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Single chip microcomputer in electric motor protector (motor temperature switches) in the role of
Date:11/06/2014   Click:1765

Motor protection device to realize the function mainly includes: three-phase current display, voice alarm, fault tripping, the fault memory, overload protection, short circuit protection, phase lack protection, three-phase unbalance protection, phase sequence protection etc.. With analog circuits to achieve such traditionalcomprehensive function, the line will become very complicated, the volume of the whole device can be very large.Therefore, at present some simulation electronic motor protector can achieve the functions are single. For example:overcurrent protection, phase lack protection device, and the emergence of SCM, which makes the motor protector is a qualitative leap in the development of hitherto unknown, reach the level in respect of intelligent, functional diversification, miniaturization, modularization, performance reliability etc..

Implementation of motor protection function by SCM system, in terms of hardware is mainly composed of a three-phase current signal sampling, sampling, leakage current and voltage sampling, keyboard interface, display part,control output, alarm output, communication interface and other parts, the following are the brief analysis of the key parts of the.

A three-phase current sampling part,

This part of the design is directly related to the current sampling precision. Therefore, must through theoretical analysis, repeated experiments to ascertain. This part of the line can be used rectifier circuit, also can use the ratio of direct AC converter circuit, which circuit is directly decided by DC or AC sampling is a sampling program, program design will be completely different.

If the use of DC sampling line is required the use of rectifier circuit, rectifier circuit comprises a half wave rectifier, a bridge type full wave rectification precision, half wave rectification, precision full wave rectifier forms, the half wave rectification of a diode and a bridge type full wave rectification are diode conduction voltage drop effect of rectifier circuit for linear problems, especially when the signal the lower voltage, the greater the impact. In order to reduce the influence of the diode conduction drop, should choose the Schottky diode rectifier devices, such as IN5819, but note that IN5819 pressure value is low (only 40V), so the sampling voltage signal should not be too large. Semi wave or full wave rectifier circuit are introduced in many data, this needless to say. Advantages of this circuit is to avoid theinfluence of the rectifying diode voltage drop, but also increase the complexity of the circuit, but also need to providepositive and negative power amplifiers, to increase the supply part of the cost. In actual use should weigh the advantages and disadvantages, consider to choose.

Two, the display part

In the industrial control instrument is often used in LED digital tube as the user interface, the utility model has the advantages of high brightness, simple drive circuit, but only show a few characters Co., to it is difficult to express therich information. At present, some high-end instruments began to use the character type liquid crystal display or LCD display, and some also contain the Chinese character module, the direct use of Chinese display, so that theinterface is very intuitive and easy to understand and operate. But at the same time, increase the cost of lines,especially the graphic liquid crystal display module price is too high, the general low priced instruments can not accept.

SCM and LED digital tube interface generally uses the way of serial communication, connected with the shift registerto drive digital tube, a different driver design can realize different display modes. For example: digital tube twinkle,the number of decimal places automatically transform, the special character display etc.. In order to reduce thepower consumption of digital tube, the general use of dynamic driving method, also is in a cycle (T) only in the (1/3 - 1/2) T time display brightness adjusting duty cycle can be changed through the digital tube. In addition, the need to pay attention to the digital tube driving voltage can not be too high, if driven by +5V power on the need to add the current limiting resistor or step-down diode. The better with step-down diode, voltage fluctuations can preventchanges occur due to stroke the digital tube light caused, thus ensuring the digital tube brightness stability.

The data line SCM and character type LCD module interface can still use the serial interface mode, and the control signal can be directly controlled by the microcontroller I/O port. If the data line in a parallel mode, will occupy a lot ofresources of MCU, and therefore should not be used. In the preparation of the driver, in the performance ofpermission should pay attention to the signal frequency to minimize the data and clock lines, which can reduce theelectromagnetic interference.

Three, the output control

The output control part adopts electromechanical relays or solid state relay. The former is cheap, the marketproducts rich, drive circuit is relatively simple, but the reliability and the service life is limited, and the contact actionwill produce a "spark", serious when can affect the normal operation of the system. Therefore, in the PCB board layout should be as far as possible from the single chip microcomputer relay output port and close to the instrument.In addition, in the relay coil should be continued flow diode in parallel, or instantly generate high induction voltage inrelay coil power, thus breaking the circuit. Solid state relay has the advantages of long service life, stable performance, no spark and other characteristics, with the perfection of its price is gradually reduced and performance has been widely used, but the small power solid state relay prices are still many times higher than ordinary relay, therefore, in the motor protector are used in most of the common relay. In many applications, people want to have two independent control output, the control output as a main loop and another for the auxiliary output and control as an alarm or other function.

The SCM system, the software is a very important part, this part can fully reflect the designer's ideas, through theprogram can realize the intelligent motor protector