Oscillations on a Soft Sart
Oscillations on a Soft Sart
(OP)
We have a 180KW soft start set at 3 x FLC supplied from a 750KVA transfomer, with standing load. We have found that just after startimg we are getting ringing, this can be so violent that the instanteous OC on the supply breaker trips.
One in four starts results in a trip. We have mitigated this by a faster start and a greater voltage start 'kick' All this has done is get through the ringing quicker but not eliminated the cause. Any ideas?
One in four starts results in a trip. We have mitigated this by a faster start and a greater voltage start 'kick' All this has done is get through the ringing quicker but not eliminated the cause. Any ideas?






RE: Oscillations on a Soft Sart
RE: Oscillations on a Soft Sart
RE: Oscillations on a Soft Sart
RE: Oscillations on a Soft Sart
(those can have torsional resonance excited by twice slip torque pulsation during startup)
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Oscillations on a Soft Sart
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Oscillations on a Soft Sart
RE: Oscillations on a Soft Sart
So some questions:
1 - sync motor?
2 - What type of load?
3 - Connected to the motor how? (belt, gearbox, rigid coupling, flex coupling
It has the smell of torsional resonance to me. But I'm sure it could be something entirely different buried within the controls also.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Oscillations on a Soft Sart
You just made sure you don't apply the field until the bypass contactor is closed.
RE: Oscillations on a Soft Sart
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Oscillations on a Soft Sart
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Oscillations on a Soft Sart
1 Iniduction motor
2 Pump
3 Direct Coupled
The soft starts are ABB
RE: Oscillations on a Soft Sart
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Oscillations on a Soft Sart
If it is the PSR, the design uses what s called "2-phase" SCR control, wherein only 2 of the 3 phases have SCRs, the third is a piece of busbar. This method works fine in a lot of applications, especially smaller motors on loads that start easily such as pumps. One drawback however is that the simpler versions like the PSR are not good at controlling low firing angles (there are more sophisticated versions available that are better at it). On these simplistic designs, the initial torque setting should never be lower than 30%. Lower settings mean that the negative torque pulsations caused by the imbalance in applied voltage will be a greater percentage of the total torque and may show up as cogging, just like what you are seeing.
Try turning up the initial torque setting (may be called initial voltage). For a pump, lower initial torque settings are somewhat meaningless anyway because the pump will not do any useful work anyway. (Assuming it is a centrifugal pump of course).
If it is another product series from ABB, you may have a bad firing board or a failing SCR gate.
RE: Oscillations on a Soft Sart
Monitoring with a FLUKE recording oscillograph
jraef
The ringing is on both motors, each connected to its own transformer
RE: Oscillations on a Soft Sart