On the design principle of ARD3 motor protector

0 Introduction With the development of electronic technology, the motor protector is developing towards the intelligent direction based on fieldbus. The ARD3 motor protector designed by our company is based on the domestic advanced level and is a motor protector with intelligent protection and communicable functions. The current range of the product series is complete, and the rated current range of the product series is 1.6 to 800A; the measurable current range is wide, which can reach 10 times the rated current of the motor; the use of advanced software algorithms and reliable hardware design, the motor overload, phase failure, three Phase unbalance, locked rotor, blocking, overvoltage, undervoltage and other faults are used for effective judgment and reliable protection. Overload protection adopts the method of calculating and analyzing the current thermal capacity of the motor, and judges the overload state of the motor according to the heat capacity. This method can maximize Give full play to the overload capacity of the motor; equipped with programmable switch input and relay output, used to realize remote signal monitoring and direct starting of the motor running status of the remote master station, auto-coupling buck, star-delta and other starting methods; with standard RS-485 interface ModBus communication protocol realizes computer networking.

1 Hardware design
ARD3 motor protector uses H8 / 3687FP single chip microcomputer to realize the motor protection function. In terms of hardware, it is mainly composed of three-phase current signal sampling, leakage current sampling, voltage signal sampling, keyboard interface, display part, control output, alarm output, communication interface, etc. The key parts are briefly introduced below.
1.1 Signal acquisition unit
ARD3 motor protector uses AC sampling algorithm to calculate the signal under test. The sampling method is to continuously sample a complete waveform of the measured signal in real time according to a certain period (referred to as the sampling period) (only half a cycle is required for the sine wave), and then perform the true effective value calculation on the sampled discrete signal, thus Obtain the true effective value of the measured signal, so as to avoid the influence of the measured signal waveform distortion on the sampled value.
The function of the signal acquisition unit is to sample, rectify, and amplify the output signals of the secondary measurement of the transformer, and convert these signals into signals that can be processed by the single-chip microcomputer. The principle of the signal acquisition and amplifying circuit for processing the three-phase current signal, residual current signal and voltage signal in the ARD3 motor protector is the same. Now we will use one current signal acquisition and amplifying circuit as an example to illustrate the working principle of the circuit.

Figure 1 Signal acquisition and amplification circuit

The signal acquisition amplifier circuit is shown in Figure 1. In the figure, the diodes A1 and A7 are bidirectional diodes, which play the role of overvoltage protection for the subsequent circuit. When the input signal is within the normal range, A1 and A7 have no effect. When the input signal exceeds the normal range (or there is a pulse interference signal), A1 and A7 are turned on to prevent signals beyond the port range of the subsequent circuit from entering the subsequent stage Circuit, destroy the A / D circuit of the latter stage. CR1 is a sampling resistor, which converts the current signal output from CT1 into a voltage signal. LM324 and CR4, CR7, CR10, CR13 form an in-phase amplifier circuit to amplify the voltage signal and input it to the A / D conversion circuit.
In Figure 1, the LM324 uses dual power supplies, which can ensure that the LM324 output voltage reaches 5V and fully utilizes A / D conversion to improve display accuracy. In Fig. 1, the input signal is divided into stages by the operational amplifier, and the small signal is output from P1.0 and the large signal is output from P1.1. This processing is because: the current range to be processed by the motor protector is very wide (from 1 times the rated current of the motor to 10 times the rated current), and the bin processing can improve the measurement accuracy.
1.2 I / O unit The switch input processing circuit is shown in Figure 2. The circuit switching value is input by IN1 ~ IN7, and IS1 ~ IS7 are generated after the light coupling. The parallel signals IS1 ~ IS7 are input to 74HC165, and the parallel signal is converted into a serial signal by 74HC165 and transmitted to the CPU. Resistors R11 ～ R18 play a current limiting role to protect the diode in the optocoupler from being damaged. RS1 ～ RS8 are pull-up resistors used in conjunction with capacitors CS ～ CS8 to stabilize the output level of the optocoupler and protect the optocoupler when power is applied.

Figure 2 Switch input circuit

Fig.2 Switching input circuit

The relay control circuit is shown in Figure 3. JDQ1 ～ JDQ4 are connected to CPU, the power supply voltage of transistors QJ11 ～ QJ14 is + 5V, and the power supply voltage of transistors QJ1 ～ QJ4 is + 24V. The control circuit of QJ11 and QJ1 is used to explain the working principle of the circuit. When the CPU outputs a high level, the transistor QJ11 is not turned on, OUT11 will not output current, the light coupling will not be turned on, and JT1 will also output high level, QJ1 will not conduct The relay will not operate. When the CPU outputs a low level, the transistor QJ11 turns on, OUT1 outputs a high level to turn on the photocoupler, JT1 becomes low, and the transistor QJ1 turns on and OUT1 outputs a low level to make the relay act. In Figure 3, diodes DJ1 ~ DJ4 are used as relay freewheeling diodes.

Figure 3 Relay control circuit

Fig.3 Relay control circuit

The control output part can use electromechanical relay or solid relay. The former is cheap, rich in market products, and the driving circuit is relatively simple, but the reliability and service life are limited, and "sparks" will occur when the contacts act, which can affect the normal operation of the system in severe cases. Therefore, in the layout of the PCB board, the relay should be as far away as possible from the microcontroller and close to the output port of the instrument. In addition, free-wheeling diodes should be connected in parallel at both ends of the relay coil, otherwise a higher induced voltage will be generated at the instant when the relay coil is de-energized, thereby destroying the circuit. The solid-state relay has the characteristics of long life, stable performance, no spark, etc. The solid-state relay is adopted in consideration of the reliability of the product in this product.
1.3 Communication unit The communication circuit is shown in Figure 4. The communication circuit realizes the conversion of the CPU serial output level to RS485 level. The ingenious point of this circuit is that the data transmission and reception are directly controlled by the hardware, without the CPU to participate in the control, which can save CPU resources and simplify programming.

Figure 4 Communication circuit

Fig.4 Communications circuit

1.4 CPU unit
The CPU unit is the core unit of the motor protector. Signal acquisition, various alarm processing, communication functions, display functions ... all are completed by it. The CPU chip used in this product is the Renesas H8 / 3687 chip. The chip functions are as follows: 62 basic instructions; RTC (on-chip real-time clock, which can be used as a free operation counter), SCI (asynchronous or clock synchronous serial communication interface ) 2 channels, 1 channel IIC interface, 8 channels 10-bit A / D, 2 8-bit timers (Timer B1, TimerV), 1 16-bit timer (TimerZ), watchdog timer, 14-bit PWM, 45 I / O pins (H8 / 3687N has 43 I / O pins), including 8 high-current pins that can directly drive LEDs (IOL = 20mA, @ VOL = 1.5V), on-chip reset power POR circuit , On-chip low voltage detection circuit (LVD). The chip has two packages: LQFP-64 (10mm × 10mm) FP-64 (14mm × 14mm). The CPU unit circuit is shown in Figure 5.

Figure 5 CPU circuit

Fig.5 Cpu circuit

Because the A / D function, IIC function, RTC, timer, watchdog and other functions have been integrated into the chip, the peripheral circuit of the CPU unit is very simple. Each pin only needs an external pull-up resistor and port drive capability to increase It is sufficient to stabilize the filter capacitor of the signal.

2 Software design The system software must complete three-phase current, one-channel residual current, three-channel voltage A / D, various protection quantities calculation, protection function judgment processing, display voltage, current, fault record, key processing, communication, transmission, etc. Features. Only when the program flow is reasonably arranged to complete these functions, the protector can work reliably. The program flow chart is shown in Figure 6:

Figure 6 program flow

Fig.6 Flow chart of software

3 Anti-interference measures As a motor protection device, the motor protector must have strong anti-interference. In the design process of the software and hardware of this product, the following measures are taken to improve the anti-interference of the product: 1 hardware: the power part is added with EMC filter, the high-frequency transformer secondary and primary are added with high voltage capacitor, the output part is added with filter circuit; the signal acquisition part is added Filter circuit; add port protection circuit at each chip input port for signal processing; add decoupling capacitor at each chip power input; freewheeling diode in parallel at both ends of the relay, add optocoupler to isolate from CPU port; unused CPU port Defined as the output state; the analog part and the digital part are partitioned when the PCB board is laid out, the analog signal is routed in the analog area, and the digital signal is routed in the digital area, the two do not enter each other's area; try to thicken the power line and ground when wiring Line, signal line, 145º line, no right-angle line; use the internal watchdog monitoring program of the CPU to run. 2 For software: all channels of signal acquisition use software filtering to increase the accuracy of sampled values. By taking a series of measures, the anti-interference performance of the product has been greatly improved, and this product successfully passed the 3C safety certification type test at one time.

4 Conclusion
ARD3 motor protector adopts advanced design scheme, integrates measurement, protection, control and communication. The product performance is safe and reliable, which can implement reliable and effective protection for the motor. ARD3 motor protector can completely replace traditional motor protection products such as thermal relays and temperature relays in actual use, replacing various conventional components such as pointer-type fuel gauges, signal lights, power transmitters, etc., simplifying the motor control circuit and reducing cables in the cabinet Connection and construction volume on site.

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