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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
Download (direct link): audelelectricalcourseforapprentices2004.pdf
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(A) Direction of windings. (B) Direction of voltages.
Figure 26-6 Additive polarity transformers.
Figure 26-7 illustrates the method of checking polarity. Connect one side of one winding to one side of the other winding as shown and mark the high-side connection H1. Connect a voltmeter between the other low-side lead and the other high-side lead. Apply the primary voltage as shown, and if the polarity is additive, the voltmeter
292 Chapter 26
V will read the primary voltage plus the secondary voltage. If this shows additive, mark the leads as shown in Figure 26-6B. A study of the direction arrows in Figure 26-6B and Figure 26-7 will show why the voltages add together.
If the voltmeter reads less than the primary voltage, the secondary leads should be marked as shown in Figure 26-8 and the transformer would be of subtractive polarity. A study of Figure 26-8 will show why the voltmeter reads less than the voltage of the power
source.
Termination Marking
Figure 26-9 illustrates a dual-voltage one-phase transformer. The high side for this coverage won’t be dual voltage, but the low side will be dual voltage. Figure
26-9A shows the basic idea, and, as noted, the X2 and X, leads are crossed internally in the transformer, as shown in Figure 26-9B.
Figure 26-10A illustrates the low-volt-age side connected with the windings in series. The crossing of X2 and X3 internally has no effect on this 120/240-volt connection, but in Figure 26-10B, the crossing of the low side internally does affect the external connections for 120 volts only. It is not necessary to cross the leads externally, which aids in avoiding mistakes in the field.
Figure 26-8 Transformer with subtractive polarity.
Transformer Facts 293
(A) Basic construction, (B) Leads Xz and X3 crossed internally.
Figure 26-9 Single-phase transformers with dual low-voltage windings.
(A) Windings in series. (B) Windings in parallel.
Figure 26-10 Connections of low-voltage windings.
Formulas
Z = 2r2 + X2
iz = 2(ir)2 + (IX)2
T(, . 100%
I (short circuit) = —---------;----
Percent Impedance
294 Chapter 26
Questions
1. Transformer cores are made of cast iron. (True or false?)
2. Why are laminations insulated from each other?
3. What losses occur in transformer cores?
4. Show by sketches three types of core formations.
5. How are core noises kept low?
6. Why are transformer cores and coils dipped in insulating varnish?
7. What makes up transformer impedance?
8. Give the formulas for short-circuit current.
9. In paralleling transformers, give the requirements that must be met.
10. Name the classes of transformers.
11. Give the ANSI insulation class temperatures.
12. Explain the effect of altitude on transformer output.
13. What is ambient temperature?
14. What markings appear on transformer leads?
15. Draw sketches of how to check polarity and explain.
Chapter 27
Transforming Polyphase Power
Two-phase power transforming requires two transformers. A schematic diagram of connections is shown in Figure 27-1. Either the high- or low-voltage side or both may be connected three-wire or four-wire as desired.
PRIMARY MAINS

PHASE A JIMMJUL , PHASE B Immmju
nnmnrm PHASE A irmw
1
: j
PHASE B 1
SECONDARY MAINS
Figure 27-1 Two
transformers connected two-phase four-wire.
Single-Phase for Three-Phase
Single-phase transformers or a single three-phase transformer may be used with three-phase power.
Three single-phase transformers are often used by utility companies on their distribution lines, whereas three-phase transformers are often used at substations.
Figure 27-2 shows three single-phase transformers in a delta-delta bank. Note that each transformer is connected as a single transformer, such as transformer No. 1 being connected independently of transformers No. 2 and No. 3. Lines A and B supply transformer No. 1 and the output is connected to X and Y, as shown in both the schematic and the vector diagram. Note in Figure 27-2 that no neutral connection is shown.
Two single-phase transformers may be connected in open-delta or
V connection as shown in Figure 27-3. This gives very satisfactory
295
296 Chapter 27
(A) Transformer connections.
Figure 27-2 Delta-delta connections and vectors.
Figure 27-3 Open-delta connections using two single-phase transformers.
voltage transformation. Phase Y-Z is often termed the phantom phase and is usually a few volts higher than the other two phases. This is widely used on distribution lines. Again, as in Figure 27-2, there is no neutral shown at this point.
Two transformers in open-delta won’t deliver 100% of their kVA ratings to a three-phase load, but will deliver only 86.6% of their total kVA ratings; and the two transformers can deliver only 57.7% of the load that could be carried if the third transformer were present.
Transforming Polyphase Power 297
One great advantage of three single-phase transformers in a three-phase bank is that under emergency conditions, two of the three transformers may be used to supply a part of the load.
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