1 Introduction As a dragging machine, the electric motor is widely used in all aspects of the national economy because of its advantages of simple structure, low price, and convenient use and maintenance. In the contemporary era, with the development of electronic technology and the maturity of smart motor protector technology, the penetration rate is getting higher and higher.
The intelligent motor protector uses microprocessor technology, which not only solves the shortcomings of traditional thermal relay setting roughness, failure to achieve phase failure protection, poor repeatability and large measurement parameter error. The protector judges the phase failure by the current, and the method of software simulation of the heat accumulation process to achieve overload protection and other methods ensure the reliable operation of the motor, and the powerful scalability of the microprocessor includes switching input, relay output, 4 ~ 20mA change Sending output, RS485 communication, etc. satisfies the "four remote" functions of the control system.
The motor protector improves the reliability of motor operation and system intelligence requirements. Therefore, the reliable operation of the protector plays an important role, and also puts forward more realistic requirements for the protector to resist external interference. The reliability design is proposed from two aspects of hardware and software.
2 Hardware reliability design
2.1 The choice of micro-processing uses Freescale's high-performance processor MC9S08AW60. MC9S08AW60 is a highly energy-efficient processor based on the S08 core of Freescale, and 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 Filtering at the power supply side The use of electromagnetic principles for hardware circuit filtering is an effective method to improve the EMC of the protector. The circuit is as shown in the figure below. The two-stage filtering process consisting of thermistor t, varistor RV1, inductance L1, L2, differential mode capacitor C1, common mode inductance L3, common mode capacitors C2, C3 is well isolated due to the power supply Input and output interference. The PTC thermistor is mainly used for overcurrent and overheat protection. It is directly connected in the load circuit. When an abnormal condition occurs on the line, it can automatically limit the overcurrent or block the current. When the fault is removed, it returns to the original state. Secondary fuse ". The choice is determined according to the maximum operating current of the line. Varistors are mainly used to absorb various operating surges and induce lightning surge overvoltage protection to prevent such overvoltage interference or damage to various circuit components. According to the surge voltage experienced by the design, choose according to the maximum allowable voltage and current capacity. Among them, L1, L2, C1 is to suppress differential mode interference, L3, C2, C3 is to suppress common mode interference. L1, L2 iron core should choose materials that are not easily saturated and have excellent MF characteristics. According to the IEC-380 safety technical index recommendation, the selection range of the component parameters in the figure is: C1 = 0.1 ~ 2uF; C2, C3 = 2.2 ~ 33uF; L3 is several or tens of millihenries, which varies with the operating current The parameter value.
Calculate the capacity of C2 and C3 according to the following formula:
Ii = 2Ï€fCyU
In the formula: Ii───Allowable AC leakage current
f───Power frequency;
U───Power supply voltage;
Figure 1 Power end processing diagram
Figure 2 Experimental results of unfiltered power supply
Figure 3 Experimental results after power supply filter processing
The picture above shows whether the power terminal uses a filter. The Swiss TRANSIENT 2000 electromagnetic compatibility tester 1000V 100KHZ 0.75mS condition EFT group pulse experiment is used. The signal captured from the TEXtronix TDS1012B is compared. A peak pulse is generated at the output end of the power supply without filter , Will cause the microprocessor to reset or even crash.
2.3 The processing of harmonics and electromagnetic radiation interference at the signal end will cause the protector to malfunction, making the measurement of electrical instruments inaccurate or even unable to work normally. The sources of such interference in the motor control loop are the frequency converter and the field intercom. The solution is as follows: First, the signal input wire is glued, and the double glued wire can reduce the common mode interference. Due to the change of the direction of the electromagnetic flux of the wire, the induction cancels each other. The second is internal line processing. As shown in the figure below, a differential amplifier with dual differential inputs has a high common-mode rejection ratio. Connect the RC filter in the input loop, use special devices for the input and output of the signal, reduce the input and output impedance, reliable grounding and reasonable shielding and other measures.
Figure 4 Signal processing circuit
2.4 Protection output processing The input and output terminals adopt photoelectric isolation method, which can also eliminate common mode interference. At the same time, a varistor is connected to the output terminal of the protection relay, which effectively improves the life of the relay and reduces the external contactor. The action interferes internally. Considering the uncertainty of the control voltage used by the customer and the capacity of the contactor coil, confirm to use MYG14D821.
Figure 5 protection output circuit
2.5 External storage technology and watchdog protection circuit use external storage chip X25043, SPI interface. The microprocessor has a built-in SPI control module, which is convenient to interface with the chip. The external storage technology guarantees the recording 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 The main body and the display unit are connected by RS485 Considering the particularity of the use environment and the diversity of requirements, the connection between the main body and the display unit is also connected by the RS485 Modbus-Rtu protocol, which improves the reliability of the display and control.
3 Software reliability design
3.1 The real-time multi-task scheduling protector plays an important role in protecting the motor. The requirement for it is that it cannot be erroneously operated or rejected, and must be fast. Real-time multi-task scheduling actually achieves the macro-multi-task effect through time slice rotation. For the protector, there are three important tasks: AC sampling at equal intervals to obtain steady-state and transient power data according to the algorithm; judging the fault based on the obtained data, fault timing, clearing and trip output; human-machine user-interface. The following figure takes a cycle T = 20mS, 32-point sampling as an example (taking into account the fast division), the total time of 32 samplings is 3.2mS, the data calculation time is 9.72mS, and the timing is 0.36mS, then the human-computer interaction time is 6.72mS . Such task scheduling not only meets the real-time protection requirements, but also responds to the parameter settings more quickly.
Figure 7 task execution relationship
3.2 AC sampling and digital filtering For AC sinusoidal signals, the effective value of voltage for one cycle
According to the definition of the effective value of the continuous periodic AC signal in the electrical engineering principle, the continuous signal is discretized, and the continuous integration is replaced by numerical integration, thereby obtaining the relationship between the effective value and the sampled value. Discretization
Empathy
On the basis of sampling the signal multiple times, the data that is closest to the true value is extracted through a software algorithm. This algorithm calculates continuous periodic AC signals with high accuracy and strong resistance to waveform distortion. When using this algorithm, digital filtering such as continuous average method and median algorithm can also be used to improve the anti-interference ability of the protector.
3.3 The software trap program is fixed in the memory of the microprocessor and is arranged by the compiler. However, when designing, the designer considers the scalability of the product, and generally has a margin, so there is always some storage space that will not be 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 error PC pointer to normal.
The method is: NOP
NOP
JSR MAIN
4 Conclusion This paper addresses various electromagnetic compatibility problems encountered in the actual use of low-voltage intelligent motor protectors. According to the characteristics of the microprocessor system, it proposes anti-interference methods from hardware and software, and obtains good EMC performance.
Shangchai Generator Set,Power Generator,Electricity Generator,Output Type Generator
Yangzhou Hengyuan Electromechanical Equipment Co., Ltd. , https://www.lchygeneratorset.com