Typical relay-contactor control circuits for asynchronous motors (IM) are built on the same principles as control circuits for DC motors.
Typical control circuits for an inferno with a squirrel-cage rotor
Motors of this type of low and medium power are usually started by direct connection to the network without limitation of starting currents. In these cases, they are controlled using magnetic starters, which at the same time provide some types of their protection.
The control circuit for an asynchronous motor using a magnetic starter (Fig. 2.1) includes a magnetic starter consisting of a contactor KM and three thermal protection relays built into it KK. The circuit provides direct (without current and torque limitation) starting of the engine, disconnecting it from the network, as well as protection against short circuits (fuses FA) and overloads (thermal relays KK).
Rice. 2.1. Blood pressure control scheme using
irreversible magnetic starter
To start the engine, close the switch QF and press the start button SIN 1. The contactor coil receives power KM, which, when turned on, connects it to the power source with its main power contacts in the motor stator circuit, and bypasses the button with an auxiliary contact SIN 1. The engine takes off according to its natural characteristics. To turn off the engine, press the stop button SAT 2, contactor KM loses power and disconnects the engine from the network. The process of engine braking by coasting begins under the influence of the load torque on its shaft.
Reversible hell control circuit.
The main element of this circuit is a reversible magnetic starter, which includes two linear contactors KM 1 And KM 2 and two thermal protection relays QC(Fig. 2.2). The circuit provides direct starting and reversing of the engine, as well as back-on braking during manual (non-automatic) control.
Rice. 2.2. IM control circuit using a reversible magnetic starter
The circuit provides protection against motor overloads (relay QC) and short circuits in the stator circuit (circuit breaker QF) and controls (fuses FA). In addition, the control circuit provides zero protection against loss (reduction) of the mains voltage (contactors KM 1 And KM 2).
Starting the engine when switched on QF in the conventional directions “Forward” or “Backward” is carried out by pressing the buttons respectively SIN 1 or SAT 2. This causes the contactor to trip KM 1 or KM 2, connecting the engine to the network and starting it up.
To reverse or brake the engine, first press the button SIN Z , which leads to the switching off of the contactor that was still switched on (for example, KM 1), after which the button is pressed SIN 2.
This causes the contactor to turn on KM 2 and serve on HELL voltage of a power source with a different phase sequence. The magnetic field of the motor changes its direction of rotation to the opposite, which leads to the beginning of the reverse process. This process consists of two stages: counter-braking and take-off in the opposite direction.
If it is only necessary to brake the engine when it reaches zero speed, the button must be pressed again SIN Z , which will lead to the motor being disconnected from the network and the circuit returning to its original position. If the button SIN Z will not be pressed, this will lead to the engine running in the other direction, i.e. to its reverse.
To avoid a short circuit in the stator circuit, which may occur as a result of simultaneously pressing the buttons incorrectly SIN 1 And SIN 2, In reversing magnetic starters, a special mechanical interlock is sometimes provided. It is a lever system that prevents one contactor from retracting if another is energized. In addition to mechanical interlocking, the circuit uses typical electrical interlocking used in reversible control circuits. It provides for cross-connection of the device's normally closed contacts KM 1 into the apparatus coil circuit KM 2 and vice versa.
It should be noted that the use of an air circuit breaker in the circuit contributes to increasing reliability and ease of use. QF. Its presence eliminates the possibility of the drive operating in the event of a break in one phase or a single-phase short circuit.
Control circuitmulti-speed HELL.
This circuit (Fig. 2.3) provides two motor speeds by connecting sections (half-windings) of the stator winding into a triangle or double star, as well as its reversal. The electric drive is protected by thermal relays QC 1 And QC 2 and fuses FA.
Rice. 2.3 . Two-speed IM control circuit
To start the engine at low speed, press the button SIN 4, after which the contactor is activated KM 2 and latching relay TOV. The motor stator turns out to be connected according to a delta circuit, and the relay TOV, by closing their contacts in the circuits of the device coils KM Z And KM 4, prepares the connection of the motor to the power source. Next button press SIN 1 or SIN 2 leads to switching on in the “Forward” or “Backward” direction, respectively.
After the engine has run up to a low speed, it can be accelerated to a high speed. To do this, press the button SIN 5, which will cause the contactor to turn off KM 2 And switching on the contactor KM 1, which ensures switching of stator winding sections from triangle to double star.
The engine is stopped by pressing a button SIN 3, which will cause all contactors to be disconnected from the network and the motor to coast down.
The use of double-circuit control buttons in the circuit does not allow simultaneous activation of contactors KM 1 and KM 2, KM 3 And KM 4. The same purpose is served by cross-connection of the normally closed block contacts of the contactors. KM 1 and KM 2, KM 3 And KM 4 in their coil chain.
IM control circuit providing direct start and dynamic braking as a function of time
The engine is started by pressing a button SIN 1 (Fig. 2.4), after which the linear contactor is activated KM, connecting the motor to the power source. At the same time, closing the contact KM V time relay circuits CT will cause it to operate and close its contact in the braking contactor circuit KM 1. However, the latter does not work, since before this the breaking contact in this circuit opened KM.
Rice . 2.4. Control circuit for starting and dynamic braking of an IM with a squirrel-cage rotor
To stop the engine, press the button SIN 2, Contactor KM switches off by opening its contacts in the motor stator circuit and thereby disconnecting it from the AC mains. At the same time the contact is closed KM V apparatus circuits KM 1 and the contact opens KM in the relay circuit CT. This causes the braking contactor to switch on. KM 1, supply of direct current to the stator windings from the rectifier V through a resistor R t and switching the engine to dynamic braking mode.
Time relay CT, Having lost power, the time delay starts counting. After a time interval corresponding to the time the engine is stopped, the relay CT opens its contact in the contactor circuit KM 1, it turns off, stopping the supply of direct current to the stator circuit. The circuit returns to its original position.
The intensity of dynamic braking is controlled by a resistor R T, with the help of which the required constant current is established in the motor stator.
To exclude the possibility of simultaneous connection of the stator to AC and DC sources, the circuit uses standard blocking using normally open contacts KM And KM 1, connected crosswise in the coil circuits of these devices.
Typical schemes for controlling IM with a wound rotor . Control circuits for a wound-rotor motor, which are designed mainly for medium and high power, must provide for current limitation during starting, reversing and braking using additional resistors in the rotor circuit. By including resistors in the rotor circuit, it is also possible to increase the starting torque up to the critical (maximum) torque level.
Scheme of single-stage IM start-up as a function of time and braking by counter-switching as a functionEMF
After voltage is applied, the time relay turns on CT(Fig. 2.5), which, with its normally open contact, breaks the power supply circuit of the contactor KM 3, thereby preventing its activation and premature short-circuiting of the starting resistors in the rotor circuit.
Fig.2.5. Control circuit for starting and braking by back-connection of an IM with a wound rotor
The engine is turned on by pressing a button SIN 1, after which the contactor turns on KM 1. The motor stator is connected to the network, the electromagnetic brake YIN The brake is released and the engine starts to take off. Inclusion KM 1 simultaneously triggers the contactor KM 4, which with its contact bypasses the back-off resistor that is unnecessary at start-up R d2, and also breaks the circuit of the time relay coil CT. The latter, having lost power, begins counting the time delay, after which it closes its contact in the contactor coil circuit KM 3, which triggers and bypasses the starting resistor R d1, in the rotor circuit, and the engine returns to its natural characteristic.
Braking control is provided by the braking relay KV, controlling the level of EMF (rotation speed) of the rotor. Using a resistor R p , it is adjusted in such a way that at start-up, when the motor slip is 0< s < 1, наводимая в роторе ЭДС будет недостаточна для включения, а в режиме противовключения, когда 1 < s < 2, уровень ЭДС достаточен для его включения.
To brake the engine, press the double button SIN 2, the normally open contact of which breaks the power supply circuit of the contactor coil KM 1. After this, the engine is disconnected from the network and the contactor power circuit is broken. KM 4 and the relay power circuit closes CT. As a result of this, contactors KM 3 And KM 4 are switched off and resistance is introduced into the motor rotor circuit R d1 + R D 2 .
Button press SIN 2 simultaneously leads to the closure of the contactor coil power circuit KM 2, which, when turned on, reconnects the motor to the network, but with a different phase rotation of the mains voltage on the stator. The engine goes into reverse braking mode. Relay TOV works even after the button is released SIN 2 will provide power to the contactor KM 2 through its contact and the closing contact of this device.
At the end of braking, when the rotation speed is close to zero and the rotor EMF decreases, the relay TOV switches off and its break contact opens the circuit of the contactor coil KM 2. The latter, having lost power, will disconnect the engine from the network, and the circuit will return to its original state. After disconnecting KM 2 brake YIN, having lost power, it will provide fixation (braking) of the motor shaft.
Scheme of single-stage IM start-up as a function of current and dynamic braking as a function of rotation speed
The diagram (Fig. 2.6) includes contactors KM 1, KM2 And KM 3; current relay CA; speed control relay S.R., intermediate relay KV; Step-down transformer for dynamic braking T; rectifier VD. Overcurrent protection is provided by fuses F.A. 1 And F.A. 2, motor overload protection – thermal relays QC 1 and QC 2.
Rice. 2.6. Control circuit for starting and dynamic braking of an IM with a wound rotor
The scheme works as follows. After feeding with circuit breaker QF voltage to start the engine, press the button SIN 1, contactor turns on KM 1, the power contacts of which connect the motor stator to the network. An inrush current in the rotor circuit will turn on the current relay CA and opening the acceleration contactor circuit KM 2. Thus, the engine will start running with a starting resistor R d2 in the rotor circuit.
Switching on the contactor KM 1 also leads to button bridging SIN 1, opening the brake contactor coil circuit KM 3 and turning on the intermediate voltage relay TOV, which, however, will not lead to the contactor turning on KM 2, since before this the relay contact opened in this circuit CA.
As the engine speed increases, the emf and current in the rotor decrease. At a certain current value in the rotor equal to the relay release current CA, it will turn off and its break contact will close the contactor power circuit KM 2. It will turn on and bypass the starting resistor R d2, and the engine will return to its natural characteristics.
It should be noted that rotating the engine will cause the speed relay contact to close S.R. in the contactor circuit KM 3, however, it will not work, since the contactor contact has previously opened KM 1.
To put the engine into braking mode, press the button SIN 2. Contactor KM 1 loses power and turns off blood pressure from AC mains. Thanks to contact closure KM 1 the braking contactor will turn on KM 3, the contacts of which close the power supply circuit of the stator winding from the rectifier VD), connected to a transformer T, and thus the engine is switched to dynamic braking mode. At the same time, the devices will lose power. TOV And KM 2, which will lead to the introduction of a resistor into the rotor circuit R D 2 . The engine begins to slow down.
When the engine speed is close to zero, the speed control relay S.R. opens its contact in the contactor coil circuit KM 3. It will turn off and stop braking the engine. The circuit will return to its original position and will be ready for subsequent work.
The principle of operation of the circuit will not change if the relay coil is current CA include the stator in phase, not the rotor.
Currently, the most common are three-phase asynchronous motors with a squirrel-cage rotor. Starting and stopping of such motors when switched on to full mains voltage is carried out remotely using magnetic starters.
The most commonly used circuit is with one starter and "Start" and "Stop". In order to ensure rotation of the motor shaft in both directions, a circuit with two starters (or a reversing starter) and three buttons is used. This scheme allows you to change the direction of rotation of the motor shaft “on the fly” without first stopping it.
Engine starting circuits
The electric motor M is powered by a three-phase alternating voltage network. The three-phase circuit breaker QF is designed to disconnect the circuit in the event of a short circuit. The single-phase SF circuit breaker protects the control circuits.
The main element of the magnetic starter is the contactor (powerful relay for switching high currents) KM. Its power contacts switch three phases suitable for the electric motor. Button SB1 ("Start") is intended to start the engine, and button SB2 ("Stop") is intended to stop. Thermal bimetallic relays KK1 and KK2 switch off the circuit when the current consumed by the electric motor is exceeded.
Rice. 1. Scheme for starting a three-phase asynchronous motor using a magnetic starter
When the SB1 button is pressed, the KM contactor is activated and contacts KM.1, KM.2, KM.3 connect the electric motor to the network, and contact KM.4 blocks the button (self-locking).
To stop the electric motor, just press the SB2 button, while the KM contactor releases and turns off the electric motor.
An important property of a magnetic starter is that if there is an accidental loss of voltage in the network, the engine turns off, but restoration of voltage in the network does not lead to spontaneous starting of the engine, since when the voltage is turned off, the KM contactor is released, and to turn it on again, you must press the SB1 button.
If the installation malfunctions, for example, when the motor rotor jams and stops, the current consumed by the motor increases several times, which leads to the activation of thermal relays, opening of contacts KK1, KK2 and shutdown of the installation. The KK contacts are returned to the closed state manually after the fault has been eliminated.
A reversible magnetic starter allows you not only to start and stop an electric motor, but also to change the direction of rotation of the rotor. For this purpose, the starter circuit (Fig. 2) contains two sets of contactors and start buttons.
Rice. 2. Scheme for starting an engine using a reversible magnetic starter
The KM1 contactor and the SB1 button with self-locking are designed to turn on the engine in the “forward” mode, and the KM2 contactor and the SB2 button turn on the “backward” mode. To change the direction of rotation of the rotor of a three-phase motor, it is enough to swap any two of the three phases of the supply voltage, which is ensured by the main contacts of the contactors.
Button SB3 is designed to stop the engine, contacts KM 1.5 and KM2.5 provide interlocking, and thermal relays KK1 and KK2 provide overcurrent protection.
Turning on the engine at full network voltage is accompanied by large starting currents, which may be unacceptable for a network of limited power.
The electric motor starting circuit with starting current limitation (Fig. 3) contains resistors R1, R2, R3 connected in series with the electric motor windings. These resistors limit the current at the moment of starting when the KM contactor is activated after pressing the SB1 button. Simultaneously with KM, when contact KM.5 is closed, the time relay KT is triggered.
The delay provided by the time relay must be sufficient to accelerate the electric motor. At the end of the dwell time, contact KT closes, relay K is activated and its contacts K.1, K.2, K.3 bypass the starting resistors. The starting process is complete and full voltage is applied to the motor.
Rice. 3. Engine starting circuit with starting current limitation
Next, we will consider the two most popular braking schemes for three-phase asynchronous motors with a squirrel-cage rotor: a dynamic braking scheme and a back-on braking scheme.
Engine braking circuits
After removing the voltage from the engine, its rotor continues to rotate for some time due to inertia. In a number of devices, for example, in lifting and transport mechanisms, forced braking is required to reduce the amount of run-out. Dynamic braking involves passing a direct current through the motor windings after removing the alternating voltage.
The dynamic braking circuit is shown in Fig. 4.
Rice. 4. Engine dynamic braking circuit
In the circuit, in addition to the main contactor KM, there is a relay K, which turns on the braking mode. Since the relay and contactor cannot be turned on at the same time, an interlocking circuit is used (contacts KM.5 and K.3).
When the SB1 button is pressed, the KM contactor is activated, supplies power to the engine (contacts KM.1, KM.2, KM.3), blocks the button (KM.4) and blocks relay K (KM.5). Closing KM.6 triggers the KT time relay and closes the KT contact without a time delay. This way the engine starts.
To stop the engine, press button SB2. The KM contactor releases, contacts KM.1 - KM.3 open, turning off the engine, contact KM.5 closes, which causes relay K to operate. Contacts K.1 and K.2 close, supplying direct current to the windings. Rapid braking occurs.
When contact KM.6 opens, the time relay KT releases, and the time delay begins. The delay time must be sufficient to completely stop the electric motor. At the end of the time delay, the KT contact opens, the K relay releases and removes the constant voltage from the electric motor windings.
The most effective method of braking is to reverse the engine, when immediately after removing power, voltage is applied to the electric motor, causing counter-torque to appear. The back-switch braking circuit is shown in Fig. 5.
Rice. 5. Scheme of engine braking by back-switching
The engine rotor speed is controlled using a speed relay with SR contact. If the rotation speed is greater than a certain value, the SR contact is closed. When the engine stops, contact SR opens. In addition to the direct contactor KM1, the circuit contains a contactor for reversing KM2.
When the engine starts, contactor KM1 is activated and contact KM 1.5 breaks the circuit of coil KM2. When a certain rotation speed is reached, the SR contact closes, preparing the circuit to engage reverse.
When the engine stops, contactor KM1 releases and closes contact KM1.5. As a result of this, the KM2 contactor is activated and supplies the electric motor with reversing voltage for braking. A decrease in the rotor speed causes SR to open, the KM2 contactor releases, and braking stops.
MINISTRY OF EDUCATION OF UKRAINE
SEVASTOPOL HIGHER VOCATIONAL SCHOOL No. 3
GRADUATION WRITTEN
EXAMINATION PAPER
“Installation of an electrical motor control circuit”
Group 7/8 student:
Levitsky Pavel Vladimirovich
By profession:
marine electrician.
Supervisor:
E.I. Korshunova
Sevastopol.
1. Introduction. The role of Electrical Engineering in the development of shipbuilding
2 Main part
2.1 Motor control circuit
2.2 Main elements of the circuit and their purpose.
2.3 Operating principle of the fan electrical circuit
2.4 Electrical circuit installation technology
3. Materials used for mounting the circuit
4. Tools
5. Safety precautions
Literature
1. Introduction. The role of electrical engineering in the development of shipbuilding
Electrical engineering in shipbuilding has a very great importance. This branch of science and technology associated with the production, transformation and use electrical energy.
Electrical and magnetic phenomena are used in shipbuilding. Many kilometers of electrical wiring arteries are laid on ships, numerous electric drives of ship mechanisms are installed, modern automatic devices, navigation and radio equipment.
The reliability and durability of a launched vessel depends on the reliability of electrical devices.
In 1832, Faraday discovered the law of electromagnetic induction and thereby laid the foundation for electrical engineering. The year of birth of the marine electric drive can rightfully be considered 1838, when the Russian scientist B.S. Jacobi created the world's first electric propulsion unit. The DC electric motor he made was installed on a small boat and tested on the Neva. The engine received power from a galvanic battery. A very weak energy base in the first half of the 19th century hampered the development of electric drives, and electricity on ships was used only for lighting.
The first serious work on the establishment of a ship's electric drive Russian courts were undertaken in the second half of the 19th century. So in 1886, electric fans were used on the cruisers “Admiral Nakhimov”, “Admiral Kornilov”, “Lieutenant Ilyin”, and in 1892 on the armored cruiser “Twelve Apostles”, for the first time in world practice, an electric steering gear was installed. The use of electric motors to drive lifting devices began in 1897 with the installation of an electric winch on the transport ship Europe. In subsequent years, electrification of steering and anchor devices was carried out on the cruisers Gromoboy, Pallada and others.
A true revolution in the development of ship energy was the work of the Russian inventor of three-phase current M.O. Dolivo-Dobrovolsky. The synchronous generators, three-phase transformer and asynchronous motors he created transformed the ship's power plant. Since 1908, alternating current began to be introduced on ships, which provided great technical and economic advantages. The cruiser Bayan and the minelayer Amur were equipped with sump pumps driven by asynchronous motors. Built according to the design of Academician A.N. Krylov's battleships of the Sevastopol type had a three-phase power plant.
Russia and Ukraine have created a huge number of ships equipped with complex automation systems with a high degree of electrification of ship mechanisms and systems. The power of generator sets of ship power plants has increased significantly.
Electrical engineering is very important on ships. To ensure normal working conditions and habitability, electric lighting is necessary. Heating devices are designed to generate heat necessary for cooking, increasing the temperature of ambient air, liquids, individual elements prone to freezing, as well as meeting the domestic needs of passengers and crew. The safety of cargo navigation, the lives of people and the safety of cargo depend on many electrical devices, for example, steering gear, fire and bilge pumps, radio station, navigation devices, emergency lighting network, etc. Electrification of mechanisms serving anchor, mooring, cargo and rescue devices makes it possible to automate these labor-intensive processes.
2. Main part
2.1 Motor control circuit
The functional diagram of control of an asynchronous motor with a squirrel-cage rotor is shown in Figure 1.
Fig. 1. Functional diagram of asynchronous motor control.
Three-phase alternating current is supplied to the circuit breaker, which is used to connect a three-phase asynchronous motor. In addition to the contact system, the circuit breaker contains combined releases (thermal and electromagnetic), which ensures automatic shutdown in case of prolonged overload and short circuit. From the circuit breaker, power is supplied to the magnetic starter. Magnetic starter is a device for remote control of a motor. It starts, stops and protects the engine from overheating and severe voltage drop. The main part of the magnetic starter is a three-pole electromagnetic contactor. From the magnetic starter, control is transferred to a three-phase asynchronous AC motor. The asynchronous motor is characterized by its simple design and ease of maintenance. It consists of two main parts - the stator - the stationary part and the rotor - the rotating part. The stator has slots into which a three-phase stator winding is placed, connected to the alternating current network. This winding is designed to create a rotating circular magnetic field. The rotation of the circular magnetic field is ensured by a phase shift relative to each other of each of the three three-phase current systems by an angle of 120 degrees.
The stator windings for connection to the 220V mains voltage are connected in a triangle (Fig. 8). Depending on the type of rotor winding, machines can be with wound and squirrel-cage rotors. Despite the fact that a wound-rotor motor has better starting and control properties, a squirrel-cage motor is simpler and more reliable to operate, as well as cheaper. I have chosen a squirrel cage motor because most of the motors manufactured in the industry these days are squirrel cage motors. The rotor winding is made like a squirrel wheel; hot aluminum is poured under pressure into the rotor grooves. The conductors of the rotor winding are connected to form a three-phase system. The motor drives the fan. Fans used on ships are differentiated depending on the pressure they create. The fan mounted in the circuit is a low pressure fan. Typically, fans are not adjustable or reversible, so their drive has a simple control circuit, which boils down to start, stop and protection.
The electrical circuit diagram of non-reversible control of a three-phase asynchronous electric motor with a squirrel-cage rotor using a circuit breaker and a magnetic starter with a bipolar thermal relay is shown in Figure 2.
From the power panel, power is supplied to the circuit breaker with thermal and electromagnetic overcurrent releases. The magnetic starter circuit is compiled in compliance with the recommended graphic symbols for the elements of automatic engine control circuits. Here, all elements of the same device are designated by the same letters.
Fig. 2. Control diagram of an asynchronous motor with a short-circuited rotor winding.
Thus, the main closing contacts of a linear three-pole contactor, located in the power circuit, its coil and auxiliary closing contacts, located in the control circuit, are designated by the letters CL. The heating elements of the thermal relay included in the power circuit, and the remaining breaking contacts with manual return of the same relay to its original position, which are located in the control circuit, are designated by the letters RT. When the three-pole switch is turned on, after pressing the KnP start button, the coil of the linear three-pole contactor CL is turned on and its main closing contacts CL connect the stator winding of the three-phase asynchronous motor IM to the supply network, as a result of which the rotor begins to rotate. At the same time, the auxiliary closing contacts of the CL are closed, shunting the KnP start button, which allows it to be released. Pressing the stop button KnS turns off the power supply circuit of the CL coil, as a result of which the contactor armature falls out, the main closing contacts of the CL open and the motor stator winding is disconnected from the supply network.
2.2 Main elements of the circuit and their purpose
Circuit breaker- device for infrequent manual switching electrical circuits and their automatic protection in case of short circuits and long-term overload. The purpose of the circuit breaker used in the circuit is described in Table 1.
Table 1 . Scope of application of the circuit breaker.
As can be seen from Table 1, the circuit breaker does not turn off when the voltage drops sharply, since there is no undervoltage release in the circuit breaker used. Protection in the event of a significant decrease or disappearance of the supply voltage is provided by a magnetic starter.
The machines are used at voltages up to 660V for rated currents from 15 to 600A, in rooms with normal environment, since they are not suitable for working in environments with caustic vapors and gases, in explosive areas and places unprotected from water. Automatic machines must be inspected, cleaned, and lubricated with instrument oil at least once a year. For my circuit, I chose an automatic circuit breaker of the AP-50 series. Appearance machine is shown in Figure 3.
1-off button, 2-on button, 3-relay, 4-spark chambers, 5-plastic casing
Fig3. Appearance and design of the AP-50 assault rifle.
It is designed for protection against overloads and short-circuit currents at U supply network up to 500V, 50 Hz on alternating current, for manual switching on and off of circuits, and most importantly for starting and protecting three-phase asynchronous motors with a squirrel-cage rotor. The switch is protected by a plastic casing. The presence of the letter B in the AP-50B series means a universal design, in which the wires enter and exit from the bottom and top through glands of the SKVrt-33 type. Marking AP-50B-3MT means the presence of electromagnetic and thermal releases and the number of poles is equal to three.
Magnetic switch- remote control switching device, for frequent switching on and off of electrical equipment, which is controlled using a separately located button. This is a device for starting, stopping and protecting electric motors. The purpose of the magnetic starter used in the circuit is presented in Table 2.
Table 2 .Scope of application of the magnetic starter.
The main part of the magnetic starter is a three-pole electromagnetic contactor, which provides switching on and off of electrical equipment. AC contactors are three-pole; they consist of an electromagnetic system of a contact and arc extinguishing device. The magnetic core of the electromagnetic system is made of a set of separate sheets of electrical steel to reduce losses due to eddy currents. Has an E-shaped core and a rotary armature. A retractor coil is located on the middle part of the fixed core. The starter is also equipped with a thermal relay – a multiple-action device that protects electrical equipment from unacceptable overheating caused by prolonged overload. To protect control circuits from short circuit currents, fuses can be installed in the starter, but this is not used in the developed circuit, since protection from short circuit currents is provided by an automatic circuit breaker. The starter used in the circuit differs in that a separately located button is not used to control it.
Magnetic starters are non-reversible and reversible. Irreversible magnetic starters provide turning on and off of motors in one direction of rotation, and reversible ones - in both directions of rotation (not used in the circuit, since fans usually do not reverse).
Depending on the size of the starter, the contacts are designed for a rated current of 3A, 10A, 25A.
For the control circuit of a three-phase asynchronous motor with a squirrel-cage rotor, I chose an irreversible magnetic starter with a forward-type contactor of the PML series. The appearance of the magnetic starter is shown in Figure 4.
Fig. 4. Appearance and structure of the magnetic starter of the PML series.
The electrical circuit of the starter is shown in Figure 5.
K - control button, L - contactor, RT - thermal relay, D - motor.
Fig. 5. Irreversible starter of the PML series.
This magnetic starter is designed for remote control of motors with power up to 75 kW, at voltages up to 500V, in a network with a current frequency of 50 Hz and provides motor overload protection (except for short circuits) and zero protection. Starters operate reliably (turn on) at mains voltage ranging from 85 to 105% of the rated voltage. Wires from the three-phase supply line are supplied to the input terminals L1, L2, L3, and wires are taken from the output terminals C1, C2, C3 to the electrical energy receiver. Automatic shutdown of the contactor in the event of a significant decrease or disappearance of voltage in the supply network provides minimum voltage protection.
Thermal relay - a multiple-action device that protects electrical equipment from unacceptable overheating caused by prolonged overload. Installed in the starter. The main part of the thermal relay is a metal plate, which is deformed under the action of a resistor-heater and, with the help of a spring, opens the relay contacts. It usually takes up to 3 minutes to cool the plate and, along with it, the object being protected from overcurrent. But this time depends on the current in the heater resistor, the load mode and the ambient temperature.
Electric motor - Which motor is needed for the production process is determined from the motor catalog in accordance with the load on its shaft under overheating conditions. It is necessary to select an engine with a rated power at which it would heat up during operation to a temperature that does not exceed the permissible one. Exceeding the permissible temperature leads to loss of electrical and mechanical strength in the insulation and to engine failure. The circuit uses a low power motor of 0.12 kW. In practice, the following nominal operating modes of electrical equipment are distinguished: a) continuous; b) short-term; c) repeatedly - short-term. The engine operating mode I have chosen is short-term. This is an operating mode in which periods of a constant rated load at a constant ambient temperature alternate with periods of shutdown. For example, the operating periods may be 15 or 30 minutes, and the shutdown periods are such that all parts of the electrical equipment are cooled to a cold state.
The engine used in the circuit (Fig. 6) is marked:
3F ~ Δ/ 220/380V 0.12kW 0.52/0.3A 2800rpm 50Hz Efficiency:83% φ=0.76
The design is protected, moisture and frost resistant.
Fig.6. Fan motor
The main components of an asynchronous motor are the stator and rotor. The stator structure of an asynchronous motor is shown in Figure 7.
Fig.7. The stator structure of an asynchronous motor.
(1-core, 2-windings, 3-frame, 4-shield)
The stator core 1 is assembled from steel plates with a thickness of 0.35-0.5 mm. The plates are stamped with grooves, varnished, collected in bags and mounted in the engine frame 3. The frame is installed on the foundation. Side shields with bearings are attached to the frame, on which the rotor shaft rests. Its winding 2 is placed in the longitudinal grooves of the stator. The beginnings and ends of the windings of each phase are brought to the shield 4, on which there are 6 clamps.
Fig.8 Connection of terminals on the motor shield when the stator winding is turned on with a triangle
The combination of three phases placed in the grooves of the stator magnetic circuit forms its three-phase winding with six terminals outward, of which three, corresponding to the beginnings of the phases, are connected to the terminals marked C1, C2, C3, and the rest, corresponding to the ends of the phases, are connected to the terminals marked C4, C5, C6.
These clamps are located in the terminal box mounted on the machine body. The presence of six accessible clamps allows you to connect individual windings to each other with metal plates in a triangle or star, which makes it possible to use the same machine at two different line voltages, the ratio of which is equal to . Figure 8 shows the position of the plate used in the circuit when connecting the windings with a triangle. In the marking of a 220/380V motor, the voltage indicated before the slash corresponds to the connection of the phases of the stator winding with a triangle, followed by a star.
The appearance of the rotor of a squirrel-cage asynchronous motor is shown in Figure 9.
Fig.9. Rotor of a squirrel-cage asynchronous motor.
a - device, b - winding
The rotor core consists of steel plates 0.5 mm thick. The plates are stamped with grooves, coated with varnish, and collected in bags that are attached to the shaft. The packages form a cylinder with longitudinal grooves into which the rotor winding is placed.
2.3 Operating principle of the fan electrical circuit
Motor control must satisfy all the requirements of production processes and provide starting, reversing, braking, speed control and continuously maintain a given mode in accordance with process conditions. Various electrical devices are used to control motors. These devices are divided according to their purpose into switching, regulating, monitoring and protective.
Typically, fans are not adjustable or reversible, so their drive has a simple control circuit, which boils down to start, stop and protection.
The start of an asynchronous motor is accompanied by a transient process associated with the transition of the rotor from a state of rest to a state of uniform rotation, in which the motor torque balances the moment of resistance forces on the machine shaft. During startup, there is an increased consumption of electricity from the supply network, which is spent not only on overcoming the braking torque applied to the shaft and covering losses in the asynchronous machine itself, but also on imparting a certain kinetic energy to the moving parts.
When using three-phase asynchronous motors of low and medium power, when the motor power is less than the power of the source supplying the network, direct starting is usually used. This launch is simple and fast.
To supply power to the electrical circuit, press the circuit breaker button. It has movable make and break contacts. Next, using the Start button, we close the magnetic starter circuit. A three-pole electromagnetic AC contactor, which is the main part of a magnetic starter, is an electromagnet with a magnetic core made of thin sheets of electrical steel, insulated from each other and tightened with studs. The principle of operation of the contactor is based on the ability of an electromagnet to attract a movable armature made of ferromagnetic material to the core. The armature is connected to movable contacts, which change their position when the armature moves. When you press the Start button, power is supplied to the contactor coil, the electromagnet core attracts an armature connected to a moving contact, which, when the armature moves, comes into contact with a fixed contact. Thus, the power contacts of the contactor are closed and the motor is connected to the network. At the same time, the blocking contact of the contactor closes and bypasses the Start button, which allows it to be released. The starter contains a thermal relay. It is triggered in the event of an overload of the motor and with its contacts opens the circuit of the contactor coil, which leads to the motor being turned off. When the power is turned off, the contactor armature returns to its original position under the action of a spring. To stop the engine, press the Stop button. In this case, the contactor coil circuit opens, its power contacts open and disconnect the engine from the network. When the air circuit breaker automatically switches off, a special device called a release is activated. The release is an electromagnetic or thermal relay that is triggered when the current increases above the permissible limit. When the release is triggered, the mechanical switch is activated and the power contacts are broken. The response time (switching off) is 0.025-0.05 s. A machine is more convenient than a switch or fuse. They provide better protection at low overloads, they are a multiple-action device.
The operating principle of the engine is based on phenomenon of electromagnetic induction- the occurrence of a current in a conducting circuit, which is either at rest in a time-varying magnetic field, or moves in a constant magnetic field in such a way that the number of magnetic induction lines penetrating the circuit changes; and also on the basis Lens's law- the direction of any induced current is such that it counteracts the cause that caused it.
The rotating magnetic field of the stator is created by a three-phase system when it is connected to an alternating current network. It crosses the rotor windings, and an electromotive force (EMF) is induced in them. Since the rotor winding is short-circuited, current flows in it under the influence of EMF. This current interacts with the rotating field of the stator and torque is created. Under its influence, the rotor begins to rotate in the direction of rotation of the magnetic field. This moment is the driving one, overcoming the resistance of the mechanism driven into rotation by the rotor.
In asynchronous motors, the working process can only occur at an asynchronous (from Greek - time-discrepant) frequency, since only with asynchrony is it possible for the magnetic lines to cross the rotor winding and induce an EMF in it. The stator field rotation frequency (n1) is usually expressed in revolutions per minute - it is not equal to the rotor rotation frequency (n2), i.e. n 1≠ n 2 . The rotor speed is always lower. The lag of the rotor from the field is characterized by slip (S). S =( n 1- n 2)/ n 1 .
As the load on the machine shaft increases, the braking torque increases, which leads to a decrease in n2 and, consequently, to an increase in slip. The stator's magnetic field will cross the rotor conductors more often, the emf and current in the rotor will increase, which will increase the torque. When reducing the load on the shaft, the process is similar.
2.4 Electrical circuit installation technology
The circuit uses automatic devices for switching, protection and control: an automatic air circuit breaker or circuit breaker, relays for various purposes, a magnetic starter, as well as various switching devices.
The first stages of developing a circuit is to work with reference books, in which ballasts and wire cross-sections are selected depending on the type and power of the engine, its purpose and its operating conditions.
After preparing the workplace, basic tools and materials, we begin marking. Vinyl plastic board 15mm thick was used as the foundation. We determine the installation locations of electrical equipment and inputs, mark the locations of holes for securing electrical equipment. We outline the places for laying electrical wiring. We mark with a soft pencil. We mark through holes, indicating their external outlines.
We use an electric drill to drill sockets and holes. We hold it in our hands with force while drilling. We pay special attention to electrical safety issues. When working, you must use dielectric gloves and mats.
We install the fastened parts exactly according to the markings. Insert screws into the holes. Tighten the nuts. Screw in by hand and then use a wrench. The part to be fixed must be pressed tightly to the base. To fasten and secure electrical wires we use special fastening brackets.
The connection points of cores and wires must be accessible for inspection and repair. We solder aluminum cores using a very hot, powerful soldering iron in compliance with electrical safety measures.
To power three-phase electric motors, it is not necessary to have a three-phase network. There are various options for starting electric motors. In villages where power lines are usually overloaded, a purely mechanical starting method is often used. The rotor is untwisted using a cord about a meter long, pre-wound on the shaft. This method is very inconvenient and is used where the engine starts without load. Most effective method starting an electric motor - connecting the third winding through a phase-shifting capacitor (capacitor starting).
Fig. 9. Scheme of capacitor starting of an electric motor
In the circuit for connecting a three-phase motor to a single-phase network, 2 capacitors of the KBG-MN type are used: Starting = 10 μF ± 5% and Running = 5 μF ± 5% (for 100 W of power, it is recommended to use a capacitor with a capacity of 8 μF.) To disconnect the starting capacitor after starting the engine a batch switch is used.
The motor housing is grounded. After securing, connecting and grounding the electrical equipment, we carry out external control and testing of the circuit. Based on the test results, it was concluded that the circuit is suitable for operation. In production, acceptance tests are usually carried out by manufacturers of marine electrical equipment in the presence of a representative of the technical control department.
3. Materials used for mounting the circuit
1. Vinyl plastic board. Viniplast is a rigid plastic based on polyvinyl chloride, which is a synthetic polymer. It has good mechanical and electrical insulating properties and sufficient heat resistance. Available in the form of sheets, plates, pipes, rods, etc. It is used as a corrosion-resistant, insulating, finishing and roofing material.
2. Fastening bolt with nut 4M - 4 pcs.
3. Bushing - 4 pcs.
4. Polyethylene fastening brackets.
5. Self-tapping screws for fastening.
6.Insulating tape.
7. Tin solder.
8. Rosin.
9. PVC insulating tube.
10. Aluminum wire, 1-core, D=1.5 sq.mm.
11. Drills.
12. Sandpaper.
13. Wiping rags.
4. Tools
1.Pliers.
2.Screwdriver.
3. Electric drill.
4. Electric soldering iron.
5. File.
6.Metal measuring ruler.
7. Wire cutters.
10.Spanners.
11. Metal scissors.
12.Hammer.
13.Kerner.
14.Hacksaw for metal
16.Soft pencil.
17.Marker.
5. Safety precautions
Accidents to people when using electrical installations mainly occur due to their violation of basic safety rules.
People who have not received appropriate safety training should not be allowed to work with electrical equipment in production or laboratory installations.
Electrical installations, if operated incorrectly and without following safety rules, even at relatively low voltage, can pose a great danger to human health and sometimes even life. Electric current passing through the human body, depending on its value, is accompanied by painful sensations, convulsions, severe pain or paralysis of individual organs. An electric arc can cause significant burns and metallization of human skin.
The degree of electric shock depends on the type, value, duration and frequency of the current, on which parts of the body the current passes through (the most dangerous through the brain and heart), as well as on the individual characteristics of the person and the climate in the room.
Safe operating conditions are ensured by a number of measures provided for by safety regulations. The main ones are: protection with the help of appropriate fences of all live parts, construction of protective grounding and grounding of equipment elements, use of insulating stands and other insulating material.
Under normal conditions, all live parts of motors are reliably isolated from metal housings. In case of insulation breakdown electrical wire through damaged insulation it will connect directly to the machine body. If a person does not stand on a rubber insulating mat or a dry wooden floor, then accidentally touching the engine will cause voltage. To eliminate this hazard, the motor frame must be grounded.
If a person is exposed to electrical current, remove voltage from the installation or area immediately electrical network with which he comes into contact. To do this, you need to turn off the nearest switch or remove the fuses. If it is not known where they are, then the wires should be taken away from the victim or separated from the electrical installation, provide him with air access, and in severe cases, begin artificial respiration until the doctor arrives. The person providing assistance must use dry clothing, rubber gloves, dry boards, etc., otherwise he himself may be electrocuted.
Engine operation is accompanied by noise and vibration, which affect the central nervous system, can lead to diseases of the cardiovascular system and even hearing loss. For a demonstration class or laboratory, the permissible noise limit is 50 decibels. In the developed installation this standard is observed.
This circuit is designed for use in rooms with a normal environment, since the automatic elements included in it are not suitable for working in environments with caustic vapors and gases, in explosive and unprotected places from water ingress.
Typically, the design of the engine provides protection of the insulation from exposure to atmospheric impurities. The room in which the engine I have chosen operates is dry, not dusty, not hot, without a chemically active environment, not fire or explosive.
To demonstrate the operation of the motor in the electrical engineering room, a motor is used that is protected from accidental contact with live parts and the ingress of foreign objects inside, protected from splashing water. In the event of a fire, water must not be used to extinguish fires in electrical installations. This may result in electrical shock and short circuit the system, which may result in new fires. If the fire does not occur in the ventilation device itself, then mechanical ventilation must be turned off immediately.
When installing the circuit, an electric drill and an electric soldering iron are used. Before connecting to the network, you must first verify by external inspection that they are in good condition. When working, make sure that the electric drill does not overheat. Protect your eyes from getting chips with glasses. Do not touch power tools with wet hands. Carry out work on a separate table, far from sources of water.
Literature
1. Dictionary-reference book for ship electricians. -L.: Shipbuilding, 1990.-392 p.
2. Samoilov Yu.S., Eidel A.S. Marine electrician: Textbook, -L.: Shipbuilding, 1985.-256p.
3. Kasatkin A.S. Basics of Electrical Engineering: tutorial for vocational schools, -M.: Higher. school, 1986.-287p.
4. Ivanov A.A. Handbook of Electrical Engineering, K.: Vishcha School, 1984.-304 p.
5. Bukhovtsev B.B., Klimontovich Yu.L., Myakishev G.Ya. Physics. Textbook for 9th grade, -M.: Education, 1986.-271 p.
6. Kitaev V.E. Electrical engineering with basics of industrial electronics. Textbook for prof.-technical taught, - M.: Higher school, 1985. - 224 p.
7. Borisov Yu.M. and others. Electrical engineering. Textbook for universities, -M.: Energoatomizdat, 1985.-552 p.
8. Karvovsky G.A., Okorokov S.P. Handbook of asynchronous motors and control gear, M.: Energia, 1969.-256 p.
9. Electric magazine No. 7, 2002, pp. 3, 4.
10. Ktitorov A.F. Basic techniques and methods for performing electrical installation work: Textbook. Allowance for secondary Prof.-techn. School - 2nd ed., M.: Higher School, 1982. - 127 p.
15.09.2014
To control asynchronous electric motors, relay contactor devices are used, which implement standard schemes for starting, reversing, braking, and stopping the electric drive.
Based on standard relay-contactor control circuits, control circuits for electric drives of production mechanisms are being developed. Starting of asynchronous motors with a squirrel-cage rotor of low power is usually carried out using magnetic starters. In this case, the magnetic starter consists of an AC contactor and two electrothermal relays built into it.
The simplest control circuit for an asynchronous electric motor with a squirrel-cage rotor. The circuit uses power and control circuits from a source of the same voltage (Fig. 4.9). To increase the reliability of operation of relay contactor devices, most of which are designed for low voltage, and to increase operational safety, circuits with control circuits powered from a reduced voltage source are used.
If switch S1 is turned on, then to start the electric motor you must press button S2 (“start”). In this case, the coil of the contactor K1M will receive power, the main contacts K1(1-3)M in the power circuit will close and the motor stator will be connected to the network. The electric motor will begin to rotate. At the same time, the closing auxiliary contact K1A will close in the control circuit, shunting the S2 (“start”) button, after which this button does not need to be kept pressed, since the contactor coil circuit KlM remains closed. Button S2 is self-resetting and, due to the action of the spring, returns to its original open state.
To disconnect the electric motor from the network, press the S3 (“stop”) button. The contactor coil K1M is de-energized and the closing contacts K1(1-3)M disconnect the stator windings from the network. At the same time, the auxiliary contact K1A opens. The circuit returns to its original, normal state. The rotation of the electric motor stops.
The circuit provides protection of the motor and control circuit from short circuits with fuses F 1(1-3), protection against motor overload by two electrothermal relays F2(1-2). The spring drive of the contacts of the magnetic starter K 1(1-3)M, K1A for opening implements the so-called zero protection, which, when the voltage disappears or significantly decreases, disconnects the motor from the network. Once normal voltage is restored, the engine will not start spontaneously.
More precise protection against a decrease or disappearance of voltage can be achieved using a low-voltage relay, the coil of which is connected to two phases of the power circuit, and its normally open contact is connected in series with the contactor coil. In these schemes, instead of installing switches with fuses at the input, air circuit breakers are used.
Control circuit for an asynchronous electric motor with a squirrel-cage rotor using a magnetic starter and an air circuit breaker. The F1 circuit breaker eliminates the possibility of one phase being broken from the protection being triggered during a single-phase short circuit, as happens when installing fuses (Fig. 4.10). There is no need to replace elements in fuses when their fuse link burns out.
In electric motor control circuits, automatic machines with electromagnetic releases or with electromagnetic and electrothermal releases are used. Electromagnetic type releases are characterized by an irregular cutoff equal to ten times the current and serve to protect against short circuit currents. Electrothermal releases have an inverse time characteristic of the current. Thus, a release with a rated current of 50 A operates at 1.5 times the load after 1 hour, and at 4 times the load - after 20 seconds. Electrothermal releases do not protect the motor from overheating at overloads of 20 - 30%, but they can protect the motor and power circuit from overheating by the starting current when the drive mechanism is stalled. Therefore, to protect electric motors from long-term overloads when using a circuit breaker with an electrothermal release of this type, additional electrothermal relays are used, as when using a circuit breaker with an electromagnetic release. Many switches, for example AP-50, protect the electric motor simultaneously from short circuit currents and overloads. The operating principles of the circuits (see Fig. 4.9, 4.10) for starting and stopping are similar. These circuits are widely used to control non-reversible electric drives of conveyors, blowers, fans, pumps, wood processing and sharpening machines.
Control circuits for an asynchronous squirrel-cage motor with a reversible magnetic starter. This scheme is used in cases where it is necessary to change the direction of rotation of the electric drive (Fig. 4.11), for example, in the drive of electric winches, roller tables, feed mechanisms of machine tools, etc. The motors are controlled by a reversible magnetic starter. The engine is turned on for forward rotation by pressing the S1 button. The coil of the contactor K1M will be energized, and the closing main contacts K1(1-3)M will connect the electric motor to the network. To switch the electric motor, you must press the S3 (“stop”) button, and then the S2 (“back”) button, which will turn off the K1M contactor and turn on the K2M contactor. In this case, as can be seen from the diagram, two phases on the stator will switch, i.e. the rotation of the electric motor will reverse. To avoid a short circuit in the stator circuit between the first and third phases due to the mistaken simultaneous pressing of both start buttons S1 and S2, reversible magnetic starters have a lever mechanical interlock (not shown in the diagram), which prevents the retraction of one contactor if the other is turned on. To increase reliability, in addition to mechanical interlocking, the circuit provides electrical interlocking, which is carried out using disconnecting auxiliary contacts K1A.2 and K2A.2. Typically, a reversing magnetic starter consists of two contactors housed in one housing.
In practice, a reverse circuit for asynchronous squirrel-cage electric motors is also used using two separate non-reversible magnetic starters. However, to eliminate the possibility of a short circuit between the first and third phases of the power circuit from the simultaneous activation of both starters, double-circuit buttons are used. For example, when you press the S1 (“forward”) button, the circuit of the contactor coil K1M is closed, and the circuit of the coil K2M is additionally opened. (The principle of operation of double-circuit buttons is shown in Fig. 4.12.) Reversing DC motors is carried out by changing the polarity of the power circuit voltage.
Control circuit for a two-speed asynchronous electric motor with a squirrel-cage rotor. Such a diagram is shown in Fig. 4.12. The drive can have two speeds. A reduced speed is obtained by connecting the stator windings to a triangle, which is done by pressing the double-circuit button S3 and turning on the short-circuit contactor with the closure of three power contacts K3. At the same time, the auxiliary contact K3A closes, shunting the S3 button, and K3A, the auxiliary contact in the K4 coil circuit, opens.
Increased speed is obtained by connecting the windings to a double star, which is realized by pressing the double-chain button S4. In this case, the coil of the contactor K3 is de-energized, the short-circuit contacts in the power circuit are opened, the auxiliary contact K3A, which bypasses the S3 button, is opened, and the auxiliary contact K3A in the circuit of the coil K4 is closed.
When you further press (move) the S4 button, the coil circuit of the contactor K4 is closed, the five K4 contacts in the power circuit are closed, the stator winding will be connected to a double star. At the same time, the auxiliary contact K4A closes, shunting the S4 button, and the auxiliary contact K4A opens in the contactor coil circuit K3. Typically AC contactors have three power contacts; the double star stator connection diagram shows five power contacts K4. In this case, the coil of the additional contactor is switched on in parallel with the coil of contactor K4.
After preliminary connection of the stator windings, the motor is started using contactors K1 and K2 to rotate forward or reverse. Contactors K1 or K2 are switched on by pressing button S1 or S2, respectively. The use of double-circuit buttons allows for additional electrical interlocking, which prevents the simultaneous activation of contactors K1 and K2, as well as K3 and K4.
The circuit provides the ability to switch from one speed to another when the electric motor rotates forward or backward without pressing the S5 (“stop”) button. When you press the S5 button, the coils of the switched-on contactors are de-energized and the circuit returns to its original, normal state.
The considered circuit is the basis for constructing control circuits for electric motors of two-speed conveyors for feeding cross-cutting units, sorting conveyors, etc.
Let's consider the issues of braking electric motors. When the stator windings are disconnected from the network, the rotor of the electric motor with a working mechanism, for example a circular saw of a sleeper cutter, continues to operate relatively for a long time rotate by inertia. To eliminate this phenomenon, in drives with asynchronous electric motors, depending on their power and purpose, counter-switch braking, friction braking and dynamic braking are used.
Control circuit for an asynchronous electric motor with a squirrel-cage rotor using back-switch braking. Such a diagram is shown in Fig. 4.13. Reverse braking circuits use an EM speed control relay (PKC) mechanically coupled to the motor shaft; its normally open contact EA closes at a certain angular speed of the motor. When the motor rotor is stationary and its rotation speed is less than 10...15% of the rated one, the relay contact EA is open. By pressing the SI button, the contactor K1M is turned on, the power contacts K1(1-3)M are closed and the engine is started, the auxiliary contact K1A.1, which bypasses the S1 button, is closed. The breaking auxiliary contact A7A.2 simultaneously breaks the power circuit of the K2M contactor coil, and somewhat later, with an increase in engine speed, the speed relay contact EA closes. Therefore, the K2M contactor does not turn on during this period.
Disconnecting the electric motor from the network with back-on braking is done by pressing the S2 (“stop”) button. In this case, the contactor coil K1M is de-energized, the power contacts K1(1-3)M are opened, and the auxiliary contact K1A.1, which bypasses the start button S1, is opened. At the same time, the breaking auxiliary contact K1A.2 closes. In this case, the engine rotates by inertia and the relay contact EA is closed, therefore, the contactor coil K2A will receive power, the main contacts K2(1-3)M will close, and the auxiliary contact K2A will open in the coil circuit K1M. The stator windings will be connected to the network to reverse the rotation of the rotor. The rotor instantly slows down and at a rotation speed close to zero, the contact of the speed relay EA opens, the coil of the contactor K2M is de-energized, the main contacts K2(1-3)M open, and the auxiliary contact K2A closes. The engine is stopped and disconnected from the mains. The diagram will be in its original position.
The considered typical back-switch braking circuit is the basis for constructing control circuits for electric motors of machines for sharpening chain saws, circular saws, frame saws, circuits for edgers, etc. Back-switch braking provides a hard, instantaneous stop of the drive and is usually used for low-power electric motors.
Scheme of friction braking of an asynchronous electric motor of a lifting mechanism. Such a diagram is shown in Fig. 4.14. According to the rules technical operation When lifting mechanisms are turned off, the drive and lifting mechanism must be reliably braked.
The simplified diagram conventionally shows a one-sided shoe brake T with a spring drive for clamping the brake pulley.
When starting the electric motor, the S1 (“start”) button is pressed, the contactor coil K1M will be energized, three contacts K1(1-3)M in the power circuit and the auxiliary contact K1A will be closed. The motor stator and the electromagnet winding Y will be simultaneously connected to the network. Electromagnet Y will simultaneously move the shoe brake away from the pulley and create spring deformation. The engine rotates disengaged.
By pressing the S2 (“stop”) button, the coil of the contactor K1M is de-energized, the main contacts in the power circuit K1(1-3)M and the auxiliary contact K1A are opened. The electric motor stator and the electromagnet winding U are disconnected from the network, a spring-driven shoe brake rigidly fixes the electric motor rotor with the lifting mechanism. The use of a reversible magnetic starter makes it possible to obtain a friction braking scheme for the electric drive of the mechanism for both lifting and lowering the load.
Scheme of friction braking of an asynchronous electric motor of machine tool equipment. Such a diagram is shown in Fig. 4.15. In the normal (off) state, the electric motor rotor is released under the action of a spring drive. This allows you to change tools and set up the machine with easy rotation of the drive shaft and electric motor rotor.
The electric motor is connected to the network using button S1, contact K1A and power contacts K1(1-3)M. Stopping the electric drive of the machine is done by pressing the double-chain button S2 (“stop”). In this case, the contactor coil K1M is de-energized, the main contacts in the power circuit K1(1-3)M and the auxiliary contact K1A are opened. The electric motor is disconnected from the network, continuing to rotate by inertia.
When you press the S2 button further, the coil circuit of the contactor K2M is closed, the contacts K2(1-2)M are closed, the electromagnet Y tightens the shoe brake. Button S2 is released and takes its original position, contactor K2M is de-energized, contacts K2(1-2)M are opened. The motor stator and electromagnet are disconnected from the network, the drive is stopped and released. This simplest scheme is the basis for the development of friction braking schemes for electric motors of machine tool equipment, which take into account the need for reverse, safety guards, and signaling.
Control circuit for an asynchronous motor using dynamic braking. Such a diagram is shown in Fig. 4.16. Dynamic braking, in contrast to counter-engagement braking and the friction method, is smooth, soft braking. The electric motor is switched on by pressing the SI (“start”) button. The contactor K1M will be turned on, the three main contacts K1(1-3)M in the power circuit will close, the auxiliary contact K1A.1 will close, the contact K1A.2 will open, the contact K1A.Z will close, after which the time relay D1M will turn on and close its RTD contact in the coil circuit of the contactor K2M, which was opened somewhat earlier by contact K1A.2.
The motor stator is disconnected from the AC mains and braking is carried out by pressing the S2 (“stop”) button. Contactor K1M loses power, main contacts K1(1-3)M open, auxiliary contacts K1A.1, K1A.3 open, and contact K1A.2 closes. The coil of the time relay D1M loses power, however, the closing contact of the RTD, being previously closed, will open with a time delay that slightly exceeds the duration of engine braking. When contact K1A.2 is closed, the coil of the contactor K2M will receive power, the auxiliary blocking contact K2A will open and contacts K2(1-2)M will close. A direct current is supplied to the stator winding. The winding creates a magnetic flux stationary in space. An EMF is induced in a rotor rotating by inertia.
The interaction of the rotor currents caused by these EMFs with a stationary magnetic flux creates the braking torque of the motor
where Mn is the rated torque of the motor; nс - synchronous speed of the motor; I"р - rotor current reduced to the stator; R"р - total active resistance of the rotor reduced to the stator; nd - relative engine speed, nd = n/nс.
After the RDT time relay contact opens, the circuit returns to its original state and the engine stops smoothly. An additional resistor Rt is used to limit the direct current. On the basis of this circuit, control circuits for electric motors of sawmill frames, sleeper cutters and other large circular saws have been created.
Scheme of thyristor control of starting and braking of an asynchronous motor with a squirrel-cage rotor. Such a diagram is shown in Fig. 4.17. In a typical open-loop control circuit for an asynchronous motor with a squirrel-cage rotor, thyristors are used as power elements included in the stator circuit of the motor in combination with relay contact devices in the control circuit. Thyristors act as power switches and, in addition, easily allow the required rate of change in voltage on the motor stator by adjusting the switching angle of the thyristors.
During startup, a smooth change in the switching angle of the thyristors makes it possible to change the voltage applied to the stator from zero to nominal, thereby limiting the currents and torque of the motor. The circuit contains a dynamic braking device in the form of a damping circuit. The use of a shunt thyristor, which closes the current circuit between two phases, leads to an increase in the DC component of the current, which creates sufficient braking torque in the high angular velocity region.
Let's consider standard diagram a complete device consisting in the power part of a group of back-to-back thyristors VS1...VS4 in phases A and C and one short-circuited thyristor between phases A and B - V5 for controlling an asynchronous motor M. The circuit includes a control unit for thyristors BU and relay control contact unit.
By pressing the S1 button, relays K1M and K2M are turned on, and pulses shifted by 60° relative to the supply voltage are supplied to the control electrodes of thyristors VS1...VS4. A reduced voltage is supplied to the motor stator windings, which reduces the starting current and starting torque. The engine rotor increases rotation speed and accelerates. The opening contact of relay K1.2 turns off relay K3M with a time delay depending on the parameters of resistor R7 and capacitor C4. The opening contacts of the K3M relay bypass the corresponding resistors in the thyristor control unit BU, and the full mains voltage is applied to the stator.
To stop the engine, button S3 is pressed, the relay control circuit is de-energized, thyristors VS1...VS4 are de-energized, and the voltage from the motor stator is removed. At the same time, due to the energy stored by capacitor C5, relay K4M is turned on during braking, which turns on thyristors VS2 and VS5 with its contacts K4.2 and K4.3. A half-wave rectification current flows through phases A and B into the stator windings of the motor, which ensures effective dynamic braking.
The current strength, and therefore the dynamic braking time, is regulated by resistors R1 and R3. This circuit also has a step mode. When button S2 is pressed, relay K5M is turned on, which, with its contacts KS.3 and K5.4, turns on thyristors VS2 and VS5. In this case, a half-wave rectification current flows through phases A and B into the motor stator windings. When button S2 is released, relay K5M and thyristors VS2 and VS5 are turned off; in this case, for a short time, due to the energy stored in the capacitor Sb, the relay is turned on, which, with its contact K6.2, turns on the thyristor VS3, and the motor rotor rotates through a certain angle due to the rotation of the resulting stator flux vector by approximately the same angle.
The turning step depends on the network voltage, the static load moment, the moment of inertia of the drive and the average value of the rectified current. The implementation of the step-by-step mode of engine operation is carried out after it has stopped, since the K5M relay can initially be turned on only after closing the normally open contacts K1.5, K4.1. The stepping mode of engine operation creates favorable setup conditions.
Control circuit for asynchronous electric motors with a wound rotor as a function of time. Such a diagram is shown in Fig. 4.18. Protection of motor power circuits from short circuit currents is carried out using maximum current relays FI, F2, F3; overload protection - electrothermal relays F4(1-2), the heating elements of which are connected through current transformers TT1, TT2. The control circuits are protected by an F5 circuit breaker, which has maximum current protection.
When the SI switch and the FS circuit breaker are turned on, the D1M time relay will receive power and its closing contacts D1A.1, D1A.2 will close, thereby preparing the switching circuit for the D2M time relay and the K1M contactor. Opening contact D1A.3 will open and turn off the circuit of acceleration contactor coils K2M, R3M, K4M.
When you next press the S2 (“start”) button, contactor K1M will turn on through the previously closed contact D1A.2, the main contacts K1(1-3) M in the power circuit will close, and voltage will be supplied to the stator winding of the motor M. All starting resistors are included in the rotor winding. The engine starts at the first rheostatic characteristic. At the same time, the auxiliary contact K1A.3, which bypasses the start button, will close, and contact K1A.2 will close, through which power is supplied to the circuit of time relay coils D2M, D3M. The breaking auxiliary contact K1A.1 will disconnect the D1M relay circuit, which releases the armature with a time delay when its coil is turned off. Therefore, D2M will not immediately turn on and its normally open contact D2A.1 will be open.
It should be noted that the normally open contact D1A.Z remains open; after the dwell time of relay D1M has expired, its normally open contact D1A.1 (as well as D1A.2) will open, and its normally open contact D1A.Z will close. As a result of these switchings, the K2M contactor will turn on in the control circuit and the first starting stage of the resistor will be bypassed - the engine will move from the first rheostatic characteristic to the second, accelerating to a higher angular speed. In addition, the time relay D2M will turn off and its opening contact with a time delay D2A.1 will close the coil circuit of the contactor K3M, which will operate and close its contacts K3(1-2)M, i.e. the second starting stage of the resistor is bypassed - the engine switches to the third rheostatic characteristic.
Finally, after opening with a time delay of the closing contact D2A.1, the D3M relay will turn off - with a time delay for which the D3M relay is configured (corresponding to the engine starting time on the last rheostatic characteristic), its contact D3A.1 will close, the K4M contactor will turn on and close its contacts K4(1-3)M. The rotor winding will be short-circuited and the motor will complete its acceleration according to its natural characteristic. This ends the stepwise start of an asynchronous motor, controlled as a function of time by electromagnetic time relays D1M, D2M, D3M.
The engine is stopped by pressing button S3. The circuit is used to drive mechanisms that do not require reverse, the duration of braking of which after turning off the engine is not significant. In particular, on the basis of this circuit, control circuits for the main electric motor of sawmill frames are created.
The article discusses the starting circuit for an asynchronous motor with a squirrel-cage rotor using non-reversible and reversible magnetic starters.
Squirrel-cage asynchronous motors can be controlled using magnetic starters or contactors. When using low-power motors that do not require starting current limitation, starting is carried out by turning them on at full mains voltage. The simplest engine control circuit is shown in Fig. 1.
Rice. 1. Control circuit for an asynchronous squirrel-cage motor with an irreversible magnetic starter
For starting, the QF circuit breaker is turned on and thereby supplies voltage to the power circuit of the circuit and the control circuit. When you press the SB1 “Start” button, the power circuit of the contactor coil KM is closed, as a result of which its main contacts in the power circuit are also closed, connecting the stator of the electric motor M to the supply network. At the same time, the KM blocking contact closes in the control circuit, which creates a power circuit for the KM coil (regardless of the position of the button contact). The electric motor is turned off by pressing the SB2 “Stop” button. In this case, the power supply circuit of the KM contactor is broken, which leads to the opening of all its contacts, the engine is disconnected from the network, after which it is necessary to turn off the QF circuit breaker.
The scheme provides the following types of protection:
From short circuits - using a QF circuit breaker and FU fuses;
from electric motor overloads - using thermal relays KK (the opening contacts of these relays during overloads open the power circuit of the KM contactor, thereby disconnecting the motor from the network);
zero protection - using a KM contactor (when the voltage decreases or disappears, the KM contactor loses power, opening its contacts, and the engine is disconnected from the network).
To turn on the engine, you must press the SB1 “Start” button again. If direct starting of the motor is impossible and it is necessary to limit the starting current of an asynchronous squirrel-cage motor, a low-voltage start is used. To do this, an active resistance or reactor is included in the stator circuit, or a start through an autotransformer is used. 
Rice. 2 Control circuit for an asynchronous motor with a squirrel-cage rotor with a reversible magnetic starter
In Fig. Figure 2 shows a control diagram for an asynchronous motor with a squirrel cage rotor and a reversible magnetic starter. The circuit allows direct starting of an asynchronous squirrel-cage motor, as well as changing the direction of rotation of the motor, i.e. reverse. The engine is started by turning on the QF circuit breaker and pressing the SB1 button, as a result of which the KM1 contactor receives power, closes its power contacts and the motor stator is connected to the network. To reverse the engine, you must press the SB3 button. This will turn off the KM1 contactor, after which the SB2 button is pressed and the KM2 contactor turns on.
Thus, the motor is connected to the network with a change in the phase order, which leads to a change in the direction of its rotation. The circuit uses blocking from possible erroneous simultaneous activation of contactors KM2 and KM1 using normally open contacts KM2, KM1. The engine is disconnected from the network using the SB2 button and the QF circuit breaker. The circuit provides all types of electric motor protection considered in the control circuit of an asynchronous motor with an irreversible magnetic starter.
