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MindMap Gallery Introduction to electrical automation
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Introduction to electrical automation
This is a mind map about getting started with electrical automation. The main contents include 1.1 Electrical Basics, 1.2 Three-phase AC and its main application parameters, 2.1 Causes of electric shock and measures to prevent electric shock, 3.1 Basic knowledge of three-phase AC asynchronous motors, etc.
Edited at 2022-11-26 21:10:15
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Introduction to electrical automation
- Avatar Character Relationship Chart
Avatar 3 centers on the Sully family, showcasing the internal rift caused by the sacrifice of their eldest son, and their alliance with other tribes on Pandora against the external conflict of the Ashbringers, who adhere to the philosophy of fire and are allied with humans. It explores the grand themes of family, faith, and survival.
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Introduction to electrical automation
1.1 Electrical basics
1.1.1 Electric energy
Electrical energy definition: refers to the ability to use electricity to do work (i.e. generate energy) in various forms.
The functions and characteristics of electric energy
Easy to convert
Easy to transfer
Easy to control
difficult to store
Classification of electrical energy
DC power supply: one-way flow or movement of charge, the direction does not change (the size can change, but the direction must not change)
AC power supply: Refers to AC power whose direction of current changes periodically with time. The moving average value within a cycle is 0. Unlike DC power, its direction changes with time, and DC power does not change periodically.
1.1.2 Commonly used physical quantities in electricians and their applications
resistance
Definition: The resistance encountered when current passes through a conductor, represented by the letter R, and the unit is: ohms (Ω). Resistors generate heat when current passes through them. We call this process of converting electrical energy into heat energy the thermal effect.
Formula: R=ρL/S
ρ: Resistivity of the material from which the resistor is made, SI unit is ohms·meter (Ω·m)
L: Length of wire wound into a resistor, SI unit is meter (m)
S: The cross-sectional area of the wire wound into a resistor, the SI unit is square meters (m^2)
The relationship between resistance and temperature: Generally speaking, the resistance of metal conductors increases as the temperature increases
current
Definition: The amount of charge passing through the cross-section of a conductor per unit time, usually I represents the current, and the unit is ampere (A)
Formula: I=Q/t
Q represents the amount of charge passing through the cross section of the conductor, the unit is: Library (C)
t represents the power-on time, the unit is: second (S)
1A=1C/1S, that is: 1A=1 bank/1 second
Voltage
Voltage, also called potential difference or potential difference, is a physical quantity that measures the energy difference produced by a unit charge in an electrostatic field due to different potentials.
The direction of voltage is from high potential to low potential. U is usually used to represent voltage, and the international unit of voltage is volts.
Formula: U=IR
Power: P=UI
Electric energy: W=PT
1.2 Three-phase alternating current and its main application parameters
1.2.1 Alternating current and its main application parameters
Alternating current definition: an electric current that continuously changes in magnitude and direction over time.
Main application parameters of alternating current
T=1/f, the number of times the alternating current completes periodic changes in 1 second, is called the frequency of the alternating current, represented by "f", and the unit is Hertz, or Hertz for short, represented by Hz. Commonly used units include kilohertz, megahertz, period and frequency, which are physical quantities that describe the speed of alternating current changes.
The instantaneous value, maximum value and effective value of the sinusoidal alternating current (if the alternating current and the direct current pass through the same resistor respectively, and the energy consumed by the two in the same time is equal, the value of the direct current is called the effective value of the alternating current)
1.2.2 Direct current and its main application parameters
Definition of direct current: The direction does not change, also known as constant current. Constant current is a type of direct current, which is a direct current that does not change in size and direction.
Commonly used DC power components: solenoid valves, indicator lights, relays, PLC controllers, various sensors, instruments, etc.
1.2.3 Star connection of three-phase circuit loads and the relationship between phase/line voltage, current and power
As shown in the figure on the right, when the load is connected in a star connection, the current (phase current) I phase in each phase load is the line current I line. The line voltage U line on the load is √3 times the phase voltage U phase (I phase = I line, U line = √3 U phase)
P=√3U line I line cosΦ=3U phase I phase cosΦ
Reactive power of three-phase symmetrical load: Q=√3U line I line sinΦ
1.2.4 Delta connection of three-phase circuit loads and the relationship between phase/line voltage, current and power
A three-phase circuit with a delta-connected load, as shown in the figure on the right. It is generally used when the three-phase load is symmetrical, such as a motor with a delta-connected winding. The voltage in each phase load (phase voltage) U phase is the line voltage U line. The line current I line on the load is √3 times the phase current I phase (U phase = U line, I line = √3I phase)
P=√3U line I line cosΦ==3U phase I phase cosΦ
Reactive power of three-phase symmetrical load: Q=√3U line I line sinΦ
2.1 Causes of electric shock and measures to prevent electric shock
1.3.1 Harm of electric current to human body
Injury classification
Electric shock: refers to the electric current passing through the human body, destroying the normal functions of the human heart, lungs and nervous system.
Electrical injury: refers to the harm caused to the human body by the thermal effect of electric current, chemical effect and mechanical effect, mainly including arc burns, burns caused by splashing of molten metal, etc.
Physiological damage from electromagnetic fields: refers to the neurological symptoms such as dizziness, fatigue, memory loss, insomnia and dreaminess that occur under the action of high-frequency magnetic fields.
The danger of electric current to the human body is related to factors such as the size of the current, the duration of the current, the path through which the current flows, and the impact of the human body's resistance.
The safe voltage value is 36V
1.3.2 Causes of electric shock and measures to prevent electric shock
Common causes of electric shock: Taking risks in violation of regulations, lack of knowledge about safe use of electricity, accidental electric shock
Electric shock method
Single-phase electric shock: refers to an electric shock caused by a human body standing on the ground or other grounded body, electric shock to a certain part of the human body, or any phase of an electrical device.
Two-phase electric shock: refers to the electric shock method in which the human body touches any two charged objects at the same time.
Step voltage electric shock: When the human body steps into the vicinity of the contact point with two feet, there is a potential difference between the front and rear feet, which is the step voltage. The resulting electric shock is called step voltage electric shock.
Measures to prevent electric shock
Insulation: Insulation uses insulating materials to isolate charged objects to achieve electrical isolation between charged objects and between charged objects and other objects.
Safety distance: The safety distance is to ensure the necessary safety distance between the human body and charged objects.
Leakage protection: A leakage protector is a device that can automatically disconnect the circuit or sound an alarm when the leakage current value in the circuit reaches or exceeds its specified value under specified conditions.
safe voltage
obstructions and obstructions
3.1 Basic knowledge of three-phase AC asynchronous motors
3.1.1 Working principle of three-phase asynchronous electric shock
It is a type of induction motor that is powered by 380V three-phase AC current (phase difference 120 degrees) at the same time. Since the rotor and stator rotating magnetic fields of the three-phase asynchronous motor rotate in the same way but at different speeds, there is a rotational speed. rate difference, so it is called a three-phase asynchronous motor. The rotor speed of the three-phase asynchronous motor is lower than the speed of the rotating magnetic field. The relative motion between the rotor winding and the magnetic field generates electromotive force and current (cutting magnetic lines of force), and interacts with the magnetic field to produce electromagnetic torque force to achieve energy conversion.
3.1.2 Structural differences between three-phase asynchronous motors
The difference between variable frequency motors and ordinary power frequency motors: different cooling fans
The rotors of wound three-phase asynchronous motors and cage-type three-phase asynchronous motors are different
Three-phase asynchronous motors have different numbers of pole pairs (different speeds, the greater the number of pole pairs, the slower the speed)
The three-phase windings are connected to the power supply in different ways (star or delta)
3.1.3 Structure of three-phase asynchronous motor
As shown on the right
3.1.4 Stator winding description
The stator winding is the circuit part of the motor. It is supplied with three-phase alternating current and generates a rotating magnetic field. The stator winding of small asynchronous motors is usually wound into various coils with high-strength enameled wire (copper wire or aluminum wire), and then embedded in the stator core slot. For large and medium-sized motors, copper bars of various specifications are insulated and then embedded in the stator core slots. In order to ensure reliable insulation between each conductive part of the winding and the iron core as well as reliable insulation between the windings themselves, many insulation measures are taken during the manufacturing process of the stator winding. The main insulation items of the stator winding of three-phase asynchronous motors are as follows: kind:
Ground insulation: the insulation between the entire stator winding and the stator core
Phase ground: the insulation between stator windings of each phase
Compartment insulation: the insulation between the turns of the stator winding of each phase
3.1.5 Precautions and related instructions when pressing the line
Generally, the factory default wiring of three-phase asynchronous motors of 5.5KW and above is delta connection; those below 5.5KW generally default to star connection.
If it is a star-connected motor with a rated voltage of 380VAC, it can be changed to a delta connection, but the input rated voltage must become a three-phase 220VAC (such as a single-phase 220VAC inverter.) I diagonal line = √3 I star line; P star = P horn
If it is a delta-connected motor with a rated voltage of 380VAC, it can be changed to a star connection. I star line=I angle line/3, P star=P angle/3
3.1.6 Control methods of three-phase asynchronous motors and their respective characteristics
AC contactor direct control: low price, poor control performance
Frequency converter control: higher price, best controllability, variable frequency speed adjustment
Soft starter control: moderate price, good controllability, no frequency conversion
3.1.7 Speed control principle of three-phase asynchronous motor
Why is it called asynchronous motor?
The speed of the rotor is always less than the synchronous speed. Otherwise, there will be no relative motion between the rotor conductor and the magnetic field, and there will be no induced current and the rotor will not rotate. Because of this, we call this AC motor an asynchronous motor. And because the rotor current of this kind of motor is generated by electromagnetic induction, it is also called an induction motor.
Through theoretical analysis, it can be concluded that the rotating magnetic field speed is: n1=60f1/P
n1: Indicates the rotation speed of the rotating magnetic field, the unit is rpm
f1: Indicates the frequency of the three-phase AC power supply, in Hertz.
P: represents the number of magnetic pole pairs
60 means 60 seconds in one minute
Note: The speed n1 of the rotating magnetic field is also called the synchronous speed. my country's three-phase AC frequency stipulates 50Hz, so the two-stage rotation speed is 3000 rpm, the fourth stage is 1500 rpm, and the sixth stage is 1000 rpm.
Slip of three-phase AC asynchronous motor
The difference between the motor rotating magnetic field speed n1 and the rotor actual speed n (n1-n) is called the rotation difference
The percentage of the ratio of the speed difference to the synchronous speed is called slip, represented by the symbol S, S=(n1-n1)*100%
Slip is an important parameter of an asynchronous motor. It is customary to use the slip to describe the running speed of the motor. When the motor is no-load, the slip rate is very small, that is, the rotor speed is close to the synchronous speed. As the load increases, the slip also increases. Generally, S<0.005 when no-load and S<0.06 when full load. That is to say, the slip of the rotor changes with the load. When the three-phase asynchronous motor is running at rated load, its slip rate is very small, about 2%~6%
3.2 Forward and reverse inching control circuit of three-phase asynchronous motor and selection and description of related components
Circuit breaker (low voltage) selection (QF)
Definition: A mechanical switching appliance that can connect, carry and break current under normal circuit conditions, and can also connect, carry and break current for a certain period of time under specified abnormal circuits (such as short circuits).
Classification
Frame circuit breaker, referred to as ACB
Plastic case circuit breaker, referred to as MCCB
Miniature circuit breaker, referred to as MCB
Low voltage circuit breaker function
Overload long-delay protection: When the line is overloaded, the circuit breaker will delay for a period of time. If there is still an overload after the delay, the circuit breaker will trip. This time is usually in the second level (usually standard)
Short-circuit short-delay protection: When a short circuit occurs, the circuit breaker will trip after a delay. This time is usually in the millisecond level (usually standard)
Short-circuit instantaneous protection: When a short-circuit current occurs, the circuit breaker trips instantly to protect the line. Generally, such short-circuit current is often very large and more catastrophic (usually standard)
Leakage protection: A protective function of the circuit breaker when the current does not pass through the conductor but is directly connected to the outside world. The purpose is to prevent electric shock. It is an additional protection function of the circuit breaker. A circuit breaker with this function is called a circuit breaker with leakage protection. (usually optional)
Undervoltage protection function: A protection function in which the circuit breaker trips when the power supply voltage is lower than or higher than a certain range of the rated voltage. This is a non-standard function of the circuit breaker and requires a single undervoltage or overvoltage coil (usually an optional match)
Low voltage circuit breaker tripping mode
Thermal magnetic trip
Thermal trip: A tripping method that uses the heat generated by the overload current to deform the bimetal and trigger the release mechanism. Because the metal sheet deforms slowly. So it is only used for overload long delay protection. Micro-breaks are all thermal-magnetic tripping, and some plastic cases are thermal-magnetic tripping. Since the thermal tripper generally directly flows through the loop current, and its operating current is greatly affected by the surrounding ambient temperature, it is generally used in low-voltage electrical circuits where the current is not very large and the overcurrent protection is not very sensitive.
Magnetic tripping: It is a tripping method that uses the magnetic field generated after the excitation coil is energized to attract the armature and trigger the tripping mechanism. Magnetic tripping can instantly disconnect the circuit, so it is used for short-circuit protection. The circuit breaker combined with the thermomagnetic principle has both overload protection and short-circuit protection. A circuit breaker with only a magnetic release is also called instantaneous tripping, and a circuit breaker with both is also called compound tripping. Miniature circuit breakers are basically in the form of thermal magnetic tripping.
Electronic trip unit
It takes a very small current from the circuit as the measured current, and its measurement accuracy is relatively high, so it is generally used on large current circuits, or circuits that require high accuracy for the operating current. An electronic tripper is a circuit composed of electronic components. It detects the main circuit current, amplifies and drives the decoupling mechanism. Electronic tripping is mostly used in ACB and MCCB circuit breakers with larger current levels.
Comparison of two tripping modes
Thermal magnetic tripping performance is stable and not affected by voltage fluctuations, has long life, low sensitivity, and is not easy to adjust
The electronic tripping function is complete, the sensitivity is high, and the setting is convenient. It is affected by the power supply and is slightly susceptible to damage.
AC contactor (KM) and its selection
Purpose: AC contactor is an electrical appliance suitable for connecting and breaking circuits and AC motors over long distances. It is mainly used to control the starting, stopping, reversing and speed regulation of AC motors, and can be combined with thermal relays or other appropriate protection devices to protect the motor from possible overload or phase failure. It can also be used to control other electrical loads such as electric heating. appliances, electric lighting, welding machines, capacitor banks, etc. It also has under-voltage and low-voltage protection functions.
Intermediate relay and its selection
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