Asynchronous electric motors are currently used very actively. They have certain advantages, thanks to which they have become so popular. To connect powerful motors to the electric network, star, triangle circuits are used. Electric motors operating on such circuits have their own advantages and disadvantages. They themselves are distinguished by reliability in operation, the ability to obtain high torque, as well as a high rate of performance.
Motor connection
As practice shows, there are two optimal schemes - “star”, “triangle”. Electric motors are connected in one of them. It is also possible to convert a "star" into a "triangle", for example.
Among the advantages of induction motors are the following:
- the ability to switch windings during operation;
- restoration of the winding of an electric motor;
- low cost of the device in relation to others;
- high resistance to mechanical damage.
The main feature that characterizes all asynchronous electric motors is the simplicity of the design. However, with all its advantages, there are some disadvantages that arise during operation:
- There is no way to control the rotor speed without losing power.
- As the load increases, the torque decreases.
- High inrush currents.
Connection Description
The star and triangle circuits for the electric motor have certain differences in the connection. "Star" means that the ends of the stator windings of equipment are assembled at one point. In this case, a network voltage of 380 V will be supplied to the beginning of each of the windings. Typically, on all wiring diagrams, this method is referred to as Y.
In the case of using a "triangle" connection circuit, the stator windings of the electric motor are connected in series. That is, the end of the first winding is connected with the beginning of the second, it, in turn, with the third. The latter will close the circuit, connecting with the beginning of the first.
Differences in wiring diagrams
The "star" and "triangle" circuits of an electric motor are the only ways to connect them. They differ among themselves, providing different operating modes. So, for example, the connection using the Y circuit provides softer operation when compared to motors connected in a "triangle". This difference plays a key role in choosing the power of an electrical device.
More powerful engines are operated only on the "triangle". The star-delta electric motor connection diagram is excellent for those cases where it is necessary to ensure a smooth start. And at the right time, switch between the windings for maximum power.
It is important to add here: connection Y guarantees soft operation, but the engine will not be able to gain its rated power.
On the other hand, the delta-star-star electric motor connection scheme will provide more power, but at the same time, the value of the starting current for the equipment will increase significantly.
It is the difference in power between the connection Y and the triangle that is the main indicator. An electric motor with a star circuit will have a power of about 1.5 times lower than through a triangle, however, such a connection will help reduce the value of the starting current. All connections that include two connection methods are combined. Usually they are used only in cases where it is necessary to start an electric motor with a large rated power.
The star-delta starting circuit for an electric motor has another advantage. The inclusion is carried out according to the circuit Y, which reduces the value of the starting current. When the device gains sufficient speed during operation, a transition to the triangle circuit occurs to achieve maximum power.
Combined Connections
The star-delta switching circuit of an electric motor is often used in cases where it is necessary to start the engine with a minimum starting current. But at the same time, all the work needs to be done at the triangle joint. To create such a switch, special contactors in three phases are used. To ensure automatic switching between circuits, two conditions must be met. Firstly, to ensure the blocking of contacts from simultaneous inclusion. Secondly, all work must be performed with a time delay.
The second point is necessary so that with a 100% probability there will be a complete shutdown of the "star" before turning on the "triangle". If this is not done, then a short circuit will occur during switching between phases. To fulfill the necessary conditions, a time relay with a delay of 50 to 100 milliseconds is used.
Time delay exercise
When using the combined star-delta connection method, the presence of a time relay for delay switching is necessary. Specialists most often choose one of three ways:
- The first option is carried out using a normally open contact of the time relay. In this case, the RT will disconnect the connection circuit with a triangle during start-up, and the current relay of the RT will be responsible for switching.
- The second option involves the use of a modern time relay with a switching delay of 6 to 10 seconds.
- The third way is to control the contactors of the electric motor with automatic devices or manually.
Switching Method Consideration
Using the classic version using a time relay for combined star-delta circuits was previously considered the most optimal. He had only one drawback, which sometimes became significant enough - the dimensions of the PB itself. These types of devices guaranteed a delay in the switching time by magnetizing the core. However, the reverse process took time.
Currently, such RVs and other devices have been supplanted by modern devices - frequency converters. Switching a motor circuit with a star-delta circuit using an inverter has great advantages. This includes more stable operation, low inrush currents.
This equipment has a built-in microprocessor responsible for changing the frequency. If we consider the essence of the inverter for an electric motor, then its principle of operation is as follows: the converter generates the desired frequency of the alternating current. To date, the industry uses special or universal inverter models for connecting asynchronous motors.
Special models are developed and used only with certain types of engines. Universal can be used with any device.
Circuit disadvantages
Despite the fact that the classical connection scheme is simple and reliable, it has its certain disadvantages.
Firstly, it is very important to accurately determine the load on the motor shaft. Otherwise, it will take too long to gain momentum, which, in turn, will exclude the possibility of quickly switching to a triangle circuit using a current relay. In this mode, it is undesirable to operate the electrical device for a long time.
Secondly, with such a connection scheme, overheating of the windings is possible, which is why experts recommend installing an additional thermal relay in the circuit.
Thirdly, when using modern time relays, it is necessary to strictly observe the rated load on the shaft of the electric motor.
Conclusion
When using a star-delta connection, it is very important to correctly calculate the load on the motor shaft. Another unpleasant fact lies in the fact that at the moment of switching from Y to a triangle, when the engine has not yet reached the desired speed, self-induction occurs. At this point, an increased voltage appears in the network. This threatens the failure of other devices and devices connected to the same network.