Low frequency transformer performance test

author: MagTop
low-frequency transformers
The research background
Installing proper shielding for low frequency transformer can effectively suppress interference and improve electromagnetic compatibility of the system. Shielding efficiency is an important index to measure shielding performance. According to the calculation formula of shielding efficiency, magnetic field tester is usually used to test the magnetic field intensity or electric field intensity before and after shielding. The direct purchase of magnetic field tester is seriously restricted by the performance and price ratio. Therefore, in view of the nature and characteristics of the transformer, a low frequency transformer shielding efficiency test system is established. The shielding efficiency of 50 VA power frequency E transformer under three shielding modes is tested by indirect method. The results show that the performance and price ratio of the shielding efficiency test system is high, the test results are accurate and reliable, and meet the requirements of electromagnetic compatibility, which provides a decision basis for choosing the appropriate shielding form and anti-jamming measures.

Composition and testing principle of low frequency transformer shielding testing system
The system consists of self-made test rack (including probe and
turntable), connecting coaxial cable and transistor millivolt meter. 
The shielding effectiveness was measured by indirect
measurement. Basic principle is: will a homemade probe sensor
space reservation point (coil) in the field, the magnetic field lines in the vertical through the probe test coil, induction signal by a
coaxial cable transmission, induction voltage RMS millivoltmeter
measured by the transistor, according to maxwell's equations, the
relationship between the magnetic induction intensity and magnetic field intensity, magnetic flux and magnetic induction intensity
relationship expression induction electric potential is derived.
In a sinusoidal magnetic field, the shielding efficiency can be
measured by the ratio of the induced potential (U0, U1) at a
point in space before and after shielding. The shielding efficiency is usually expressed as decibel (DB)

The research conclusion
The shielding efficiency of double-layer shield is better than that
of single-layer shield, and the shielding efficiency of single-layer
shield is better than that of short-circuit ring. Double shielding
shielding shielding effectiveness of shielding effectiveness is not a single simple sum, but in the transformer to be measured for the first time provide low magnetic resistance pathways on the basis of the leakage magnetic field, again through the first layer of
shielding the magnetic flux leakage of low magnetic resistance
pathway as the second layer shielding, so compared with the
single layer shielding, double layer shielding outside leakage
magnetic field has been greatly reduced. Short-circuit ring shieldinguses the magnetic leakage field to generate eddy currents in the
short-circuit ring of good conductor, which in turn generates a
magnetic field opposite to the magnetic leakage field to counteract its effect. The shielding efficiency of this structure will increase
with the increase of the frequency of magnetic leakage field and
the increase of induced eddy currents, and the shielding effect of eddy currents is not obvious at low frequency. This result is in
good agreement with electromagnetic theory.