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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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Joseph Henry devised the first transformer in 1832. The first practical transformer was patented in England in 1882 by Gaulard and Gibbs. The American rights were purchased by Westinghouse in 1886. The first transformer to be built in this country was built by William Stanley in 1885, an employee of the Westinghouse Company.
Transformer Types
The efficiency of induction coils is very low, but the efficiency of transformers is in the range of 92 to 99%. Large transformers often are nearly 99% efficient. Beyond a doubt, the transformer is one of the most efficient pieces of electrical equipment ever made.
Transformers are for transforming AC voltages from one voltage to another. This voltage transformation may be either up or down in value. Both step-up and step-down transformers are alike: One is for stepping up the voltage, and the other is for stepping down the voltage. Figure 25-3 illustrates both a step-up transformer and a step-down transformer, and they may be identical types of transformers.
In Figure 25-3, 13.8 kV (kilovolts) are delivered to transformer Tv which steps up the voltage to 115 kV for transmission to a distant point, transformer T2 (which could be an identical transformer to Tj), where the voltage is transformed down from 115 kV to 13.8 kV for distribution.
280 Chapter 25
Principles of Operation
Transformers are sometimes termed static transformers since they have no moving parts. The principle of a transformer is illustrated in Figure 25-4.
Assume that 100 volts AC are impressed on winding A, wound around a soft iron ring C, and the coil contains 300 turns. A magnetic flux is set up by the current in A within the soft iron core C. This flux is set up around each turn or convolution of coil A. The flux lines are like rubber bands that expand outward, cutting each turn of winding A, expanding until they cut each turn of coil B, before they eventually occupy the cross-section of core C.
As the impressed voltage and the current in coil A rise and fall, the flux in turn rises and falls, cutting the turns of coil B. From the principle of induction, the rising and falling of the flux cutting coil B induces an emf in coil B. There is also a counter emf induced in coil A. This emf and the emf induced in coil B are in opposition to the impressed emf in coil A.
Now, if coil B is open, that is, has no load connected, there will be no flux generated by coil B in opposition to the flux generated by the impressed emf and current in coil A. The counter emf in coil
A, however, in opposition to the impressed emf on coil A, opposes the changes in current and tends to keep the flux oscillating with the frequency of the impressed emf.
This may be compared to the shunt motor as covered in Chapter 19, where the effect of counter emf was illustrated in Figure 19-5 with the accompanying explanations. Namely, the rotating armature generated a counter emf almost equal to the impressed emf, so that the impressed emf delivers just enough current to keep the armature going.
Transformer Principles 281
The difference here is that the transformer windings are stationary, while the flux is oscillating.
Windings and Voltages
The winding that receives the impressed emf is always termed the primary winding, regardless of the impressed voltage; and the winding that receives the induced emf, in this case winding
B, is the secondary winding. There is very often confusion in that the high-voltage winding is called the primary and the low-voltage winding is called the secondary. As may be seen, this is not the case. The input side is the primary, and the output side is the secondary.
The voltages of a transformer are proportional to the number of turns in the windings. Thus, if in Figure 25-4 winding A has 300 turns and an impressed voltage of 100 volts, and winding B has 150 turns, then winding B will have an induced voltage of 50 volts.
Each convolution or turn of transformer coils is cut by magnetic flux four times per cycle: first, when the flux is rising; second, when the flux is falling to zero; third, when the flux is rising in the reverse direction; and fourth, when the flux is falling to zero.
The average voltage that is induced in each winding with a flux having a maximum value of f will be
Eavg = 4fnT/108 where
Eavg = average emf induced in each winding f = maximum magnetic flux n = frequency in hertz T = number of turns in coil
4 = constant: number of times each turn is cut by flux per cycle
108 = constant to reduce absolute lines of force for conversion of terms to practical volts
The rms voltage or effective voltage, as read on the voltmeter, equals 1.11 times the average voltage. So, for the effective voltage instead of the average voltage, the 4 in the above formula becomes 4 X 1.11, or 4.44. Thus,
Erms = 4.4fnT/108
282 Chapter 25
It was shown that for the transformer in Figure 25-4, with no load on winding B, the current into winding A was limited to a low value by the counter emf induced in that winding.
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