Renesas MCU in AC Motor Design

　　This paper introduces a high-performance, low-cost frequency conversion controller for speed regulation of AC induction motors in household appliances. According to the control characteristics of AC induction motors, the power drive circuit, speed detection, circuit protection and EMC are analyzed. The core of the controller is composed of a low-cost, high-performance Renesas microcontroller and a three-phase power drive chip IRS2136S, which reduces the cost of the frequency conversion controller. The speed detection uses a position sensor to shorten the software development cycle. The EMC measures added to the circuit greatly increase the reliability of the system. The circuit has been used in some household appliances. The whole machine has good stability and anti-interference, and has passed the UL and national household appliance safety standards.

　　0 Preface

　　With the rapid development of power electronics technology and microcontroller technology, the application of modern AC variable frequency speed regulation system technology in motor control systems is becoming more and more widespread. The fully digital controlled variable frequency speed regulator using fully controlled device IGBT has been universalized, with the advantages of wide speed regulation range, high speed regulation accuracy, fast dynamic response, high operating efficiency, and convenient operation. The popularization and application of variable frequency speed regulators has improved the automation level of modern industry, improved product quality and labor efficiency, and saved energy to the maximum extent, thus conforming to the national policy of developing and building a harmonious and economical society.

　　This article uses the cost-effective microcontroller R5F21246 seties launched by Renesas, which is embedded with the R8C/Tiny series CPU core. All MCUs in the R8C/Tiny series have integrated on-chip flash memory, which has high performance and good ease of use, 1 MB address space, 2 KB on-chip data storage FLASH, which is very suitable for home appliances such as air conditioners, washing machines, refrigerators, and industrial control applications such as AC servo motors.

　　1 Basic structure of AC induction motor system

　　The principle structure of the AC induction motor control system is shown in Figure 1, which includes a main circuit, a control unit, a power drive unit, a protection unit, and a signal feedback acquisition unit.

　　

　　The AC induction motor control unit includes speed controller, torque and current controller, etc. Renesas single-chip microcomputer R5F21246 has become a trend of becoming the control core of AC induction motor control system with its advantages of low price, low power consumption, high performance and fast processing speed. The power drive unit adopts the driver chip IRS2136S, which not only integrates the drive circuit, but also has fault detection and protection circuits such as overvoltage, overcurrent, overheating and undervoltage. When the system has problems, it can protect itself in time, improving the reliability of the system. Each collected signal is transmitted to the R5F21246 through feedback, and is accurately and quickly processed and then output to realize real-time control of each component. A soft start circuit is also added to the main circuit to reduce the impact of the startup process on the driver.

　　2. Drive circuit design of AC induction motor system

　　The power drive module, as the strong power part of the AC induction motor control system, consists of two units: the power supply unit provides power for the digital and analog circuits of the entire system; the power drive unit IRS2136S is used to drive the motor (see Figure 2).

　　

　　The design of the power board includes four hardware protection functions, namely overcurrent (forward and reverse current) protection, bus overvoltage protection, bus undervoltage protection and IGBT overheat protection. Each protection has an LED alarm display. In order to enhance reliability, after the protection action occurs, a shutdown signal will be sent to the MCU to protect the power device IRS2136S and prevent damage to the components. These signals must be canceled by a separate reset signal, otherwise the protection circuit will remain effective to prevent misoperation from damaging the equipment and affecting personal safety.

　　2.1 Power Circuit

　　The main circuit of the power supply is shown in Figure 3. Input voltage range: AC 187 ~ 264 V, frequency range: 48 ~ 53 Hz. The DC bus voltage output is stable at a normal value of about 310 V. Schottky diode D110 is connected in parallel at both ends of the bus voltage to prevent the bus from overvoltage. The filter capacitor C110 filters out interference signals. Resistors R104 ~ R109 are connected in parallel with C107 and C106 to stabilize the voltage at both ends of the capacitor. The DC bus voltage output adopts a pre-charging circuit. At the moment of power-on, the current limiting resistor R102 provides current to the rectifier bridge D101 and filter capacitors C107 and C108, reducing the impact on the rectifier bridge. When the filter capacitor is almost fully charged, the current limiting resistor is bypassed by relay K101. The DC bus voltage generates 16 V and 5 V stable DC voltages through VIPER12 (U101) and LM78L05 (U102) chips, which respectively power the drive circuit and other circuits, as shown in Figure 4.

　　

　　2.2 Temperature Detection Circuit

　　Figure 5 is the temperature detection circuit of IGBT. R379 in the circuit is a negative temperature coefficient thermistor NTC. When the temperature is too high, the resistance of R379 becomes smaller. The voltage output fault signal of the detection circuit is fed back to the microcontroller. After the microcontroller analyzes and processes, measures are taken to protect the system, such as stopping the driver chip from working, and releasing the protection when the temperature returns to normal.

　　2.3 Current Detection Circuit

　　In order to reduce system cost, the circuit adopts single resistor sampling technology. Generally speaking, the vector control algorithm needs to collect at least two-phase current of the motor, but the single resistor sampling only needs to collect the current of the negative bus.

　　As shown in FIG. 2 , a single resistor R236 samples the current ishunt. For each switching state of the lower bridge arms Q202 , Q204 , and Q206 , the current state flowing therethrough is shown in Table 1.

　　

　　In Table 1, "0" indicates that the switch is off, and "1" indicates that it is on. Since the current is almost constant in one PWM cycle, it is only necessary to sample twice in one PWM cycle to obtain the state of each phase current of the motor at that moment, because the sum of the three-phase current is zero.

　　Single resistor sampling will encounter some challenges. The space vector pulse width modulator (SVPWM) will have two long and one short duty cycles, two short and one long duty cycles, and three almost equal duty cycles at the sector boundary and low-speed modulation area of ​​the space vector. In this way, if the effective vector lasts less than the current sampling time, an error will occur. The solution adopted in this solution is to insert an effective vector of a fixed time at adjacent boundaries, and in the low-speed modulation area, the effective vector is inserted in turn. Inserting an effective vector will cause distortion to the current waveform, which needs to be compensated by software in this case.

　　The advantages of single resistor sampling are not only to reduce the cost of the system, but also that it detects three-phase current based on the same gain and offset, with good consistency. The disadvantages are also obvious. For the MCU, the algorithm is complicated and its operation time is increased, and the code is longer than the three-resistance method; for current detection, its waveform distortion is slightly larger than the three-resistance method. The performance of single resistor sampling is fully competent for the application of variable frequency air conditioners, and the cost is low, which is why most home appliance manufacturers are willing to choose single resistor sampling.

　　2.4 Speed ​​Detection Circuit

　　The position sensor plays a role in determining the rotor position and speed in the AC induction motor. It provides correct position information as a logic switch circuit, that is, converting the magnetic pole position signal of the rotor magnet into an electrical signal. Then it controls the stator winding phase change. This design uses FUS1881 (Hall sensor). The motor rotor rotates one circle and inputs 8 pulses to the microcontroller, thereby controlling the movement of the AC induction motor and making the control program simple. The Hall detection circuit is shown in Figure 6.

　　

　　3 EMC measures

　　The electromagnetic compatibility (EMC) anti-interference design of this hardware system mainly includes the following aspects: lightning protection and filtering processing of power input, grounding technology, shielding technology, isolation technology, and printed circuit board design.

　　3.1 Power Input Filtering

　　As shown in Figure 7, L101, C102 and C103 form a common-mode choke. This is a power filter composed of a low-pass filter circuit. It implements the initial attenuation of the high-frequency components in the transient voltage waveform. It allows DC or 50 Hz current to pass through, and has a greater attenuation on interference signals with higher frequencies. Since there are two types of interference signals, differential mode and common mode, the power filter must have an attenuation effect on both types of interference. Note that when the operating current exceeds the rated current, the inductor core in the filter will saturate, reducing the actual inductance and resulting in a decrease in the high-frequency filtering performance of the filter. Therefore, when determining the rated operating current of the filter, take 1.5 times the actual current value. The varistor V101 plays a role in suppressing differential mode overvoltage and increasing the leakage current limiting voltage.

　　

　　3.2 IGBT protection circuit

　　As shown in Figure 2, the power drive amplifier circuit, if the forward voltage rise rate dv/dt applied to the IGBT is too large, a large displacement current will be generated due to the existence of the junction capacitance, which may act as a trigger current, reducing the forward blocking ability of the IGBT, and causing mis-conduction in severe cases. In order to suppress the effect of dv/dt, a resistance absorption loop is connected in parallel at both ends of the IGBT gate level and the gate, and a bidirectional freewheeling diode loop is added to suppress the large displacement current, and use the resistor to consume this part of the energy. Similarly, this protection loop can also attenuate and suppress high-frequency interference signals in the circuit.

　　3.3 Others

　　Since the system has a lot of power supply, the analog ground and digital ground are properly handled. In addition, attention is paid to the grounding of the shell, and a small capacitor is added to each input end of the driver chip IRS2136S to improve reliability. When designing the PCB board, attention is paid to the routing method, line width, via processing, etc.

　　4 Conclusion

　　The control circuit of the AC induction motor based on Renesas microcontroller designed in this paper, combined with the advanced three-phase power driver chip IRS2136S, adopts 6 single-tube IGBTs and single resistor sampling, which reduces the production cost; adds EMC measures to make the system work more stable and reliable. This paper makes full use of the advantages of the R8C/Tiny series CPU core - rich firmware library functions, shortening the development cycle. Practical application shows that the AC induction motor control circuit based on Renesas microcontroller has good dynamic performance, stable and reliable system operation, high efficiency, low cost, energy saving and low noise, which can fully meet the actual use requirements of household appliances.