Stray field transformer
A stray field transformer is a transformer that specifically has a relatively loose magnetic coupling between the primary and secondary windings.
Basics
While transformers usually strive for the best possible or strong magnetic coupling between the windings through a high mutual inductance , with leakage field transformers a looser coupling is achieved by spatially separating the primary and secondary windings from each other and thus increasing the magnetic leakage flux or a magnetic one Establishes a shunt in the space between the primary and secondary winding.
In this way, an output voltage with a "softer" current-voltage characteristic is obtained, ie a voltage source with a higher and sometimes also adjustable source impedance. The leakage field transformer combines the function of a transformer (voltage transformation and galvanic isolation) and a current-limiting choke in one component. In this case, the value that actually acts as the current limiting inductance is the short circuit inductance .
Layout and function
If an ordinary transformer is short-circuited, very high primary and secondary currents flow, which result from the low leakage inductance and the winding resistances. This is associated with high losses.
In the case of a stray field transformer, this current is reduced by offering the magnetic flux an escape path and thus increasing the stray inductance:
- Accommodation of the windings on different legs (two-leg construction; the magnetic shunt leads through the air)
- Creation of a magnetic shunt made of iron between the wound legs (three-leg structure; the shunt leads over this additional leg, which may have a variable air gap)
When idling, the primary voltage is translated almost like a normal transformer, since the shunt represents a comparatively high magnetic resistance .
If the secondary current increases, the associated increase in the magnetic resistance of the secondary leg increases the magnetic flux in the shunt until it has reached the limit value set by the primary voltage and the short-circuit inductance in the event of a short circuit. Almost all of the magnetic flux now runs through the shunt. In the leg on which the secondary winding is located, the flux is now almost zero, and the secondary current has in turn reached its upper limit value - the short-circuit current. This short-circuit current is essentially determined by the properties of the magnetic shunt or the resulting short-circuit inductance: if it is higher, the short-circuit current is lower. If, on the other hand, the magnetic resistance in the shunt is increased (e.g. by increasing the air gap), the short-circuit current also increases.
The magnetic flux is divided according to the ratio of the magnetic resistances between the shunt and the secondary leg. The leakage field transformer behaves like a normal transformer with a current-limiting choke connected in series. Therefore, the heat losses are low even in the event of a short circuit.
Stray field transformers naturally have increased magnetic flux densities in their environment. As a result, screen devices with cathode ray tubes , among other things, can show a flickering or fluctuating picture or audio devices can be disturbed. This may have to be taken into account by setting up the devices appropriately or by changing the alignment of the devices.
Application and designs
- Ballast for neon advertising tubes (task: transformation of the mains voltage to a few kilovolts and current limitation), designs with fixed or adjustable magnetic shunt
- So-called bell transformer (task: short-circuit resistance and low no-load losses ), enlarged iron core , dynamo sheet inserted between the windings as a shunt
- Welding transformer (task: adjustable current limitation for the welding arc ), current via taps or variable coupling or adjustable magnetic shunt can be selected - typically with adjusting crank and pointer in slot
- particularly safe power supplies (task: short-circuit resistance and particularly high reliability), two-leg design with enlarged iron core volume
literature
- A. Senner: Electrical engineering. 4th edition. Verlag - Europa - Lehrmittel, 1965
- Günter Springer: Expertise in electrical engineering. 18th edition, Verlag - Europa - Lehrmittel, Wuppertal, 1989, ISBN 3-8085-3018-9