Common problem points of low frequency transformers

author: MagTop
29/07/2022
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1>The problem of DCR:
  1. DCR is too high: because the silicon steel sheet  low frequency transformer
 almost wire diameter is very thin, the number of turns is many,
the choice of enameled wire becomes the most important part.
 Most US customers prefer to use AWG lines to design and
structure DCR. For some reasons, we have to use MM line
approximation to replace, Taiwan enamel wire bare diameter valuea little bit larger, the film is a little bit thicker, so thousands of
circles down often DCR will be super high, and the line package
fat. The other point is the problem of line wrap fat, neat close
winding and disorderly thin winding, the DCR will be different
with different line length, neat close winding will be a little lower.
  1. Differences in DCR values between the two groups: Some
specifications require that the DCR difference between the two
UI CORE twins should be within a certain amount or SPEC
requires that the voltage of the two twins should not exceed 0.1V when the secondary output of the twin is idle. In this case, the two products wound by the same axis of the same winding
machine should be used to assemble, so as to get a good successrate. 
3>The excitation current problem:
General power transformer, excitation current specification is not
very strict, refers to the secondary side without load when the
primary current. If the excitation current exceeds the possible
cause:
a. The number of turns is too small, or the wire diameter is too large
b. The silicon steel sheet is not assembled properly
c. Silicon steel sheet material is not good enough
d. The silicon steel sheet is too thick
e. Instrument error
f. The input condition is correct. If the excitation current exceeds much, the coil should be checked for short circuit.
 
3>Loss is too high
Our wattmeter reading on the test board is out of specification (copper and iron damage) because the winding has been completed, it is customary to improve from iron damage:
A. Silicon steel sheet is not installed properly (rough edge is not unified, not installed tightly)
B. Improve core material
C. Select thin silicon steel sheets.
4>The voltage adjustment rate is too large
Δ U % = (U20 - U2)/U20
U20: no-load output voltage; U2: The load output voltage is involved in the design of copper loss, iron loss, number of turns.
5>Temperature rise
The resistance method can be used to calculate (234.5+T1)/R1 = (234.5+T2)/R2
Hi-pot and IR (insulation resistance) HI-POT test is the transformer must be 100% full test requirements, many customers also require each shipment to be attached with HI-POT full test statement.
Causes of bad HI-POT:
A. The leading wire between the two windings is in contact or too close
B. Short circuit of tin bridge between pins
C. The insulation tape is not completely covered or the number of layers is too small
D. Insufficient distance between windings and CORE
E. During assembly, CORE will wipe BOBBIN or CASE. When the cut-off current on SPEC is 3mA, do not use 2mA or 1mA to test HI-POT, 2mA is more strict than 3mA.
6> Containing leaching
In principle, silicon steel sheet transformers do not need to be vacuumed and immersed. Too much vacuum pressure will cause Vanili water to penetrate between the cores, creating gaps and reducing power.