Re: Explain This! More information included



The transformer as shown is not an autotransformer but appears to be ( hard to tell from the diagram except that there is primary/secondary isolation and no neutral on the primary) a delta star 2-winding transformer. Please clarify this. Autotransformers do not provide isolation but have part of the winding common to both sides. Please check the wiring diagram. I have a suspicion with regard to what is happening -if it is a true autotransformer- but the diagram doesn't provide such information.
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Don Kelly snipped-for-privacy@shawcross.ca
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hard
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That's due only to my knoweledge of how a general purpose transformer looks in a schematic. This is a true autotransformer, with some portion of the output tapped from the primary; as opposed to pure induction. Though I didn't sketch them, the internal wiring diagram was availible at another x-former, (these are about half the size of a standard 70 KVA model). There was no intentional bonding of the neutral at the connections. However; a path was created through the conduit due to the wiring mistake.
So Don, Does your suspicion relate at all to the theory I hinted at? the one sparked by the electrics of a classic car?
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----------------- It may not. After disconnection, the motor acts as a generator for a short period. This could result in a few things- Triplen harmonics will return to the motor by any path available including some that one may not suspect-if this path included a ground fault relay (depending on where the CT's supplying the relay are-not a situation that occurs in automotive wiring- then the relay may have ppicked up on these. I have also seen ground fault tripping due inrush currents in transformers which caused saturation in current transformers resulting in a net DC component in the ground relay. Again only an AC related condition. There could be another ground loop depending on the motor internal connection and the impedance to ground of this motor so it could be a backfeed from the motor in which some of the current was through the relay- again this depends on the relay connections. This is a situation which would resemble anything that happens in a DC automotive system- oddball ground loops.
To resolve this would require a full diagram of the actual wiring including ground/neutral connections, intentional or otherwise, as well as the protective relay wiring- neutral current measured directly or by summing the phase currents (and where measured with respect to the relay that tripped). Do you have that data?
Alternatively, I could be talking through my hat.
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Don Kelly snipped-for-privacy@shawcross.ca
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including.....
More complete diagrams/data may not be required after you read what I believe may have occurred. A couple of points I'd like to mention first: *Except for emergencies; operators & set-ups are well disciplined to remove loads first , a written procedure is to be followed for a normal shut down. *The fact that I was told this had occurred on other machines when they were initially installed by an outside contractor: it is even more certain that the bus-duct tripped with everything switched off, except for the disconnect feeding the autotransformer. Which caused the trip when it was switched off. *So at the moment before disconnect was switched off we had: -the energized 3 phase autotransformer feeding an open 3 pole switch. -a transformer derived neutral miss wired to the machine ground lug. -an autotransformers' neutral is not normally bonded to case. -conduit completed machine ground path back to electrical system ground. The relation to a classic car electrical system I had in mind was the ignition circuit. The old points & coil; when the points "opened", the energy of the coils' collapsing field was directed to the spark plug. Since we had a path, (incorrectly), from the autotransformers' neutral back to the ground at ground-fault breaker; I believe the collapsing field induced a voltage that was detected by it. The disconnect would have been my points. What do you think? Anyone else buy into this theory?
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---------- OK- no motor load? This is somewhat different than what the diagram indicated. do you see my reason for wanting a diagram of what was there rather than a rather generic diagram. Was the motor connected? -----------

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In this scenario, while the unloaded autotransformer was the only load, any
neutral current through of this transformer would be through the accidental
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