From a Wiki,
Magnetic shunts can be used to limit current or regulate voltage.
This regulation is accomplished by inserting a ferrous magnetic shunt into
the transformer core, such that the magnetic flux from the primary winding
has an alternate (although high impedance) path around the secondary
As the current draw on the transformer secondary winding increases, more
primary magnetic flux diverts through the magnetic shunt.
Selecting different materials with suitable saturation characteristics it is
possible to make transformers with a range of regulation functions.
Also in microwave oven transformers - small magnetic shunts provide enough
leakage inductance to resonate the doubler capacitor
73s Tracey G5VU
Just out of curiosity, what is the purpose of those magnetic shunts when
microwave oven transformers are used for their intended application ?
73 Alberto I2PHD