A 3 phase electric motor has 3 coils. These coils can be configured or connected either end to end (as a triangle) in Delta configuration or with 3 ends together as a Star.
Why use 2 types of coil windings?
If a stationary electric motor were started directly in Delta mode the current drawn would be very high. To handle this high current (up to 4 times the normal running current) we’d often need to use very thick 3-phase power service cables and heavier duty switchgear. All this would prove to be an expensive and inefficient method of getting a motor up to normal running speed.
A more efficient starting method is to use a clever device that can flip between the two types of motor coil configurations once the motor is up to running speed. This device is called a Star Delta Motor Starter.
You can find diagrams and more info for Electric Motors Star and Delta on- http://www.electriciansblog.co.uk/2012/08/electric-motor-star-and-delta-wiring-and-link-connection-diagram/
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