AC Generator Problem

hi all,

i have problem with three phase ac generator 200kva/380v/50HZ. exciter voltage is 17v/4amps, these informations are from the name plate. i checked the resistance of the exciter stater and it was 4.2 ohms.

the problem is: i fitted new 8 amps AVR and run the generator, i got output but is not steady, it goes from 330v to 440v, and when i turn the stability adjust rheostat it wont effect, also when i turn the volt adjust rheostat also nothing happened.

i thought the problem is the AVR so i replaced it with another new AVR and it still exactly the same problem.

i replaced the AVR with another new 5 amps AVR, the same manufacturer, and it start getting right. i got an output 380v and its steady now.

my question is: why it does not work right with higher amps AVR while it works fine with smaller one? for my knowledge, the higher amps one is an AVR as well but its different from the small one that it can carry more amps! so why this happened.

your help is highly appreciated