Asynchronous electric motors are currently used very actively. They have certain advantages due to which they have become so popular. To connect powerful motors to the electrical network, "star", "triangle" schemes are used. Electric motors operating on such schemes 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 performance indicator.
Motor connection
As practice shows, there are two optimal schemes - "star", "triangle". Electric motors are connected on one of them. It is also possible to convert "star" to "triangle", for example.
Among the advantages of asynchronous motors, the following stand out:
- switchablewindings during operation;
- recovery of electric motor winding;
- 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 design. However, with all its advantages, there are some disadvantages that arise during operation:
- No ability to control rotor speed without wasting power.
- When the load increases, the torque decreases.
- High starting currents.
Description of connections
The "star" and "delta" circuits for the electric motor have certain differences in connection. "Star" means that the ends of the stator winding of the equipment are assembled at one point. In this case, the mains voltage of 380 V will be applied to the beginning of each of the windings. Usually, on all wiring diagrams, this method is indicated as Y.
In the case of using the "delta" connection scheme, the stator windings of the electric motor are connected in series. That is, the end of the first winding is connected to the beginning of the second, which, in turn, is connected to the third. The latter will complete the circuit, connecting with the beginning of the first.
Differences in connection schemes
The "star" and "triangle" circuits of the electric motor arethe only way to connect them. They differ from each other, providing different modes of operation. So, for example, connecting using the Y scheme provides softer operation when compared with delta-connected motors. 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 motor connection is excellent for applications where a soft start is required. And at the right time, switch between windings for maximum power.
It's important to add here: connecting Y guarantees smooth operation, but the engine will not be able to reach its nameplate power.
On the other hand, the delta-star-wye motor connection will provide more power, but the starting current for the equipment will also increase significantly.
It is the difference in power between the Y connection and the triangle that is the main indicator. An electric motor with a star circuit will have about 1.5 times less power than a delta motor, however, such a connection will help reduce the starting current. All connections that incorporate two connection methods are combined. Usually they are used only in cases where it is necessary to start an electric motor with a large nameplate power.
Start-up scheme "star-delta" for the electric motor has another advantage. Turning on is done in a Y-pattern, which reduces the starting current. When the device picks up enough speed during operation, it switches to a delta scheme to achieve maximum power.
Combined connections
The star-delta switching scheme 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 must be done on the "triangle" connection. To create such a switch, special three-phase contactors are used. Two conditions must be met to enable automatic switching between schemas. Firstly, to ensure that the contacts are blocked from being turned on simultaneously. Secondly, all work must be done 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 the switching between phases. To fulfill the necessary conditions, a time relay with a delay of 50 to 100 milliseconds is used.
Implementation of time delay
When using the combined star-delta connection method, the presence of a time relay for switching delay is necessary. Specialists most often choose one of three methods:
- First optionis carried out using a normally open contact of the time relay. In this case, the RT will turn off the delta connection during start-up, and the current relay 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 motor contactors by automatic devices or manually.
Consideration of switching method
The use of the classic version with the use of a time relay for combined star-delta circuits was previously considered the most optimal. He had only one drawback, which sometimes became quite significant - the dimensions of the RV itself. These types of fixtures ensured that the switching time was delayed by the magnetization of the core. However, the reverse process took time.
At present, such RV and other devices have been supplanted by modern devices - frequency converters. Switching the star-delta motor circuit with an inverter has great advantages. This includes more stable operation, low starting 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 AC frequency. To date, the industry uses special or universal inverter models forconnection of asynchronous motors.
Special models are developed and used only with certain types of engines. Universal can be used with any device.
Scheme flaws
Despite the fact that the classic connection scheme is simple and reliable, it has certain drawbacks.
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 the delta 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 passport load on the shaft of the electric motor.
Conclusion
When using the 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 triangle, when the engine has not yet gained the necessary speed, self-induction occurs. At this point, there is increased voltage in the network. This threatens to damage other devices and devices connected to the same network.
