Electrical Engineering ⇒ Topic : Star-Delta Starter

Star-Delta Starter. Star-delta switching method is based upon the principle that with three windings connected in star, voltage across each 1/√3 winding is i. e., 57.7% of the full line to line voltage whereas the same windings connected in delta will have full-line-to-line voltage across each.

   The star-delta starter connects the three stator windings in star across the rated supply voltage at the starting instant. After the motor attains speed the same windings through a change over switch are re-connected in delta across the same supply voltage.

    The basic connection diagram of a star-delta starter is shown in Fig.(a). An actual starter incorporates under-voltage and over-load coils. The starter is also provided with a mechanical inter-locking device to prevent the handle from being put in the 'Run' position first. Such starters are employed for starting 3-phase squirrel-cage induction motors of rating between 4 kW and 15 kW.

   When star-connected, the applied voltage over each motor phase is reduced by a factor 1/√3 and hence the torque becomes 1/3 of that which would have been developed if motor were directly connected in delta. The line current is reduced to 1/3. Hence, during starting period when motor is star-connected, it takes 1/3rd as much starting current and develops 1/3rd as much torque as would have been developed were if directly connected in delta.

                                                                       Fig. (a) Star-delta starter

Star-delta starter

Figure (a) shows the schematic diagram of a star-delta starter. In position 1, a, b, c are short circuited. In position 2, a is connected to B, b is connected to C, and c is connected to A to form delta connection.

Let V₂ be the line voltage. The per phase motor starting current I_st.y with stator winding in star is given by

If stator windings are delta connected, the per phase motor starting current I_st.d with direct switching is given by

Star-delta starter is widely used as it is cheaper than autotransformer.

                                   figure (a)