Electric motors are widely used in industry and everyday life. When operating some mechanisms, it is necessary to ensure rotation of the motor shaft in different directions, that is, it is necessary to reverse. To do this, use a specific control circuit and use an additional magnetic starter (contactor) or a reversing starter.
The type of engine reverse starting circuit depends on the following factors:
Therefore, reverse circuits can vary greatly, but once you understand the principles of their construction, you can assemble or repair any similar circuit.
Before disassembling engine reverse circuits, you need to define the concepts which will be used when describing the work:
In order for an electric motor to change its rotation, its magnetic field must be changed. To do this, you need to make some switches, which depend on the type of electrical machine.
The electric motor can operate in both three-phase and single-phase modes. The operating principle of the circuits changes slightly, but there are some additions to the power supply from a single-phase network.
The electrical circuit diagram of the reversible start of a three-phase electric motor with a squirrel-cage rotor is as follows (the diagram is shown in Fig. 1) The entire circuit is powered from a three-phase alternating current network with a voltage of 380 V through an automatic circuit breaker.
In order to reverse such an electric machine (M), you need to change the alternation of any two phases connected to the stator. In the diagram, magnetic starter Mp1 is responsible for forward rotation, and Mp2 is responsible for reverse rotation. The figure shows that when Mp1 is turned on, the phases on the stator A, B, C alternate, and when Mp2 is turned on - C, B, A, that is, phases A and C change places, which is what we need.
When voltage is applied to the circuit, the coils Mp1 and Mp2 are de-energized. Their power contacts Mn1.3 and Mn2.3 are open. The electric motor does not rotate.
When you press the Start1 button, power is supplied to the MP1 coil, the starter is triggered and the following happens:
If there is a need to stop the engine or reverse, you need to press
Stop button. In this case, the power circuit Mn1 opens, the contactor is turned off, its contacts return to their original state shown in the figure, and the electric motor stops.
In order for the engine to start rotating in the opposite direction, you need to press the Start button2. By analogy with Mp1, contacts Mp2.3, Mp2.1, Mp2.2 will operate, a phase switch will occur on the stator winding and the motor will begin to rotate in the opposite direction.
The control circuit is powered from two phase wires. With this connection, contactors with 380 V coils must be used. Fuses Pr1 and Pr2 provide protection against short circuit currents. In addition, removing these fuses completely de-energizes all controls and avoids the risk of electrical injury during maintenance and repair.
Protection of the electric machine from overloads is provided by thermal relay RT. When an increased current flows in any of the three stator windings, the bimetallic plate RT heats up, causing it to bend. At a certain current, the plate heats up so much that its bending causes the thermal relay to operate, due to which it opens its normally closed contact PT in the control circuit of the coils Mp1 and Mp2 and the motor is disconnected from the network.
The response time depends on the current value: the higher the current, the shorter the response time. Due to the fact that the RT operates with a certain delay, inrush currents, which can be 7-10 times higher than the rated ones, do not have time to trigger the protection.
Depending on the type of device and settings, after the thermal relay is triggered, there are two options for returning the circuit to working condition:
The considered three-phase motor reverse circuit can be modified depending on conditions and needs. For example, the control circuit can be powered from a 12 V network, in which case all control elements will be under safe voltage and such an installation can be safely used in high humidity.
Reversing the motor can only be done when the motor is completely stationary, otherwise the starting currents will increase several times, which will lead to the protection being triggered. In order to monitor the fulfillment of this condition, time relays can be added to the control circuit, the contacts of which are connected in series to MP2.2 and MP1.2. Thanks to this, after pressing the Stop button, the engine can only be started in the opposite direction after a few seconds. which are necessary to completely stop the mechanism.
In order for a three-phase asynchronous motor with a squirrel-cage rotor to operate from a single-phase 220 V network, a connection diagram with starting and running capacitors is used.
Three wires come from the stator winding of the electric motor. Two wires are connected directly to the phase and neutral wires, and the third is connected to one of the supply wires through a capacitor. In this case, the direction of rotation depends on which of the supply conductors the capacitor is connected to.
If you want to turn such a connection diagram into a reversible one, it needs to be supplemented with a toggle switch that will switch the capacitance from one power wire to another.
Reversing starting of a DC motor can be accomplished by changing the polarity of the connection of the armature winding or field winding. Depending on how these two windings are connected, DC motors have the following types of excitation:
DC motors can run out - a condition of machine operation in which the speed increases so much that it leads to mechanical damage.
When using a commutator motor with parallel or independent excitation, this mode may occur when the excitation winding breaks. Therefore, the connection diagram of the reversible motor in this case is constructed in such a way that the armature winding is switched, and the field winding must be directly connected to the power source. That is, it is unacceptable to connect the excitation circuit through any contacts or fuses.
Otherwise, the control circuit differs from the reversible connection of a three-phase motor only in that two DC supply wires are switched instead of three AC phases.
The main element in reversible electric motor connection circuits is a magnetic starter. The use of these devices allows us to solve a number of problems:
During installation, adjustment and repair, safety regulations must be strictly observed..
In the case of working with an electric motor control circuit, to completely turn it off, you need to de-energize the power section and control circuits. Some electric motors can be powered by two independent power sources, so be sure to study the connection diagram. Make the necessary shutdowns and check with an indicator that there is no voltage not only on the power contacts, but also on the auxiliary contacts.
If capacitors are installed in the circuit, they should be given time to discharge after turning off the power. before touching live parts.
All electrical circuit diagrams of machines, installations and machines contain a certain set of standard blocks and assemblies that are combined with each other in a certain way. In relay contactor circuits, the main elements of motor control are electromagnetic starters and relays.
Most often used as a drive in machines and installations. These engines are easy to design, maintain and repair. They satisfy most requirements for the electric drive of machine tools. The main disadvantages of asynchronous motors with a squirrel-cage rotor are large starting currents (5-7 times higher than the rated current) and the inability to smoothly change the motor rotation speed using simple methods.
With the advent and active introduction into electrical installation circuits, such motors began to actively displace other types of motors (asynchronous with a wound rotor and DC motors) from electric drives, where it was necessary to limit starting currents and smoothly regulate the rotation speed during operation.
One of the advantages of using squirrel-cage induction motors is the ease of their connection to the network. It is enough to apply three-phase voltage to the motor stator and the engine starts immediately. In the simplest version, you can use a three-phase switch or batch switch to turn it on. But these devices, despite their simplicity and reliability, are manual control devices.
In the diagrams of machine tools and installations, the operation of one or another engine in an automatic cycle must often be provided, the sequence of switching on several engines, automatic change in the direction of rotation of the engine rotor (reverse), etc. must be ensured.
It is impossible to provide all these functions with manual control devices, although in a number of old metal-cutting machines the same reverse and switching of the number of pairs of poles to change the speed of rotation of the motor rotor is very often performed using packet switches. Switches and package switches in circuits are often used as input devices that supply voltage to the machine circuit. Still, motor control operations are performed.
Switching on the engine via an electromagnetic starter provides, in addition to all the convenience of control, zero protection. What this is will be described below.
Three electrical circuits are most often used in machines, installations and machines:
control circuit for a non-reversible motor using one electromagnetic starter and two “start” and “stop” buttons,
control circuit for a reversible motor using two starters (or one reversing starter) and three buttons.
control circuit for a reversible motor using two starters (or one reversing starter) and three buttons, two of which use paired contacts.
Let's look at the operating principle of all these schemes.
The diagram is shown in the figure.
When you press SB2 “Start”, the starter coil is supplied with a voltage of 220 V, because it turns out to be connected between phase C and zero (N). The moving part of the starter is attracted to the stationary part, thereby closing its contacts. The starter's power contacts supply voltage to the engine, and the locking contact closes parallel to the "Start" button. Thanks to this, when the button is released, the starter coil does not lose power, because In this case, the current flows through the blocking contact.
If the blocking contact were not connected in parallel with the button (for some reason it was missing), then when the “Start” button is released, the coil loses power and the power contacts of the starter open in the motor circuit, after which it turns off. This mode of operation is called “jog”. It is used in some installations, for example in crane-beam schemes.
Stopping a running engine after starting in a circuit with a blocking contact is performed using the SB1 “Stop” button. In this case, the button creates a break in the circuit, the magnetic starter loses power and, with its power contacts, disconnects the engine from the supply network.
If the voltage disappears for any reason, the magnetic starter is also turned off, because this is equivalent to pressing the "Stop" button and creating an open circuit. The engine stops and restarting it in the presence of voltage is possible only by pressing the SB2 “Start” button. Thus, the magnetic starter provides the so-called. "zero protection". If it were absent from the circuit and the engine was controlled by a switch or batch switch, then when the voltage returned, the engine would start automatically, which poses a serious danger to operating personnel. See more details here -.
An animation of the processes occurring in the diagram is shown below.
The scheme works similarly to the previous one. Changing the direction of rotation (reverse) the motor rotor changes when the phase order on its stator changes. When the KM1 starter is turned on, phases A, B, C arrive to the motor, and when the KM2 starter is turned on, the phase order changes to C, B, A.
The diagram is shown in Fig. 2.
The engine is turned on for rotation in one direction using the SB2 button and the KM1 electromagnetic starter. If it is necessary to change the direction of rotation, you must press the SB1 “Stop” button, the engine will stop and then, when you press the SB 3 button, the engine begins to rotate in the other direction. In this scheme, to change the direction of rotation of the rotor, an intermediate press on the “Stop” button is necessary.
In addition, the circuit requires the use of normally closed (breaking) contacts in the circuits of each starter to provide protection against simultaneous pressing of two “Start” buttons SB2 - SB 3, which will lead to a short circuit in the motor power supply circuits. Additional contacts in the starter circuits prevent the starters from turning on at the same time, because When you press both “Start” buttons, any of the starters will turn on a second earlier and open its contact in the circuit of the other starter.
The need to create such a blocking requires the use of starters with a large number of contacts or starters with contact attachments, which increases the cost and complexity of the electrical circuit.
An animation of the processes occurring in a circuit with two starters is shown below.
3. Scheme for controlling a reversible motor using two magnetic starters and three buttons (two of which have mechanically coupled contacts)
The diagram is shown in the figure.
The difference between this circuit and the previous one is that in the circuit of each starter, in addition to the common SB1 “Stop” button, 2 contacts of the SB2 and SB 3 buttons are connected, and in the KM1 circuit, the SB2 button has a normally open contact (no contact), and SB 3 has a normally open contact - closed (break) contact, in the KM3 circuit - button SB2 has a normally closed (break) contact, and SB 3 has a normally open contact. When each button is pressed, the circuit of one of the starters is closed, and the circuit of the other is simultaneously opened.
This use of buttons makes it possible to avoid the use of additional contacts to protect against the simultaneous activation of two starters (this mode is impossible with this scheme) and makes it possible to perform reverse without intermediately pressing the “Stop” button, which is very convenient. The "Stop" button is needed to completely stop the engine.
The diagrams presented in the article are simplified. They do not have protection devices (circuit breakers, thermal relays) or alarm elements. Such circuits are also often supplemented with various contacts of relays, switches, switches and sensors. It is also possible to power the coil of the electromagnetic starter with a voltage of 380 V. In this case, it is connected from any two phases, for example, from A and B. It is possible to use a step-down transformer to reduce the voltage in the control circuit. In this case, electromagnetic starters with coils for voltages of 110, 48, 36 or 24 V are used.
Reverse- This is a change in the direction of rotation of the electric motor. Reversing can be done by changing the polarity of the supply voltage coming to the starter. These may be regulators used for DC motors.
Reversal can be performed using a change in phase rotation in the AC network. This action is performed automatically when the polarity of the reference signal is changed, or after a certain command is received at the desired logical input.
Reversing can be accomplished using information that is transmitted via the field bus; this capability is included in a certain set of standard functionality and is characteristic of most modern regulators used in AC circuits.
Fig No. 1. Tesus U (magnetic starter) with reversing block
To change the direction of the motor, the polarity of the voltage coming to the armature of the motor changes.
Currently, quite rarely, the contactor method is used.
There is a static method, it consists in changing the polarity at the output of the converter in the armature winding or by changing the direction of passage of the excitation current. This method is characterized by the presence of a large time constant of the excitation winding, which is not always convenient.
Rice. No. 2. Reversing the motor using a magnetic starter.
During controlled braking of mechanisms with a high moment of inertia of the load, it is necessary to return the energy generated by the electrical machine back to the main electrical network.
Using the braking process, the regulator acts as an inverter, the energy produced has a negative charge... thus the regulator can perform two operations, one is reverse, the other is regenerative braking. The regulator is equipped with two bridges that are connected back-to-back.
The bridges used invert the voltage and current.
Fig. No. 3. Reverse of an asynchronous electric motor with a direct frequency converter; a) speed and components of the vector of stator currents of the IM, b) phase voltages of the electrical network and load current.
Reverse can be carried out by a frequency converter used for asynchronous electric motors.
Reversing control is performed using vector control in a closed loop system using a feedback sensor. With its help, the current components Id and Iq are independently controlled; they serve to determine the flux and rotating torque of the motor. Controlling an asynchronous motor is similar to carrying out operations to control and regulate a DC motor.
Fig.No. 4 . Functional diagram of a speed controller with vector control and feedback sensor.
To implement the reverse function, an external signal appears at the logical input of the controller intended for executing this command. It changes the order of switching the power switches of the inverter and motor reverse. Reverse can be performed in several ways.
When the phase sequence changes on a motor in operation, the field rotation changes. As a result, a large slip appears, which creates a sharp increase in the current of the inverter (frequency converter) to the highest value (internal limitation of the inverter current). When the slip is large, the small braking torque and the internal controller of the inverter will reduce the speed command. When the electric motor reaches zero speed, a reverse occurs, which corresponds to the acceleration curve. Excess energy not spent on friction and load is dissipated in the rotor.
The torque of the mechanism is directly opposite to the torque of the engine and exceeds it in magnitude, that is, natural deceleration occurs many times faster than the deceleration curve set by the regulator. The speed value gradually decreases and the direction of rotation changes.
At a torque when natural braking is less than that set by the regulator, the motor begins to operate in a state of regenerative braking and returns energy to the converter. Diode bridges do not allow energy to pass into the network, the filter capacitors are charged, the voltage increases and a safety device is turned on, protecting against the release of energy.
To prevent overvoltage, a braking resistor is connected to the capacitor unit via a brake switch. The braking torque is limited by the capacitance in the DC link of the converter, the speed value drops and a rotation change occurs. Various modifications of resistors with different ratings ensure compliance with the engine power and energy dissipation. In the vast majority of cases, the brake key in models is located in the regulator itself.
The presence of a braking resistor is typical for regulators designed to provide controlled braking; this method is one of the most cost-effective. With its help, the engine can slow down the rotation until the movement stops, without changing the direction of the working rotation.
This option is typical for test benches. The energy released is too large; the resistors cannot cope with its dissipation, because the temperature will rise. For this purpose, systems are provided that make it possible to return energy back to the electrical network. In this case, the diode bridge is not used; instead, a semiconductor bridge made of IGBT transistors is used. The performance of operating functions is determined using multi-level control; it makes it possible to obtain a current characteristic close to the form of a pure sine.
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IMPORTANT! Before connecting the electric motor, you must ensure that it is correct in accordance with its specifications.
(hereinafter referred to as the starter) is a switching device designed to start and stop the engine. The starter is controlled through an electric coil, which acts as an electromagnet; when voltage is applied to the coil, it acts with an electromagnetic field on the movable contacts of the starter, which close and turn on the electrical circuit, and vice versa, when the voltage is removed from the starter coil, the electromagnetic field disappears and the starter contacts are under the action of the spring returns to its original position, breaking the circuit.
The magnetic starter has power contacts designed for switching circuits under load and block contacts which are used in control circuits.
Contacts are divided into normally open- contacts that are in their normal position, i.e. before applying voltage to the coil of the magnetic starter or before mechanical impact on them, are in an open state and normally closed- which in their normal position are in a closed state.
The new magnetic starters have three power contacts and one normally open block contact. If it is necessary to have a larger number of block contacts (for example, during assembly), an attachment with additional block contacts (contact block) is additionally installed on the magnetic starter on top, which, as a rule, has four additional block contacts (for example, two normally closed and two normally open).
Buttons for controlling an electric motor are included in push-button stations; push-button stations can be one-button, two-button, three-button, etc.
Each button of the push-button post has two contacts - one of them is normally open, and the second is normally closed, i.e. Each of the buttons can be used both as a “Start” button and as a “Stop” button.
This diagram is the simplest diagram for connecting an electric motor; it does not have a control circuit, and the electric motor is turned on and off by an automatic switch.
The main advantages of this scheme are its low cost and ease of assembly, but the disadvantages of this scheme include the fact that circuit breakers are not intended for frequent switching of circuits; this, in combination with inrush currents, leads to a significant reduction in the service life of the machine; in addition, this scheme does not include Possibility of additional motor protection.
This scheme is also often called simple motor starting circuit, in it, unlike the previous one, in addition to the power circuit, a control circuit also appears.
When you press the SB-2 button (the “START” button), voltage is applied to the coil of the magnetic starter KM-1, while the starter closes its power contacts KM-1 starting the electric motor, and also closes its block contact KM-1.1 when the button is released SB-2 its contact opens again, but the coil of the magnetic starter is not de-energized, because its power will now be provided through the KM-1.1 block contact (i.e. the KM-1.1 block contact bypasses the SB-2 button). Pressing the SB-1 button (the “STOP” button) leads to a break in the control circuit, de-energizing the magnetic starter coil, which leads to the opening of the magnetic starter contacts and, as a result, to stopping the electric motor.
To change the direction of rotation of a three-phase electric motor, you need to swap any two phases supplying it:
If it is necessary to frequently change the direction of rotation of the electric motor, the following is used:
This circuit uses two magnetic starters (KM-1, KM-2) and a three-button post; the magnetic switches used in this circuit, in addition to a normally open block contact, must also have a normally closed contact.
When you press the SB-2 button (START 1 button), voltage is applied to the coil of the magnetic starter KM-1, while the starter closes its power contacts KM-1 starting the electric motor, and also closes its block contact KM-1.1 which bypasses the button SB-2 and opens its block contact KM-1.2 which protects the electric motor from turning on in the opposite direction (when the SB-3 button is pressed) until it stops first, because An attempt to start the electric motor in the opposite direction without first disconnecting the KM-1 starter will result in a short circuit. To start the electric motor in the opposite direction, you need to press the “STOP” button (SB-1), and then the “START 2” button (SB-3), which will power the coil of the KM-2 magnetic starter and start the electric motor in the opposite direction.
10Reverse is a mechanism for directing part of the jet or air stream in the direction of movement of the aircraft and creating reverse thrust. In addition, reverse is the name used for the operation mode of an aircraft engine, which uses a reversing device.
The device is mainly used after landing, during the run or for emergency braking. In addition, reverse is used for reversing without the help of a towing vehicle. Some planes turn on the reverse while in the air. Most often, the device is used in transport and commercial aviation. After landing, the reverse is characterized by noise. It is used in conjunction with a wheel braking system, which reduces the load on the main braking system of the aircraft and shortens the distance, especially when the runway friction coefficient is low, as well as at the very beginning of the run. The contribution of reverse thrust varies greatly in different situations and aircraft models.
Reverse is produced by deflecting all or part of the jet that comes from the engine using different shutters. In various power plants, the reversing device is implemented in different ways. Special shutters are capable of blocking the jet, which is created purely by the external circuit of a turbojet engine (as on the A320), or the jet of all circuits (Tu-154M). The design features of the aircraft affect the equipment of the reverse gear. This can be either all engines or a specific part. For example, on the three-engine Tu-154, only the outer engines can create reverse, while the Yak-40 aircraft can create reverse.
Bucket flaps are a special mechanism that redirects the air flow. There can be two or more similar valves on engines. Outwardly they look like buckets. For example, in an engine with a high bypass ratio with flow over the entire plane, like the D-30Ku-154 (Tu-154M).
The reverse method, in which a special metal profile is installed in the nozzle and the rear part of the engine, is called profiled grilles. The engine is operated in direct thrust, and the flaps in the grilles redirect the passage of exhaust gases. A similar design is used in many aircraft engines, in particular in power plants with a low bypass ratio with shutoff of the entire flow (Tu-154, Boeing 727).
But the reverse system has its drawbacks. Possible troubles include the use of reverse at low speeds (less than 140 km/h). The jet can lift debris from the runway surface, which, when the aircraft runs at low speeds, can enter the air intake and cause damage. At high speeds, raised debris does not create interference due to the fact that it does not reach the height of the air intake.
The reverse device is installed on four engines, but in practice the 2nd and 3rd engines do not use reverse, because the process can damage the fuselage skin.
Reverse in propeller-driven aircraft is realized by turning the propeller blades (the angle of attack of the blades changes to negative), namely, with the direction of rotation unchanged. Therefore, the propeller creates reverse thrust. This type of reversing device can be used on piston and turboprop engines. Reverse is often provided on amphibians and seaplanes.
The first use of reverse began in the 30s. Passenger aircraft Douglas DK-2 and Boeing 247 were equipped with reverse.
A huge number of aircraft do not use reverse due to its uselessness or technical complexity. For example, due to some wing mechanization capabilities and the high efficiency of air brakes in the tail of the BAe 146-200, turning on the reverse is not required. Accordingly, all 4 engines do not work in reverse mode. For the same reason, the Yak-42 aircraft does not need a reverse device.
Most aircraft with afterburners do not have a reverser due to the magnitude after the landing roll. This circumstance forces the construction of long runways, at the end of which emergency braking devices should be installed. In this case, aircraft are equipped with effective wheel brakes and parachutes. It should be noted that the pneumatics and brakes of such aircraft are subject to severe wear and tear and often require replacement.
Some aircraft allow the possibility of using thrust reverser directly in the air, but such inclusion depends on the type of aircraft. In some situations, the reverse is turned on before landing, and in others - at the time of descent, which significantly reduces the vertical braking speed or makes it possible to avoid permissible excess speeds during a dive, emergency descent or combat maneuvers.
The ATR 72 is a turboprop airliner, a prime example of the use of reverse in the air. In addition, the air reverse can be used by the Trident turbojet airliner, the Concorde supersonic airliner, the C-17A military transport aircraft, the Saab 37 Wiggen fighter, the Pilatus RS-6 turboprop and others.
