· This method is used in the case of motors which are built to run normally with a delta-connected stator winding.
• By means of a triple- pole- double- throw switch, the winding is connected in star at starting and delta connected when running as shown in figure.
• When start connected, the voltage applied per phase is reduced by 3 times.
• The current taken from the line is 58 percent of that at normal voltage.
• But the starting torque is reduced to 0.582 or 33% of that at normal voltage.
Effects of low frequencies on three phase induction motor performance operating in close proximity to rated speed.(Report): An article from: American Journal of Applied Sciences
• When the motor reaches sufficient speed the switch is thrown over thereby connecting the motor
in delta across the line.
• It is clear that star-delta switch is equivalent to an auto-transformer of ratio 1/3 or 58% approx.
• This method is cheap and effectively provides the starting torque required in not more than 1.5 times the full load torque.
• When the operating handle is put downwards the terminals A1, B1, C1 are shorted. Now the stator windings are connected in star and energized by three-phase supply.
• When the motor has picked up the required speed the operating handle is put upwards and the stator windings are connected in delta across the three-phase supply.
• In star-delta starter also the no-volt release and over load release are provided to protect the motor against the failure of supply and overload.
• This type of starters is used to start the motors from the range of 7.5 hp to 20 hp squirrel cage induction motors.