Shield winding
A shield winding (also called static shielding or static shield , English shield winding ) is an additional winding between the primary and secondary windings of a transformer .
Basic principle
In addition to the inductive coupling between the primary and secondary windings, there is also a capacitive coupling . While the inductive coupling is used for the desired power transmission, the capacitive coupling is rather undesirable. They can transmit higher- frequency interference. The shield winding is a metal layer between the windings. It therefore acts like a Faraday cage as a shield for the electric field . In contrast, the magnetic field built up by the windings is hardly influenced.
The shield winding between the primary and secondary windings is connected to ground on one side , so it must not be a closed turn.
Technical
The shield winding usually only has one connection. If there is a second connection, this is only used to test the shield winding.
The shield winding can consist of a single-layer wire winding or foil (metal foil). The wire winding is easier to manufacture, but does not offer as good shielding as the foil version (a voltage is also induced in the shield winding, which reduces the shielding effect). A shield winding made of foil can, depending on the frequency range, reduce the capacitive coupling between the primary and secondary circuit by 20 dB (i.e. to 10%). The wire or foil must be electrically insulated and open at one end so that the shield winding does not act like a short-circuit turn.
The shield winding is connected to the protective earth , the connection cable is colored green / yellow.
Second shield winding
A second shield winding can be used for special requirements. This is then connected to the ground on the secondary side. The connection cable is black. It is possible to use different materials (sensibly with complementary properties) for the two shield windings.
It is also possible to split the first shield winding. This does not create a “second shield winding” in the sense of the previous paragraph, but both parts are brought together on the primary side in such a way that the voltage induced in the partial windings is canceled out. In terms of manufacturing effort and shielding, this represents a compromise between wire winding and foil shield.
Other uses for shield windings
The shield winding can also be viewed as additional protection against the breakdown of voltages from the primary to the secondary circuit. For medical devices in particular, this is advertised as part of a whole range of protective mechanisms.
A shield winding increases the costs for the transformer and also requires a connection for the protective conductor (e.g. by means of a three-core mains lead with a protective contact plug). This is why most devices do not have a shield winding.
Individual evidence
- ↑ Archived copy ( memento of the original from October 6, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ a b c Archived copy ( memento of the original dated June 5, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ a b http://www.patent-de.com/19870402/DE3533649A1.html
- ↑ http://www.reo.de/files/medizintechnik.pdf ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.
