Oscillations on a Soft Sart

[Rodmcm]

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?

 

[dpc]

Capacitors anywhere nearby?

 

[Rodmcm]

No

 

[jraef]

Not all soft starters are created equal. Some have a lot of trouble maintaining control at low firing angles, as you appear to show here. Some need to "test and correct" in the early moments of a ramp profile before they get settled into a control method, i.e. "torque ramping". What brand is it?

 

[electricpete]

It's not a syncronous motor by any chance, is it?
(those can have torsional resonance excited by twice slip torque pulsation during startup)

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[electricpete]

It was probably a silly comment. I'd doubt anyone would apply a soft start to a sync motor.

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[jraef]

On the contrary, it's done all the time. Synch motors start as induction motors, so all the same issues apply.

 

[electricpete]

I was thinking about torque considerations and the possibility of pole slip. But I'll take your word for it.

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.

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[jraef]

Side bar...

I was thinking about torque considerations and the possibility of pole slip.

You just made sure you don't apply the field until the bypass contactor is closed.

 

[electricpete]

I came back to correct myself but you beat me to it. You're right of course. The sync motor usually starts by the armotisseur (sp?) windings.

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[electricpete]

What threw me a little bit (besides just not paying attention) was thinking about torque pulsations that occur during starting of sync motor. They do in fact occur for salient pole rotors (generally slow speed machines). But it has nothing to do with the rotor fields. The reason is reluctance effects. Each time the stator field passes the rotor poles, the torque varies. The frequency of that variation is twice slip speed. That frequency starts high and decreases to zero during the start. Any torsional resonance in between will be excited.

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[Rodmcm]

To clarify

1 Iniduction motor
2 Pump
3 Direct Coupled

The soft starts are ABB

 

[itsmoked]

How are you monitoring this ringing?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Which ABB model of soft starter?

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.

 

[Rodmcm]

itsmoked

Monitoring with a FLUKE recording oscillograph

jraef

The ringing is on both motors, each connected to its own transformer

 

[jraef]

The ringing is on both motors, each connected to its own transformer

Huh. In re-reading your original post I realize your application is too large for the PSR Series starter, so it wouldn't be the 2-phase control issue anyway. Being that it would be highly unlikely to have 2 damaged soft starters, this is more likely to be an issue with whatever ABB does to sense and control the low end of the firing angle. You might want to consider trying a different brand.