Speed fluctuation in VSD driven pump

[nesamani05]

we have 3 LPG pumps(2+1) operated by separate VSD Motors(690 KW).
Nowadays we have the below problem.For ex.
Pump A & B runs at speed of 3000 rpm, then if small voltage dip occurs, then pump A speed goes to 100 rpm, then automatically comes to 3000 rpm. This cycle occurs within 50 secs. Pump B runs in same speed without any problem.

Why it is happening..? how to rectify the problem...?

VSD is by ANSALDO.

 

[ozmosis]

it depends what you mean by 'small' when it comes to the voltage dip. If the DC link drops below a certain threshold set within the drive, it will cut out.
If the drive has auto-restart then when the volts come back again, it will restart.
Check the settings between drive A and B. Maybe drive B has a function called 'kinetic buffering' or whatever they might call it. This will maintain control voltage longer than normal and ensure the drive does not cut out if sustaining a dip > the 'normal' 4-5 cycles.

 

[DickDV]

Are these motors running independent pumps or are the two pumps piped in parallel? If in parallel, are the VFD's running independently against independent setpoints or are the two VFD's linked together in some way? If linked, please describe how.