Application of Remote Temperature Monitoring Instrument in Motor Multi-point Temperature Monitoring

This article combines the application examples of the multi-channel temperature remote monitor in the multi-point temperature monitoring of the motor, and introduces in detail the principle, main modules of the Changhui temperature remote monitor and the method of building a multi-point temperature measurement system with the DCS system.
Through the analysis of a large number of accidents of motor manufacturers, it is found that accidents often occur in the link of motor maintenance, which puts forward higher requirements for motor maintenance. In the past, due to insufficient attention to the protection of motors, accidents of motor damage occurred frequently, causing great economic losses. Therefore, how to more effectively ensure the reliability of motor equipment is an important issue that must be resolved in safe production. The use of a two-wire multi-channel temperature remote monitor to form a DCS temperature control system can effectively solve the problem of difficult motor maintenance.

Scheme comparison
If the method of measuring the current and voltage of each phase of the motor is used to protect the motor, it is necessary to install a current and voltage transformer to convert the large current and high voltage into a small signal to the power transmitter, and the power transmitter converts the signal into 4-20mA Pass it to the DCS system collector for display control. In practical applications, a large number of motors are distributed in various plant areas. Due to the scattered distribution in each plant area, the motor power is relatively large and the installation site is relatively complicated. Obviously, this method is not easy to operate, and the cost is high, so it is not desirable. If the motor temperature is measured to protect the motor, the motor manufacturer usually embeds 2~3 Pt100 temperature sensors inside the motor coil during production, and the motor's working status can be grasped by measuring the temperature rise of the motor. As long as the resistance change of the Pt100 temperature sensor is converted into 4-20mA and passed to the DCS system, the temperature can be monitored. This type of transmitter is divided into two types: with power supply and without power supply. Among them, the two-wire transmitter without power supply is more suitable for on-site use. Only two wires can collect a Pt100 signal. However, if a motor has 2~3 sensors, it needs 2~3 transmitters, which brings about problems such as more wiring, more DCS collection points, high cost, troublesome installation and so on. In this regard, this article will introduce a two-wire multi-channel remote temperature monitor developed by Changhui Instrument.

Advantages of two-wire transmitter
The two-wire transmitter provides the driving power through the signal line, so the use of the two-wire transmitter can save the power cord, solve the inconvenience of special power supply scattered in the huge factory area, and reduce the construction cost. The signal of the two-wire transmitter is a 4-20mA DC current signal, which has strong anti-noise signal ability and can ensure the stable transmission of the signal even in an environment with power equipment. If a voltage signal is used, for example, a noise voltage of 10V is added to a 0-10V signal line, the signal on the signal line will be doubled. The output terminal of the two-wire transmitter measured the internal impedance of the transmitter to be at least 1MΩ, so the current value (I≈0.01mA) added to the signal impedance by the noise signal voltage (10V) has only an influence on the range 20mA 1/2000, this is the advantage of the two-wire transmitter.

Changhui XTRM temperature remote monitor
Changhui Instrument Manufacturing Co., Ltd. has opened up the setting interface of the internal parameters of the XTRM temperature remote monitoring instrument to users, allowing users to have full autonomy, changing the passive situation of users relying on manufacturers for a long time, and making users more convenient To carry out secondary development. This product uses Pt100 platinum resistance and thermal resistance to measure temperature, the output signal is the highest temperature in each temperature measurement loop, and the 4-20mA adopts two-wire international standard signal output, which can be used with DCS, PLC and various secondary instruments Connected, and has a strong anti-interference ability.
XTRM temperature remote monitor
Working principle of temperature remote monitor
The XTRM temperature remote monitor is a temperature transmitter with multiple inputs and a 4-20mA two-wire output. It is installed near the measured point, and the two-wire output serves as the power supply line for the signal and the instrument at the same time. The output current is divided into two parts, the constant 4mA current is used as the measurement zero point, and at the same time the electronic circuit and power supply current in the instrument, and the 0-16mA changing current is proportional to the change of the measured input signal. Changhui temperature remote monitor is based on a micro-power single-chip microprocessor, which converts the temperature signal measured by the thermal resistance or thermocouple into a digital signal through an A/D converter through the input signal circuit. The microprocessor performs high-precision correction according to the sampling result and sends it to D/A. D/A converts the digital signal into an analog signal, and the analog signal is converted into a 4-20mA constant current output by V/I. The principle block diagram is shown as in Fig. 1.
The principle block diagram of the multi-channel temperature remote monitor
Figure 1 Block diagram of the multi-channel temperature remote transmission monitor
The design of the main module of the temperature remote transmission monitor
XTRM multi-channel temperature remote transmission monitor adopts a modular design idea, which mainly includes input circuit, amplifying circuit, CPU circuit, button circuit, display drive circuit, V/I output circuit, power module circuit, etc.

1. Input circuit
The Pt100 resistance signal can be converted into a voltage signal by the bridge circuit, and X1 is connected to the input end of the 4051 multiplexer. There are 4 such circuits in this temperature remote monitor, namely X1~X4. The input circuit is shown in Figure 2.
Multi-channel temperature remote transmission monitor input circuit diagram
Figure 2 Input circuit diagram of multi-channel temperature remote monitor
2. Amplifying circuit
After the four signals of X1~X4 are selected by the electronic switch, because the signal is small, linear amplification is required. This watch uses an instrumentation amplifier composed of operational amplifiers, which has excellent performance and has been widely used in measurement amplifiers. The three operational amplifiers are low-drift functional blocks. They are composed of a dedicated integrated operational amplifier plus a discrete component input stage. Because the symmetry of the discrete component input stage has been selected, it can be used for general operational amplifiers. Performance indicators have been improved, with high common-mode rejection and low output temperature drift. The amplifying circuit is shown as in Fig. 3.
 Multi-channel temperature remote transmission monitor amplifier circuit
Figure 3 Amplifying circuit of multi-channel temperature remote monitoring instrument
3. Button circuit
The man-machine dialogue is carried out by means of buttons. There are 5 buttons, namely the parameter function setting button (SET), the parameter increase button ▲, the parameter decrease button, the left shift button and the right shift button. The user can view any temperature through the panel buttons when needed.

4. Display drive circuit
The display driver adopts 6-bit dynamic scanning display, and the scanning frequency is designed to be about 70Hz. The LED display window displays the temperature measurement value, and the channel status indicators CH1~CH4 indicate the highest of the 4 input signals currently displayed in the LED display window. The display drive circuit is shown in Figure 4.
Multi-channel temperature remote transmission monitor display drive circuit
Figure 4 Multi-channel temperature remote transmission monitor display drive circuit
5. CPU circuit
The single-chip microcomputer is a highly integrated, high-precision single-chip system SoC produced by Texas Instruments (IT). It is the 16-bit Flash RISC microcontroller with the lowest power consumption in the industry. With abundant on-chip peripherals, it is a very cost-effective single-chip microcomputer. Using it as the control core of the system not only greatly simplifies the system hardware circuit, but also greatly improves the cost-effectiveness of the system.

It embodies the advanced low-power design concept of modern single-chip computers, and its clock system provides a rich combination of software and hardware. It includes an on-chip DOC and two crystal oscillators, can generate three kinds of clocks suitable for the system, supports six working modes, and has five low-power modes. The internal clock system can be set by software to control the chip and make it work in different modes, so that the whole system can reach the lowest power consumption and exert the best performance. Therefore, this single-chip microcomputer is very suitable for XTRM multi-channel temperature remote transmission monitor. Design requirements.

6. V/I output circuit
The V/I output circuit adopts PWM pulse width modulation method to output, which has the advantages of simple circuit, low cost, high precision and strong anti-interference ability. The output adopts full software debugging, no potentiometer adjustment, which is convenient and practical. The multi-channel temperature remote monitoring instrument can also transmit and output 4-20mA analog signals corresponding to the measured value according to the measurement range of the instrument, and provide it to paperless recorders, other instruments, PLC and DCS systems. The V/I output circuit is shown in Figure 5.
V/I output circuit of multi-channel temperature remote monitor
Figure 5 The output circuit of a multi-channel temperature remote monitor
 
7. Power module circuit
The power module circuit enters the voltage stabilizing circuit through the protection circuit (overvoltage, overcurrent, anti-interference), and provides the working power of CUP and other devices. The power module circuit is shown in Figure 6. This circuit has the following characteristics:
①High output voltage stability;
②Low temperature coefficient;
③The price is cheap and the structure is simple.
Multi-channel temperature remote transmission monitor power supply module circuit
Figure 6: Multi-channel temperature remote transmission monitor power supply module circuit
System configuration
DCS system is a comprehensive control system that emerged with the continuous rise of modern large-scale industrial production automation and the increasing complexity of process control requirements. It is a combination of computer technology, system control technology, network communication technology and multimedia technology. It can provide Friendly man-machine interface and powerful communication function. It is a modern equipment that realizes process control and process management. Two-wire transmitters are used frequently in the field of industrial automation. Like other industrial field devices, two-wire transmitters also have problems with interference and anti-interference. According to the DCS system AI card interface mode (current type or loop power supply type), the isolation distributor with different functions is selected. In principle, it is required to provide isolated power to the transmitter and ensure that each transmitter has an independent power supply. It can also isolate the transmitter signal and transmit it to the DCS. XTRM multi-channel temperature remote monitoring instrument composes a computer DCS monitoring system, with animation display, report, alarm, trend, printing and other functions, which can realize centralized monitoring, control and recording of temperature parameters.
Multi-channel temperature remote transmission monitor
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The XTRM multi-channel temperature remote monitor from Changhui Instrument Manufacturing Co., Ltd. can be easily connected to the DCS control system. You can intuitively observe the operating temperature of each motor in the central control room. Once you find that the temperature rise exceeds the standard, you can immediately take measures to prevent accidents. This method is currently widely used in cement plants, power stations, paper mills and other enterprises. Practical applications show that the use of XTRM multi-channel temperature remote monitor can be widely implemented in multi-point temperature monitoring.