Design of frequency converter based on TMS320F2808 direct torque control

Direct Torque Control (DTC) technology is a new type of AC speed control technology developed in the past two decades following vector control technology. Due to its advantages such as simple structure, fast torque response and good parameter robustness, it has become the main control strategy for AC motor speed regulation in the field of modern transmission control. However, since it is a new technology, most of it is only in theoretical research and discussion.
Based on the basic principle of direct torque control, this paper uses TMS320F2808 produced by TI as the main control chip to design a fully digital direct torque control inverter system, and gives the current and voltage detection circuits and data analysis results.

1 Basic principles of direct torque control
Direct torque control technology is to obtain the actual speed n of the induction motor through the speed sensor; compare it with the given speed n* and obtain the torque given value through the speed regulator. The speed regulator generally uses a PI controller; the motor stator current and voltage values ​​are obtained by the sensor, and the flux and torque estimator of the induction motor are input to obtain the stator flux amplitude |ψs| and the actual torque value Te;
the actual torque Te and the torque given value are processed by the torque regulator to obtain the torque switch signal TQ; the stator flux given value and the flux calculation value |ψs| are processed by the flux regulator to generate the flux switch signal ψQ; the interval judgment obtains the stator flux interval signal SN according to the stator flux component, and the switch signal selection unit integrates the flux switch signal ψQ, the torque switch signal TQ and the flux position signal SN, and obtains the correct voltage switch signal by looking up the table to control the accurate operation of the motor.
The basic principle of the direct torque control system is shown in Figure 1.

2 System Hardware Design
2.1 Main Circuit
In order to make the control system simple in structure, reliable in operation and easy in maintenance, this system adopts modular structure design. Figure 2 is the block diagram of the asynchronous motor direct torque control system based on TMS320F2808.

The main circuit of the system adopts a typical AC-DC-AC voltage-type frequency conversion structure, and the rectifier link adopts a single-phase bridge uncontrolled rectifier circuit. The rated voltage of the rectifier diode in the design is 800 V. The intelligent power module (IPM) used in the inverter circuit is Cyntec's IMl3400, which encapsulates 6 IGBTs. The main components of the DC intermediate circuit are large-capacity electrolytic capacitors. Considering that the fluctuation range of the grid voltage is 220 V±10%, the withstand voltage of the filter capacitor should be greater than 1.1×220×1.414. Therefore, an electrolytic capacitor with a rated voltage of 400 V is selected.
2.2 Control loop
TMS320F2808 produced by TI is used as the main control chip. TMS320F2808 has no specific requirements for the power-on sequence, and provides 4 high-precision ePWMs, which provides more convenience for the design of digital motor control systems.
2.2.1 Power supply circuit of TMS320F2808
The power supply circuit of TMS320F2808 is shown in Figure 3. TPS70151 produced by TI is selected to power the TMS320F2808 processor. TPS70151 is a dual-channel LDO integrated circuit regulator with power-on sequence control and power-on reset functions, and the output is 500 mA/3.3 V and 250 mA/1.8 V.

2.2.2 PWM signal isolation circuit
The PWM signal of this design is output by the EPWMxA unit of TMS320F2808. There is a voltage signal isolation problem between the DSP and the IPM module. The PWM signal isolation circuit is shown in Figure 4, which is completed by SN74LVCC4245A. DIR is connected to the digital ground (N1), and the signal transmission direction is from A to B. UP, VP, WP, UN, VN, and WN are respectively connected to the PWM signal input pins of the IPM module, and U+, V+, W+, U-, V-, and W- are respectively connected to the EPWMlA, EPWMlB, EPWM2A, EPWM2B, EPWM3A, and EPWM3B pins of the DSP.

2.2.3 Voltage and current detection circuit
In direct torque control, in order to calculate the actual stator flux, it is necessary to detect the stator voltage and current. Generally, two Hall voltage sensors are required for stator voltage detection, and the circuit design is relatively complicated, so this article changes to detect the DC bus voltage. The voltage detection circuit is shown in Figure 5. HCPL-788J isolation amplifier optocoupler is selected for DC bus voltage detection. The isolation amplifier optocoupler is relatively cheap, and the circuit design structure is simple. It can not only collect voltage signals, but also realize overvoltage protection.

The current detection uses two current sensors, the selected model is TBC-05SY, which is a closed-loop Hall current sensor. Its rated input current is 5 A, the measurement current range is ±15 A, the rated output voltage is ±(4+0.5%)V, the zero current offset is ±20 mV, and the linearity is ≤0.1%FS (FS is the range of the current sensor). The current detection circuit is shown in Figure 6.

3 System Software Design
The control program is mainly divided into two parts: the main program and the interrupt service subroutine. The main program mainly completes the definition of various constants and variables of the system and the initialization of various registers, including the definition of various control variables, the definition and initialization of constants, as well as the initialization of CPU clock, I/O port, interrupt vector table, timer, A/D converter and on-chip registers. The interrupt service subroutine mainly completes data sampling, stator flux calculation, electromagnetic torque calculation, voltage space vector selection and PWM pulse formation. Figure 7 shows the flow of the main program and the interrupt service subroutine.

4 Experimental results
After the field test, Matlab7.2 software was used to plot the test data and make the corresponding fitting straight line. The comparison of the voltage and current detection circuit data detection curve and the fitting straight line is shown in Figure 8 and Figure 9. The experimental results show that the system, as a hardware platform for the direct torque control strategy, has the advantages of strong anti-interference ability, good voltage and current protection measures, and strong software portability.

5 Conclusion
This paper completes the design of a direct torque control inverter based on TMS320F2808. It has the advantages of simple circuit, strong anti-interference ability, low power consumption and reliable performance. The designed system has perfect protection measures, which can provide effective protection for various faults and greatly extend the service life of the inverter.