Transformers & Capacitive Coupling

[BigJohn1]

I've heard a lot mentioned lately concerning capacitive coupling as it relates to power transformers. Needless to say, I don't understand how it comes into play. I was under the impression that transformers were inductively coupled, that the change in current and corrosponding change in magnetic feild in the pri. induces currents in the sec.

When things are capacitively coupled, does the change in the intensity of the elctric feild in one object induced a changing electric feild in nearby objects?

How much does capacitive coupling actually relate to transformer design/function?

Also, I seem to recall reading something about transformers adversly affecting nearby equipment through capacitive coupling. If a transformer were encased in metal, and the frame was properly grounded, wouldn't this do a great deal to eliminate any E-feilds beyond the transformer?

Thanks a bunch.
-John

 

[mpparent]

I think what you're referring to is a ungrounded power system, in which the transformer is capacitively coupled to ground.

Mike

 

[ScottyUK]

Only other thing I can think of is the inter-winding capacitance, which at 50/60Hz should be of academic interest as far as normal transformer operation is concerned.

Can't imagine how the windings within an earthed transformer can capacitively couple to anything outside of the tank. Are you sure the coupling wasn't inductive?

 

[aishu]

Refer IEC70076-3capacitively coupled and inductively coupled windings and associated transferred voltage phenomena for tertiary windings

 

[busbar]

Anecdotal comment… "Topaz" used to make 480-208Y/120V “isolation” transformers that, IIRC, were labeled with a primary-to-secondary capacitance of a few picofarads. The idea was that they would pass 60Hz efficiently, but significantly attenuate higher frequencies and fast-rise-time transients. They also contained a floating intertwinding shield that was supposed to be connected to some mythical “quiet” ground.

From a sane electrical-safety standpoint, it was virtually impossible to meet NEC 450 & 250 while following the manufacturer’s installation instructions. The US military was a big customer for these things, and in a fair number of cases basic installation codes could be conveniently waived for a research/instrumentation/experimental setting.