The Principle and Working of a sStar –Delta Starter for a 3-phase Induction Motor

Star-delta starter


· 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.

Fuzzy Logic Speed Controllers Using FPGA Technique: For Three-Phase Induction Motor Drives
•     This type of starters is used to start the motors from the range of 7.5 hp to 20 hp squirrel cage induction motors.


  1. thanks for this
    useful message

  2. ohh nice dear frnd...keep watching this blog for more contents...

  3. can u put a clearer image please


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