Abstract : A vector control variable frequency speed regulation system for a three-phase asynchronous motor was designed, using Mitsubishi's PM25RSB-120 intelligent power module as the core of the main circuit and TI's TMS320LF2407A DSP chip as the core of the control circuit. Experimental results show that the entire system has fast dynamic response, small overshoot, and high steady-state accuracy.
Keywords : DSP; three-phase asynchronous motor; intelligent power module; vector control
Chinese Library Classification Number: TP273
Document Identifier: A Design and Realization on Vector Control System of Three-phase Asynchronous Motor Based on DSP LUO Hui1 HU Ze1 WANG Wen-jing2 SHI Lei1 (1. Southwest Petroleum University 2. Northwest Oilfield Company, Sinopec)
Abstract: We design a vector control system of Three-phase Asynchronous Motor by taking the PM25RSB-120 IPM of MITSUBISHI & TMS320LF2407A DSP of TI as the key controller of the main circuit and control circuit. The experimental results show that the whole system run well with quick dynamic response, small overshoots and high accuracy of steady-state.
Keywords : Digital Signal Processor; Three-phase Asynchronous Motor; Intelligent Power Module; Vector Control
1. Introduction
In the petroleum and petrochemical industry, the operation of drilling rigs in oil drilling, pumping units, fans, water pumps, oil pumps and mud pumps in production sites consumes a lot of electrical energy. How to make full and reasonable use of electrical energy is very important. After adopting variable frequency speed regulation technology, the energy saving effect is very obvious. Many oil pumps in the Karamay Oilfield in Xinjiang have adopted variable frequency speed regulation devices. For example, the No. 3 Oil Production Plant has applied a frequency converter on the oil pump. After operation, the effect is good. According to instrument testing, after adopting variable frequency speed regulation, the active power saving is 65.73%, the reactive power saving is 78.79%, and the power factor reaches 0.99. According to actual operation statistics, the variable frequency speed regulation oil pump power saving rate is 46.83%, and the entire investment can be recovered after 316 days. After adopting a 100kVA frequency converter, the output power of the high-pressure pump of the catalyst plant microsphere unit in Changling Refinery, Hunan Province, was reduced from 18.6kW to 7.2kW, saving 61.3% of electricity[3]. In my country, the variable frequency speed regulation system of asynchronous motor has huge market potential.
This paper presents the hardware and software design of a high-performance vector control variable frequency speed regulation system using the TMS320LF2407A DSP chip as the control core, and finally conducts experimental research on the entire system.
2. Vector control technology for asynchronous motors
The mathematical model of an asynchronous motor in a three-phase stationary coordinate system is very complex, mainly due to its intricate magnetic flux linkages. Therefore, to simplify the mathematical model, it is necessary to transform the mathematical model of the asynchronous motor from the three-phase stationary coordinate system to a two-phase synchronous rotating coordinate system through coordinate transformation. The transformation from the three-phase stationary coordinate system (ABC coordinate system) to the two-phase stationary coordinate system (Oab coordinate system) is called the Clarke transformation, and the transformation from the two-phase stationary coordinate system to the two-phase synchronous rotating coordinate system (OMT coordinate system) is called the Park transformation.
Vector control, also known as field-oriented control, decomposes the current vector into the excitation current component ism that generates magnetic flux and the torque current component ist that generates torque in a two-phase synchronous rotating coordinate system through coordinate transformation. The two components are made perpendicular to each other and independent of each other, and then adjusted separately. In this way, the torque control of AC motor is similar to that of DC motor in principle and characteristics. This is the core idea of vector control [1].
3 System Composition
Figure 1 shows the structure diagram of a DSP-based vector control system for a three-phase asynchronous motor. It comprises three main parts: the main circuit, the control circuit, and the protection circuit. Specifically, it consists of a rectifier and filter module, an inverter module, an IPM protection module, a three-phase asynchronous motor, voltage, current, and speed detection modules, a display module, a main control module, and a DSP-PC communication module.
Figure 1. Block diagram of the DSP-based vector control system for a three-phase asynchronous motor
The main circuit of the system adopts a typical AC-DC-AC voltage-type frequency converter structure. The rectifier stage uses a three-phase bridge uncontrolled rectifier module, and the inverter circuit uses Mitsubishi's Intelligent Power Module (IPM) PM25RSB-120 as the power device. The intermediate DC stage utilizes a large capacitor for filtering. The system control circuit consists of two parts: the TMS320LF2407A DSP core circuit and external expansion circuits based on the core circuit. The DSP core circuit is responsible for the overall system control and specific algorithm implementation functions. The external expansion circuit mainly performs functions such as voltage, current, and speed signal detection, data display, and communication between the DSP and the PC. It also comprehensively processes various fault signals from the IPM to form a total fault signal, which is sent to the fault interrupt entry point of the TMS320LF2407A. The host computer (PC) part uses Visual Basic to write the communication interface, mainly responsible for the setting of speed (frequency) and magnetic flux, as well as the display of speed control system faults.
4 System Hardware Design
4.1 Main Circuit Design
The main circuit consists of three parts: a rectifier circuit, a filter circuit, and an inverter circuit. The rectifier circuit uses a 6RI15G-120 (15A, 1200V) three-phase full-bridge rectifier module from Fuji Electric Corporation of Japan. Theoretically, the larger the filter capacitor value, the better; however, considering size and price, two 1000μF/450V electrolytic capacitors are used in series, with a total withstand voltage of 900V and a capacitance of 500μF. The inverter circuit uses a third-generation intelligent power module IPM from Mitsubishi Electric Corporation of Japan, model PM25RSB-120 (25A, 1200V), which integrates overvoltage, undervoltage, overcurrent, overheat, and short-circuit alarm functions.
The PM25RSB-120 has two DC input ports (P, N), three AC output ports (U, V, W), one pump-brake port (B), and 19 control input ports. The six PWM signals generated by the TMS320LF2407A are input to the PM25RSB-120's UP , VP , WP and UN, VN , WN control pins after passing through a Toshiba optocoupler isolator TLP521 . These control pins manage the switching on and off of the upper and lower arms of the three-phase inverter bridge. The DC voltage signal output from the three-phase rectifier bridge is input to the PM25RSB-120 via the P and N terminals.
Figure 2 IPM Inverter Circuit
When the IPM malfunctions, fault signals are emitted from the UFO , VFO , WFO , and FO ports. These signals are processed to form a total fault signal, which is then sent to the TMS320LF2407A port to promptly block the DSP's PWM port and protect the entire system. The IPM drive power supply must use four sets of +15V isolated power supplies. Each phase of the upper bridge arm uses one power supply, while the three lower bridge arms and the pump-up voltage control share one set. The 10μF and 0.1μF capacitors connected to each drive power supply are for decoupling the wiring impedance between the power supply and the IPM. The IPM inverter circuit diagram is shown in Figure 2.
4.2 Control Circuit Design
The TMS320LF2407A DSP controller from TI is a high-performance digital motor control chip. As a general-purpose programmable microprocessor specifically designed for digital control systems, it integrates not only strong digital signal processing capabilities, but also peripherals such as input, output, A/D conversion and event capture necessary for digital control systems [2].
The control circuit consists of two PCBs: one for the DSP core circuit and the other for external expansion circuits based on the DSP core circuit. In this design, the control circuit primarily performs functions such as voltage, current, and speed signal detection, communication between the DSP and the PC, data display, and comprehensive fault signal processing.
This design uses a CHB5-P Hall effect current sensor provided by the South China University of Technology Technology Development Company to detect current. It operates at 15V, can measure currents up to 10A, and has a maximum output current of 10mA. The TMS320F2407A incorporates a 10-bit A/D converter, with each converter having a fastest conversion time of 375ns. When detecting stator current, two input currents need to be converted via A/D conversion, therefore two channels are required for parallel conversion to complete signal transmission.
Figure 3 Current detection and signal conditioning circuit diagram
As shown in Figure 3, one output signal from the Hall current sensor is converted into a corresponding bipolar voltage signal Ua after passing through resistor R26. Ua is then converted into a unipolar voltage signal U2 after passing through a level-offset amplifier circuit. This signal can be directly fed into channel 0 of the DSP A/D converter for current sampling. In this design, the level-offset amplifier circuit consists of dual operational amplifiers LM358, resistors R27 , R28 , R30 , and LM366. The LM366 is a voltage reference chip capable of providing a 2.5V reference voltage (U0). Diodes D6 and D7 form a limiting circuit, ensuring that the DSP input is between 0 and 3.3V. In this design, the A/D conversion reference voltage is 3.3V, provided by the TI level conversion chip REF3033. The REF3033 is a high-precision voltage reference chip with an error of 0.2% and a maximum output current of 25mA.
This design uses an Omron OVW2-2048-2MD rotary encoder for speed detection. It is powered by +5V and has three outputs: A, B, and Z phases. A and B phases are used for speed measurement, with a 90-degree phase difference, outputting 2048 pulses per revolution. The Z phase output output is one pulse per revolution and is used for reference point positioning. The A and B phase outputs of the rotary encoder are isolated by high-speed optocouplers and connected to the incremental photoelectric encoder interface (QEP) pin of the DSP. By counting the pulses, the actual measured speed of the motor can be calculated.
The measured speed of the motor is displayed by a 4-digit common cathode LED digital tube, and the display mode adopts dynamic scanning display. The LED is driven by MAX7219, and the DSP's SPI (Serial Peripheral Module) is responsible for data communication with MAX7219. MAX7219 is a high-performance, low-cost multi-digit LED display driver. Its interface adopts the popular synchronous serial peripheral interface (SPI), which can drive 8 LEDs at the same time. It contains 8 display RAMs and 6 special function registers on the chip, which can be easily addressed [4].
This design uses the MAX232 driver chip, which conforms to the RS-232 standard, to implement serial asynchronous communication between the DSP's SCI (Serial Communication Module) port and the PC's serial port. The host computer portion uses Visual Basic to develop the communication interface. In this design, the host computer is mainly responsible for sending system start and stop commands, setting the speed (frequency) and magnetic flux during speed adjustment, and displaying system faults.
5 System Software Design
The block diagram of the DSP implementation of the vector control algorithm is shown in Figure 4.
Figure 4 Block diagram of DSP-implemented vector control structure for asynchronous motor
The software design employs a modular approach, primarily comprising a main program module and an interrupt service subroutine module. The main program module mainly includes system initialization and communication between the DSP and the PC. The initialization section includes hardware initialization and the initialization of various vector control algorithm modules; interrupt initialization; setting initial values for various control registers; allocating addresses and setting initial values for various variables used during computation, etc.
The entire vector control algorithm is implemented within the underflow interrupt service routine of EVA's timer/counter T1. The T1 underflow interrupt service routine is the core of the entire software system, and its flowchart is shown in Figure 5. It mainly includes photoelectric encoder speed detection, phase current detection, coordinate transformation, rotor flux linkage position calculation, current and speed PI regulation, and voltage space vector SVPWM generation. The interrupt service routine is woken up from the main program's wait loop and executed after each timer/counter T1 underflow event. During the execution of the interrupt service routine, the given speed (or frequency) and flux are read from the main program.
Figure 5 Flowchart of the T1 underflow interrupt service subroutine
6. Experimental Results and Analysis
During the experiment, the step speed of the motor was given as 1000 r/min. The motor was started under no-load, and at t=0.4s, a load of 5 N·m was suddenly applied.
Figure 6. Rotor speed and torque waveforms
Figure 7. Stator current in a two-phase synchronous rotating coordinate system
Figure 6 shows the correlation between electromagnetic torque and speed changes. During motor startup, the starting torque is large due to the motor system's high inertia; the speed cannot change abruptly. Initially, the speed feedback is zero, resulting in a large speed deviation. The speed regulator quickly saturates, reaching its limit, thus producing a large electromagnetic torque. As the speed increases, it exceeds the set value within 0.2 seconds, causing the speed regulator to desaturate, the ASR output to decrease, and the ATR setpoint to decrease, consequently reducing the electromagnetic torque. As the speed stabilizes, the torque also tends to stabilize. Since the motor operates under no-load, the output electromagnetic torque is very small, close to zero. The situation is similar when a load is suddenly applied at 0.4 seconds, similar to the motor startup scenario. In steady state, the electromagnetic torque equals the load torque. The system's current loop uses current-tracking hysteresis comparison control, so there is some pulsation in the electromagnetic torque during steady state.
In Figure 7, after the motor starts, the M-axis current quickly reaches its steady-state value. At the moment of sudden load application, ism experiences a small pulsation, but quickly returns to its steady-state value and remains essentially constant, unaffected by load changes. The T-axis current waveform shows that ist changes with the load; when a sudden load is applied, ist increases rapidly with the load increase, indicating successful decoupling of the stator current.
Figure 8. PWM1 and PWM2 pin waveforms Figure 9. PWM1 and UP pin waveforms
When the given frequency is 40Hz, the output control waveforms of each pin of the DSP's PWM1 to PWM6 were measured using an oscilloscope. Figure 8 shows the output control waveforms of the PWM1 and PWM2 pins, and Figure 9 shows the output waveforms of the PWM1 and UP signals. Figure 8 shows that the amplitudes are the same, but the phases are opposite, and there is a dead time between the two waveforms. Figure 9 shows that the amplitudes are different, but the phases are the same. All output waveforms match the theoretical waveforms.
7. Conclusion
Leveraging the powerful computing capabilities and fast real-time processing of the high-performance TMS320LF2407A dedicated DSP chip for motor control, complex control algorithms in variable frequency speed control systems can be more easily programmed and implemented. Experimental results show that the vector control system exhibits high steady-state accuracy, short dynamic adjustment time, small overshoot, and strong anti-interference ability, achieving speed control performance comparable to that of a DC motor.
References
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[2] Wang Xiaoming, Wang Ling. DSP Control of Electric Motors—TI DSP Applications. Beijing: Beijing University of Aeronautics and Astronautics Press, 2004, pp. 120-184.
[3] Wu Zhongzhi, Wu Jialin. Inverter Application Manual. Beijing: Machinery Industry Press, 2005. 556-557.
[4] Wang Jianhua et al. MAX7219 principle and application. Electronic Technology, 2003, (12): 36-38
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[9] Datasheet of PM25RSB-120.MITSUBISHI Electric.1998
First Author's Biography
Luo Hui, male, is a master's student at the School of Telecommunications Engineering, Southwest Petroleum University. His main research areas are intelligent instruments and computer measurement and control.
Author's contact information: Southwest Petroleum University
Title: Design and Implementation of a DSP-Based Vector Control System for Three-Phase Asynchronous Motors
Postal code: 610500
Mailing Address: Class 4, 2005 Master's Program, Graduate School, Southwest Petroleum University, Xindu District, Chengdu, Sichuan Province
Recipient: Luo Hui
Telephone: 13880267853 028-83030525
Email: [email protected]
