Design of Switched Reluctance Motor Drive System Based on Single Chip Microcomputer

1 Introduction

The switched reluctance motor is a control device composed of a reluctance motor and an electronic switch drive controller, also known as a switched reluctance motor drive system (Switched Reluctance Motor drive, referred to as SRD). The motor has a simple and sturdy structure and reliable operation. The system has high starting torque, low starting current, wide speed regulation range, and high operating efficiency. It is particularly suitable for frequent start-stop and forward and reverse operation, making SRD a strong competitor for AC, DC motor drive systems and brushless DC motor drive systems. At present, SRD has been used in many fields, such as electric vehicle drive, household appliances, servo and speed regulation systems, and many other fields.

This paper designs a SRD control system with 80C196 single-chip microcomputer as the control core, making full use of the flexible characteristics of SRD motor control mode, using digital control system to control SR motor, simplifying the hardware circuit and improving the reliability of the system.

2 SR motor working principle

The operating principle of SR motor follows the principle of minimum magnetic resistance - the magnetic flux always closes along the path with the minimum magnetic resistance, and the tangential magnetic pull is generated due to the distortion of the magnetic field. When the iron core with a certain shape moves to the position of minimum magnetic resistance, its main axis must coincide with the axis of the magnetic field. The typical structural principle of SR motor is shown in Figure 1.

The specific process is as follows: When the A-phase winding current control switches S1 and S2 are closed, the A-phase is excited, and the magnetic field force generated causes the rotor to rotate to the position where the rotor axis coincides with the stator axis, thereby generating an electromagnetic torque of magnetic resistance. If the ABCD phase windings are energized in sequence (the B, C, and D phase windings are not drawn in the figure), the rotor will rotate continuously in the counterclockwise direction. Conversely, if the BADC phase windings are energized in sequence, the rotor will rotate in the clockwise direction.

Figure 1 SR motor structure schematic

Since it is an electromagnetic torque of magnetic resistance nature, the direction of the SR motor is independent of the current direction of the phase winding and only depends on the power-on sequence of the phase winding. This makes it possible to simplify the power converter circuit. When S1 or S2 is closed, the A-phase winding absorbs electrical energy from the power supply, and when S1 or S2 is disconnected, the winding current feeds the remaining energy back to the power supply through diodes VD1 and VD2. Therefore, the SR motor has the characteristics of energy feedback and high system efficiency.

2 SRD System Hardware Design

2.1 Basic components of the system

The SRD system mainly consists of four parts: switched reluctance motor, power converter, controller and sensor. The relationship between them is shown in Figure 2.

Figure 2 Basic structure of SRD

The SR motor is the executive component of the entire system. The double-pole magnetic resistance motor and power converter provide the energy required for the SR motor to operate. The position detector is the provider of signals such as rotor position and speed. The controller is the center of the system. It comprehensively processes speed instructions, speed feedback signals, and feedback signals from current sensors and position sensors, thereby controlling the working state of the main switch device in the power converter and achieving ultimate control of the operating state of the SR motor. [page]

2.2 Power conversion circuit

This paper uses a four-phase 8/6-pole reluctance motor as the experimental platform for system control, and the power circuit uses an H-type power circuit, as shown in Figure 3. A, B, C, and D are the four-phase windings of the SR motor, VD1~VD4 are freewheeling diodes, and AB, CB, CD, and AD are turned on at the same time and work in a cycle. If V1 and V3 are turned on at the same time, phases A and B are energized. If V1 is turned off and V2 is closed, phase A is disconnected and phase C begins to turn on. At this time, the A-phase winding will continue to flow through the diode VD1, and the C-phase winding will also form a loop, increasing the current of phase B. The neutral point potential will inevitably increase, prompting the A-phase continuous current to decay rapidly and forcibly change phases. Then turn off V1 and turn on V4, then phases C and D are turned on, and they are turned on in sequence, and the motor works continuously. Its * frequency switch uses a PWM control signal to control its on and off through the drive circuit.

Figure 3 Power circuit schematic

2.3 Control circuit structure

The control circuit part mainly includes 80C196 single-chip microcomputer, angle position detection circuit, display keyboard circuit and interface circuit. In the control circuit, the single-chip microcomputer is the core, responsible for executing instructions and processing data. The angle position detection circuit detects the rotor position through a photoelectric sensor, and sends the position signal to the power converter and display circuit to display system information after processing. The 8279 interface circuit is responsible for the information exchange between the CPU and the display keyboard part. When the system is in working state, the user gives the speed through the keyboard. After receiving the speed command, the single-chip microcomputer compares it with the actual measured speed, and outputs a PWM signal with adjustable duty cycle through the internal control algorithm. This PWM signal causes the voltage at both ends of the phase winding of the SR motor to change, thereby achieving the purpose of speed regulation.

3 SRD system software design

The system software mainly consists of single chip microcomputer processing software and PC host computer control software.

3.1 MCU Software Design

The switch reluctance motor control software is based on the control system hardware. It completes the detection of position signals, input given signals and current operating status, makes judgments and calculations, and outputs appropriate current control and commutation control signals. It consists of a foreground program and a background program, as shown in Figure 4. The background program includes an initialization program, a keyboard scanning program, a speed control program, a display program, and an idle cycle program. The foreground program consists of four interrupt programs, which are position interrupt programs, software timer interrupts, external interrupts, and serial port interrupt programs.

Figure 4 Switched reluctance motor control program flow

The initialization program completes the initialization of relevant units, selects the working mode of the external interface, initializes the functional components of each part of the CPU, enables interrupts, etc., so that the entire system is ready to run. The keyboard scanning program checks the input of the keyboard and performs corresponding software de-jitter processing. When a key is pressed, it makes corresponding judgments, such as forward, reverse, and braking. The speed control program completes the starting and braking of the motor according to the given speed and the state of the motor. When running, it outputs the corresponding PWM signal according to the speed deviation. The display program mainly completes the information display of the motor under the panel control mode. The given speed is displayed flashing before the motor starts, and the actual speed is given after starting. If a fault occurs during operation, the fault nature code is displayed. The idle cycle program consists of a power supply detection program, hardware fault detection, fault handling, and a command analysis program for the host computer.

The position interrupt program is that the rising and falling edges of the square wave signals output by the two position sensors both cause interrupts, that is, the motor generates interrupts every time it rotates 15°. This program mainly completes the running speed detection, phase change control and the switching of the running mode. The timer interrupt realizes the main loop timing, current sampling, current chopping and other functions. The external interrupt program realizes the external keyboard input function. The serial port interrupt realizes the communication between the host computer and the single-chip microcomputer.

The working process of the system is as follows: When the single-chip microcomputer receives the start command, when the detection system status is normal, according to the rotor position signal provided by the position sensor, the corresponding output signal is given according to the start logic, and the signal is supplied to the motor winding through the power converter to make the motor start to rotate. When the rotor rotates through a certain angle, the control circuit changes the motor power-on phase through the power converter according to the change of the position sensor signal. During operation, a PWM control signal with an adjustable duty cycle is output according to the error between the given speed and the actual speed. When the operation command changes, such as reversing, stopping, and braking, the control circuit changes the working logic again, and the motor realizes the operation requirements through the power converter. If a fault occurs during operation, the control circuit will protect by shutting down the power converter and display the fault code through the display circuit.

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2 PC software design

The system host computer control software is developed using VB language. Its function is to control the operation of the single-chip microcomputer through serial port commands and display the actual operating speed. Using this software, the PC can realize the forward, reverse, stop, brake and set specific rotation speed of the motor. The software operation interface is shown in Figure 5.

Figure 5 Software running interface

4 Summary

This system uses 80C196KC16-bit single-chip microcomputer as the core, and is equipped with peripheral interface chips such as 8279 to form peripheral circuits such as detection, setting, display, and protection. In addition, the drive circuit, main circuit, and reluctance motor form a complete SRD system. A speed regulation experiment was conducted on a 2KW switched reluctance motor. The experimental results show that the switched reluctance motor has a simple and firm structure, a wide speed regulation range, excellent speed regulation performance, and high efficiency in the entire speed regulation range. The system has high reliability and meets the design requirements.