Today, relay-contactor control circuits are most often used. In such systems, the main devices are electromagnetic starters and relays. In addition, a device such as a three-phase asynchronous squirrel-cage induction motor is most often used as a drive for machine tools and other installations.
Description of engines
These types of drives began to be actively used due to the fact that they are easy to operate, maintain, repair and design. They have only one serious minus, which consists in the fact that the starting current exceeds the rated current by about 5-7 times, and there is also no possibility to use the simple control methods to smoothly change the rotor speed.
This type of machine began to be actively used due to the fact that such devices as frequency converters began to be actively introduced into electrical installations. Another significant advantage of an induction motor with a three-phase current and a short-circuited rotor is that it has a fairly simple network connection scheme. To turn it on, you only need to apply a three-phase voltage to the stator, and the device will immediately start. In the simplest control circuits, a device such as a packet switch or a three-phase switch is used to start it. However, these devices, despite their simplicity and ease of use, are elements of manual control.
This is a huge minus, since in the schemes of most installations it is necessary to use the engine switching circuit in automatic mode. It is also necessary to provide for an automatic change in the direction of rotation of the rotor of the engine, that is, its reverse and the order of inclusion of several engines in operation.
Basic wiring diagrams
In order to provide all the necessary functions that have been described above, it is necessary to use automatic modes of operation, rather than elements of manual control of the drive. However, it is fair to say that some older metal cutting machines still use batch switches to change the number of pole pairs or to provide reverse.
The use in the wiring diagrams of asynchronous motors (HELL) not only batch switches, but also circuit breakers is possible, but they perform only one function - connecting the circuit to the voltage supply. All other operations that the engine control circuit provides are carried out under the guidance of an electromagnetic starter.
When you connect the circuits HELL with squirrel-cage rotor through this type of starter provides not only a convenient control mode, but also creates zero protection. Most often, three switching methods are used as engine control circuits in machines, plants and other machines:
- the first circuit is used to control a non-reversible motor, uses only one electromagnetic type starter and two buttons - "Start" and "Stop";
- the second reversible motor control scheme involves the use of three buttons and two conventional type starters or one reversible type;
- the third control scheme differs from the previous one only in that of the three control buttons, two have paired contacts.
Electromagnetic type starter circuit
Starting an induction motor in such a connection scheme is carried out by pressing the corresponding button. When it is pressed, a current with a voltage of 220 V is supplied to the starter coil. The starter has a moving part, which, when voltage is applied, is attracted to the fixed part, as a result of which the device contacts are closed. These power contacts supply input voltage to the motor. In parallel to this process, the interlock contact is also closed. Its inclusion is carried out parallel to the "Start" button. It is thanks to this function that when the button is released, the coil is still energized and continues to power the engine for it to function.
If, for any reason, during the start of the induction motor, that is, when you press the "Start" button, the interlock contact would not close or, for example, would be absent, then immediately upon release the current would cease to be supplied to the coil, the power contacts of the starter would open, and the engine immediately stopped. This mode of operation is called “jogging”. It takes place, for example, when controlling a crane beam.
In order to stop the three-phase asynchronous motor with squirrel-cage rotor, it is necessary to press the "Stop" button. The principle of operation in this case is quite simple and based on the fact that pressing the button creates a break in the circuit, disconnecting the power contacts of the starter, thereby stopping the operation of the engine. If the voltage at the power source disappears at the time of operation, the engine will also stop, since such a defect is equivalent to pressing "Stop" and further creating a break in the device circuit.
After the device has been stopped by power off or loss of power, you can restart it only with the button. This is what is called zero protection in engine control circuits. If instead of a starter, a switch or a switch were installed here, then when the voltage appeared again in the source, the engine would automatically start and continue to work. This is considered unsafe for maintenance personnel.
The use of two starters in a reversing device
The control circuit of this type of induction motor, in fact, works in the same way as the previous one. The main difference here is that it becomes possible, if necessary, to change the direction of rotation of the rotor. To do this, it is necessary to change the working phases available on the stator winding. For example, if you press the "Start" button KM1, then the order of the working phases will be A-B-C. If you turn on the device from the second button, that is, with KM2, then the order of the working phases will be reversed, that is, C-B-A.
Thus, it turns out that in order to control an asynchronous motor with a circuit of this type, two Start buttons, one Stop button and two starters are needed.
When you press the first button, which is usually indicated as SB2 in the circuit, the first contactor will turn on and the rotor will rotate in one of the sides. If there is a need to change the direction of rotation to the opposite, you must click on "Stop", after which the engine is started by pressing SB3 and turning on the second contactor. In other words, in order to use this scheme, an intermediate press on the stop button is necessary.
Since the control of the engine with such a circuit is complicated, there is a need for additional protection. In this case, we are talking about operating normally-closed (opening) contacts in the starter. They are necessary in order to provide protection against simultaneous pressing of both Start buttons. Pressing them without stopping will cause a short circuit. Additional contacts in this case prevent the simultaneous inclusion of both starters. This is due to the fact that when pressed simultaneously, one of them will turn on a second later than the second. During this time, the first contactor will have time to open his contacts.
The disadvantage of controlling an electric motor with such a circuit is that starters must have a large number of contacts or contact attachments. Any of these two options not only complicates the entire electrical structure, but also increases the cost of its assembly.
The third kind of control scheme
The main difference between this scheme of the engine control system from the previous one is that in the circuit of each of the contactors, in addition to the common "Stop" button, there are two more contacts. If we consider the first contactor, then in its circuit there is an additional contact for SB2 - it is normally open (closing), and SB3 has a normally closed (opening) contact. If we consider the connection diagram of the second electromagnetic starter, then its "Start" button will have the same contacts, but located opposite to the first.
Thus, it was possible to achieve that when one of them is pressed while the engine is running, the circuit already in operation will open, and the other, on the contrary, will be closed. This type of connection has several advantages. Firstly, this circuit does not need protection against simultaneous switching on, which means that there is no need for additional contacts. Secondly, it becomes possible to perform reverse without an intermediate click on "Stop". With this connection, this contactor is used only to completely stop the working HELL.
It is worth noting that the considered control schemes for starting the engine are somewhat simplified. They do not consider the presence of various additional protection devices, alarm elements. In addition, in some cases it is possible to power the starter's electromagnetic coil from a source of 380 V. In this case, it becomes possible to connect only from two phases, for example A and B.
Direct start control circuit with time function
The engine is started as usual - with a button, after which voltage will be supplied to the starter coil, which will connect the BP to the power source. The peculiarity of the circuit is as follows: together with the closure of the contacts at the starter (KM), one of its contacts in the other circuit (CT) will be closed. Because of this, a circuit is closed in which the brake contactor (KM1) is located. But its operation at this moment is not carried out, since in front of it is the opening contact of the CM.
To disconnect, use another button that opens the KM circuit. At this time, the device is disconnected from the AC mains. However, at the same time, a contact is closed that was in the circuit of the braking relay, which was previously referred to as KM1, and the circuit is also disconnected in the time relay, which is designated as CT. This is what leads to the fact that the contactor KM1 is turned on. In this case, the engine control circuit is switched to direct current. That is, the supply voltage is supplied from the built-in source through the rectifier, as well as the resistor. All this leads to the fact that the unit provides dynamic braking.
However, the work of the circuit does not end there. The circuit has a time relay (CT), which starts the countdown of the braking time immediately after it is disconnected from the power supply. When the allotted time for shutting down the motor expires, the CT opens its contact, which is in the circuit KM1, it turns off, which is why the supply of constant current to the motor also stops. Only after this a complete stop occurs, and it can be considered that the engine control circuit has returned to its initial position.
As for the braking intensity, it can be regulated by the direct current, which follows through the resistor. To do this, set the necessary resistance in this area.
Scheme for the operation of a multi-speed engine
Such a control scheme may provide the ability to obtain two engine speeds. To do this, the stator half-winding sections are connected to a double star or to a triangle. In addition, in this case, it is also possible to reverse. To avoid malfunctions of the engine control system, in such a complex circuit there are two thermal relays, as well as a fuse. In the diagrams, they are usually marked as KK1, KK1 and FA, respectively.
Initially, it is possible to start the rotor with a low speed. For this, a button is usually provided in the circuit, which is marked as SB4. After pressing it, it starts at a low frequency. The stator of the device in this case is connected according to the scheme of a regular triangle, and the existing relay closes the two contactors and prepares the motor for connecting power from the source. After that, you need to press the button SB1 or SB2 to determine the direction of rotation - "Forward" or "Back", respectively.
When the run to low frequencies is carried out, it becomes possible to accelerate the engine to high rotation rates. To do this, press the SB5 button, which disconnects one of the contactors from the circuit and connects the other. If we consider this action from the point of view of the chain, then a command is issued to switch from a triangle to a double-type star. In order to completely stop the work, there is a "Stop" button, which on the diagrams is marked as SB3.
Push-button post
This equipment is intended for switching, that is, connecting circuits in which alternating current flows with a maximum voltage of 660 V and a frequency of 50 or 60 Hz. Such devices can also be used in DC networks, but then the maximum operating voltage is limited to 440 V. It can even be used as a control panel.
An ordinary button post has the following design features:
- Each of its buttons is deprived of fixing.
- There is a Start button, which most often has not only a green color, but also contacts of a normally-divorced type. Some models even have a backlight that turns on after pressing. Purpose - the introduction of a mechanism.
- "Stop" is a button that has a red color (most often). It is located on closed contacts, and its main purpose is to disconnect any device from the power source in order to stop its operation.
- The difference between some devices is the material used to make the frame. It can be made of metal or plastic. In this case, the case plays an important role, since it has a certain degree of protection, depending on the material.
Main advantages
Among the main advantages of such devices are the following:
- the complete set of this device can not always be standard, it can be adjusted according to the wishes of the customer;
- the case is usually made of non-combustible refractory plastic or metal;
- there is good sealing, which is achieved due to the presence of a rubber gasket between the cover and the contacts inside;
- the sealant for this button post is under good protection from the influence of any aggressive environmental factors;
- there is an additional hole on the side so that it is convenient to insert the desired cable;
- All fastenings available at the post are made of high-strength stainless steel.
Post Type
There are three types of fasting - these are PKE, PKT and PKU. The first is usually used for working with woodworking machines for industrial or home use. PKU is used in industry, but only at those facilities where there is no danger of explosion, and the concentration of dust and gas does not rise above the level that the device can damage. FCT are precisely those posts that can be used in control circuits of three-phase asynchronous motors with a squirrel-cage rotor, as well as other motors of the electrotechnical type. In addition, they are also actively used to control equipment such as beam cranes, bridge cranes and other devices designed to lift heavy loads.