MODBUS, developed by Modicon (now Schneider Electric) in 1979, is a means of communication with multiple devices through a single twisted pair. At first, it worked on RS232, but then it used for both RS485 to get higher speed, longer distance, and multi-drop network. MODBUS has quickly become a common standard in the automation industry, and Modicon has launched to the public as a free protocol.

MODBUS is a Master-Slave system, Master is connected to one or more Slave. The master is usually a PLC, PC, DCS, or RTU. Slave MODBUS RTU is usually field devices, all connected to the network in multi-drop configuration. When a MODBUS RTU owner wants information from the device, he will send a message about the data needed, summarizing error detection to the device address. All other devices on the network will receive this message but only the specified device will respond.

What modbus standards are in common use?

Currently, there are 03 modbus standards that are commonly used in industry – automation: Modbus RTU, Modbus ASCII, Modbus TCP

All data are sent in the same format. The only difference between the 3 MODBUS types is how the data is encrypted. Detail:

• Modbus ASCII:
  All data is encrypted using hexadecimal, using 4-bit ASCII properties. For every byte of information, two communication bytes are needed, double that of MODBUS RTU or MODBUS/TCP. However, MODBUS ASC II is the slowest of the three types of protocols, but it is suitable when the telephone modem or the connection uses radio waves because ASC II uses message identification features. Because of this delineation feature, any trouble in the transmission medium will not cause the receiver to misinterpret information. This is important when it comes to slow modems, cell phones, noisy connections or other fastidious media.
• Modbus RTU:
  Data is encoded in binary, and only needs one communication byte per data byte. It is ideal for RS 232 or RS485 multi-point networks, ranging from 1200 to 115 baud rates. The most popular speeds are 9600 to 19200 baud. MODBUS RTU is the most widely used industrial protocol, so most of this article will only focus on its basis and application. More details on the Modbus RTU protocol can be found here.
• Modbus TCP:
  MODBUS / TCP is simply MODBUS over Ethernet. Instead of using this device for connecting to slave devices, so the IP addresses are used. With MODBUS / TCP, MODBUS data is simply summarized in a TCP / IP packet. Therefore, any Ethernet network that supports MODBUS / IP will immediately support MODBUS / TCP.

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Communication standard RS422 is a serial communication standard. RS422 supports high speed communication and long data transmission distance. Each signal is transmitted on a twisted pair (2 wires) and that is different from other communication standards. With a distance of 40 feet (equivalent to 12m), the maximum data rate is 100 Kilobytes per second. A 120 – Ohm resistor is connected in series at the end of the line to prevent reflection and interference. RS422 is often used between one transceiver and another. However, at each output, up to 10 receivers can be transmitted.

Features of RS422 communication standard:

+ 1 Driver up to 10 Receivers

+ Maximum line length and speed:

• 40 Feet = 12m 10 Mbits/sec
• 400 Feet = 122m 1 Mbits/sec
• 4000 Feet = 1219m 100 kbits/sec

What is the difference between RS485 and RS422?

The RS485 communication standard is a communication standard built on the standard RS422 platform. The main difference is that RS485 allows up to 32 transceiver pairs to be on the line at the same time. Similarly, RS485 also needs a 120 Ohm impedance at the end of the line to prevent signal reflection and loss. If more than one device needs data transmission, the RTS line is used as a control line, allowing data transmission.
Features of RS485 communication standard:

+ Up to 32 Driver/Receiver Pairs

+ Maximum line length and speed:

• 40 Feet = 12m 10 Mbits/sec
• 400 Feet = 122m 1 Mbits/sec
• 4000 Feet = 1219m 100 kbits/sec

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The figure below will give us a basic view of the physical differences such as: line length, communication mode, physical logic level, communication speed range, �/p>

In addition, these two standards have a fundamental difference in the mode of signal transmission:

• RS-232 allows to use at least 3 wires: Tx (transmit), RX (receive) and GND (ground). In particular, the logic state of the signal uses the pressure difference between TX and RX compared to the ground wire GND.
• RS-485 uses voltage difference between 2 wires A and B to distinguish logic 0 and 1, not with ground. Especially, when the signal is far away, if there is a voltage drop, it also drops on both wires so the signal is still guaranteed.

>>> Because of this, RS485 allows for a longer signal transmission and a higher transfer rate than RS232.
RS485 can also allow multi-point communication, including multiple devices that can communicate in a network and RS232 only transmits in a point-to-point mode, ie when two devices are directly connected to each other, No third party devices can participate in data exchange.

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1. What is RS-485 communication?

In 1983, the Electronic Industry Association (EIA) approved a new balanced transmission standard called RS-485. Has been widely accepted and used in industry, medical, and civil. The RS485 standard can be considered as a development of RS232 in serial data transmission. These RS232 / RS485 converters allow users to communicate with any device that uses the RS232 serial link via RS485. RS485 link is formed for long distance data acquisition and control for applications. The outstanding feature of RS485 is that it can support a network of up to 32 base stations on the same line, the baud rate can be up to 115,200 for a distance of 4000feet (1200m).
With the balanced transmission pattern and the wires are twisted together, when the noise occurs in one wire, it also happens in the other wire, which means that the two wires have the same noise. This makes the voltage difference between the two wires do not change significantly, so the receiver still receives the correct signal thanks to the special feature of the receiver that eliminates noise. The RS485 link is widely used in industry, where the noise environment is quite high and confidence in system stability is important. Besides, the ability to communicate over long distances at high speed is also very interested, especially in places where many communication stations are spread out over a wide area.

2. Characteristics of RS-485 communication

2.1 Balance transmission

Balanced transmission system consists of two signal wires A, B but no ground wire. The reason is called balance because the signal on one line is opposite to the signal on the other. This means that one wire is playing high, the other is playing low and vice versa.

2.2 Signal level

With two wires A and B being balanced, the TTL high-level signal is specified when the voltage of wire A is at least 200mV from wire B, the TTL low-level signal is specified when the pressure of wire A is smaller than wire B minimum is also 200mV. If the VAB voltage is within -200mV <VAB <200mV then the signal is now considered to fall into the indefinite region. The voltage of each signal wire compared to the signal ground on the receiver side must be within – 12V to + 12V.

2.3 Twisted-pair wire

+ As its name suggests, twisted-pair wire is simply a pair of wires of equal length and twisted together. Using twisted pair will minimize noise, especially when traveling over long distances and at high speeds.

+ Twisted pair wire characteristic impedance:
Depending on the shape and insulation of the wire, it will have a Characteristic impedence (Zo), which is usually indicated by the manufacturer. The recommended characteristic impedance for a line is between 100 and 120 ohm, but this is not always the case.

2.4 Common type voltage

The transmission signal consists of two wires without a mass wire, so they need to be referenced to a common point, the common point at this time can be the mass or any given voltage level. The common-mode voltage (VCM) is mathematically stated as the average of the two signal voltages referenced to mass or to a common point.

2.5 Grounding

The signal on the two wires when referenced to the common ground is then it needs to be scrutinized. At this time the receiver will determine the signal by reference that signal to the ground of the receiver, if the ground between the receiver and the transmitter has a voltage difference that exceeds the allowed threshold, the received signal will be wrong. or damage the device. This shows that the RS485 network consists of two wires but up to three voltage levels are considered. Because ground is an imperfect electrical conductor, it has a definite resistance, which causes a potential difference from point to point, especially in areas with lots of thunder, large current consuming machines, These converters are installed and grounded.
The RS485 standard allows a difference of earth potential up to 7V, which is greater than 7V. So the ground is an unreliable reference point and a better way for signaling now is to add a third wire, which will be grounded mass at the source to use as a reference voltage.

2.6 Terminal resistors

Terminal resistors are simply resistors located at the two ends of the line. The ideal terminal resistance value is equal to the characteristic impedance value of the helix, usually around 100-120Ohm.

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RS 232 is a standard communication protocol mainly used for serial communication between two devices. It was first introduced by the EIA (Electronic Industries Association) in 1960 with the purpose of providing a roadmap for connecting one device to other peripherals for perfect digital communication.

The advent of USB has greatly reduced the need for the RS232 protocol, but we still cannot rule out its importance in some industrial applications where Programmable Logic Controllers and Computerized Numerical Control Equipments (CNC) are programmed individually using the RS 232 connector which is an interface between DTE (Data terminal equipment) and DCE (Data communication equipment).
RS 232 is a standard protocol proposed for serial data transfer between electronic devices. It mainly operates on three signals: Transmission line, Receive line and Ground.

2. Features of RS-232

The logic level of RS232 is defined differently from the TTL logic:

• DCE: a high level (corresponding to logic 0) is a voltage of +5 to +15 V, while a low level (corresponding to logic 1) is a voltage of -5 to -15 V
• DTE: High levels are defined as from +3 to +15 V (called space), and low levels are defined as from -3 to -15 V (called mark).

+ Line impedance: The wire resistance is up to 3 – 7 ohms and the maximum cable length is 15 meters, but within the capacitance limit per unit length.

+ Operating voltage: 250VAC max

+ Rated current: Maximum 3 Amps

+ Insulation: 1000 VAC

+ Slew rate: The rate of change of signal levels is called Slew rate. With variable rates up to 30V / ms and the maximum bit rate will be 20 kbps.

3. Connector Pinout

In fact, the RS232 port, also known as the COM port, has two types of connectors, DB-25 and DB-9. In DB-25, there are 25 pins available for many applications, but some applications do not use all 25 pins. Therefore, the 9-pin connector is made to facilitate the connection of devices.

4. Types of serial communication

There are two main types of serial communication: Half Duplex and Full Duplex

+ Half Duplex only transmits information in one direction. It comes with two lines in which one is a data line and the other is a signal ground. In this communication, the terminal has the ability to send or receive data, but not at the same time. This method is an old method and is no longer practiced.

+ Full Duplex can transmit and receive data in both directions, requiring three main paths: data transmission, data reception and signal ground.

5. Application of RS-232

+ Connect devices such as PLC, HMI, Inverter, Motor,�together easily

+ Many applications in measuring equipment, electronic scales

+ Used a lot in CNC, Bar code machines

+ Low cost, easy integration should be widely used in civil and industrial

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-Data communication refers to the conversion of information or data, mainly in digital format, from a transmitter to a receiver via a link (be it copper wire, coaxial cable, optical cable or whatever Any other means) connect these two devices.
-Traditional communication networks are used to enable data transfer between computers, computers and its peripherals and other devices. On the other hand, industrial communication networks are a special type of network made to handle real-time control and data integrity in harsh environments on large installations.

-Examples of industrial communication networks include Ethernet, DeviceNet, Modbus, ControlNet, etc.
Three important control mechanisms are used in the field of industrial automation: Programmable Logic Controllers (PLC), Supervisory Control and Data Acquisition (SCADA) and Distributed Control System (DCS). All of these components communicate with field meters, smart field devices, monitoring control computers, distributed I / O controllers and HMI interfaces.
-The means of transmission for data transmission and control signals may be wired or wireless. In the case of wired transmission, the cable used may be a twisted pair, coaxial cable or fiber optic. Each network cable has its own electrical characteristics that may be less or more suitable for a particular type of network or environment. In the case of wireless transmission, communication is done via radio waves.

2. Decentralized in industrial communication network

In the manufacturing industry or processes, information or data flows from the school level to the management-operation level (bottom to top) and vice versa. Different levels have to handle different requirements of a specific level. Therefore, it is clear that no unique communication network address requirements are required for each level. Therefore different levels may use different networks based on requirements such as data volume, data transfer, data security, etc. Based on function, industrial communication network is classified into three main levels:

Device Level:

+ This lowest level includes field devices such as sensors and actuators of processes and machines. The task of this level is to transfer information between these devices and technical process components such as PLC. The transfer of information can be digital, analogue or hybrid. The measured values may last for a long time or over a short period of time.

+ To provide decentralized field communication, 4-20 mA current, point-to-point communication protocols are widely used. These networks include parallel, multi-wire cables as a means of transmission. Common serial communication protocol standards used at this level include RS232, RS422 and RS485. There are many other school-level communication networks available that describe various factors such as response time, message size, etc.

+ Today, fieldbus technology is the most sophisticated communication network used at the field level because it facilitates distributed control between various smart field controllers and controllers. This is a two-way communication system in which many variables are taken care of by single transmission. Different fieldbus types include HART, ControlNet, DeviceNet, CAN Bus, Profibus and Foundation Fieldbus.

Control Level:

-This level includes industrial controllers such as PLC, distributed control units, and computer systems. The tasks of this level include configuring automation devices, downloading program data and processing data variables, adjusting variables, monitoring control, displaying variable data on the HMI, calendar storage history, etc. Therefore, the control level requires characteristics such as short response time, high speed transmission, short data length, machine synchronization, continuous use of important data, etc.
-Local area networks (LAN) are widely used as communication networks at this level to achieve the desired properties. Ethernet with TCP/IP protocol is mainly used as control level network to connect control units with computers. In addition, this network acts as a control bus to coordinate and synchronize between different controllers. Some fieldbuses are also used at this level as control buses such as Profibus and ControlNet.

Information Level:

This is the highest level of the industrial automation system that gathers information from lower levels, such as the control level. It handles large volumes of data that are not used continuously or limit time. Large-scale networks exist at this level. Therefore, Ethernet WANs are often used as information level networks for factory planning and management information exchange. Sometimes these networks can connect to other industrial networks through gateways.

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1. Serial Communication

The serial protocol is the basic communication system provided for all controllers such as PLCs. This protocol is implemented using standard protocols such as RS232, RS422 and RS485. The abbreviation RS stands for Recommended Standard – The proposed standard which defines the serial communication characteristics in terms of electrical, mechanical and practical features.

The serial communication interface is either integrated into the CPU or the processing module (in the Programmable Logic Controller) or it may be a separate communication module. These RS interfaces are mainly used to transmit data reasonably at a high data rate between the PLC and the remote device. Barcode readers, printers and vision systems are examples of these interfaces.
The RS-232 serial protocol is designed to support a transmitter and a receiver and thus it provides communication between a controller and a computer. Maximum cable length must be up to 50 feet. The serial communication standards RS 422 (1Tx, 10 Rx) and RS485 (32Tx, 32 Rx) are designed to communicate between a computer and multiple controllers. These standards are limited to lengths of 1650 feet (in the case of RS422) and 650 feet (in the case of RS485).

2. HART (Highway Addressable Remote Transducer)

HART is an open process control network protocol, superimposing digital communication signals on top of 4-20mA signals using the Bell 202 Frequency Shift Keying (FSK) technique.

It is the only communication network that facilitates both analog and two-way digital communication at the same time by the same wiring, and thus these networks are also called hybrid networks. This digital signal, called the HART signal, carries diagnostic information, device configuration, calibration and other additional process measurements.
HART network operates in point-to-point or multi-point mode. In point-to-point mode, 4-20 mA current signals are used to control the process while HART signals remain unaffected. HART multipoint network is used when devices are widely spaced. HART-compatible multivariable smart field devices are widely used in many industries. HART communication network is mainly used in SCADA applications.

3. DeviceNet

This is an open device hierarchy based on CAN technology. It is designed to interface field devices (such as sensors, switches, barcode readers, control panel displays, etc.) with higher level controllers (such as PLCs) with acceptance. unique basic CAN protocol. It can support up to 64 nodes and supports up to a total of 2048 devices.

It reduces network costs by integrating all devices on four-wire cables that carry both data and power. Power on the network allows devices to be powered directly from the system and thus reduces the physical connection points. This network is commonly used in the automotive and semiconductor industries.

4. ControlNet

It is an open control network that uses Common Industrial Protocol (CIP) to combine the function of peer-to-peer and I/O networks by providing high-speed performance. This network is a combination of Data Highway Plus (DH +) and remote I/O. It is used for real-time data transmission of time-critical and non-time-critical data between I/O or microprocessors on the same network.

It can communicate with up to 99 nodes at a data rate of 5 million bits per second. It is designed for use on both equipment and field levels of industrial automation systems. It provides media and media redundancy at all nodes of the network.

5. Modbus

It is an open system protocol that can run on many different physical layers. It is the most widely used protocol in industrial control applications. It is a serial communication technique that provides a master / slave relationship to communicate between devices connected on the network. It can be done on any transmission medium, but is often used with RS232 and RS485.

Modbus serial with RS232 or RS485 (as physical layers) facilitates connection of Modbus devices with controllers (such as PLCs) in the bus structure. It can communicate between a master and a number of slaves up to 247 with a data transfer rate of 19.2 kbits/s.
A newer version of Modbus TCP / IP uses Ethernet as a physical layer to facilitate data exchange between PLCs in different networks. Regardless of the physical network type, it facilitates the method of accessing and controlling one device by another device.

6. Profibus

Profibus is one of the widely known and widely implemented field networks. These networks are mainly used in process automation and factory automation fields. It is best suited for complex communication tasks and time-critical applications. There are three different versions of Profibus, Profibus-DP (Decentralized Periphery), Profibus-PA (Process Automation) and Profibus-FMS (Fieldbus Message Specification).

7. Foundation Fieldbus

It is an open fieldbus standard specifically designed to meet the mission-critical needs of a safe environment. It is a type of LAN for fieldbus compatible devices and controllers used in manufacturing and processing industries.

It is an open fieldbus standard specifically designed to meet the mission-critical needs of a safe environment. It is a type of LAN for fieldbus compatible devices and controllers used in manufacturing and processing industries.
This is a two-way digital protocol standard, defined by safety standard IEC 61158-2 (for FF H1) and compatible with Ethernet equipment (in the case of FF HSE). The three types of FF networks include low speed H1, high speed H2 and high speed Ethernet HSE.

• The H1 network supports a speed of 31.25 kbps.
• The H2 network supports 2 speeds of 1.0 Mbps and 2.5 Mbps.
• HSE network supports 10 or 100 Mbps speeds because it uses the Ethernet protocol.

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What is an IP rating?

IPxy

+ x: is a number representing the level of dust protection.

+ y: is a number representing the level of waterproof.

Regarding the specific figures in the structure, thelivingchristmascompany.com will provide quite a full meaning as follows:

For X:
0: No protection
1: Protect from objects> 50mm.
2: Protect from objects> 12.5 mm.
3: Protect from objects> 2.5 mm.
4: Protect from objects> 1 mm.
5: Protection against dust ingress.
6: Protection against dust completely.

For Y:
0: No protection
1: Protection against vertical water droplets.
2: Protection against direct injection water with angles> 15′
3: Protection against direct injection water with angles> 60′
4: Protected against water splashes from all directions. Tested for a minimum of 10 minutes with an oscillating spray (limited ingress permitted with no harmful effects).
5: Protection against low-pressure jets (6.3 mm) of directed water from any angle (limited ingress permitted with no harmful effects).
6: Protection against direct high pressure jets.
7: Protection against full immersion for up to 30 minutes at depths between 15 cm and 1 metre (limited ingress permitted with no harmful effects).
8: Protection against immersion in water from 1m to 2m for long periods.
9:(K): Protection against high-pressure, high-temperature jet sprays, wash-downs or steam-cleaning procedures – this rating is most often seen in specific road vehicle applications (standard ISO 20653:2013 Road Vehicles – Degrees of protection).

While we cover a huge range of electrical enclosures, our most common IP ratings are probably 65, 66, 67 and 68. So for quick reference, these are defined below:

+ IP65 Enclosure – IP rated as “dust tight” and protected against water projected from a nozzle.

+ IP66 Enclosure – IP rated as “dust tight” and protected against heavy seas or powerful jets of water.

+ IP 67 Enclosures – IP rated as “dust tight” and protected against immersion. for 30 minutes at depths 150mm – 1000mm

+ IP 68 Enclosures – IP rated as “dust tight” and protected against complete, continuous submersion in water.

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CB has the function of protecting overload and short circuit in electric system. Some CB lines have additional leakage protection function called aptomat against leakage or shock.

1. Aptomat Classification:

1.1 Classification by structure:

+ Miniature Circuit Breaker (MCB)

+ Moulded Case Circuit Breaker (MCCB)

1.2 Classification by function:

+ CB Overload protection and short circuit: MCB, MCCB.

+ CB leakage protection voltage: Residual Current Circuit Breaker (ECCB), Residual Current Circuit Breaker with Overcurrent Protection ( RCBO ), Earth Leakage Circuit Breaker (ELCB).

1.3 Sorting by phase number / number of poles:

+ 1 Phase / 1 Pole CB

+ 1-Phase + Neutral CB (1P + N): 2 poles

+ 2 Phase / 2 Pole CB: 2 poles

+ 3 Phase / 3 Pole CB: 3 poles

+ 3-Phase + Neutral CB (3P + N): 4 poles

+ 3 Phase / 3 Pole CB: 4 poles

1.4 Classification by short-circuit current:

+ Low cut current: commonly used in civilian.

+ Standard cutting line: often used in industry.

+ High shear line: commonly used in industry and special applications.

1.5 Classification according to the ability to adjust the current:

+ CB has a constant current rating. For example, Mitsubishi’s MCCB NF400-SW 3P 400A has a constant current rating of 400A.

+ CB adjust the rated current. For example, the MCCB NF400-SEW 3P 400A from Mitsubishi has an adjustable rating of 200A – 400A.

In this article, thelivingchristmascompany.com will introduce about MCB and MCCB.

2. MCB / MCCB structure

MCB / MCCB is composed of parts: contact, arc stamping box, MCB shear drive mechanism, protective hook.

+ Contact: MCB / MCCB usually has two types of contacts (main contact and arc) or 3 contacts (main contact, auxiliary contact, arc). The operation of the contact is as follows: when closing the arc contact circuit closed first, followed by the auxiliary contact, finally the main contact. When the main contact circuit breaker opens first, the auxiliary contact opens after and finally the electric arc.

+ Arc stamping box: There are 2 types of arc stamping equipment: semi-closed type arc and open type arc. Features of 2 different types: semi-closed type is located in the enclosure of MCB and has air holes. Open type is used for voltages greater than 50KA or large voltages of 1000V (high voltage). The arc has many steel plates arranged in a mesh into different sections to facilitate the extinguishing of the arc.

+ MCB / MCCB shear transmission mechanism: There are two ways of MCB / MCCB shear drive (manual and electromechanical). For manual shear drives which are carried out with MCB / MCCBs, the rated current is not large. As for the type of mechanical control in the MCB / MCCB has a larger current.

+ Protective hook: Protective hook works to protect electrical equipment from overload and short circuit. There are 2 types of protection hook: electromagnetic hook and thermal relay hook.

3. Principle of operation MCB / MCCB

MCB / MCCB uses temperature-sensitive device (thermal element), current-sensitive electromagnetic device (magnetic element) to provide shut-off mechanism for protection and isolation purposes.

In the case of overloads, a current more than the rated current is driven through the MCB. As the current flows through the bimetallic strip, it gets heated up and deflects by bending and releases the mechanical latch. Deflection time of bimetallic strip depends on the amount of current flowing through the strip. Higher the current faster will be the deflection of the bimetallic strip.

During short circuits, a transient current flowing through the solenoid forces the plunger towards the latch. This action instantaneous releases the mechanical latch and opens the contacts immediately.

Specifications of MCB / MCCB

+ In: Rated current. For example: MCCB 3P 250A 36kA, In = 250A.

+ Ir: the operating current is adjusted within the permissible range of Aptomat. For example, the current regulator 250A can be adjusted from 125A to 250A.

+ Ue: Rated working voltage.

+ Icu: The short-circuit current is able to withstand the maximum current of the contact for 1 second.

+ Icw: Ability to withstand short-circuit currents in 1 unit of time.

+ Ics: the actual ability to cut off when a device malfunction occurs. This ability depends on the manufacturer due to different manufacturing technology. For example, the same manufacturer has two types of MCCB, Ics = 50% Icu and Ics = 100% Icu.

+ AT: Ampe Trip

+ AF: Frame Ampere. For example, NF250A 3P 200A and NF250A 3P 250A both have AF = 250A but one will work when the current exceeds AT = 200A, one will work when the current exceeds AT = 250A. The AT / AF specification indicates the strength of the switch contact. For example, the Aptomat 250AT / 400AF will be more durable than the Aptomat 250AT / 250AF, the size of Aptomat 400AF is also larger, the price is higher.

+ Characteritic cuver: is the protective curve of CB (selected curve of CB). This is a very important parameter, which determines the position of CB in the electrical system.

+ Mechanical / electrical endurace: The number of permitted mechanical switches / permitted number of electrical switches.

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Application of Thermal Relays

Thermal relays are fitted with Contactor to protect electrical equipment, especially electric motors when overcurrent, overload during operation.

Note: Thermal relays only work to change the contact state, but not to disconnect the power so it must be combined with another switch.

The characteristic of thermal relays is that it takes a certain amount of time to operate based on the thermal expansion mechanism and not act as fast (instantaneously) as electromagnetic switches. Therefore, thermal relays are only used for overload protection, not for short circuit protection. To protect against short circuit, it must be used with aptomat, fuse.

Thermal relays operating at alternating voltages up to 500V, frequency 50Hz, have a range of effects from a few hundred mA to a few hundred A. Thermal relays of Mitsubishi, LS, Schneider have a range of 0.1A to 800A.

Structure of thermal relay

Note:

1. Leverage
2. The contact is normally closed
3. The normally open contact
4. Screw adjusting the impact current
5. Bimetallic bar
6. Heating wire
7. The lever
8. Reset button

It can be said that the thermal relay is not too complicated and the usage is very simple.

The working principle of thermal relays

True to its name, thermal relays operate based on the change of temperature of the current. When the current is overloaded, a huge amount of heat is produced which causes the metal plate of the relay to heat up, leading to the expansion. In the composition of the thermal relay, the double metal plate plays an extremely important role for the device to work effectively. This dual metal plate is made of two metal bars with different elongation index.

Typically, the first metal bar will have less expansion coefficient and often invar (including 36% Ni + 64% Fe). The second metal bar is usually made from brass or chromium – nickel steel because of its expansion index about 20 times greater than the invar. The two slabs are assembled into one sheet by hot rolling or welding.

When the current has a sudden change, the temperature will impact on the double steel rod so that it bends in the direction of the metal rod with less expansion coefficient now that we can use directly for the current or surrounding resistance wire. . The amount of bend is more or less depending on the length and thickness of the metal rod.

Classification of Thermal Relays

– According to the structure of thermal relays divided into two types: open type and closed type.

– On request use: Single and double pole type.

– By heating method:

+ Direct heating: Electricity flows directly through the double metal plate. This type has a simple construction, but when changing the rated current, the plate must be changed Double metal, this type is not convenient.

+ Indirect heating: The electric current flows through the independent heating element, the heat radiated indirectly causes the metal plate to bend. This type has the advantage of wanting to change the rated current we only need to change the heating element. The disadvantage of this type is that when there is a large overload, the heating element can reach quite high temperatures, but because of poor heat transfer air, the metal sheet has not been able to toxic and the heating element has been burnt off.

+ Combined heating: This type is relatively good because it burns directly and indirectly. It has relatively high thermal stability and can work at multiple overloads
big.

How to choose a thermal relay

Thermal relays are used to protect the motor overload, so when choosing a thermal relay, the right motor type must be selected for protection. In many cases, the user selects a thermal relay according to the current of the contactor or the aptomat which is incorrect and leads to the motor to be burnt when overloaded.

Below is the Table of thermal relays according to motor power:

Some notes when selecting thermal relays:

+ Choose a thermal relay with an adjustable threshold corresponding to the operating range of the motor or slightly higher. The lowest regulating threshold of a thermal relay should be lower than the middle of the motor operating range. The highest adjustable threshold of the thermal relay must be higher than the upper limit of the motor operating range.

+ Some types of thermal relays have a pin for contactor (usually small thermal relays). Therefore it can only install the right type of contactor compatible with it.

+ Some high-end thermal relays have built-in phase failure protection.

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