Set in motion and Turn on

Set in motion verb - Get going; give impetus to.

Turn on and set in motion are semantically related in begin topic. In some cases you can use "Turn on" instead a verb phrase "Set in motion", when it comes to topics like start a function.

Turn on verb - To take sudden, violent action against.

Set in motion and turn on are semantically related in start the operation of topic. You can use "Set in motion" instead a verb "Turn on".