Open Transition Motor Starter
Open Transition Motor Starter
(OP)
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.
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.






RE: Open Transition Motor Starter
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.
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RE: Open Transition Motor Starter
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
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"Why not the best?"
Jimmy Carter
RE: Open Transition Motor Starter
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
RE: Open Transition Motor Starter
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
RE: Open Transition Motor Starter
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?
RE: Open Transition Motor Starter
Rafiq Bulsara
http://www.srengineersct.com
RE: Open Transition Motor Starter
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
RE: Open Transition Motor Starter
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)
RE: Open Transition Motor Starter
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Open Transition Motor Starter