Run submerged waterfall pump without water, what will happen

[herbolaryo]

In the manual of the submerged waterfall pump...
It says, don't run motor without water.
If the motor run without water, what will happen to the pump? :?:

[sparx]

Hi, depends how long! a few secs. to try out may be OK but pump relies on water to lubricate impellor/casing so if run for long dry then either siezes up or wears badly so poor performance,
regards SPARX

[herbolaryo]

The pump is located downstream the falls with a float switch to top off the water when it gets low due to normal evaporation of water. There is a pond at the lowest point of the falls where the pump is submerged.

However, there are unexpected electricity blackout which empties the whole pond. Since the float switch can keep up , during the time when electricity just came back, it sucks up air.

The pump sucking air only happen during occassional electricity blackout.

The humming noise occurred after that time eventhough I cleaned the filters and the motor "blade" of debri.

So what can be the cause of the hum noise in the pump and how to fix that?

[sparx]

Hi maybe humming cause it can't sing??? sorry!
motor humming usually because it cannot spin freely, it may be still jammed too tight,