Direct-on-line starting of a Grundfos 7.5 k W motor installed on a Grundfos CR pump The star-delta starting method features a lower locked-rotor current, but peaks during the starting process as the changeover from star to delta is made.
When starting in star (t = 0.3 s), the current is reduced.
A motor controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and faults.
Every electric motor has to have some sort of controller.
After a time period – which depends on individual needs – it connects the motor in delta contactor K3 open and contactor K2 close.
Star-delta starter curve – Synchronous speed / Full-load current Starting torque and current are considerably lower at star-delta starting than at direct-on-line starting: one third of the equivalent DOL value.
The flexibility provided by this design can also be used to start the motor with a lower voltage.
Star-delta connections give a low starting current of only about one third of that found with direct-on-line starting.
This method can only be used with induction motors that are delta connected to the supply voltage.However, when switching over from star to delta (at t = 1 .7 s), the current pulse reaches the same level as the locked-rotor current seen with direct-on-line starting.The current pulse can even get higher, because the motor during the switching period is un-powered which means it reduces speed before the full voltage (delta voltage) are supplied.Direct-on-line starting (DOL)// As the name suggests, direct-on-line starting means that the motor is started by connecting it directly to the supply at rated voltage.Direct-on-line starting, (DOL), is suitable for stable supplies and mechanically stiff and well-dimensioned shaft systems – and pumps qualify as examples of such systems.