What happens if the AC frequency generated by local power companies drops below 50-60 Hz?

If the frequency of the AC mains decreases, then all of the electric clocks in

your house will run slow and lose time. For example, if the 60 Hz utility drops

to 59.9 Hz and stays there, then your clocks will be 2minutes and 24seconds

behind the correct time after one day. This is a fairly serious amount, and a

tight restriction on the required accuracy. Several years ago, when I was

working briefly in the dispatch and control center of the electric utility in a

major US city, they showed me the frequency monitor, and explained that

if the frequency goes low, for example, for some period of time, then they

were responsible to then run it high, for just the right length of time so that

all the clocks in the city would return to the correct time. I'm quite sure that

all of that is totally obsolete now, since the major utilities now buy and sell

and share each others' power, and in order to do that, they all have to be

exactly at the same frequency and phase.

If the AC mains were to go down significantly in frequency, then all of

the motors in your house ... like the ones in your furnace, air conditioner,

refrigerator, freezer, ceiling fans, etc. ... would operate weakly, overheat,

and possibly burn out if things stayed like that for too long.