CONNECTION TYPES AND FEATURES OF ELECTRIC MOTORS
In three-phase asynchronous motors, after the stator windings are completed, six terminals are usually attached to the terminal board for mains connection outside the motor. These are the winding input and output ends. It is possible to start (i.e. start up) the motors in different ways depending on the connection types of these terminals.
Direct Start (Direct Online DOL)
As the name suggests, this method is to connect the motor directly to the mains. Although it is the fastest and easiest method, it has some problems. Motors draw 7-10 times more current from the mains when starting with DOL. This may not be a problem in small motors (<5.5 kW), but in larger motors it may cause mains faults, power losses and voltage imbalances.
Yıldız-üçgen Yol Verme
The main purpose of this type of starting is to prevent the inrush current we mentioned above. The motor is operated in star connection during start-up, thus providing initial movement with a lower current. Once the engine is started, it immediately switches to the normal connection, which is the delta connection. The most important points in this connection are the operating time in star connection and the transition time from star connection to delta connection.
In star connection, the voltage falling on the motor windings decreases to V/√3 and the current decreases to 1/3. If the engine switches to delta connection before it reaches its full speed, it starts as DOL and a starting current occurs, as we mentioned above. On the other hand, since the current and voltage are low in the star connection, the torque of the motor is also low. Therefore, this starting type cannot be applied to motors that are already under load. However, if the triangle connection is not switched on even though the engine is revving, the engine will not be able to lift the connected load. In this connection type, star-delta transition is very important and this transition must be sudden.
Starting Using Soft Starter (Softstarter)
Softstarters slowly gradually increase the voltage applied to the motor. Therefore, it prevents a sudden load on the motor and inrush current. The use of a soft starter extends the life of equipment and system components as it does not transmit stress to both the motor and the electrical system.
