Reliability Design of Low Voltage Intelligent Motor Protector

1 Introduction

　　As a traction machine, the electric motor is widely used in all aspects of the national economy due to its advantages of simple structure, low price, easy use and maintenance, etc. In modern times, with the development of electronic technology and the maturity of intelligent motor protector technology, the popularity rate is getting higher and higher.

　　The intelligent motor protector adopts microprocessor technology, which not only solves the shortcomings of the traditional thermal relay setting rough, unable to achieve phase failure protection, poor repeatability, and large measurement parameter errors. The protector judges the phase failure through the current, and the software simulates the heat accumulation process to achieve overload protection, etc. to ensure the reliable operation of the motor , and the powerful expansibility of the microprocessor includes switch input, relay output, 4~20mA transmission output, RS485 communication, etc., which well meet the "four remote" functions of the control system .

　　The motor protector improves the reliability of motor operation and the intelligent requirements of the system. Therefore, the reliable operation of the protector plays a pivotal role. At the same time, it also puts forward more realistic requirements for the protector to resist external interference. The following proposes reliability design from both hardware and software aspects.

2 Hardware Reliability Design

　　2.1 Selection of microprocessor

　　Adopt Freescale's high-performance processor MC9S08AW60. MC9S08AW60 is a highly energy-saving processor based on the S08 core of Freescale. It is the first microcontroller approved for the automotive market. It can be used in home appliances, automobiles, industrial control and other occasions, and has the best EMC performance in the industry.

　　2.2 Power supply filtering

　　Using electromagnetic principles to filter hardware circuits is an effective way to improve the EMC of the protector. The circuit is shown in the figure below. After two-stage filtering consisting of thermistor t, varistor RV1, inductor L1, L2, differential mode capacitor C1, common mode inductor L3, common mode capacitor C2, C3, the input and output interference caused by the power supply end is well isolated. PTC thermistors are mainly used for overcurrent and overheat protection. They are directly connected in series in the load circuit. When an abnormal condition occurs in the line, they can automatically limit the overcurrent or block the current. When the fault is eliminated, they return to their original state. They are commonly known as "ten thousand times fuse". The selection is determined according to the maximum working current of the line. Varistors are mainly used to absorb various operating surges and inductive lightning surge overvoltage protection to prevent such overvoltage interference or damage to various circuit components. According to the surge voltage to be endured by the design, it is selected according to the maximum allowable use voltage and current capacity. Among them, L1, L2, C1 are used to suppress differential mode interference, and L3, C2, C3 are used to suppress common mode interference. L1 and L2 iron cores should be selected from materials that are not easy to saturate and materials with excellent MF characteristics. According to the IEC-380 safety technical index recommendation, the selection range of component parameters in the figure is: C1=0.1~2uF; C2, C3=2.2~33uF; L3 is several or tens of millihenries, and different parameter values ​​are taken depending on the working current.

　　Calculate the capacity of C2 and C3 according to the following formula:

　　Ii=2πfCyU

　　Where: Ii - Allowable AC leakage current

　　f───power frequency;

　　U───power supply voltage;

　　                                                              Figure 1 Power supply processing diagram

　　                                                   Figure 2 Experimental results of unfiltered power supply

　　                                                 Figure 3 Experimental results after power supply filtering

　　The above figure shows whether a filter is used at the power supply end. The Swiss TRANSIENT 2000 electromagnetic compatibility tester 1000V 100KHZ 0.75mS condition EFT group pulse experiment is used. Compared with the signal captured by TEXtronix TDS1012B, the power supply output end without filtering produces a spike pulse, which will cause the microprocessor to reset or even crash.

　　2.3 Signal processing

　　Harmonic and electromagnetic radiation interference can cause the protector to malfunction, making the electrical instrument measurement inaccurate or even unable to work properly. The sources of this type of interference in the motor control circuit are the frequency converter and the on-site intercom. The solutions are: First, glue the signal input line. The glued double-glued wire can reduce common-mode interference. Since the direction of the magnetic flux of the electromagnetic induction of the wire is changed, its induction cancels each other out. Second, internal line processing. As shown in the figure below, a differential amplifier with dual differential input has a very high common-mode rejection ratio. Connect an RC filter in the input circuit, use special devices at the input and output ends of the signal, reduce input and output impedance, reliably ground and reasonably shield, etc. [page]

　                                                            　Figure 4 Signal processing circuit

　　2.4 Protection output processing

　　The input and output terminals use photoelectric isolation to eliminate common mode interference. At the same time, a varistor is connected in parallel to the output terminal of the protection relay, which effectively increases the life of the relay and reduces the internal interference caused by the external contactor. Considering the uncertainty of the control voltage used by the customer and the contactor coil capacity, it is confirmed to use MYG14D821.

　　                                                                 Figure 5 Protection output circuit

　　2.5 External Storage Technology and Watchdog Protection Circuit

　　Use external memory chip X25043, SPI interface. The microprocessor has built-in SPI control module, which is convenient to interface with the chip. External storage technology ensures the record of operating status and events. Low voltage reset and external watchdog improve the reliability of the protector.

　　                                        Figure 6 External memory and watchdog circuit

　　2.6 Main unit and display unit connected via RS485

　　Taking into account the particularity of the use environment and the diversity of requirements, the connection between the main body and the display unit also adopts RS485 Modbus-Rtu protocol connection, which improves the reliability of display and control.

3 Software Reliability Design

　　3.1 Scheduling of Real-time Multitasking

　　The protector plays an important role in protecting the motor. The requirements for it are that it should neither malfunction nor refuse to operate, and it must be fast. The scheduling of real-time multi-tasks actually achieves a macroscopic multi-task effect through the rotation of time slices. For the protector, there are three important tasks: equally spaced AC sampling, obtaining steady-state and transient power data according to the algorithm; judging faults based on the obtained data, fault timing, clearing and tripping output; and human-computer interaction interface. The figure below takes a cycle T=20mS and 32-point sampling as an example (taking into account fast division). The total time of 32 samplings is 3.2mS, the data calculation time is 9.72mS, and the timing is 0.36mS, so the time for human-computer interaction is 6.72mS. Such task scheduling not only meets the real-time protection requirements, but also responds to parameter settings quickly.

　                                                           　Figure 7 Task execution relationship

　　3.2 AC sampling and digital filtering

　　For an AC sinusoidal signal, the effective value of the voltage in one cycle is

　　According to the definition of the effective value of a continuous periodic AC signal in the electrical principle, the continuous signal is discretized and the continuous integral is replaced by numerical integration to obtain the relationship between the effective value and the sampling value.

　　Same reason

　　Based on multiple sampling of the signal, the data closest to the true value is extracted through the software algorithm. This algorithm calculates the continuous cycle of the AC signal with high accuracy and strong ability to resist waveform distortion. When using this algorithm, digital filtering such as the continuous average method and the median algorithm can also be used at the same time to improve the anti-interference ability of the protector.

　　3.3 Software Traps

　　The program is fixed in the memory of the microprocessor and arranged uniformly by the compiler. However, when designing, the designers consider the scalability of the product and generally leave a margin, so there is always some storage space that is not used. When the PC pointer of the microprocessor is misplaced due to interference, the system will fail. The software trap is to add a forced jump instruction after the unused storage space, interrupt entry, and subroutine to restore the faulty PC pointer to normal.

　　The method is: NOP

　　NOP

　　JSR MAIN

4 Conclusion

　　Aiming at various electromagnetic compatibility problems encountered in the actual use of low-voltage intelligent motor protectors, this paper proposes anti-interference methods from both hardware and software aspects according to the characteristics of the microprocessor system, and obtains good EMC performance.