Design principle of main body and measuring module of ARD3T intelligent motor protector

0 Introduction There are many types of motor protection devices in the market, from simple electronic thermal relays to high-end multi-functional, intelligent and networked products, but there are few independent modular products, especially domestic motor protector manufacturers There is no real modular product. Generally, the domestic motor protector concentrates all functions in the main body. The function module of this type of product is a circuit board module, which is only suitable for production and use inside the factory. When the function needs to be expanded in the use place, the main unit should be removed, or the new product can be replaced. To achieve function expansion, if a certain function on the circuit board needs to be repaired, the entire product needs to be removed, which is not conducive to on-site use.
ARD3T is a modular product with the following modules: measurement module, temperature module, analog module, communication module, switch module, display module, and main module. The basic module is composed of the main body and the measurement module to realize basic measurement, protection and control functions. The other modules are optional modules. The basic modules and optional modules are used to achieve complex functions. After adding and deleting modules, you can directly invest in the product configuration Use, no need to return to the factory, which is conducive to production and use. This article mainly introduces the design principles of the main body and the measurement module.

1    Function introduction of main body and measuring module The main body module is the brain of ARD3T, coordinate the work of each module, analyze, judge and process the information of each module. The main hardware resources included are: digital input, digital output, Rs485 communication circuit, PTC measurement circuit, fault record, etc. Available programming functions include: digital input, output programming, timer, counter, truth table, various starting methods, protection actions, 4 ~ 20mA transmission output, etc.
The measurement module completes the measurement of current, voltage and residual current, and sends the measurement structure to the main body through the internal data bus. The current measurement range of the measurement module is 0.4A to 800A, the voltage is AC380V, AC660V, the frequency is 45Hz to 65Hz, and the leakage current is 50mA to 5A, 3A to 30A. When the motor current is less than or equal to 100A, an integrated transformer module (the transformer is installed inside the measurement module) is used, and when it is greater than 100A, an external current transformer + measurement module is used.

2 Introduction to the design principles of the main body and measurement module 2.1 Features of the main body and measurement module The composition of conventional motor protector products usually includes: transformer module, main body module and display module. The transformer module is a protection-level current transformer, which directly transmits the secondary side signal of the transformer to the main body module, and the current sampling processing circuit is inside the main body module. The design of this processing method is simple, but has the following deficiencies: each current specification of the transformer module and the main module correspond to each other. If the customer replaces the used motor or the production process changes, the original product rated current setting cannot be When the requirements are met, the two parts of the transformer and the main body need to be returned to the factory for replacement; after the transformer module fails, the main body cannot give a prompt; the transformer module is not connected to the main body or the connection is unreliable, and the high voltage generated by the secondary side is open. Most conventional motor protectors can only program digital inputs and digital outputs, with few programmable functions, without timer, counter, and truth table functions, and cannot perform related programming, usually with only one starter It is not possible to set the starting mode on site.
The main body of the modular ARD3T product and the measurement module are connected through an internal data bus. The power supply of the measurement module comes from the internal data bus. The main body module can automatically identify the type of measurement module through the internal communication protocol. When the main body module detects that the measurement module is faulty, it automatically issues an alarm prompt. When the actual use of the motor needs to be replaced for various reasons, and the new motor's rated current, power and other parameters are not within the range of the original protector parameters, only the ARD3T measurement module needs to be replaced, the main module does not need to be changed, and the new measurement module After connecting with the main body, it can be directly produced through reconfiguration. The measurement module itself has a CPU unit and a signal sampling processing circuit, which is used to complete the measurement of electrical parameters. Digital communication is used between the main body and the measurement module, and the information is transmitted reliably. When the main body and the measurement module are not connected by a connecting line, the interface between the measurement module and the main body will not cause high-voltage danger.
Compared with conventional motor protectors, the programmable functions of the ARD3T main module's digital input and digital output include, in addition to conventional motion control, various alarm outputs, trip outputs, counter outputs, timer outputs, and truth table outputs , Communication output, etc., can achieve more complicated motor operation through programmable functions. The main body of ARD3T is also equipped with: protection mode, direct start, two-way start, star-delta start, auto-coupling step-down start, two-speed single winding start, two-speed double winding start and other starting methods. Switching between different starting methods is more in line with on-site requirements. The ARD3T main body has a module expansion interface, and field personnel can realize module function expansion through simple external wiring.

         2.2 The hardware principle of the main body and measurement module According to the functions of the main body and measurement module: the main module hardware must be able to provide the following functions: power supply, digital input circuit, digital output circuit, PTC measurement circuit, communication processing circuit, CPU unit, etc .; The measurement module must have voltage, current, residual current sampling circuit, signal processing circuit, CPU unit, communication circuit, etc. The hardware block diagram of the main body and measurement module is shown in Figure 1 and Figure 2.

Figure 1 Main hardware block diagram

Fig.1 main module hardware block diagram

      Figure 2 Hardware block diagram of the measurement module

Fig.2 measurement module hardware block diagram

The power supply of the ARD3T module comes from the internal data bus, that is, the power supply of the entire product comes from the power supply part of the main body. The stability of the main power supply determines the reliability of the product. The switching power supply has the advantages of high efficiency and wide application range. This project adopts the switching power supply as the main power supply, and the power supply adopts PINY's TENY series chips.
The main body switch input is used for external start, stop and other control buttons or as a process chain. When field wiring, such as binding the control line and the power line or laying the cable, pay attention to the layering treatment. The connection line is too long and does not The use of shielded cable will easily cause the misoperation of the digital input signal, which directly affects the safety and reliability of production. Therefore, when designing the digital input circuit, there must be corresponding anti-interference processing measures to make the circuit have strong anti-interference ability. The ARD3T digital input signal circuit is shown in Figure 3. The resistors R71 and R94 are current limiting resistors and the capacitor C31 The first-stage filter circuit is composed, and the resistance and capacitance parameters are calculated according to the filtering requirements. The diode D1 is a TVS tube, which has two functions: one is for misconnection protection; the other is used as a second-stage filter. U12 is an optocoupler used for signal isolation. Capacitor C35 is a filter capacitor at the subsequent stage of the optocoupler, which can eliminate high-frequency pulse signals and further ensure the accuracy of the detection of the digital input signal.

Figure 3 Switch input circuit

Fig.3 Switching input circuit

The main switch output is used to control the contactor, intermediate relay, and signal output (running signal, fault signal, etc.). The accuracy of the switch output signal also affects the reliability of production. When designing the switching output circuit, first select the relay. Generally, the parameters we are most concerned about are: contact load, maximum switching voltage, maximum switching power, maximum switching current, surge voltage, dielectric voltage, etc. These parameters can be found in the technical manual, but the "contact load" given in the manual usually refers to a purely resistive load. The actual load in use is mostly inductive. When inductive load is used, the contact capacity needs to be multiplied by The corresponding derating factor is the value we need. If the exact derating factor cannot be known, it can be measured by the relay test. The ARD3T switch output circuit is shown in Figure 4. In Figure 4, C44, C20, R10, and R11 form a filter and current limit circuit to prevent external interference from causing relay output to malfunction. At the same time, R10 and R11 form a voltage divider circuit that can also absorb small Pulse interference signal; D6 is a freewheeling diode, used to release the self-induced electromotive force generated by the coil when the relay is turned off.

The main body of ARD3T is equipped with RS485 communication circuit, and the communication protocol is MODBUS. The principle of the communication circuit is shown in Figure 5. The ingenious point of this circuit is that the data transmission and reception are directly controlled by the hardware, without CPU control. . When designing the RS485 communication circuit, the optocoupler should be selected according to the highest baud rate of the communication. When the highest baud rate is 38400, it is usually selected

Figure 4 switching output circuit

Fig.4 Switching output circuit

Take high-speed optocouplers such as 6N137 and TLP115. When the highest baud rate is 9600/19200, you can use conventional optocouplers such as TLP181, but pay attention to the matching of the resistance values ​​R56, R6, R3, R4, otherwise when the high and low temperature test, the communication line length is 1200 meters, the communication Reliability will be reduced.

Figure 5 Communication circuit

Fig.5 Communication circuit

As one of the necessary modules of ARD3T, the measurement module is responsible for measuring the current, voltage, residual current, frequency and power, and timely transmitting the calculation results to the main module through the internal bus. The measurement module adopts the true effective value algorithm of AC sampling to complete the measurement of current, voltage, and residual current. The true effective value can ensure the accuracy of the measurement when the signal is distorted.

The measurement of current, voltage, and residual current are all analog signal measurements. The principle in hardware design is the same, and all have the following components: signal sampling circuit, filter circuit, signal amplification circuit, and A / D circuit. The current signal sampling is isolated by the transformer, and the secondary side signal of the current transformer is taken by the sampling resistor. The voltage signal is collected by the resistor divider. The sampling resistor needs to select a metal film resistor with high accuracy and good temperature drift coefficient. The filter and the signal amplification circuit together form a signal processing circuit. In view of the advantages of the RC low-pass filter circuit, which is simple in structure, high in reliability, and able to withstand a large overload and surge impact, the filter circuit in the circuit design uses RC low-pass filter Circuit. The frequency of the signal filtered by the RC filter circuit is related to the sampling frequency. When designing, the signal with more than half the sampling frequency is filtered out.

The signal processing circuit of the measurement module is shown in Figure 6. In the figure, RB5 is the sampling resistor, and the signal sampling is completed. R15 and C19 form an RC filter; Q3 is a protection circuit to prevent large signal input and damage to the rear pole chip. U3B realizes reverse signal amplification and outputs the signal to the rear pole AD.

The current and voltage frequency measured by the measurement module are usually the power frequency frequency. When the frequency deviates from the power frequency, the measurement will still cause a large measurement error. For this reason, the ARD3T measurement module uses frequency tracking technology for automatic frequency tracking , The tracking range is 45Hz ~ 65Hz, the basis of frequency tracking is frequency measurement, frequency measurement can be divided into two methods of software frequency measurement, hardware frequency measurement, the most common method of hardware frequency measurement is hardware phase locked loop, software frequency measurement is commonly used The methods include least squares method, Newton algorithm, discrete Kalman filter algorithm, discrete Fourier algorithm and its improved algorithm, orthogonal demodulation method, etc. Various frequency measurement methods have their own characteristics. ARD3T frequency measurement is achieved by using a hysteresis comparator on the hardware and a method of timing the output of the hysteresis comparator on the software. In pure sine wave signals, frequency measurement is easy to implement, but it is difficult to ensure the accuracy of frequency measurement when there is harmonic interference. The hardware circuit of ARD3T frequency measurement has a hysteresis comparison function to ensure the accuracy of frequency measurement Sex.

Figure 6 Measurement signal processing circuit

Fig.6 Signal acquisition amplifier

2.3 ARD3T current and voltage measurement data The voltage and current data measured by the measurement circuit in 2.2 are shown in Table 1 and Table 2. The test environment is divided into normal temperature and humidity (+ 25 ℃, 60%) Humidity), low temperature (-20 ℃), high temperature and high humidity (+ 65 ℃, 95% humidity), three kinds of ambient temperature, the signal is divided into pure sinusoidal signal without power frequency, harmonic frequency signal, 45Hz / 65 sinusoidal signal, 45Hz / 65 has harmonic signal, the measurement results under various conditions are compared with the output parameters of the instrument. It can be seen that the measurement module can ensure the accuracy of the measurement under various environments and various signals tested .

Table 1 Measurement data of measurement module 1

Tab.1 measured data 1 measurement module

Test environmental conditions	type of data	Instrument output parameters
		I = 25A U = 380V F = 50Hz Harmonic	I = 100A U = 380V F = 50Hz Harmonic	I = 100A U = 380V F = 45Hz Harmonic	I = 25A U = 380V F = 65Hz Harmonic
+25 ℃, 60% humidity	Ia	25.00	100.0	100.0	25.00
	Ib	25.00	99.9	99.9	25.00
	Ic	24.98	100.0	100.0	24.98
	Uab	379.9	379.9	379.8	380.0
	Ubc	379.8	379.8	379.8	379.8
	Uca	380.0	380.0	380.0	380.0
+65 ℃, 95% humidity	Ia	24.92	99.9	99.9	24.93
	Ib	24.92	99.8	99.8	24.93
	Ic	24.93	99.9	99.9	24.93
	Uab	379.9	380.0	379.9	379.9
	Ubc	380.0	379.8	379.8	379.8
	Uca	379.8	380.0	380.0	380.0
-20 ℃	Ia	25.17	100.1	100.1	25.16
	Ib	25.16	100.2	100.1	25.16
	Ic	25.15	100.2	100.1	25.16
	Uab	381.6	381.8	381.6	381.8
	Ubc	381.7	381.7	381.6	381.8
	Uca	381.6	381.5	381.6	381.8

Table 2 Measurement data of measurement module 2

Tab.1 measured data 2 measurement module

Test environmental conditions	type of data	Instrument output parameters
		I = 25A ; U = 380V F = 50Hz	I = 100A U = 380V F = 50Hz	I = 100A U = 380V F = 45Hz	I = 25A U = 380V F = 65Hz
		Harmonics (1-31st order combination, maximum 40%)
+25 ℃, 60% humidity	Ia	24.98	100.0	100.0	25.00
	Ib	25.00	99.9	99.9	25.00
	Ic	24.98	100.0	100.0	24.98
	Uab	380.1	380.0	379.9	380.0
	Ubc	380.0	380.0	379.9	379.9
	Uca	380.1	380.1	380.0	380.1
+65 ℃, 95% humidity	Ia	24.92	99.8	99.8	24.91
	Ib	24.92	99.8	99.8	24.91
	Ic	24.93	99.9	99.9	24.91
	Uab	380.1	380.0	379.9	380.0
	Ubc	380.0	380.0	379.9	379.9
	Uca	380.1	380.0	380.0	380.0
-20 ℃	Ia	25.15	100.2	100.1	25.17
	Ib	25.16	100.2	100.2	25.15
	Ic	25.15	100.2	100.1	25.15
	Uab	381.5	381.7	381.6	381.6
	Ubc	381.7	381.7	381.8	381.6
	Uca	381.5	381.7	381.6	381.6

3 Anti -interference measures of ARD3T main body and measuring module

         When designing in the main body and measurement module of ARD3T , a variety of anti-jamming measures for software and hardware are adopted. In the hardware, there is a filter circuit composed of common mode inductance, X capacitance, and differential mode inductance. Through this circuit, the product can pass the 4-level electrical fast burst test; the product is designed with a dedicated surge protection circuit to ensure The product passes the 4 level surge test; adding the necessary filter circuit at the switch input can effectively prevent the switch from being misleaded by the interference signal; adding a reasonable protection circuit to the signal processing chip pin to prevent the abnormal signal from causing chip damage; signal A properly designed filter circuit at the processing circuit effectively attenuates the measurement error caused by high-frequency interference; adding digital filter technology to the measurement software can ensure the measurement accuracy.

4 Conclusion With the popularization of the use of motor protectors, the market's demand for products is constantly changing. Product research and development should focus on the future market development. The ARD3T motor protector is based on summing up the market development direction and combining the characteristics of its original products. Introduced from the top, the product design ideas and design concepts are advanced and the functions are reliable, which is more in line with the needs of the future market. The ARD3T motor protector can not only replace traditional motor protection products such as thermal relays and temperature relays in actual use, but also replace various conventional components such as pointer-type fuel gauges, signal lights, power transmitters, etc., and can also be used as the core component of MCC Complete certain logical operations, simplify the motor control circuit, and add and delete product modules on site, which is convenient to use and more in line with future market requirements.

The article comes from: "Power System Protection and Control" 2012 Issue 12.

[1] Sun Li. An improved method for frequency measurement of power system software [J]. Relay, 2006, (19): 38-41

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