CONNECTION TYPES AND CHARACTERISTICS OF ELECTRIC MOTORS
In three-phase asynchronous motors, after the stator windings are made, six terminals are generally brought out to the terminal box outside the motor for connection to the mains. These are the winding input and output terminals. Depending on how these terminals are connected, it is possible to start (i.e. initially energize) the motor in different ways.
Direct-On-Line Starting (DOL)
As the name suggests, this method involves connecting the motor directly to the mains. Although it is the fastest and easiest method, it carries certain problems of its own. With DOL starting, motors draw 7-10 times the rated current from the mains at initial start-up. This may not be a problem for small motors (<5.5 kW), but in larger motors it can cause mains faults, power losses and voltage imbalances.
Star-Delta Starting
The main purpose of this starting method is to prevent the inrush current mentioned above. During start-up, the motor is run in a star connection, which provides initial movement with a lower current. Once start-up is complete, the motor immediately switches to the normal delta connection. The most important points in this connection method are the running time in the star connection and the transition time from the star connection to the delta connection.
In the star connection, the voltage across the motor windings drops to V/√3 and the current drops to 1/3 of its value. If the motor switches to the delta connection before reaching full speed, it starts as if it were DOL, as mentioned above, and an inrush current occurs. On the other hand, since the current and voltage are low in the star connection, the motor torque is also low, which means this starting method cannot be applied to motors that are already under load. However, if the motor reaches full speed but is not switched to the delta connection, it will then be unable to carry the connected load. In this connection type, the star-to-delta transition is very important, and this transition must be instantaneous.
Starting Using a Soft Starter
Soft starters gradually and progressively increase the voltage applied to the motor. This prevents sudden loading of the motor and inrush current. Because the use of a soft starter does not transmit stress to either the motor or the electrical system, it extends the service life of the equipment and system components.
