Michael had a mild doubt:

"So either the unit will remain off or may start a relay chatter as it oscillates OFF and ON."

Let me clarify - it chatters like hell if you try to switch of the primary !!

Natin, the only simple way to implement this is with a window comparator built around the likes of LM311. You just feed a rectified and smoothed DC from your secondary and compare this with a High and Low setting that are presettable with good quality pots ( which are powered by regulated DC from another secondary winding that normally feeds your power circuits ) To avoid relay chatter you need to have good hysterisis ( +ve feedback) and if you want to save your motor coils have some kind of time delay ( 1 minute or so ) before cutting in after a fault has cleared.

And finally do not try to save on secondaries by taking the sample voltage from the section feeding the power circuits - hope you can figure out why.

I have implemented very decent Hi-Lo cutouts with the above basic guidelines. ( And in one case implemented a scheme whereby if the fault reset happens 3 (or n) times in succession, then the circuit raises an alarm and waits for manual intervention.)

And if you want to avoid comaparators, drop the project or quite simply provide a power line stablizer ;-)

Raghu