Starting method for three-phase squirrel-cage motors.
The star-delta starting method is used for motors with lower and medium power ratings, if direct switching on is not possible because of the high starting current; in general from a rated power of 5 kW onwards.
For star-delta starting, the stator winding with an insulation matching the phase-to-phase voltage is first of all operated in star connection. In this way, each phase gets 1/√3 times the value of its rated voltage. Starting current and starting torque are thus reduced to a third of their normal value.
The delta connection must only be established after the machine start-up. Otherwise, the intended effect (reduction of the starting current) will get lost. Therefore, the torque generated in star connection must be high enough to accelerate the driven machine approximately up to the rated speed. For this reason, motors with double squirrel-cage rotor are particularly suited for starting motors via wye-delta switching.
Manual or automatic switching is possible.
