overfluxing in generator

Home work question? Well! You see any magnetic material has got a certain magnetic flux density 'B' and this is proportional to the ratio "V/f". (Remember, the EMF equation of a generator?). When the machine's operating voltage and frequency are known, its magnetic core's maximum flux density can be fixed previously. Normally the maximum flux density for magnetic cores is about 1.2 Tesla. When during service, the V/f ratio exceeds the design ratio (may be due to high voltage or low frequency), this maximum flux density is exceeded and this is called "over-fluxing". Sustained operation under such conditions would cause the core to saturate.

equivalent of over speeding.

No! Overspeeding happens because of over frequency. Over-fluxing happens because of under-frequency.