On Mon, 7 Aug 2017 13:56:27 -0700 (PDT), misterroy wrote:

Am I correct in thinking that large wind turbines do not produce power
when the wind speed is below 12mph?

Varies.

Why on some days do they stop altogether and others they keep turning,
even though there is hardly any wind at all?

Those blades are big, heavy and flexable. They motor them round to
prevent damage to the bearing(s) and to not have the blades take a
set. If you see a turbne going slow watch a blade and how much it
flexes as it goes past 12 o'clock.

--
Cheers
Dave.