Open Transition Motor Starter

[davva]

I took these waveform measurements of an open transition autotranformer, 65% tap, motor starter (1700kW rating).

The generation system is a ships network with two 2.4MW 440V generators.

The two waveforms are the start and the run circuit breakers closing. Run closes 40secs after start and the motor is fully up to speed after 16 secs and remains unloaded.

There is a very severe voltage drop when the run contactor closes.

I am aware that open transition motor starters shouldn't be used in weak generation networks. But this system has been in place for 7 years and they are only noticing problems recently with some newly installed downstream equipment falling over as it can't handle the drop.

I made severly repeat starts and the voltage drop on RUN changeover was much less (410V).

I made the assumption that the inherent starter design is at fault with the system that it is installed in. However, I don't understand what is happening in the circled area where the voltage flattens out then drops further. Could this be an intermittent AVR problem or the poor contacts/contact bounce of the circuit breaker?

Appreciate any help/comments.

 

[electricpete]

That's an interesting trace and I don't know the answer.

However when I set about to try to figure it out, I find the motor equivalent circuit is embedded in my brain. The induction motor equivalent circuit is actually valid for steady state analysis but most definitely not valid for transient analysis. When you actually look at transient behavior using a more detailed transient model (such as Krauss), there are many things that happen that we would never have guessed from our equivalent-circuit-based intuition. For example an induction motor can overshoot sync speed upon startup and a few other interesting quirks discussed here:

http://www.eng-tips.com/viewthread.cfm?qid=248895

Maybe (?) this is another one of those things that is a result of transient behavior that is not at all obvious from the equivalent circuit.

If I have energy and time this weekend I may try to do a computer simulation of wye delta start (but don't hold your breath). If any other modelers out there want to try, I'd be interested to see your results.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[waross]

It sounds as if you have had the issue from day one, but have installed new equipment that is more sensitive to voltage dips than the existing equipment.
You have not indicated the nature of the new equipment. It may be possible to feed the new equipment with a constant voltage transformer or a UPS.
Star delta start is not much used anymore because of the transient current issues.
Look at this thread.
Wye-delta start – peak current and instantaneous setting

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davva]

New equipment was detector system for boiler that detected loss of flame. The low input tolerance for detector was 190V and it was supplied by an offline UPS. On the second stage of the starter the 220V into the offline UPS dropped to 182V which was above the default switchover voltage of UPS to batteries but below the detector low input voltage. Seem fairly confident that have found the problem (which I too think has probably always existed). I just couldn't explain the waveform and the vessel crew are sure that they've never seen the lights dim so much in the past when the motor starts so I wanted to check that it couldn't be something else wrong.
Just to confirm it is open transition autotransformer starter not star/ delta. Also motor was fully up to speed by the time of change to full voltage.

Thks for help so far

 

[waross]

The open transition time doesn't have to change very much to give an out of sync closure. Try increasing the transition timer a tiny amount. check the control circuit to see if someone has defeated the lockout contacts that prevent starting and running contact overlap. Look for broken, bent or worn parts in the contactors and the operating mechanism.
It's embarrassing to state definitively that it is an existing problem and then have someone replace a broken spring and cure the issue.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davva]

Waross.

Thanks for advise. I did repeat a number of starts of the motor 8 in total in the time frame i had available on only 1 start produced the severe voltage drop. Which made me think that it is just dependent on the point of wave switching when the run contactor closes.
I inspected the circuit breakers (I mentioned contactors above but it is actually two ABB Emax breakers). They looked like they could do with a bit of TLC (photo attached of right hand contacts of the run breaker). This made me think that perhaps poor contacts could be the fault. Or is the state of the contacts a result of the open transition starter itself?

 

[rbulsara]

There may be more to the story. Get some one experienced on the site, an experienced engineer. I do tend the agree with Bill that the voltage dip issue probably was always there, just found out by more sensitive equipment now. Also wear and tear takes its toll.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Can you tell us a little about the control scheme? Is trhis manual or automatic? Is there any intentional delay in the transition?

I suspect that the contact condition may be a result rather than a cause.
Look up some closed transition diagrams. You may want to consider changing to closed transition. Even if you don't change, when the customer sees the cost to convert, they may decide that the problem is not that bad and they can continue to live with it.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davva]

Yes I had already assessed the space in the compartment with view to change to a closed transition circuit using another circuit breaker.

Attached is the current circuit (which as you can see is very simple). There is no intentional delay between start and run contactor (only the inherent delay of the aux contacts of k652)

 

[waross]

I'm sorry. My laptop quit and I am using an old clunker that won't open .docx files.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davva]

attached again. thanks