Reliability Design of Low Voltage Intelligent Motor Protector

1 Introduction

As a kind of traction machinery, the motor is widely used in all aspects of the national economy because of its advantages such as simple structure, low price, easy use and maintenance. 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 rough setting of traditional thermal relay, inability to realize phase-break protection, poor repeatability and large measurement parameter error. The protector judges the phase-break fault by current, and realizes overload protection by software simulation of heat accumulation process, etc., which ensures the reliable operation of the motor. 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, and also puts forward more realistic requirements for the protector to resist external interference. The following proposes reliability design from the two aspects of hardware and software.

2 Hardware reliability design
2.1 Selection of microprocessor
Freescale's high-performance processor MC9S08AW60 is used. MC9S08AW60 is a highly energy-saving 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 Power supply end filtering
Using electromagnetic principles for hardware circuit filtering is an effective way to improve the EMC of the protector. The circuit is shown in the figure below. After the two-stage filtering treatment composed 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 due to the power supply end is well isolated. PTC thermistors are mainly used for overcurrent and overheating protection. They are directly connected in series in the load circuit. When the line is abnormal, 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". Determine the selection 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. Select according to the maximum allowable operating voltage and current carrying capacity based on the surge voltage that the design withstands. Among them, L1, L2, and C1 are used to suppress differential mode interference, and L3, C2, and C3 are used to suppress common mode interference. The L1 and L2 cores should be made of materials that are not easy to saturate and have excellent MF characteristics. According to the IEC-380 safety technical indicators, 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 supply 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 conditional 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 end processing

Harmonic and electromagnetic radiation interference will 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 loop are the frequency converter and the on-site intercom. The solutions are: First, the signal input line is glued. The glued double glue line can reduce the common mode interference. Because the direction of the magnetic flux of the electromagnetic induction of the wire is changed, its induction cancels each other. The second is internal line processing. As shown in the figure below, the differential amplifier with dual differential input has a very high common mode rejection ratio. Connect RC filters in the input loop, use special devices at the input and output ends of the signal, reduce input and output impedance, reliable grounding and reasonable shielding.

Figure 4 Signal processing circuit

2.4 Protection output end processing
The input and output ends use the method of photoelectric isolation, which can also eliminate common mode interference. At the same time, a varistor is connected in parallel at the output end of the protection relay, which effectively increases the life of the relay and reduces the interference to the internal due to the action of 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 storage chip X25043, SPI interface. The microprocessor has a built-in SPI control module, which is convenient for interfacing with the chip. External storage technology ensures 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 unit and the display unit are connected via RS485
Considering the particularity of the use environment and the diversity of requirements, the connection between the main unit and the display unit also adopts the RS485 Modbus-Rtu protocol connection, which improves the reliability of display and control.

3 Software Reliability Design
3.1 Scheduling of Real-time Multitasks
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 multitasks is actually to achieve a macroscopic multitask 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 following figure 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, the timing is 0.36mS, and the human-computer interaction time is 6.72mS. Such task scheduling not only meets the real-time requirements of protection, 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 software algorithms. 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 at the same time to improve the anti-interference ability of the protector.

3.3 Software Traps
The program is solidified 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 erroneous 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.