Why you use the star delta connection?

Answer 1

Advantage:

Star Connection:

• In a star connection, phase voltage Vph = VL/ (3)1/2. Since the induced emf in the primary winding of an alternator is directly proportional to the number of turns, a star connected alternator will require less number of turns than a delta connected alternator for the same voltage.
• For the same line voltage, a star connected alternator requires less insulation than a delta connected alternator. Due to the above reasons three phase alternators are generally star connected.
• In star connection, we get 3-phase and 4-wire system. This permits the use of two voltages (phase voltages as well as line voltages). Single phase loads can be connected between any one lie and neutral wire while the 3-phase loads can be put across the three lines. Such a flexibility is not available in delta connection
• In star connection, the neutral point can be earthed. Such a measure offers many advantages. For example, in case of line to earth fault (L-G fault), the insulators have to bear 1/31/2 (57.7%) times the line voltage. Earthing of neutral also permits the use of protective devices (relays) to protect the system in the case of ground faults

Answer 2

Motors pull a lot of starting current, sometimes as much as four times their running current. This can damage components and/or make the motor overheat.

Star-delta is a technique that can reduce this problem. The motor is started in star configuration, where each winding gets only 0.577 (1/1.732) of it normal running voltage. This cuts the power by 2/3. (0.577 squared is 0.333) Once the motor is running, the contactor switches over to delta configuration, giving the windings full voltage and power.

Even using this technique, big motors generate a lot of startup heat. One particular circulating water pump that I recall, which used a megawatt to run, could only be started twice in a 30 minute period of time without being given time to fully cool.