What is the main difference between star and delta connection for 3-phase machines?

Answer 1

The difference is where they are grounded in the supply transformer. If you're the same guy that continues to ask questions regarding delta and star or "Y" connections, then you probably already know the answer.

High voltage transmission lines supply power to step down Transformers to supply buildings. The power will likely go through a switchgear to control overcurrent and distribute the electricity to a number of transformers. Those transformers will typically step down the voltage through a "Delta" (primary) and "Y" secondary.

The Delta primary is grounded at a mid-point between two of the three phases, this creates the so called high leg out of the third phase. The Y secondary is grounded at the center of the Y.

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Answer 2

The main difference between star and delta is that in star, the phase voltage is root 3 multiplied by the line voltage making the speed of the machine to run at low speed while in delta, the phase voltage is equal to the line voltage making the machine to run at high speed.