5. Master electrical appliances
The master electrical appliance is used to issue control commands in the form of switching contacts in the control circuit, so that the control circuit can perform the corresponding control tasks. Master electrical appliances are widely used and have a wide range of types, including buttons, travel switches, proximity switches, universal transfer switches, master controllers, selector switches, foot switches, etc.
button
The button is one of the most commonly used master electrical appliances, which has a simple structure and convenient control.
1. The structure, type and common model of the button
The button is composed of a button cap, a return spring, a bridge contact and a shell, etc., and its structural schematic diagram and graphic symbols are shown in Figure 3. The contacts are bridge contacts, and the rated current is less than 5A. There are two types of contacts: normally open contacts (dynamic breaking contacts) and normally closed contacts (dynamic contacts).
Button can be divided into flat button and emergency stop button from the shape and operation mode, emergency stop button is also called mushroom head button, as shown in Figure 3 (c), in addition to key buttons, knobs, pull buttons, universal joystick type, light type and other types.
Figure 3 Schematic diagram and graphic symbols of the button structure
The contact action mode of the button can be divided into two types: direct action type and micro action type, and the buttons shown in Figure 3 are all direct action type, and the contact action speed is related to the speed of hand pressing. The contact action of the micro-motion button changes quickly, regardless of the speed of hand pressing, and its operating principle is shown in Figure 4. The moving contact is composed of deformed reeds, when the downward movement of the bent reed is lower than that of the flat reed, the bent reed is rapidly deformed, and the flat reed contact is bounced upwards to achieve the instantaneous action of the contact.
Small micro buttons are also called micro switches, micro switches can also be used in various relays and limit switches, such as time relays, pressure relays and limit switches.
Figure 4 Schematic diagram of the operation of a micro-motion button
The most commonly used button is the reset type flat button, as shown in Figure 1-20(a), the button is flush with the shell, which can prevent foreign objects from being accidentally touched.
2. The color of the button
The red buttons are used for "Stop", "Power Off", or "Accident".
The green button is preferentially used for "start" or "power-up", but black, white, or gray buttons are also allowed.
"Start" and "stop" or "power-on" and "power-off" with dual use of one button, that is, if the function is changed after alternate pressing, the red button cannot be used, nor can the green button be used, but the black, white or gray button should be used.
Movement when pressing, stop movement when lifting (e.g., joging, micro-movement), apply black, white, gray or green buttons, preferably black buttons, not red buttons.
For a single reset function, use a blue, black, white, or gray button.
At the same time, there are "reset", "stop" and "power off" functions with red buttons. The light button must not be used as an "incident" button.
3. Button selection principle
(1) According to the use occasion, select the type of control button, such as open type, waterproof type, anti-corrosion type, etc.
(2) According to the use, choose the appropriate type, such as key type, emergency type, light type, etc.
(3) According to the needs of the control loop, determine the number of different buttons, such as single button, double button, three buttons, multiple buttons, etc.
(4) According to the requirements of the working status indication and the working situation, select the color of the button and the indicator light.
Table 1 gives the meaning of the button color.
Table 1 Meanings of button colors
The buttons for the alternating "start" and "stop" movements must be black or white or gray, no red or green, the "jog" button must be black, the "reset" button must be blue, and the reset button must be red when it has a stop function, and the "allow start" button must be white
Travel switch
The travel switch is also called the limit switch, there are many kinds of it, according to the form of movement can be divided into direct action, micro action, rotation type, etc., according to the nature of the contact can be divided into contact type and non-contact type.
1. There is a contact travel switch
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 the main command electrical appliance that uses the contact action to issue control instructions by using the parts of the production machinery to move and press. It is used to control the direction of movement, speed, stroke size or position of production machinery, etc., and its structure forms are varied.
Figure 5 shows a schematic diagram and graphical symbols of the operation principle of the travel switch for several types of operation.
The main parameters of the travel switch are type, action stroke, working voltage and contact current capacity. At present, the domestic production of travel switches include LXK3, 3SE3, LXl9, LXW and LX series.
Commonly used travel switches are LX19, LXW5, LXK3, LX32 and LX33 series.
Fig.5. Schematic diagram and graphic symbol of the travel switch
2. Contactless travel switch
The non-contact travel switch, also known as the proximity switch, can replace the contact travel switch to complete the stroke control and limit protection, and can also be used for non-contact switches such as high-frequency counting, speed measurement, liquid level control, part size detection, and automatic connection of processing programs. Because it has the characteristics of non-contact triggering, fast action speed, can act in different detection distances, stable and pulsation-free signals, stable and reliable work, long life, high repeat positioning accuracy and can adapt to harsh working environments, it is widely used in machine tools, textiles, printing, plastics and other industrial production.
The non-contact travel switch is divided into two types: active type and passive type, most of the non-contact travel switch is active, mainly including 3 parts: detection element, amplification circuit and output drive circuit, generally using 5V~24V DC current, or 220V AC power supply, etc. Figure 6 shows the block diagram of the three-wire active proximity switch.
Fig.6. Block diagram of active proximity switch
According to the working principle of the detection element, the proximity switch can be divided into high-frequency oscillation type, ultrasonic type, capacitive type, electromagnetic induction type, permanent magnet type, Hall element type and magnetic sensitive element type. Different types of proximity switches detect different objects to be detected.
Capacitive proximity switch can detect a variety of solid, liquid or powder objects, which is mainly composed of capacitive oscillator and electronic circuits, its capacitance is located at the sensing interface, when the object approaches, it will oscillate due to changing the capacitance value, thus generating an output signal.
Hall proximity switches are used to detect magnetic fields, and magnets are generally used as the detected body. The magnetic sensing device inside is only sensitive to the magnetic field perpendicular to the end face of the sensor, when the pole S pole is facing the proximity switch, the output of the proximity switch produces a positive jump, and the output is high, and if the pole N pole is facing the proximity switch, the output is low.
Ultrasonic proximity switches are suitable for detecting targets that cannot or cannot be reached, and their control functions are not interfered with by sound, electricity, light and other factors, and the detection objects can be solid, liquid or powder objects, as long as they can reflect ultrasonic waves. It is mainly composed of several parts, such as piezoelectric ceramic sensor, electronic devices for emitting ultrasonic waves and receiving reflected waves, and program-controlled bridge switches for adjusting the detection range.
The high-frequency oscillating proximity switch is used to detect various metals, mainly composed of three parts: high-frequency oscillator, integrated circuit or transistor amplifier and output, and its basic working principle is that when a metal object is close to the coil of the oscillator, the eddy current generated inside the metal object will absorb the energy of the oscillator, causing the oscillator to stop. The oscillation and stop signals of the oscillator are transformed into switching signal output after shaping and amplification.
There are several kinds of two-wire, three-wire and four-wire proximity switch output forms, and the transistor output type has NPN and PNP two, and the shape has a variety of square type, round type, slot type and separate type, and Figure 7 is the working principle diagram of slot-type three-wire NPN photoelectric proximity switch and the working schematic diagram of long-distance separation photoelectric switch.
Fig.7. Slotted and discrete photoelectric switches
The main parameters of the proximity switch include type, operating distance range, operating frequency, response time, repeatability, output type, working voltage and output contact capacity. The pictorial symbol of the proximity switch can be represented in Figure 8.
Figure 8 Graphical symbol of the proximity switch
The product range of proximity switch is very rich, the commonly used domestic proximity switch is LJ, 3SG and LXJ18 and other series, foreign imported and imported products also have a large number of applications in China.
3. There is an option for a contact travel switch
The following points should be paid attention to in the selection of contact travel switches:
(1) Application and control object selection.
(2) Select the protection form for the installation environment, such as open type or protected type.
(3) Control the voltage and current of the loop.
(4) The force transmission and displacement relationship between the mechanical and travel switch should be selected in the appropriate head form.
4. Selection of proximity switches
(1) Working frequency, reliability and accuracy.
(2) Detection distance and installation size.
(3) Contact form (with contact, without contact), number of contacts and output form (NPN type, PNP type).
(4) Power supply type (DC, AC) and voltage level.
Transfer switches
The transfer switch is a kind of multi-gear, multi-contact, multi-circuit main electrical appliance, mainly used for the replacement of lines in various control equipment, remote control and commutation measurement of ammeter and voltmeter, etc., and can also be used to control the starting, commutation and speed regulation of small-capacity motors.
The working principle of the transfer switch is the same as that of the cam controller, but the place of use is different, the cam controller is mainly used for the main circuit, directly controlling the motor and other electrical equipment, while the transfer switch is mainly used for the control circuit, indirectly controlling the motor through relays and contactors. There are two main types of commonly used transfer switches, namely universal transfer switches and combination switches. The structure and working principle of the two are basically similar, and they can be substituted for each other in some applications. According to the structure type, the transfer switch is divided into ordinary type, opening combination type and protection combination type, etc., and is divided into two kinds according to the use of the main order control and control motor. The graphical symbol of the transfer switch is the same as that of the cam controller, as shown in Figure 2.
The contact on-off state of the transfer switch can also be represented in a graph, as shown in Table 2 for the 4-pole 5-position transfer switch in Figure 2.
Table 2 The status table of the on-off contact of the transfer switch
The main parameters of the transfer switch include type, handle type, contact on-off status table, working voltage, number of contacts and its current capacity, which are detailed in the product manual.
Commonly used transfer switches are LW2, LW5, LW6, LW8, LW9, LWl2, LWl6, VK, 3LB and HZ and other series, of which LW2 series is used for the control of high-voltage circuit breaker operation loop, LW5, LW6 series are mostly used in the power drive system to control the line or motor, LW6 series can also be installed into a double-row type, between the columns with gear meshing, and operated by the same handle, this switch can be installed up to 60 pairs of contacts.
The selection of transfer switch can be carried out according to the following aspects:
(1) Rated voltage and working current.
(2) Handle type and positioning features.
(3) The number of contacts and the number of wiring diagrams.
(4) Panel type and logo.
6. Resistors
Resistance is an indispensable electrical component in electrical products, which can be divided into two categories, one is resistive elements, which are used for weak current electronic products, and the other is industrial resistive devices (referred to as resistors), which are used for the current regulation of low-voltage and strong AC and DC electrical circuits and the starting, braking and speed regulation of motors.
Commonly used resistors are ZB type plate and ZG type tubular resistors, which are used for current regulation in low-voltage circuits. ZX type resistors are 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 external dimensions. The graphic symbol of the resistor is shown in Figure 10.
Figure 10 Resistor and rheostat symbols
7. Rheostat
The function of the rheostat is similar to that of the resistor, the difference is that the resistance of the rheostat is continuously adjustable, while the resistance of each section of the resistor is fixed, and the resistance value can be adjusted by series and parallel connection or the method of selecting different sections of resistance in the control circuit, and the resistance value is intermittently adjustable.
Commonly used rheostats include BC type slip line rheostats, which are used for current and voltage regulation of circuits, control or regulation of electronic equipment and instrumentation circuits, etc. BL type excitation rheostat is used for excitation or speed regulation of DC motor, BQ type starting rheostat is used for starting DC motor, BT type rheostat is used for excitation or speed regulation of DC motor, BP type frequency sensitive rheostat is used for starting control of three-phase AC wound asynchronous motor. The main technical parameters of a rheostat are similar to those of a resistor. The graphical symbol of the rheostat is shown in Figure 10.
8. Voltage regulator
There are fewer types of voltage regulators, and the TD4 carbon resistance voltage regulator is used to automatically regulate the voltage in small and medium-capacity AC or DC generators.
9. Electromagnets
Commonly used solenoids include MQ type traction solenoid, MW type lifting solenoid, MZ type brake solenoid, etc.
MQ type traction solenoid is used for long-distance control of mechanical equipment and various automation system operating mechanisms in low-voltage AC circuits.
MW type hoisting solenoids are used to attract magnetic materials such as steel when installed on lifting machinery.
MZD type single-phase braking solenoid and MZS type three-phase braking solenoid are generally used to form electromagnetic brakes, and the schematic diagram of TJ2 type AC electromagnetic brake composed of braking solenoids is shown in Figure 10, usually electromagnetic brakes and motor shafts are installed together, and their electromagnetic brake coils and motor coils 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 electrified. The electromagnetic brake coil is energized to attract the armature to compress the spring, the brake shoe and the brake wheel fixed on the motor shaft are loosened, the motor rotates, when the motor and the electromagnetic brake lose power at the same time, the brake shoe holds the brake wheel tightly under the action of the compression spring, so that the motor moves.
The graphic symbol of the electromagnet is the same as that of the electromagnetic brake, and the text symbol is YA.
The graphic symbol of the electromagnetic brake is shown in Figure 11.
Fig.11 Schematic diagram and graphic symbols of electromagnetic brakes