the difference between underspeed and under frequency ?
 

they seem to be the same :S i mean isnt frequency directly related to the speed of the CSD ?
so if underspeed occurs under frequency would occure as a consequence for it .
so why there is '' underspeed '' protection ?
makes no sense

The axial gear differential/hydraulic speed control section in the IDG converts a varying engine input speed of 4,650 - 9,200 rpm, to a constant output speed of 12,000 ±150 rpm. The constant output speed allows the generator section of the IDG to produce ac power with a frequency of 400 ±5 Hz.

Is there an underspeed protection or is the underspeed trip the protection ? Think about it.:ok:

after researching and asking people i think i understood it now, so correct me if im wrong.
underspeed means if we're shutting off the engine the frequency of the generator frequency goes down and that would trip all the bus tie breakers in a parrallel system due to under frequency protection activation so a speed switch is situated in the CSD to monitor the oil pressure if it goes low due to underspeed ( engine shut off ) it transmits a signal to GCU which trips the generator line contactor of that particular generator only without tripping all the bus tue contactors and all the other generators .
please someone correct me if im wrong.

Plus or Minus 5 (Volts/Hz) is the definition of Quality Power - along with correct phase rotation.

yea im talking about the aircrafts that run the generators in parralel eg boeing 727 .

An underspeed trip is due to mechanical failure if the CSD or input shaft (or normal engine SD) the result is to only open the GCB. The UF trip will, in a parallel bus system, first trip the Bus tie breaker then GB and Field Relay.
During normal engine SD the Underspeed trips prevents unnecessary loss of bus power and prevents storing nuisance faults.