Firstly, due to under 1st critical speed, I input trial weight at 0o of xe8237y which has highest vibration, as calculation of 10% rotor weight, the trial weight would be 5g but i put 8g to see response of rotor. When started, the vibration and phase almost the same, it strange to me ( this is not first time i do field balancing but first time i see).
so, i try to rotate weight 45o as rotation to see what rotor response but changing not much. Reconsider to cause of high vibration but it looks like the same unbalance. So, i decide to put 15g on 180o and it works with reducing vibration value. Final calculation with split weight at 180o and 210o with total 39g. And vibration is fine now (3 probes under 25um, 1 probe is 43um) but i think we can not do better and accepted this.
this is not a first time i see that the trial weight put at coupling hub with phase different to rule under 1st critical speed. Last year, i do for motor of boiler feed water 1.8MW, the situation is the same.
i know that it could be effect of rotor shape mode but under 1st critical, is this usually happened? Can you share your experience on this?
i do not know how to analyse the rotor model. If able, i hope to hear your sharing.