9 Common Knowledge About Low-Voltage Electrical Apparatus

Circuit breaker

Low voltage circuit breaker, commonly known as automatic switch or air switch, is used for infrequent on-off control in low-voltage distribution circuit.

It can automatically break the fault circuit in case of short circuit, overload or undervoltage. It is a control and protection electrical appliance.

There are many kinds of circuit breakers. According to their uses and structural characteristics, they can be divided into DW frame circuit breaker, DZ plastic shell circuit breaker, DS DC fast circuit breaker, DWX and DWZ current limiting circuit breakers, etc.

The frame circuit breaker is mainly used as the protection switch of the distribution line, while the plastic shell circuit breaker can be used not only as the protection switch of the distribution line, but also as the control switch of the motor, lighting circuit and electrothermal circuit.

Taking the molded case circuit breaker as an example, this paper briefly introduces the structure, working principle, use and selection method of the circuit breaker.

Structure and operation of circuit breaker

The circuit breaker is mainly composed of three basic parts, namely contact, arc extinguishing system and various releases, including overcurrent release, voltage loss (undervoltage) release, thermal release, shunt release and free release.

The following figure 1-8 is the schematic diagram and graphical symbols of the working principle of the circuit breaker. The breaker switch is closed manually or electrically by the operating mechanism. After the contact is closed, the free tripping mechanism locks the contact in the closing position

The following figure 1-8 is the schematic diagram and graphical symbols of the working principle of the circuit breaker. The breaker switch is closed manually or electrically by the operating mechanism. After the contact is closed, the free tripping mechanism locks the contact in the closing position

The overcurrent release is used for short circuit and overcurrent protection of the line. When the current of the line is greater than the set current value, the electromagnetic force generated by the overcurrent release will trip the hook, and the moving contact will be quickly disconnected under the tension of the spring to realize the tripping function of the short circuit release.

schematic diagram and graphic symbols of circuit breaker working principle

Fig. 1-8 schematic diagram and graphic symbols of circuit breaker working principle

The thermal release is used for overload protection of the line, and its working principle is the same as that of the thermal relay.

The voltage loss (undervoltage) release is used for voltage loss protection. As shown in Figure 1-8, the coil of the voltage loss release is directly connected to the power supply and is in the pull in state, and the circuit breaker can be closed normally;

In case of power failure or low voltage, the suction force of the voltage loss release is less than the reaction force of the spring, and the spring makes the moving iron core upward to trip the hook, so as to realize the tripping function of the circuit breaker.

The shunt release is used for remote tripping. When the button is pressed remotely, the shunt release is powered on to generate electromagnetic force to trip.

The protection of different circuit breakers is different, and they shall be selected according to needs. The protection mode can also be marked in the graphic symbol, as shown in Figure 1-8. Three protection modes of voltage loss, overload and overcurrent are marked in the graphic symbol of circuit breaker.

Selection principle of low voltage circuit breaker

When selecting low-voltage circuit breaker, the following aspects should be considered

  • Selection of circuit breaker type: it shall be selected according to the application occasion and protection requirements.

For example, plastic shell type is generally selected; Current limiting type shall be selected when the short-circuit current is large; Frame type shall be selected when the rated current is relatively large or there are selective protection requirements; When controlling and protecting DC circuits containing semiconductor devices, DC fast circuit breakers shall be selected.

  • The rated voltage and current of the circuit breaker shall be greater than or equal to the normal working voltage and current of the line and equipment.
  • The limit making and breaking capacity of the circuit breaker is greater than or equal to the maximum short-circuit current of the circuit.
  • The rated voltage of the undervoltage release is equal to the rated voltage of the line.
  • The rated current of the overcurrent release is greater than or equal to the maximum load current of the line.

Controller

The controller is a kind of manual operation, which directly controls the high current (10a ~ 600A) of the main circuit.

Common controllers include KT type cam controller, kg type drum controller and KP type plane controller. The functions and working principles of various controllers are basically similar. The following takes the common cam controller as an example.

Cam controller is a large-scale manual controller, which is mainly used to directly control the start, stop, speed regulation, commutation and braking of small and medium-sized wound asynchronous motor in hoisting equipment. It is also suitable for other electric driving occasions with the same requirements.

The cam controller is mainly composed of contact, rotating shaft, cam, lever, handle, arc extinguishing cover and positioning mechanism.

Figure 1-9 shows the structural principle diagram and graphical symbols of cam controller.

There are multiple groups of contacts in the cam controller, which are controlled by multiple cams respectively, so as to realize the simultaneous control of multiple contacts in a more complex circuit.

Because there are many contacts in the cam controller, the connection of each contact at each position is different, so it can not be represented by ordinary normally open and normally closed contacts.

Fig. 1-9 (a) shows the schematic diagram of 1-pole 12 position cam controller.

The graphic symbol shown in Fig. 1-9 (b) indicates that there are 12 positions of this contact, and the small black dot in the figure indicates that the position contact is connected.

As can be seen from the schematic diagram, when the handle is turned to positions 2, 3, 4 and 10, the contact is connected by the cam.

Figure 1-9 (c) shows a 5-pole 12 position cam controller, which is composed of five 1-pole 12 position cam controllers.

Figure 1-9 (d) shows the graphical symbol of 4-pole 5-bit cam controller, indicating that there are 4 contacts, and each contact has 5 positions. The small black dot in the figure indicates that the contact is connected at this position. For example, when the handle is turned to position 1 on the right, contacts 2 and 4 are connected.

Because the cam controller can directly control the motor, it has large contact capacity and arc extinguishing device.

The advantages of cam controller are simple control circuit, few switching elements and convenient maintenance. The disadvantages are large volume, heavy operation and unable to realize remote control. At present, the cam controllers used include kt10, ktjl4, ktjl5 and ktjl6 series.

structural principle diagram and graphic symbols of cam controller

Fig. 1-9 structural principle diagram and graphic symbols of cam controller

Contactor

Contactors are mainly used to control motors, electric heating equipment, electric welding machines, capacitor banks, etc.

They can frequently connect or disconnect AC and DC main circuits to realize remote automatic control.

It has the function of low voltage release protection and is widely used in the automatic control circuit of electric drive.

There are two types of contactors: AC contactor and DC contactor. The following describes the AC contactor.

the structural diagram and graphical symbols of AC contactor

Figure 1-10 shows the structural diagram and graphical symbols of AC contactor.

Components of AC contactor

Electromagnetic mechanism

The electromagnetic mechanism is composed of coil, moving core (armature) and static core.

Contact system

The contact system of AC contactor includes main contact and auxiliary contact. The main contact is used to make and break the main circuit, with 3 or 4 pairs of normally open contacts; Auxiliary contact is used for control circuit and plays the role of electrical interlocking or control. It usually has two pairs of normally open and two pairs of normally closed contacts.

Arc extinguishing device

All contactors with a capacity of more than 10A have arc extinguishing devices. For contactors with small capacity, double break bridge contacts are often used to facilitate arc extinguishing; For large capacity contactors, longitudinal seam arc extinguishing cover and grid arc extinguishing structure are often used.

Other parts

Including reaction spring, buffer spring, contact pressure spring, transmission mechanism and shell, etc.

The contactor is marked with terminal number, the coils are A1 and A2, the main contacts 1, 3 and 5 are connected to the power side, and 2, 4 and 6 are connected to the load side.

The auxiliary contact is represented by two digits. The first digit is the sequence number of the auxiliary contact, the last digit 3 and 4 represent the normally open contact, and 1 and 2 represent the normally closed contact.

The control principle of the contactor is very simple. When the coil is connected to the rated voltage, electromagnetic force is generated to overcome the spring reaction force and attract the moving iron core to move downward.

The moving iron core drives the insulating connecting rod and moving contact to move downward, so that the normally open contact is closed and the normally closed contact is disconnected.

When the coil loses power or the voltage is lower than the release voltage, the electromagnetic force is less than the spring reaction force, the normally open contact is disconnected and the normally closed contact is closed.

Main technical parameters and types of contactors

Rated voltage

The rated voltage of the contactor refers to the rated voltage of the main contact. AC has 220V, 380V and 660V, the rated voltage applied in special occasions is up to 1140V, and DC mainly has 110V, 220V and 440V.

Rated current

The rated current of the contactor refers to the rated working current of the main contact. It is specified under certain conditions (rated voltage, service category and operating frequency). At present, the commonly used current level is 10A ~ 800A.

Rated voltage of suction coil

AC 36V, 127V, 220V and 380V, DC 24V, 48V, 220V and 440V.

Mechanical life and electrical life

Contactors are frequently operated electrical appliances, which should have high mechanical and electrical life. This index is one of the important indexes of product quality.

Rated operating frequency

The rated operating frequency of the contactor refers to the allowable operation times per hour, generally 300 times / h, 600 times / h and 1200 times / h.

Action value

The action value refers to the pull-in voltage and release voltage of the contactor. It is specified that when the pull-in voltage of the contactor is greater than 85% of the rated voltage of the coil, it shall be pulled in reliably, and the release voltage shall not be higher than 70% of the rated voltage of the coil.

Common AC contactors include cjl0, cjl2, cj10x, cj20, cjxl, CJX2, 3TB and 3td series.

Selection of contactor

(1) Select the type of contactor according to the nature of the load.

(2) The rated voltage shall be greater than or equal to the working voltage of the main circuit.

(3) The rated current shall be greater than or equal to the rated current of the controlled circuit. The motor load shall also be increased or reduced appropriately according to its operation mode.

(4) The rated voltage and frequency of the suction coil shall be consistent with the selected voltage and frequency of the control circuit.

Starter

Starter is used for starting and stopping control of three-phase asynchronous motor. It is a complete set of low-voltage control device.

Starter is used for starting and stopping control of three-phase asynchronous motor. It is a complete set of low-voltage control device.

QJ type decompression starter adopts autotransformer to reduce voltage, which is used to control the infrequent decompression starting control of three-phase cage asynchronous motor; QX starter is star delta step-down starter.

Various starter control circuits vary according to model and motor capacity

Master appliance

The master electric appliance is used to issue control commands in the form of on-off of switch contacts in the control circuit to enable the control circuit to perform corresponding control tasks.

The master electrical appliance is widely used and has a wide variety. The common ones are button, travel switch, proximity switch, universal transfer switch, master controller, selector switch, foot switch, etc.

Button

Button is one of the most commonly used master appliances, which has simple structure and is easily controlled.

Structure, type and common model of button

The button is composed of button cap, return spring, bridge contact and shell. Its structural diagram and graphic symbols are shown in Figure 1-20.

The contact adopts bridge contact, and the rated current is below 5A. Contacts are divided into normally open contacts (dynamic breaking contacts) and normally closed contacts (dynamic closing contacts).

Buttons can be divided into flat button and emergency stop button in terms of shape and operation mode. Emergency stop button is also called mushroom head button, as shown in Figure 1-20 (c).

In addition, there are many types, such as key button, knob, pull button, universal lever type, illuminated type and so on.

schematic diagram of button structure and graphic symbols

Figure 1-20 schematic diagram of button structure and graphic symbols

The contact action mode of the button can be divided into direct action type and micro action type. The buttons shown in Figure 1-20 are direct action type, and the contact action speed is related to the speed of hand pressing.

The contact action transformation speed of the inching button is fast, which has nothing to do with the speed of hand pressing. Its action principle is shown in Figure 1-21.

The moving contact is composed of a deformed reed. When the curved reed moves downward under pressure and lower than the flat reed, the curved reed deforms rapidly and bounces the flat reed contact upward to realize the instantaneous action of the contact.

Small micro button is also called micro switch. Micro switch can also be used in various relays and limit switches, such as time relay, pressure relay and limit switch.

action principle diagram of inching button

Fig. 1-21 action principle diagram of inching button

The buttons are generally reset and self-locking. The most commonly used button is the reset flat button, as shown in Figure 1-20 (a). The button is flush with the shell to prevent foreign matters from touching by mistake.

Button color

The red button is used for “stop”, “power off” or “accident”.

Green buttons are preferred for “start” or “power on”, but black, white or gray buttons are also allowed.

One button dual-purpose “start” and “stop” or “power on” and “power off”, that is, if the function is changed after alternately pressing, the red button or green button shall not be used, but the black, white or gray button shall be used.

If it moves when pressing and stops when lifting (such as inching and inching), use black, white, gray or green buttons, preferably black buttons, not red buttons.

For single reset function, use blue, black, white or gray buttons.

Red buttons are used for those with “reset”, “stop” and “power off” functions at the same time. The light button shall not be used as an “accident” button.

Selection principle of buttons

(1) Select the type of control button according to the application occasion, such as open type, waterproof type, anti-corrosion type, etc.

(2) According to the purpose, select appropriate types, such as key type, emergency type, lamp type, etc.

(3) According to the needs of the control circuit, determine the number of different buttons, such as single button, double button, three button, multi button, etc.

(4) Select the color of buttons and indicator lights according to the requirements of working status indication and working conditions.

Table 1-1 gives the meaning of button color.

Colour Meaning An example
Red Handling accidents Emergency stop
Extinguish combustion
Stop or “power off” Normal shutdown
Stop one or more motors
Local shutdown of the unit
Cut off a reset switch with “stop” or “power off” function
Green Start or “power on” Normal start
Start one or more motors
Local start of the device
Turn on a switching device (put into operation)
Yellow Participate in Prevent accidents
The parameter suppresses the abnormal state
Avoid unwanted changes (accidents)
Blue Any specified intention not included in the above color All meanings not included in red, yellow and green: blue can be used
Black, grey, white No specific intention Any function other than the “stop” or “power off” button of single function

Table 1-1 meaning of button color

Travel switch

Travel switch is also called limit switch.

It has many kinds.

According to the movement form, it can be divided into direct action type, micro motion type, rotary type, etc;

According to the nature of the contact, it can be divided into contact type and contactless type.

Travel switch with contact

The travel switch with contact is referred to as the travel switch for short.

The working principle of the travel switch is the same as that of the button.

The difference is that it is not pressed by hand, but a master appliance that uses the contact action of the moving parts of the production machinery to send out control commands.

It is used to control the moving direction, speed, stroke size or position of production machinery, and its structural forms are diverse.

Figure 1-22 shows the action principle diagram and graphical symbols of several operation types of travel switches.

The main parameters of travel switch include type, action travel, working voltage and current capacity of contact.

At present, domestic travel switches include lxk3, 3se3, lxl9, LXW and LX series.

Commonly used travel switches include LX19, LXW5, lxk3, lx32 and lx33 series.

structural diagram and graphic symbols of travel switch

Figure 1-22 structural diagram and graphic symbols of travel switch

Contactless travel switch

Contactless travel switch, also known as proximity switch, can replace the contact travel switch to complete travel control and limit protection.

It can also be used as a non-contact switch for high-frequency counting, speed measurement, liquid level control, part size detection, automatic connection of machining program, etc.

It is widely used in machine tool, textile, printing, plastics and other industrial production because it has the characteristics of non-contact trigger, fast action speed, action in different detection distances, stable and reliable signal, long service life, high repeated positioning accuracy and can adapt to harsh working environment.

Contactless travel switches are divided into active type and passive type.

Most contactless travel switches are active type, mainly including detection element, amplification circuit and output driving circuit. Generally, 5V ~ 24V DC current or 220V AC power supply are adopted.

Figure 1-23 shows the structural block diagram of three wire active proximity switch.

structural block diagram of active proximity switch

Figure 1-23 structural block diagram of active proximity switch

According to the working principle of detection elements, proximity switches can be divided into high-frequency oscillation type, ultrasonic type, capacitance type, electromagnetic induction type, permanent magnet type, Hall element type and magnetic sensor type.

Different types of proximity switches detect different objects.

Capacitive proximity switch can detect various solid, liquid or powder objects. It is mainly composed of capacitive oscillator and electronic circuit. Its capacitance is located at the sensing interface.

When the object approaches, it will oscillate due to changing the capacitance value, resulting in output signal.

Hall proximity switch is used to detect magnetic field.

Generally, magnetic steel is used as the detected body. The internal magnetic sensitive device is only sensitive to the magnetic field perpendicular to the end face of the sensor.

When the magnetic pole s is facing the proximity switch, the output of the proximity switch will jump positively and the output will be high level.

If the magnetic pole n is facing the proximity switch, the output will be low level.

The ultrasonic proximity switch is suitable for detecting inaccessible or inaccessible targets.

Its control function is not disturbed by acoustic, electrical, optical and other factors.

The detected objects can be solid, liquid or powder objects, as long as they can reflect ultrasonic waves.

It is mainly composed of piezoelectric ceramic sensor, electronic device for transmitting ultrasonic wave and receiving reflected wave, and program-controlled bridge switch for adjusting detection range.

The high-frequency oscillating proximity switch is used to detect various metals. It is mainly composed of high-frequency oscillator, integrated circuit or transistor amplifier and output.

Its basic working principle is that when a metal object approaches the coil of the oscillator, the eddy current generated inside the metal object will absorb the energy of the oscillator and stop the oscillator.

The oscillation and stop signals of the oscillator are converted into switching signals after shaping and amplification.

The output forms of proximity switches are two-wire, three wire and four wire, the output types of transistors are NPN and PNP, and the shapes are square, round, slot and separated,

Figure 1-24 shows the working principle of slot three wire NPN photoelectric proximity switch and the working diagram of remote separation photoelectric switch.

slot type and separate type photoelectric switch

Figure 1-24 slot type and separate type photoelectric switch

The main parameters of proximity switch include type, action distance range, action frequency, response time, repetition accuracy, output type, working voltage and output contact capacity.

The graphical symbols of the proximity switch can be shown in Figure 1-25.

graphical symbols of proximity switch

Figure 1-25 graphical symbols of proximity switch

There are many kinds of proximity switches. The commonly used domestic proximity switches include LJ, 3sg, lxj18 and other series. Imported and imported products are also widely used in China.

Selection of contact travel switch

The following points shall be paid attention to in the selection of contact travel switch:

(1) Application and control object selection.

(2) The installation environment shall be protected, such as open type or protective type.

(3) Voltage and current of control circuit.

(4) The appropriate head form shall be selected according to the force transmission and displacement relationship between machinery and travel switch.

Selection of proximity switch

(1) Operating frequency, reliability and accuracy.

(2) Inspection distance and installation dimension.

(3) Contact form (with contact and without contact), number of contacts and output form (NPN type and PNP type).

(4) Power type (DC, AC), voltage level.

Transfer switch

The transfer switch is a master electrical appliance with multi gear, multi contact and multi loop control.

It is mainly used for line replacement, remote control and commutation measurement of ammeter and voltmeter in various control equipment.

It can also be used to control the starting, commutation and speed regulation of small capacity motor.

The working principle of the change-over switch is the same as that of the cam controller, but the use places are different.

The cam controller is mainly used for the main circuit to directly control the electrical equipment such as motor, while the change-over switch is mainly used for the control circuit to indirectly control the electric machine through relays and contactors.

There are two main types of transfer switches commonly used, namely universal transfer switch and combination switch.

The structure and working principle of the two are basically similar, and they can replace each other in some applications.

The transfer switch is divided into ordinary type, open combination type and protection combination type according to the structure type;

According to the purpose, it is divided into two types: main command control and motor control.

The graphic symbols of the change-over switch are the same as those of the cam controller, as shown in Figure 1-26.

Table 1-2 on-off state of change-over switch contact

on-off state of change-over switch contact

The main parameters of the transfer switch include type, handle type, contact on-off state table, working voltage, contact quantity and current capacity, which are described in detail in the product manual.

The commonly used change-over switches include lw2, lw5, LW6, LW8, LW9, lwl2, lwl6, VK, 3lb and Hz series.

Among them, lw2 series is used to control the operation circuit of high-voltage circuit breaker, lw5 and LW6 series are mostly used to control the line or motor in the electric drive system, and LW6 series can also be installed into a double column type.

The columns are meshed with gears and operated by the same handle,

The switch can be equipped with up to 60 pairs of contacts.

The change-over switch can be selected according to the following aspects:

(1) Rated voltage and operating current.

(2) Handle type and positioning features.

(3) Number of contacts and wiring diagram number.

(4) Panel type and mark.

Resistor

Resistance is an indispensable electrical component in electrical products, which can be divided into two categories: one is resistance component, which is used for weak current electronic products, and the other is industrial resistance device (resistor for short), which is used for current regulation of low-voltage strong current AC and DC electrical lines, starting, braking and speed regulation of motors.

Commonly used resistors are ZB plate and ZG tube resistors, which are used for current regulation in low-voltage circuits. ZX resistor is mainly used for starting, braking and speed regulation of AC and DC motors.

The main technical parameters of the resistor include rated voltage, heating power, resistance value, allowable current, heating time constant, resistance error and overall dimension. The graphic symbols of resistors are shown in Figure 1-27.

graphical symbols of resistors and rheostats

Figure 1-27 graphical symbols of resistors and rheostats

Rheostat

The function of rheostat is similar to that of resistor.

The difference is that the resistance of rheostat is continuously adjustable, and the resistance of each section of resistor is fixed.

In the control circuit, the resistance value can be adjusted by series parallel connection or selecting different sections of resistance, and the resistance value is intermittently adjustable.

Commonly used varistors include BC sliding wire varistors, which are used for current and voltage regulation of circuits, control or regulation of circuits such as electronic equipment and instruments, etc.

BL type excitation rheostat is used for excitation or speed regulation of DC motor; BQ starting rheostat is used for starting DC motor;

BT rheostat is used for excitation or speed regulation of DC motor;

BP frequency sensitive rheostat is used for starting control of three-phase AC wound asynchronous motor.

The main technical parameters of rheostat are similar to those of resistor.

The graphic symbols of rheostat are shown in Figure 1-27.

Voltage regulator

There are few types of voltage regulators. TD4 carbon resistance voltage regulator is used to automatically adjust the voltage in small and medium-sized AC or DC generators.

Electromagnet

Commonly used electromagnets include MQ traction electromagnet, MW lifting electromagnet, MZ braking electromagnet, etc.

MQ traction electromagnet is used for remote control of mechanical equipment and various automatic system operating mechanisms in low-voltage AC circuit.

MW lifting electromagnet is used to be installed on lifting machinery to attract magnetic materials such as steel.

Mzd single-phase brake electromagnet and mzs three-phase brake electromagnet are generally used to form electromagnetic brake. The schematic diagram of Tj2 AC electromagnetic brake composed of brake electromagnet is shown in Figure 1-28.

Generally, the electromagnetic brake and the motor shaft are installed together, and the electromagnetic brake coil and the motor coil are connected in parallel, and the two are energized at the same time, or the electromagnetic brake coil is energized first, and then the motor is energized.

The electromagnetic brake coil is energized to attract the armature to press the spring, then the brake shoe and the brake wheel fixed on the motor shaft are released, and the motor rotates.

When the motor and the electromagnetic brake lose power at the same time, the brake shoe will hold the brake wheel tightly under the action of the compression spring to brake the motor.

The graphic symbol of electromagnet is the same as that of electromagnetic brake, and the text symbol is ya.

The graphic symbols of electromagnetic brake are shown in Figure 1-28.

schematic diagram and graphic symbols of electromagnetic brake

Figure 1-28 schematic diagram and graphic symbols of electromagnetic brake

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