On Friday 01 January 2016 12:29:16 Bertho Stultiens wrote: > On 01/01/2016 05:47 PM, Dave Cole wrote: > > That's done all of the time. In fact it is part of the NEC > > (National Electric Code) that is followed (for the most part) in the > > US. Pretty much every house in the US is wired like that. (I'm not > > making this stuff up. :-) ) > > I agree with the connection at power entry in the house. That is quite > a different story. That is actually the same here in EU (most > countries). > > I think that we should separate two things: > - house installation > - machine wiring > > House installation is pretty much standardized with a lot of rules and > reasons. Specifically to ensure referencing and protection. > > Machine wiring is different in that you can have scenarios where > references are moved, especially in a 2-phase system where you are not > using the neutral, which is the scenario we have here. > > > Now, if you don't want to do that inside the panel for some reason > > (which I might have missed), that may be a different story. > > I'm just saying that is standard practice in the US and on the > > European machines I have worked on as well. > > The reason for /not/ connecting the ground on a secondary winding is > to prevent a capacitively coupled ground path. Especially if you > cannot guarantee a 100% balanced primary-to-secondary side wrt. ground > (*). > > What happens is that there will be a current in the ground connection > which causes an imbalanced current on the primary side (remember: > primary is 2-phase circuit without neutral reference). This is a > differential current discrepancy on the primary side. If you have an > RCD (which you should), then it can or will trip due to the current > imbalance. > > > (*) and from the example measurements presented by JT, we can be > assured that there is an imbalance, as seen from the phase shift > causing a difference in voltage measured wrt. ground on the secondary. > > > There are a lot of good reasons to tie one leg the transformer to > > ground besides to establish the safety ground and neutral as is > > common on the US. Intermittent faults to ground, with an ungrounded > > system, can cause the secondary of the transformer to fly way above > > absolute ground causing connected devices, or the transformer to > > suffer from insulation breakdowns. That's the extreme, but it can > > happen. > > Actually, the secondary should normally be floating. Most transformer > setups are to ensure galvanic separation and that means you may never > connect neutral to the secondary side. > > Connecting ground on a secondary is a different scenario, where you > most often see the 0V (DC) potential connected to protective ground. > This is often done in the PC's power supply. The 0V (DC) to protective > ground connection prevents the scenario to which you refer to. > > The 0V (DC) to protective ground connection is repeated in many > devices, which have both power and signal connections. This gives rise > to ground-loops, which are to be prevented at all times and it is the > cause of the whole thread. > > > One of way too many references on the web. > > http://ecmweb.com/bonding-amp-grounding/basics-bonding-and-grounding > >-transformers > > It looks like the link is about a 3-phase system, where you would > connect the center point in a star-configured secondary to ground. > That is a different scenario.
So is this Bertho, this is a single phase system delivering 240+ volts AC to the two ends of a single core transformers primary winding. While I personally am not 100% happy with a no neutral hook up, it is legal as long as it never leaves the confines of the load machine. The machine itself is grounded well. The fact that the core/frame of the control transformer isn't grounded probably enhances the capacitative coupling, which might amount to something in the 100 pF neighborhood. If the frame/core were also grounded, I would expect some of that un-balance to go away and the total, added up, would be less than the delivered voltage primarily because the measureing meter is going to absorb some of it when hooked up to measure it, causing its own imbalance. In short, this is not a 2 phase system where the phase angles are +- 90 degrees, so for practical purposes the 2 phase math is irrelevant. The reason I wrote 240+ is that 127+127=254, which if Johns meter is a decent one, he should be reading within say 2 volts of that from L1 to L2. With no neutral, only the secondary of that control transformer is to be considered the src of a 127 volt AC feed, and either end of it can and should be tied to the well grounded machine frame, but not both of course. Having it grounded gives a direct path to ground for the noises the VFD might induce from close proximity. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
