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Introduce Commonly Used Control Appliances

The use of relays and contactors to realize the control and protection of motors and production equipment is called relay contact control. A relay control system is composed of control appliances that undertake different functions, such as buttons, switches, relays, and contactors.

1. Combination switch

Combination switch structure

The combination switch is also called a transfer switch.

1. Purpose

It is generally used as a switch for the power supply of electrical equipment, and also used for start/stop control of small-capacity motors.

2. Structure

For a commonly used three-pole switch, each pole has a pair of static contact pieces connected to the outer terminal of the box, and the movable contact piece can be rotated under the control of the handle to achieve the on/off control of the circuit.

3. Type

There are single pole, double pole, three pole and four pole, etc., and the rated current is 10A, 25A, 60A and 100A.

As shown in the figure below, when rotating the shaft with the handle, three electric shocks can be connected or disconnected at the same time.

Combination Switch

The following picture shows the application of the combination switch: controlling the start and stop of a small-capacity motor.

Use the combination switch to directly control the start and stop of the motor.

Second, the button

Buttons are often used to connect and disconnect control circuits.

The outline drawing, structure and symbols of the button are shown in the figure. Usually the red button represents stop and the green button represents start.

Three, AC contactor

It is used to switch on and off high-current circuits frequently.

1. Structure and principle

When the coil is energized, the static iron core generates electromagnetic attraction and pulls in the moving iron core. Since the contact system is linked with the moving iron core, the moving iron core drives the three moving contact pieces to run at the same time, and the contacts are closed to connect power supply. When the coil is de-energized, the suction force disappears, and the moving iron core linkage part is separated by the reaction force of the spring, so that the main contact is disconnected and the power supply is cut off.

Principle of AC Contactor

2. Symbols

Coils and contacts belonging to the same device are indicated by the same text.

Commonly used AC contactors include CJ10, CJ12, CJ20 and 3TB series.

Technical indicators of the contactor: rated working voltage, current, number of contacts, etc.

Four, intermediate relay

It is usually used to transmit signals and control multiple circuits at the same time. It can also be used directly to control small-capacity motors or other electrical actuators.

The intermediate relay has a small capacity and a large number of contacts, and is used to control the circuit.

Several commonly used intermediate relays:

Five, thermal relay

Used for motor overload protection.

1. Working principle

The heating element is connected to the main circuit of the motor. If it is overloaded for a long time, the bimetal will be heated. Due to the large expansion coefficient of the lower layer of the bimetallic sheet, it bends upwards, the lever is pulled back by the spring, and the movable break contact is disconnected.

2. Symbols

Commonly used thermal relays:

Six, fuse

Used for short-circuit protection in low-voltage lines.

1. Classification

Commonly used fuses are: plug-in fuse, screw fuse, tube fuse and filled fuse.

2. Symbols

3. Selection of the rated current IF of the fuse

(1) Resistive loads such as electric lamps and electric furnaces

IF>IL

(2) Single motor

Fuse rated current ≥ motor starting current/2.5

(3) Motors that start frequently

Fuse rated current ≥ motor starting current/1.6-2

Seven, circuit breaker

Circuit breaker refers to a switching device that can close, carry and break current under normal circuit conditions and can close, carry and break current under abnormal circuit conditions within a specified time.

Circuit breakers are generally composed of contact system, arc extinguishing system, operating mechanism, trip unit, shell and so on.

Circuit breaker symbol:

Introduce Commonly Used Control Appliances 1

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