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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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Figure 27-11 Voltage and current in a wye transformer.
In Figure 27-11, for the wye connection, the line voltage is 1.73 times the winding voltage, and the line current is the same as the winding current.
302 Chapter 27
The above descriptions, of course, assume balanced loads and are representative only.
1. Sketch a schematic of three single-phase transformers connected in delta-delta.
2. Sketch a schematic of an open-delta connection.
3. Add a neutral connection to the drawing for question No. 2.
4. Sketch a schematic of a three-phase, delta-delta transformer.
5. Sketch a schematic of a three-phase wye-wye transformer.
6. What problems are encountered in wye-wye connections?
7. Sketch a schematic of a four-wire delta-wye.
8. Sketch a Scott connection. Tell what it is used for, and describe it.
Chapter 28
Autotransformers differ from conventional transformers in that they have only one continuous winding. There are many NEC restrictions to their use, but they have a place in the electrical industry.
Autotransformers and Conventional Transformers
In the conventional transformer, the high-voltage winding and the low-voltage winding are electrically isolated. Thus, they are termed isolation transformers.
Figure 28-1 illustrates the type of transformer covered in Chapter 27 and gives voltages and current for this particular transformer. An autotransformer of the same capacity and voltage is illustrated in Figure 28-2. Particularly note that the two windings are as one and not isolated.
Figure 28-1 Ordinary transformer with voltages and current shown.
Figure 28-2 Autotransformer.
304 Chapter 28
Figure 28-3 illustrates what happens to the output voltage if the connections are not made correctly. Figure 28-3A shows the right connections, and Figure 28-3B shows the connections to G-E of the winding reversed. As a result there is zero voltage at C-D.
(A) Right connection. (B) Wrong connection.
Figure 28-3 Right and wrong connections of an autotransformer.
Induction Voltage Regulator
The reversal of a part of the winding is shown to clarify the operation of an induction voltage regulator. These regulators are used as voltage boosters on long power lines and work automatically as the load increases or decreases to take care of voltage drops at the load caused by IR loss in the conductors. (See Figure 28-4.) They have built into them RXLXC networks, which may be set to conform to
Figure 28-4 Induction voltage regulator.
Autotransformers 305
the characteristics of the power line and thus automatically compensate for changes of load at the load end of the line and keep a stable voltage there.
The induction regulator consists of two windings. The secondary, S, is connected in series with the load. The primary, P, is connected in shunt across the line and is mounted so it will turn so that its inductive relationship with the secondary may be varied. The primary is the moving coil, because it is wound with finer, more flexible conductors. A regulator is an autotransformer with a variable ratio of transformation.
If the primary is rotated 90° in one direction, the line voltage will be boosted by the amount of emf induced into the secondary. If the primary is rotated 90° in the other direction, the voltage of the line will be lowered to the extent of the induced emf in the secondary. Thus, the amount of rotation and the direction of rotation, when controlled automatically, will compensate for the voltage drops in the load side of the line from the induction regulator. A phantom impedance circuit is built-in to simulate the line characteristics.
Transformer Booster
The conventional type of transformer is sometimes used on power lines to boost or buck line voltage, as an emergency measure. Great care should be taken in connecting it up, as will be explained.
Figure 28-5 shows such a transformer booster. The high-side (A) winding is for 1000 volts, and the low side (B) is for 100 volts. These are used as merely representative figures. The values used will depend on the line voltage and the amount of voltage boost required.
Figure 28-5 Transformer booster connection.
This connection will act as a current transformer, which will be explained in the next chapter. A word of caution, however, is necessary. Unless handled properly, the connecting of the booster may be extremely hazardous. A very high voltage may be produced in winding A if it is not connected to the line.
306 Chapter 28
Thus, winding A must be connected to the line before winding B is connected and, also, winding A must be connected solid to the line and no fuses inserted in this connection. After winding A is connected, then winding B may be connected in series with the line, so as to add to the line voltage. See the material on checking polarity in Chapter 27. Winding B must be large enough to handle full line current without damage. Therefore, the full-load current of the low side of the transformer must be checked with the anticipated full-load line current, and the high-side voltage rating of this transformer must be that of the line being connected to.
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