A transformer consists of two or more electrical windings that are linked together by a magnetic ﬁeld. Except for special-purpose transformers, the coupling is enhanced with a ferromagnetic core. Transformers are often drawn as shown in Figure 1, with the primary coil on one leg and the secondary coil on the other leg, although they are not actually built that way. When AC voltage is applied to the primary winding, magnetic ﬂux is established, which links the secondary winding. If the ﬂux is sinusoidal, a sinusoidal voltage will be induced in the secondary.
By deﬁnition, the primary is the side that is connected to the source (generator), and the secondary is the side that is connected to the load. Either side may be the high-voltage or low-voltage side.
The core is constructed of a high-Permeance (low-reluctance) material to minimize the magnetizing current. To keep eddy current losses down, the core is made of laminations, the thickness of which is inversely proportional to the rated frequency of the transformer.
Eddy current losses are proportional to the lamination thickness squared. Thus, halving the thickness would reduce the eddy current losses by 75%; however, the cost of the core increases with decreasing lamination thickness.
The cost of building the core must be traded off against the cost of the losses. Typically, for 60 Hz operation, silicon steel laminations with thicknesses of 0.010″ to 0.020” are used. While for very high-frequency operation, wound ribbon (r = 0.001″) or compressed-powder cores are required. Obviously, if eddy currents are to be conﬁned to a lamination, the laminations must be insulated from each other. The insulation may be nothing more than the oxidation on the surface of the lamination.
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Since some of the space occupied by the core is actually insulation, not iron, the flux density will be somewhat higher. This is accounted for by a so-called stacking factor, which is the percentage of the core’s cross-sectional area that is actually iron. The stacking factor is usually above 0.9 for 60 Hz operation, but it decreases as the lamination thickness decreases.
Two types of transformer cores are used. The core type is shown in Figure 2, and the shell type is shown in Figure 3. In the core type, there are windings on each leg of the core-the windings surround the core. We typically draw a core-type transformer such as Figure 1, but the shell type is also commonly used. In the shell type, the windings are on the center leg of the core, and the core surrounds the windings.
Fig.2: Core-type transformer, with barrel windings
The cross-sectional area of the core depends on the amount of flux required to operate the transformer and the ﬂux density at which the iron can operate. Later we will see the relationship between the voltage, flux, and frequency. The dimensions of the core window are determined by the size of the winding, which in turn is a function of the number of turns (voltage level) and the size of the wire (ampacity). Thus, the window size is related to the power level of the transformer.
Fig.3: Shell-type transformer, with pancake windings.
Putting the coils on the core requires either splitting the core or winding the coils directly on the core. Because it is cheaper to assemble the coil onto the core, the core is generally cut. Figure 4 shows the so-called E-I and C laminations that result from cutting shell-type and core-type cores, respectively.
Figure 5 shows photographs of two transformer core and coil assemblies. The one on the left is made of stamped laminations; the one on the right is made of wound METGLAS tape.
Fig.5: Two transformer core and coil assemblies.
Electrical sheet steel tends to elongate and contract in the presence of an alternating magnetic ﬁeld. This phenomenon is called magnetostriction (MS). These movements cause 120 Hz sound vibrations in a 60 Hz transformer, but because the relationship between MS and B is nonlinear, there are harmonics at 240, 360, 480, etc. Other parts of the transformer may vibrate with the core, amplifying the noise. This is particularly important when mounting dry-type transformers indoors. They should always be mounted where the sound is least objectionable.
Copper provides the best conductivity and, therefore, the minimum volume for the coil; however, aluminum is occasionally used to reduce the cost. The conductor must carry current without overheating, and there must be room for the insulation and possibly cooling ducts in the core window.
The conductors may be round, square, or rectangular, and there may be several conductors in parallel to reduce the I2R losses. The operating temperature of the coil is extremely important because the insulation may deteriorate at increased temperature and the resistance of the coil also increases with temperature.
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The insulation in a transformer is really a system of insulation, as different types are used in different places. The conductors have insulation on them, and there is insulation between coils and between the coils and the transformer tank.
Insulation from turn to turn only sees the voltage of a single turn, which may be only a few volts. The insulation between layers of a coil must withstand higher voltages, and the insulation between the coil and the core or tank must withstand the full voltage of the transformer.
There are basically two types of windings-the barrel and the pancake. Figures 2 and 3 show the barrel and pancake, respectively, although either type of winding may be used on either type of core.
In the barrel winding, the high-voltage coil typically surrounds the low-voltage coil to reduce the insulation requirements between the coil and one core. When the barrel type is used, half of the primary and secondary windings are placed on each leg of the core to maximize the mutual coupling and reduce leakage ﬂux. Note that this differs from the way it is drawn in Figure 1.
In the pancake winding, thin primary and secondary windings are alternated on the center leg of the shell core. If used on a core type, half of each would be placed on each side. The coils shown in Figure 5 are both barrel type.
Transformer coils bear fairly standard markings. The high voltage (HV) coils are denoted by the letter H, and the low voltage (LV) coils are denoted by the letter X. Thus, if there is one HV and one LV coil, the terminals would be H1 and H2 on the HV side and X1 and X2 on the LV side, as shown in Figure 6(a). Terminals with the same number (e.g., H1 and X1) have the same instantaneous polarity. That means that current entering these terminals will produce ﬂux in the same direction. The HV coil could be on either the primary or secondary side, depending on whether it is a step-up or step-down transformer. In Figure 6, we will let the primary side be the HV side.
Fig.6: Transformer markings and connections.
There can be more than one coil on a transformer side. A second LV coil would be designated X3 and X4, as shown in Figure 6(b). These low-side coils can be connected in series or parallel to give two different voltages-for example, 240×120.
Figures 6(c) through (e) show other symbols and coil arrangements. The transformer in Figure 6(c) has a shield on the secondary to help eliminate transient noise. This is called an isolation transformer; and it usually has the same voltage on both sides, such as 120-120.
The grounded shield between the coils eliminates capacitive coupling of high-frequency noise. To allow for voltage variations, transformers may have multiple tap settings on one or both sides, as shown in Figure 6(d). This one might offer the choice of 110, 115, or 120 volts on the secondary.
Finally, the transformer in Figure 6(e) has three terminals on the secondary to provide two voltages. For example, 120 V is available from either line to the grounded neutral and 240 V line-to-line. This arrangement is used in modern residential services and would be designated as 240/ 120.
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For a transformer of substantial size, the assembly must be packaged for use. A power transformer (substation level) is always mounted in a steel enclosure and ﬁlled with oil. The oil insulates the coils from the tank and also provides heat transfer for cooling. The oil is circulated from the transformer tank to external radiators that may have fans to blow air through the radiator.
The transformer has one power rating with natural convection cooling and a higher rating with forced cooling. Figure 7 shows a power transformer rated 2500 kVA. The transformer is approximately 10 ft tall (to the top of the bushings).
Fig.7: 2500 kVA power transformer.
There are two types of distribution transformers. Outdoors or in a vault, oil-ﬁlled transformers are used. Indoors, so-called dry-type (air-cooled) transformers are used, but if they are rated above 1125 kVA or 35 kV they must be in a vault. Figure 8 shows a cutaway view of a pole-mounted distribution transformer; Figure 9 shows a three-phase, dry-type transformer.
Fig.8: Cutaway view of a pole-mounted transformer.
Transformer coils may be subjected to large mechanical forces when short circuits occur on the system, so they must be rigidly braced inside the tank to avoid being torn apart under fault conditions. The dry-type transformer coils in Figure 9 are completely encased in rigid epoxy.
Fig.9: Dry-type transformer.
Finally, the coils inside the transformer must be connected to the outside world. These connections are made through bushings, which are porcelain insulators with a conductor through the center. The size of the bushing varies with the voltage level. Bushings are Visible on the top of the transformer in Figure 7.
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