When a motor is initially connected in the star delta starting, its phase voltage is 58% of the line voltage. Motor current is decreased as a result of the reduced voltage.
As a result of the reduced current, the motor starts more reliably and without any mechanical or electrical strain. Star Delta Starter is hence recommended.
When first turned on, the induction motor can draw up to six or seven times their rated current. The rotor impedance (XL) is extremely inductive when it first starts.
XL=2 x pi x sfs x L
as the stator supply frequency and the frequency of rotor current are equal.
The motor uses a lot of electricity and produces very little torque when starting in this condition.
A lower supply voltage must be used to lower the voltage induced in rotor circuit in order to limit an induction motor’s beginning current.
The supply voltage will drop as the motor current decreases.
Small rating motors may be started on DOL-Direct on Line, though when the motor rating increases, a star delta starter must be used to start the motor.
The stator winding initially connects in the star of a star delta starter to lower the stator phase voltage by 42% of the supply voltage.
The starting current will decrease since the voltage across winding will be 58% of the supply voltage. The phase voltage across stator winding is 0.58*440=255 volts if the line voltage is 440 volts.
The timer activates the delta contactor after the motor has run in star mode for a predetermined amount of time, and the motor winding is now linked in delta.
When coupled in a delta configuration, the stator winding receives the full supply voltage.
The high ranking If the fluid coupling is utilized or the motor is powered by VF Drive, the squirrel cage motor may be started on DOL.
The motor starts in soft starting mode with a lower voltage and frequency due to the VF drive.