On Sunday, 1 March 2020 14:12:27 UTC, Roger Hayter wrote:
Jethro_uk wrote:

On Sun, 01 Mar 2020 13:37:47 +0000, AnthonyL wrote:

Are there any conditions when the phase needs to be known? If so how is
it detected?

If you connect a 3-phase motor up wrong, it won't work. Does that count ?

From a position of no theoretical knowledge and little practical
experience, I thought it would go backwards. Am I wrong?




This has the suggestion of leading to a bridges of Koenigsberg puzzle ...
what's the minimum number of wires you'd need to swap to get it
working



It depends.
A 3ph motor can be connected in "star" or " delta/mesh" (Mesh=USA)
Reversing any two phases reverse the motor.

However if you swop the connections on one of the windings, (ie reverse the Live and Neutral connections,) it won't run.