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CAN bus based motor protection device

In Electronic Infomation Category: C | on April 15,2011

Abstract: Three-phase Induction Motor Protection System hardware and SP486CT datasheet and software have been studied to Freescale DSP 56F807 microprocessor as control core, together with the CAN bus, LCD display and SP486CT price and sampling, and SP486CT suppliers and other functional modules. The field bus technology to measure the specific microprocessor control devices placed in the individual measurement and control device into a decentralized network of nodes connected to each other to exchange information and jointly to complete the network control tasks. Because DSP algorithms on the strong data processing capability, the instantaneous voltage, current and amplitude of the negative sequence current accurate calculations without regard to a matter of time, with the alternative method of computing hardware, software, logic, reducing the waste of hardware resources .

Is the industries most widely used motor of power equipment, but because in the course of protection is not enough, often the following issues: the low efficiency devices, protection devices often refuse to move so that the motor burned, due to malfunction of the trip . In recent years, with computer technology, automatic control theory and the continuous development of signal processing theory, there has been a microprocessor as the core, the combination of protection and the formation of computer technology, computer protection device.

1 system hardware design

Dual CPU system architecture, design and implement a set by the digital signal processor 56F807 MCU W78E516 constitute additional computer protection monitoring device. FREESCALE digital signal processor 56F807 (thereafter referred to as the 56F807) as the main chip to complete the signal acquisition, signal processing, protection and communication functions. The chip A / D conversion, to open into and out of circuits and serial communication ports and other functions, the signal input voltage is 0 V ~ 3 V, the fastest conversion speed required for each scan at the same time 5.3 s, acquisition of large ones, * median and rate of flow of sample completely filled the requirements. MCU W78E516 complete all the features of man-machine interface. Between the two modules MODBUS protocol based on RS -485 bus real-time communication. The dual CPU architecture has to work in parallel, the advantages of division of labor, both to ensure the protection of the liquid, selectivity, sensitivity and reliability, but also to achieve real-time measurement precision. CAN bus by real-time remote monitoring and debugging. Therefore, the user can choose according to the site network communication interface and flexible way. This truly the intelligent motor protection, centralized monitoring and management. The system hardware block diagram in Figure 1. According to their functions into two modules: the data acquisition module, the module out into the open, EEPROM module and DSP 56F807 protection module comprising; by the CAN bus and RS-485 bus connection via the display circuit monitoring module.

Figure 1 System hardware diagram

1.1 protection module features

Protect the modules main function is to complete the data collection, processing, calculation, logic and export protection logic and action. Hardware circuit diagram shown in Figure 2.

Figure 2 Hardware connection diagram

1.1.1 Data Acquisition Module

Acquisition of the analog system used 56F807 own 12 bit, 16-channel A / D converter, voltage and current measurement using time sampling frequency 1 600 Hz, sampling interval of about 0.625 ms. For the 50 Hz power frequency AC signal, each cycle of the sample is 32 points. After the external current and voltage input isolation transformer, low-pass filter input to the converter module, the A / D conversion by a number of discrete sequence samples, and then Fourier algorithms are Ia, Ib, Ic, I0, Ua, Ub , Uc, and the amplitude of U0. Also calculated derived motor active power, reactive power, power factor and other parameters.

1.1.2 out into the open module

Open out into the module under development requirements, the device designed to open into the 12-volume, 12-out volume. Open intake for motor start, stop and alarm status feedback information collection. 12-out volume is used mainly for fault tripping and alarm use. The system uses the eight I / O port given the amount of signal out of 4 and 8 bit through the serial input, serial / parallel output shift register 74HC595 control group 8 out of 8 output volume control. In order to prevent malfunction caused by interference, the use of two related I / O port while the output coupler with different power usually moves off the signal to achieve open and close function. When the condition is met out, and then out the amount of optical coupling from the post after TIL113 output to drive external relays to achieve protection of the export action.

1.1.3 data storage unit module

X5043 chip is produced by the United States XICOR set on-reset, "watchdog" timer, voltage monitoring and serial E2PROM four functions in one of the specific integrated chip, to reduce system cost, save board space . X5043 in the power-on reset, "watchdog" timer, power supply voltage monitoring function of the system can play a protective role; 512 8 bit of E2PROM can be used to store important data within the system.

1.2 Monitoring Module

1.2.1 CAN communication module There are currently many

microcontroller embedded into a system CAN controller, DSP 56F807 is also integrated within the CAN controller, which supports standard and extended message frames, just connect the external CAN transceiver that can easily be CAN CAN bus controller connected to the network, the network office at any time a node can take the initiative to other nodes on the network to send information, receive and send data in real time.

1.2.2 Other modules

RS-485 Communication: Communication with 485 master-slave network, the use of MAXIM balance produced by differential transmit / receive chip MAX485, MAX485 chips with half-duplex communication, you can achieve more comprehensive protection of the networking device. Each IC chip includes a driver and a receiver, consistent with RS-485/RS-422 communication standards.

6N137 optical coupler is a single channel for high-speed optocouplers, with temperature, current and voltage compensation, in this design, the use of the DSP in the TXD1 6N137 optocoupler and RXD1 signal TX and RX isolated.

Display circuit: as the motor protection system, the display module, the system uses the LCD is TG160128A1, it is assembled by the manufacturer of the LCD driver, and provides an interface drive circuit, through DSP56F807 the I / O ports can be read and write operations on the LCD to achieve.

Power modules: DSPs operating voltage is 3.3 V, while the development boards power supply voltage is 5 V, it must be a 5 V ~ 3.3 V voltage conversion.

Use AS1117M5 -33 to 5 V-chip 3.3 V voltage conversion to voltage. Between digital and analog power supply is connected with the beads, between digital and analog ground are also connected with the beads. Between analog and analog power supply with small capacitors, digital and digital power connection between the small capacitor.

2 system software design

In the motor protection devices, each node periodically collected on-site detection of voltage and current signals, and then collected by Fourier algorithm to analyze and process the data to calculate the voltage and current RMS value and the sub-harmonic and the amplitude, phase, positive and negative sequence parameters such as real-time computing, real-time to determine whether the value received exceeds the limit value, that is, the failure to determine whether, and through the CAN bus to send data to the host computer.

2.1 overall system software

Design of the system software design is modular by design, is divided into three parts: the initialization module, system control module and communication module. Initialization module of complete DSP systems, peripherals, components, and system management initialization and so on. As the motor protection system is demanding real-time system, which uses the main program module and the interrupt routine method of combining modules.

Mainly by the protection module interrupt subroutine and communication module. Main flow and the protection of the module flow chart shown in Figure 3.

Figure 3 System flow chart

2.2 each phase current, voltage amplitude algorithm

The 56F807 chip has the following advantages: in a single instruction cycle can be completed within one addition and one multiplication, separate program and data space, can access instructions and data to support pipelining, so that fetches, decoding and execution operations can overlap. In addition the high frequency, can be used in the design of complex algorithms to provide accurate time guarantee protection. Therefore, the magnitude of strike current and voltage are used Fourier filtering algorithm. First find the Fourier sine and cosine coefficients, and then square, square root formula calculated amplitude.

Set input power is:

If one week after the wave

discrete points within N consecutive samples Y1, Y2, Y3, Yn, you can find the whole week Fourier sine and cosine function as follows:

Where: N sampling points for the week of waves, 600 Hz sampling frequency, N = 12.

K = 1 Shi be fundamental positive, cosine coefficient:

Then according to (6) to calculate the exact magnitude of the value of accurate and reliable fault diagnosis.

By (6) the calculation of the amplitude has two square and one square root, greater than calculated, so use with powerful computing capabilities of the DSP, can not be considered a matter of time and to ensure the accuracy of the amplitude, thus ensuring the reliability of protection.

2.3 negative sequence current algorithm selection

Negative sequence current as a criterion for motor protection, there is asymmetry in the fault and determine whether the type of asymmetric fault, has a very important role. The use of DSP has a very powerful data processing capability, consider alternative software method of calculation method of hardware logic, the connection may not only reduce the hardware, but also can improve the reliability and accuracy of the protection.

The three-phase current XA, XB, XC symmetrical components can be decomposed into positive, negative and zero sequence of three components, while the negative sequence symmetrical components compared with the amount of three-phase fundamental relationship:

The sampling rate 12 points per week, three-phase current sampling instant value XA (k), XB (k), XC (k), the negative sequence through the fast algorithm in practice to take the instantaneous negative sequence current is:

By (8) can be seen that the instantaneous value of the negative sequence current in the k-point sampling phase A, B relative to the first k and k-4 and C-phase point sampling value of the first k-4 point sampling value of the relevant , the use of current amplitude formula can accurately calculate the magnitude of the negative sequence current.

2.4 CAN communication module

In a variety of field bus, CAN bus not only has outstanding reliability, timeliness and flexibility. But also has many other features not available in the bus:

(1) by the message identifier (11 bit or 29 bit) to determine the bus access priority; (2) using non-destructive arbitration technique, when two nodes send messages simultaneously to the bus, the priority level than low node will take the initiative to send out, the higher priority node can not be affected; (3) uses a short frame structure, transmission time is short, the probability of interference is low, with good error detection results, and each frame CAN Information has a CRC checksum to ensure that the very low data error rate; (4) serious errors in the CAN node has an automatic shut case of output, so that the operation of other nodes on the bus will not be affected; (5) CAN only can be achieved through packet filtering point, multipoint, and global broadcast, send and receive data in several ways.

Has 14 B each transmit buffer register structure. The register data frame structure includes an identifier, waiting to send data, send data and send the length of the frame buffer priority register.

2.5 CRC checksum algorithm in the 56F807

Order to be able to fast and reliable and timely information to pass each other, consider the transmission distance, site conditions, interference, and many other factors, generally used in the communication method of data validation. Cyclic redundancy check is a common verification methods.

Cyclic redundancy check code CRC (Cyclic Redundancy Check Code) is a branch of linear block codes, error detection capability is a strong cyclic code.

Cyclic redundancy check for error detection of data transmission (Error Detecting) is the use of the principle of division and remainder. Encoding and decoding method is simple, easy, error detection ability, false positive probability is almost zero, and this method to obtain parity information on the way with strong coverage, is a highly efficient error correction method . Figure 4 shows the basic principle of parity.

Figure 4 CRC check

basic schematic Production

CRC polynomial G (x) by the agreement, there are a variety of production polynomial included in the international standards, such as:

CRC-12 G (x) = x12 + x11 + x3 + x2 + x +1.

CRC-16 G (x) = x16 + x15 + x2 +1, etc., used in this design is the CRC-16. The codec used

CRC polynomial modulo 2 division, and polynomial division can be used with a feedback shift register, therefore, with the DSP to achieve the CRC encoding and decoding key is to simulate by a DSP shift register (that is, simulated handwriting polynomial division.) Taking into account the 56F800 series DSP accumulator A and B are 32 bit, therefore, can use a 32 bit accumulator A as a shift register. In the CRC encoding and decoding are related to the code shift and XOR operations, this can 56F800 series of LSR, LSL (logical shift) and EOR (XOR) two instructions to achieve. CRC, the flow chart in Figure 5.

flow chart of Figure 5 CRC checksum This design is the use of DSP56F807

powerful chip, together with the peripheral modules to realize the motor current, voltage signal rectifier, filter and converted to DC signal to the DSPs A / D port through the protection algorithm to determine whether the action, fault handling, and parameter settings, LCD, and on through the field bus network for state of all real-time monitoring of the motor, run control, data processing and parameter adjustment, and its function is a simple digital protection before the device can not be compared. Through the protection device designed to debug and prototype analysis showed that the normal protection action, and other protective equipment testing requirements are met, the initial validation of the system hardware and software part of the design is correct.

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