Star-Delta Starting



      The induction motor must be built to run normally with a mesh-connected stator winding. At starting, the winding is connected temporarily in star.  The phase voltage is thus reduced to 1/√3 = 0.58 of normal, and the motor behaves as if the auto-transformer were employed with a ratio x = 0.58. The starting current per phase is  Is = 0.58Ix ,  the line current is  (0.58)2Ix =0.33Ix ,  the starting torque is one-third of short-circuit value.

        The method is cheap and effective, so long as the starting torque is not required to exceed about 50 percent of full load torque. It can therefore be used for machine-tools, pumps, motor generators etc. The method is unsuitable for motors at voltages exceeding 3000 V. because of the excessive number of stator turns needed for delta running.  Where induction motors are required to run for considerable periods on small loads, a star-delta switch permits the machine to be star-connected during these periods, with reduction of magnetiz­ing current and increase in efficiency.