak washing", "soft washing" and other washing procedures only change by changing The duration of forward and reverse. And in order to take care of the requirements for the rotation speed during washing, the rotation speed during dehydration is often low, generally only 400 to 600 rpm.

The efficiency of single-phase pole-changing two-speed induction motors is very low, generally below 30%. The starting current is more than 7 to 8 times of the rated current, which will impact the power grid.

If you use a switched reluctance speed-regulating motor instead of a single-phase pole-changing two-speed induction motor, you can achieve very satisfactory results.

Nanjing Ruipeng Technology Co., Ltd. has successfully developed a special SRD system for 5kg drum washing machines, which has achieved exciting results.

The system "standard washing", the drum rotation speed is 57 rpm; while the "gentle washing", "velvet washing" drum rotation speed is 25 rpm. Really do not damage high-grade fabrics. During "dehydration", the rotation speed of the drum can be arbitrarily set between 400 and 1200 rpm.

The system also designed special programs for various actions of the washing machine. If specific start, acceleration and deceleration programs are designed for forward rotation and reverse rotation washing, the washing rate of clothes can be effectively improved. Specific starting and uniform speed-up programs are designed for rinsing and dewatering, which effectively avoids vibration and noise caused by uneven distribution of laundry on the drum during dehydration; for laundry that is not evenly distributed at all, it can Intelligently select a lower spin speed for it.

After testing and comparing, the same clothes use the same "standard washing", the power consumption of this system is only 44% of the ordinary drum washing machine (powered by two-speed induction motor); its power consumption, water consumption, cleanliness, dehydration A series of indicators such as frequency, noise, etc. have reached the European A-class washing machine standards.

As a new type of speed regulation drive system, the switched reluctance motor is more and more recognized and applied for its simple structure, low cost, high efficiency, excellent speed regulation performance and flexible controllability. Has been successfully used in electric vehicle drive systems, household appliances, industrial applications, servo systems, high-speed drives, aerospace and many other fields, becoming AC motor speed control systems, DC motor speed control systems, and brushless DC motor speed control systems Strong competitors.

The United States, Canada, Egypt and other countries have carried out research and development of SRD systems. In foreign applications, SRD is generally used in traction, such as battery cars and electric cars. At the same time, high-speed performance is a special direction of SRD. According to reports, the United States has developed a 25000r / min, 90kW high-speed SRD prototype for space technology. The research of SRD system has been included in the "Eighth Five-Year", "Ninth Five-Year" and "Tenth Five-Year" scientific research planning projects of small and medium-sized motors in China.

The research group of Switched Reluctance Motor of Huazhong University of Science and Technology developed a pure electric car using SRD in the "Ninth Five-Year Plan", and applied the SRD to the hybrid city bus in the "10th Five-Year Plan", all of which achieved good results. . The Textile Machinery Research Institute has applied SRD to towel printing machines, cloth winders, coal mine traction and electric vehicles, and has achieved significant economic benefits.

Nowadays, 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-price digital signal processing chips (DSPs), use high-performance DSPs to develop various complex algorithms. The indirect position detection technology, without the need for additional external hardware circuits, 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. Computer-controlled material and component supply management achieved high-efficiency, high-quality, fully automated and balanced production throughout the plant. The level of design and manufacturing continued. Improved, special control chips and integrated power devices have been continuously developed, and the performance and applicability of switched reluctance motors have been continuously enhanced. With the increasing development of the national economy, the mechanization and automation of various industries are becoming higher and higher, providing a huge potential market for switched reluctance motors.

As the latest generation of stepless speed regulation system, the switched reluctance motor is still in the stage of deepening research and development and continuous improvement, and its application field is also constantly expanding. Due to the good speed regulation performance and low cost of SRM, popularization can produce very good economic and social benefits, which requires the joint efforts of scientific research, development and manufacturing.