Keywords: Intelligent motor protector, MCF51EM256, ARD2L, protection time
0 Preface
In modern industrial and mining enterprises, the proportion of motor-driven power accounts for more than 90% of all power. They are already the most important motive power and driving devices in production activities and daily life [1,2]. To this end, the normal operation of the motor is detected and protected. Operation is very important. The protector has experienced thermal relays, fuses, electromagnetic current relays, analog electronic motor protectors, and finally developed into digital electronic motor protectors, which are today's intelligent motor protectors. This article designs an ARD2L intelligent motor protector that protects against motor start-up timeout, overload, underload, short circuit, phase failure, unbalance, grounding / leakage, stall, blockage, external faults, etc. (Hereinafter referred to as ARD2L), which can effectively improve the safety of motor operation and reduce production losses, and is an ideal substitute for traditional thermal relays [3].
1 Hardware design
The hardware circuit of ARD2L includes main control chip MCU, frequency signal, current signal, zero sequence current signal acquisition circuit, switch input module, relay output module, transmission output module, RS-485 communication interface, human-computer interaction unit (status indication Lights, nixie tube / LCD display), hardware circuit block diagram shown in Figure 1.
Figure 1 ARD2L hardware circuit block diagram
1.1 Main control chip
The MCU chip adopts Freescale's Coldfire-V0 architecture core 32-bit processor MCF51EM256, the clock frequency can reach 50.33MHz, built-in 256K Flash, 16K RAM, 4 independent 16-bit A / D channels, 3 timers, 3-way SCI communication interface and built-in RTC clock, I2C, SPI, KBI interface and other resources, with extremely high cost performance.
1.2 Power
The power supply is the key part of whether the equipment can work normally, stably and reliably. ARD2L adopts Ankerui's universal switching power supply module. The input voltage of this module is AC85V ～ 265V, the input frequency is 45Hz ～ 60Hz, and it has multi-channel isolated voltage output to meet the requirements of different functions for different power supply voltages. The output voltage is stable, the failure rate is small, and the output ripple is less than 1%; the power input part is designed to include voltage thermistor, TVS tube, anti-reverse diode and other devices, which has a certain protection against overvoltage and overcurrent. Can make the product pass severe EMC test. This module has high stability, reliability and anti-jamming capability after field use [4].
1.3 Signal acquisition circuit
The signal acquisition circuit is responsible for collecting current signals, frequency signals and zero-sequence current signals. Among them, the current signal adopts the transformer to isolate the input, and the AC signal is raised to the CPU for software differential operation. The current sampling circuit is shown in FIG. 2. Taking the A-phase 6.3A specification as an example, the current transformer ratio used is 100A: 20mA, 5P10 protection type. The current measurement of this scheme achieves 0.5S accuracy in the range of 1.2 times, and meets the accuracy of 5S in the range of 8 times, and its overload capacity is calculated as 8 times, that is, 50.4A current is added to the transformer, and the current through the sampling resistor R1 is 10.08 mA, the voltage across the terminal is 0.886V. At the same time, raise the voltage of the sampled signal UREF = 1.2V, so that the amplitude of the AC signal is greater than zero, which is convenient for A / D sampling; add a voltage limiting diode at the output of the circuit to limit the input voltage to 3.3V or less. The / D sampling channel plays a very protective role.
Figure 2 Current sampling circuit diagram
The frequency sampling circuit is shown in Figure 3. This circuit uses MCP6002 dual op amp for two-stage amplification, the primary amplification factor is small, and filter processing is performed between the primary and secondary. The secondary op amp shapes the AC signal into a square wave signal, which is captured by edge triggering, and then The measurement frequency is calculated inside the CPU.
Figure 3 Frequency sampling circuit diagram
1.4 Human-computer interaction interface
The display of the human-computer interaction interface adopts digital tube or liquid crystal. The user can choose the display mode according to the actual needs, and the input mode is the button. Among them, the digital tube display adopts dynamic scanning mode, and its driving circuit is composed of 74HC595 and triode; the liquid crystal display uses Topview's LM12832BCW 128 dot matrix Chinese liquid crystal, and its data transmission uses SPI serial port, which can greatly save CPU resources. At the same time, the LED and LCD displays are controlled by the same SPI interface, making the two display methods universal.
1.5 Control module
The control module is mainly composed of switch input and output, as shown in Figure 4. Among them, the switch input is used to monitor the switch status of the circuit breaker and contactor and the industrial interlock status of the collection site, and can also be used to start and stop the motor according to customer requirements; the switch output is mainly used to output trip signals and alarms Signal and remote start / stop signal.
Figure 4 Switch input and output circuit
1.6 Communication / transmission module
The communication module adopts the RS-485 module Modbus RTU communication protocol, which can realize the functions of telemetry, remote control, and remote communication. The transmission is to convert the current signal we need into a DC 4 ~ 20mA analog output, which is convenient to form a network system with PLC, PC and other control machines to achieve remote monitoring of motor operation.
2 Software design
The software design of ARD2L mainly uses embedded C language. The software design of the protector includes the initialization of the configuration of the power-on system each time, the reset of the key register, the judgment of whether the display unit is digital or liquid crystal, the initial state of the relay setting, the initialization of A / D sampling and Calculation and protection of electrical parameters. The main functions of the software are as follows:
void main (void)
{
DisableInterrupts;
MCU_initi (); // CPU initialization
if (_RES == 0) // Judge whether it is digital tube or LCD display led_or_lcd = 0;
else
{
led_or_lcd = 1;
PTBDD_PTBDD2 = 1;
PTDDD_PTDDD4 = 1;
lcd_init2 ();
}
recover_FIRSTFLAG (); // Recover memory calibration data
if (FIRSTFLAG! = 0x1234) {init_flash ();}
else recover_byte ();
initi_uart2 ();
relay_all_initi (); // Initial state of relay in place
EnableInterrupts // Start interrupt
Vref_init ();
sampling_init (); // AD sampling initialization
for (;;)
{
__RESET_WATCHDOG ();
if (over_flag == 1) // Calculation and protection {
over_flag = 0;
measure_ABC ();
protect ();
sent (); // send output}
measure_frequency_a (); // measure frequency rtc_time_deal ();
warning_deal ();
trouble_deal ();
program1 (); // Programmable relay processing
event_deal ();
getkey ();
DI_read ();
reset (); // reset lamp_deal ();
stat (); // Statistics total running time, stop time display (); // Measurement data and protection event display
}
}
The software flow of ARD2L mainly includes A / D signal acquisition program, TPM frequency measurement program, electrical parameter calculation program, protection processing program, various communication protocol processing programs, etc. Part of the program flow is shown in Figure 5.
Figure 5 Main program flowchart (part)
3 Test results and accuracy verification
3.1 Current accuracy test results
The current accuracy test source adopts Nanjing Dandick's DK-34B1 AC sampling transmitter. The test of the fundamental wave is carried out by adding the third harmonic of 40% distortion rate. Table 1 tests the effective value and fundamental value of the three-phase current of the ARD2L intelligent motor protector of 6.3A specification. From the data in the table, it can be seen that the accuracy of the ARD2L intelligent motor protector in the measurement range of 10% ~ 120% Ie Class 0.5, Ie is the rated motor power [5].
Table 1 ARD2L intelligent motor protector three-phase current test results
3.2 Protection time test results
The ARD2L intelligent motor protector has protection functions such as starting overtime, overload, underload, short circuit, phase failure, unbalance, grounding / leakage, locked rotor, blocking, external fault, etc., and has been tested for protection time according to the JB / T 10736-2007 standard , See Table 2.
Table 2 ARD2L protection time test results
It can be seen from Table 2 that the protector meets the accuracy requirement of the trip delay protection time error of ± 10% or 100mS [6].
4 Typical applications
The wiring of ARD2L intelligent motor protector using direct start mode is shown in Figure 6. Among them, the start and stop of the motor is controlled by the on-site button (the protector does not control the start and stop of the motor), and the attraction coil of the contactor KM is connected into the normally closed contact of the trip relay. After the power is turned on, when the start button SF is pressed, the KM suction coil is energized, which closes the KM main contact and the motor starts to work; when the stop button SS is pressed, the KM suction coil is de-energized, the KM main contact is released, and the motor stops . The remote start must be controlled by the host computer, the protector itself is not controlled.
Figure 6 ARD2L motor protector direct start mode wiring diagram
5 Conclusion
This paper uses EM256 to design a high-performance, multi-function ARD2L intelligent motor protector, and introduces its hardware circuits such as power supply, signal acquisition, input and output control in detail, and analyzes the operation of the protector through the software main function and flowchart process. The test results of current accuracy and protection time show that the protector has excellent measurement and protection functions.
Article Source: "Electrical Technology", Issue 3, 2014
references:
[1] Ma Xinjun, Design of Motor Protector, Master Degree Thesis, Northeastern University, 2005.
[2] Ding Jinlei, ARM-based integrated motor protector device design, master thesis, 2008.
[3] Ankerui Electric Co., Ltd., ARD2 Intelligent Motor Protector Selection Manual, 2013.
[4] Ren Zhicheng, Zhou Zhong, Principles and Application Guide of Electric Power Measurement Digital Instruments, China Electric Power Press, 2007.
[5] ARD3 series intelligent motor protector, Shanghai enterprise standard, Q / TDEI 27-2011
[6] JB / T 10736-2007 Low voltage motor protector.
About the Author:
Yu Jing, female, undergraduate, engineer of Wuhan Ankerui Electric Co., Ltd., the main research direction is intelligent power monitoring and power management system
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