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Audel electrical course for apprentices and journeymen - Rosenberg P.

Rosenberg P. Audel electrical course for apprentices and journeymen - Wiley & sons , 2004. - 424 p.
ISBN: 0-764-54200-1
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Chapter 26
Transformer Facts
Transformer cores are made up of thin strips of a very high grade of electrical steel. Usually it takes 40 to 50 such strips to make up one inch in thickness. These strips are called laminations. Each lamination has an oxide coating on one side for insulation, or one side may be coated with a varnish. The purpose of laminations, which is to reduce eddy currents, was covered in Chapter 16.
The losses in a transformer cause heating. In Chapter 25 it was explained that transformers have very high efficiencies, but there are losses that have to be considered:
1. Iron losses, namely, eddy currents and hysteresis.
2. Copper losses, which are I2R and I2Z losses.
3. There are also magnetic flux losses, due to leakage of the flux and the magnetic reluctance of the steel core.
Transformer losses must be considered, as they enter into the cost of electricity. Large industrial users are usually on what is termed a primary rate. This means that they may own the transformer, and the electricity is metered at the primary side of the transformer.
When the transformers have a voltage that is too high to meter, then the power will be metered on the secondary side and the transformer losses will be calculated and added to the power consumption.
Sometimes the transformer losses are figured in the rate structure.
The most common configurations for transformer cores are E and I, U and I, and I laminations. In cutting or stamping out the laminations, these three types make 100% use of all the steel sheets from which the laminations are being cut.
The laminations for small and medium-size transformers are usually made up of E and I laminations. See Figures 26-1 and 26-2. Figure 26-1A shows the final stamping of E and I laminations. Figure 26-1B shows proportions (these are not meant to be dimensioned) of the laminations. Also shown are staggering of laminations during stacking (Figure 26-1C), with the windings and the flux paths, while Figure 26-2 shows the layout of the stampings. Note that there is no steel lost from the stampings.
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(A) Pieces. (B) Stacked laminations.
Figure 26-1 E and I laminations.
Figure 26-2 How E and I laminations are laid out for stamping.
The design must be such that the core is not saturated with magnetic flux. Operation at too high a voltage will cause flux saturation and this will affect the transformer operation. Note that in the E and I laminations, the core is twice the dimensions of the outer legs.
Figure 26-3A shows the shape of the U and I laminations. Figure
26-3B shows the staggering of the laminations on assembly, and Figure 26-3C shows the flux path.
(A) Laminated pieces.
Figure 26-3 U and I laminations
Transformer Facts 287
Figure 26-4A shows the I laminations. Figure 26-4B shows their staggered joints while being assembled, and Figure 26-4C shows the flux path of the I-laminated core.
(B) Staggering of (A) Laminated pieces. laminations.
Figure 26-4 I laminations.
There are other core configurations, one of which is the circle or doughnut core.
During the core assembly, it is essential that care be taken in assembly and the joints be as tight as possible, wedged and secured or clamped to prevent noise from vibrating laminations, to keep the sound levels of the transformer low and to keep the path for the flux at as low a magnetic reluctance as possible. Burr-free laminations also aid in keeping sound levels low.
During each half-cycle the laminations tend to change their dimensions. This is called magnetostriction and is due to the molecules of the steel acting as very small magnets and changing their positions.
After the coils are assembled on the core, the core and windings are dipped into insulating varnish and baked. Before dipping they must be thoroughly dried.
There are many facts concerning transformers that are common to all transformers. These will be covered here not in any particular order of importance, as they are all important.
Transformer Impedance
Impedance in a transformer is composed of ^ both resistance and the inductive reactance of the winding: z = 2(R2 + Xj) '
The voltage drop in a transformer from no load to full load is due mostly to the resistance of the windings (see Figure 26-5), as the resistive component of the winding is in phase with the
i---------------------------------------------------------1
I I
. I T J
(C) Flux path of transformer.
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