Why use Y-Δ connection?

Answer 1

The starting current of any heavy electric motor can be more than 4 times the normal load current it draws when it has gained speed and has reached its normal running output power and temperature.

So, if it were started simply when connected in DELTA, the starting current would be huge and - just to be able to start the motor, not to run it normally - would require:

large circuit breakers, big enough to allow the start-up surge current to pass without immediately shutting it off. (But the breakers would then be much too big to be able to protect the motor from over-current faults whilst it is running normally.)

very thick 3-phase power service cables. (But the cable would then be much bigger than is necessary whilst the motor is running normally.)

very large coils and contacts on the relays or contactors used to control the motor. (But they would then be much bigger than is necessary whilst the motor is running normally.)

One solution to this problem is to start the motor in STAR and then, when the motor has gained sufficient speed, change its connections to DELTA to allow the motor to run at its full speed and torque from then on. It's a bit like using the gears of an automobile.

Update: Electronic motor-control systems, which offer soft-starts in DELTA configuration, are now replacing the use of manual or semi-automatic star-delta starters.

Technical explanation

When the windings of a 3-phase motor are connected in STAR:

the voltage applied to each winding is reduced to only (1 /.'/'3) [1 divided by root three] of the voltage applied to the winding when it is connected directly across two incoming power service line phases in DELTA.

the current per winding is reduced to only (1 /.'/'3) [1 divided by root three] of the normal running current taken when it is connected in DELTA.

so, because of the Power Law V [in volts] x I [in amps] = P [in watts],

the total output power when the motor is connected in STAR is:

PS = [VL x (1/.'/'3)] x [ID x (1/.'/'3 )] = PD x (1/3) [one third of the power in DELTA]

where:

VL is the line-to-line voltage of the incoming 3-phase power service

ID is the line current drawn in DELTA

PS is the total power the motor can produce when running in STAR

PD is the total power it can produce when running in DELTA.

a further disadvantage when the motor is connected in STAR is that the total output torque is only 1/3 of the total torque it can produce when running in DELTA.