Prune
2006-07-29 11:20:08 UTC
I see some transformers use a copper band parallel to the windings but
outside the core, in addition to the usual magnetic shielding around the
core sides or fully enclosing the transformer. My question is if it should
be grounded or not. Here's the information I found on the web:
"There is usually also a circumferential copper shield (or 'flux band')
around the entire transformer. It serves primarily as a radiation shield.
It is often left floating in low cost designs, though it may be connected
to the Secondary ground if desired. If so connected, safety issues may need
to be considered in regards to the requirement of reinforced insulation
between Primary and Secondary, and also the required Primary to Secondary
'creepages' (distance along the insulating surface) and 'clearances'
(shortest distance through air) as applicable. When the transformer uses an
air gap on its outer limbs, the fringing flux emanating from the gap causes
severe eddy current losses in the band. So this band is also usually only
2-4 mil thick. Note that the ends of this band can, and should be, soldered
together, because this is an outer shield, and can never constitute a
shorted turn for the transformer."
It's not clear for me from this. On the one hand, they say it's left
floating in low cost designs, implying it's better to be grounded; on the
other hand, wouldn't it send a bunch of noise into ground? And does
grounding at all affect the shielding ability? Also their mention about
connecting to secondary ground, why put more noise there; shouldn't it go
to chassis ground/earth instead (in case the two grounds are partially
decoupled such as by a ground lifting circuit as used for breaking ground
loops)?
outside the core, in addition to the usual magnetic shielding around the
core sides or fully enclosing the transformer. My question is if it should
be grounded or not. Here's the information I found on the web:
"There is usually also a circumferential copper shield (or 'flux band')
around the entire transformer. It serves primarily as a radiation shield.
It is often left floating in low cost designs, though it may be connected
to the Secondary ground if desired. If so connected, safety issues may need
to be considered in regards to the requirement of reinforced insulation
between Primary and Secondary, and also the required Primary to Secondary
'creepages' (distance along the insulating surface) and 'clearances'
(shortest distance through air) as applicable. When the transformer uses an
air gap on its outer limbs, the fringing flux emanating from the gap causes
severe eddy current losses in the band. So this band is also usually only
2-4 mil thick. Note that the ends of this band can, and should be, soldered
together, because this is an outer shield, and can never constitute a
shorted turn for the transformer."
It's not clear for me from this. On the one hand, they say it's left
floating in low cost designs, implying it's better to be grounded; on the
other hand, wouldn't it send a bunch of noise into ground? And does
grounding at all affect the shielding ability? Also their mention about
connecting to secondary ground, why put more noise there; shouldn't it go
to chassis ground/earth instead (in case the two grounds are partially
decoupled such as by a ground lifting circuit as used for breaking ground
loops)?