Tìm hiểu về khởi động từ, khởi động mềm và biến tần

Contactors, soft starters and inverters are known as important devices in electric motor control systems, helping to optimize performance and protect the motor during operation. Contactors are often used to turn electric motors on and off, helping to control the power supply safely and effectively. Soft starters help reduce the starting current, protect the motor from electric shocks, and allow smooth starting and stopping of the motor. Inverters not only regulate the speed and torque of the motor, but also save energy by controlling the input frequency and voltage. This article will help you understand in depth the functions and operation of contactors, soft starters and inverters so that you can make the right choice to buy the right product for your work needs!

1. Contactor

Concept of contactor

Contactor is a low voltage electrical device, extremely important in electrical systems, capable of frequently closing and opening power circuits.

Structure of contactor

The structure of the starter includes an electromagnet, a contact system and an arc extinguishing system, specifically:
Electromagnet: There is a steel core, a spring to push the core cover to move back to its original position, and a coil to create a magnetic attraction. The function of the electromagnet is to create a magnetic field.
Contact system: There is a main contact (a normally open contact, which closes when power is supplied to the magnetic circuit of the starter in the electrical cabinet, causing the magnetic circuit to attract again, capable of allowing large currents to pass through) and an auxiliary contact (with 2 states of normally closed and normally open, capable of allowing currents to pass through contacts less than 5A).
Arc extinguishing system: Due to continuous switching, an electric arc will appear, making the contacts susceptible to burning and gradually wearing out, so an arc extinguishing system is needed.

Working principle of contactor

The contactor operates on the principle of supplying power to the circuit, controlled by the rated voltage value of the contactor and starting. At this time, the current will reach the two ends of the coil fixed on the magnetic core, a magnetic field is generated, the magnetic force appears to attract the core to move and form a closed magnetic circuit.
At that time, the contactor is in the operating state, the main contacts will close, the auxiliary contacts will change state and maintain (when normally open, it will close and when normally closed, it will open).
When we cut off the current to the circuit, the contactor is also cut off. Under the effect of the compression spring, the movable core returns to its original position. At the same time, the contacts will return to the normally open state and the motor will stop working.

Wiring diagram of 3 phase starter

MCB: Miniature Circuit Breaker

: Main contact of contactor

: auxiliary contact of contactor

OL: overload relay

: coil-contactor is the coil of the contactor

: Normally closed contact of relay

Classification of contactors

• According to the driving principle: The types of contactors are divided into many different types such as: electromagnetic contactors, hydraulic contactors, and pneumatic contactors. Of which, electromagnetic contactors are the most commonly used.

• According to the type of current: This type will have both DC contactors and AC contactors.
• According to the structure: Contactors used in places with limited height and contactors used in places with limited width.
• According to the voltage: Divided into low-voltage contactors, medium-voltage contactors.
• According to the rated current: There are small currents from small currents such as: 6A, 9A, 12A, 18A, ... or large currents such as: 600A, 800A, 900A.
• According to the function: There are contactor types specialized for capacitors, for motors, dustproof or waterproof types, ...

The effect of the contactor

In industry, magnetic starters play an important role in the control and safe operation of motors and electrical equipment. This is an electromechanical automation solution, providing simplicity, high stability, and easy repair without the need for complicated procedures.

• Automatic lighting system: Using PLC or thermal relay with timer setting, this system can automatically turn on and off the lighting according to the set day and time schedule.
• Ensure the motor starts directly or combine with thermal relay to protect the motor in case of overload, helping to increase durability and operating efficiency.Motor control:
• Capacitor control: Automatically close or cut off the levels of the capacitor to adjust to the load level, optimize efficiency and protect the electrical system.

Advantages of contactor

Optimal safety: With remote on/off mode, the contactor ensures operator safety, especially with a protective shell that prevents arc discharge in hazardous environments.
Compact, sturdy design: Easy to install in any location, even in tight spaces, helping to optimize space in the working environment.
Reasonable price: With affordable price, the contactor becomes an accessible and popular choice in the market.
Energy saving: Fast on/off time helps save energy effectively, improving operating efficiency.
High durability: The main and auxiliary contacts of the contactor are resistant to corrosion and abrasion, prolonging the service life.
Wide application: Contactors are commonly used in industrial zones, factories, and workshops.

Disadvantages of contactor

Although the magnetic starter method is effective, it also has some disadvantages that need to be noted:
Large starting current: The load current with high inertia causes the starting current to be prolonged, which can put pressure on the system and affect performance.
Heat up the motor: When operating continuously, the magnetic starter can cause the electric motor to heat up, not start smoothly, leading to reduced efficiency and life of the voltage system due to prolonged voltage drop.
Complicated in operation and maintenance: For high-demand motors such as squirrel cage rotors, this method makes operation and maintenance more difficult.

2.Thermal relay (Support device of magnetic contactor)

What is thermal relay

Thermal relay is understood as a device used to protect electrical capillaries to avoid damage to the electrical system when the current is overloaded, suddenly increased or leaked outside. Thermal relay is an electronic device used in electrical applications to protect the system from temperature overload. They have the function of monitoring the temperature of the system and automatically disconnecting or shutting down the system when the temperature of the system exceeds the set limit. This helps to minimize the risk of fire and damage to the system, thereby ensuring safety for users.

Structure of Thermal Relay

Thermal relays have a simple structure with main components including:

• The lever is present in the thermal relay.
  The normally closed contact is denoted as NC.
  The normally open contact is denoted as NO.
  The adjusting screw prevents the current from operating.
  The bimetallic bar of the thermal relay.
  The heating wire is present in the thermal relay.
  The thermal relay lever.
  The reset button is denoted as reset.

Operating principle of thermal relay

The operating mechanism of the thermal relay is that when the current is overloaded or the electrical device operates continuously, it will generate a large amount of heat that affects the metal bar. This will cause the relay to expand, open the capillaries and at the same time cut off the power to limit the level of danger when people use it.

Classification of Thermal Relays

Phase:
1-phase thermal relay: Single-phase thermal relay devices are used by many households for 220V motor currents to ensure safety for all members as well as the electrical system in the house. In addition, this product line can also monitor operating processes to prevent risks that may occur when there is an internal motor short circuit. 3-phase thermal relay: The 3-phase product line is one of the electrical devices with the largest capacity from 3kw, 4kw to 4.5kw. 3-phase thermal relays are often used in industry to ensure the efficiency that the current can provide. In addition, when a problem occurs, the relay will have the task of cutting off the power source from the motor to minimize all dangers.
Follow:1-pin relay: This type of relay usually has only one pin to connect to the power source, not very popular in industrial applications.
2-pin relay: Usually used in simple circuits, with the function of closing or opening a single electrical circuit.
3-pin relay: Can be connected to a power circuit and a load circuit, often used to control a device.
4-pin relay: This type allows control of multiple circuits, with the ability to simultaneously close/open multiple devices.
5-pin relay or more: Used in complex applications, can have many different functions and operating modes, such as multi-point relays or automatic control relays.

Application of Thermal Relay

Thermal relay is an important device in large electrical systems, including industrial machinery, air conditioning systems and residential electrical systems. When the temperature of the system exceeds the specified limit, the relay will detect and turn off the input power. This power shutdown will help the system stop and prevent it from being destroyed. Moreover, the relay is also used to reduce the risk of fire. In some cases, temperature overload can lead to an explosive environment and cause a fire. When this condition is detected, it will immediately turn off the power to reduce the risk of fire. Thanks to this function, the thermal relay supports the protection of the electrical system and the safety of the user.

3. Soft Start

Concept of soft starter

Soft starter is an electrical device used to support the starting process of medium and large capacity 3-phase asynchronous motors. Helps protect the motor from damage due to large and sudden starting currents, and avoids power grid system voltage drops that damage other electrical equipment on the same grid and mechanical effects.

Uses of soft starter

• Reduced maintenance costs: The soft starter's precise torque adjustment effectively responds to heavy or light loads, optimizing performance for all applications.
• Precise torque adjustment: Soft starters support the soft stop function, which is especially useful for pumping systems where water hammer is common when stopping suddenly using star-delta or direct starting methods.Safe Soft stop: With the soft stop function, the conveyor system transporting fragile materials will be protected, avoiding damage caused by stopping too quickly.
• Fragile material protection: In the event of a sudden voltage interruption, the motor can still stop by rotational inertia, helping to maintain safety and stability for a short period of time.Inertia stop:
• Flexible voltage regulation: Soft start allows the motor to gradually reduce voltage in about 1–20 seconds (depending on requirements), making the shutdown process smoother and protecting the motor better. Energy saving: When operating at low load, soft start automatically reduces voltage current to the optimal level, saving energy and reducing operating costs.

Structure of soft starter

The structure of soft start includes:
Control unit: Depending on the type of soft starter, the control unit may include a display and keyboard or be adjusted by screws or potentiometers. This makes it easy for users to configure according to operating needs.
Thyristor or SCR (Silicon Controlled Rectifier): This important component performs the task of adjusting and switching the current smoothly, helping to reduce sudden loads when starting.
Heat dissipation system and cooling fan: Ensure stable operation of the device by reducing the temperature, helping the soft starter operate effectively for a long time.
Protective housing: Diverse designs according to dustproof and waterproof standards depending on the usage environment, helping to protect the device from external impacts.
Contactor Bypass: Some soft starters have built-in bypass contactors, helping to optimize operating efficiency and save energy during operation.
Multi-function controller: Includes connection ports such as Modbus, Profibus, status relay, protection against overheating, overload, and the ability to adjust start-up time via potentiometer or screen, ensuring flexibility and safety in use.