What is a star delta starter connection?

A device called a “star-delta motor starter” is used to make three-phase motors start and run efficiently.

During motor start-up, 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 direct online, 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.)

The 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.