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18-5 A solenoidal-core
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18-6 A toroidal-core
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A toroidal core provides considerably more inductance per turn, for the same kind of ferromagnetic material, than a solenoidal core. It is common to see toroidal coils or transformers that have inductance values as high as 100 mH.
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Pot Core Even more inductance per turn can be obtained with a pot core. This is a shell of ferromagnetic material that is wrapped around a loop-shaped coil. The core is manufactured in two halves (Fig. 18-7). You wind the coil inside one of the halves, and then bolt the two together. The final core completely surrounds the loop, and the magnetic flux is confined to the core material.
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292 Transformers and Impedance Matching
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18-8 Schematic symbols for autotransformers. At A, air core, step-down.
At B, laminated iron core, step-up. At C, ferrite or powdered iron core, step-up.
Like the toroid, the pot core is self-shielding. There is essentially no coupling to external components. A pot core can be used to wind a single, high-inductance coil. Inductance values of more than 1 H are possible with a reasonable number of wire turns. In a pot-core transformer, the primary and secondary must be wound next to each other. This is unavoidable because of the geometry. Therefore, the interwinding capacitance of a pot-core transformer is high. Pot cores are useful at AF and the lowest-frequency parts of the RF spectrum. They are rarely employed at high radio frequencies.
Autotransformer In some situations, there is no need to provide dc isolation between the primary and secondary windings of a transformer. In a case of this sort, an autotransformer can be used. It has a single, tapped winding. Figure 18-8 shows three autotransformer configurations. The unit shown at A has an air core, and is a step-down type. The unit at B has a laminated iron core, and is a step-up type. The unit at C has a powdered iron core, and is a step-up type. You ll sometimes see autotransformers in radio receivers or transmitters. Autotransformers work well in impedance-matching applications, and also perform well as solenoidal loopstick antennas. Autotransformers are occasionally, but not often, used in AF applications and in 60-Hz utility wiring. In utility circuits, autotransformers can step the voltage down by a large factor, but they aren t used to step voltages up by more than a few percent.
Power Transformers
Any transformer used in the 60-Hz utility line, intended to provide a certain rms ac voltage for the operation of electrical circuits, is a power transformer. Power transformers exist in a vast range of physical sizes, from smaller than a tennis ball to as big as a room.
At the Generating Plant The largest transformers are employed at the places where electricity is generated. Not surprisingly, high-energy power plants have bigger transformers that develop higher voltages than low-energy, local power plants. These transformers must be able to handle high voltages and large currents simultaneously.
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When electrical energy must be sent over long distances, extremely high voltages are used. This is because, for a given amount of power ultimately dissipated by the loads, the current is lower when the voltage is higher. Lower current translates into reduced loss in the transmission line. Recall the formula P = EI, where P is the power (in watts), E is the voltage (in volts), and I is the current (in amperes). If you can make the voltage 10 times larger, for a given power level, then the current is reduced to 1 10 as much. The ohmic losses in the wires are proportional to the square of the current. Remember that P = I 2R, where P is the power (in watts), I is the current (in amperes), and R is the resistance (in ohms). Engineers can t do much about the wire resistance or the power consumed by the loads, but they can adjust the voltage, and thereby the current. Suppose the voltage in a power transmission line is increased by a factor of 10, and the load at the end of the line draws constant power. This increase in the voltage reduces the current to 1 10 of its previous value. As a result, the ohmic loss is cut to (1 10)2, or 1 100, of its previous amount. That s a major improvement in the efficiency of the transmission line, at least in terms of the loss caused by the resistance in the wires and it is the reason why regional power plants have massive transformers capable of generating hundreds of thousands of volts.
Along the Line Extreme voltage is good for high-tension power transmission, but it s certainly of no use to an average consumer. The wiring in a high-tension system must be done using precautions to prevent arcing (sparking) and short circuits. Personnel must be kept at least several meters away from the wires. Can you imagine trying to use an appliance, say a home computer, by plugging it into a 500,000-V rms electrical outlet Medium-voltage power lines branch out from the major lines, and step-down transformers are used at the branch points. These lines fan out to still lower-voltage lines, and step-down transformers are employed at these points, too. Each transformer must have windings heavy enough to withstand the product P = EI, the amount of VA power delivered to all the subscribers served by that transformer, at periods of peak demand. Sometimes, such as during a heat wave, the demand for electricity rises above the normal peak level. This loads down the circuit to the point that the voltage drops several percent. This is called a brownout. If consumption rises further still, a dangerous current load is placed on one or more intermediate power transformers. Circuit breakers in the transformers protect them from destruction by opening the circuit. Then there is a temporary blackout. At individual homes and buildings, transformers step the voltage down to either 234 V rms or 117 V rms. Usually, 234-V rms electricity is provided in the form of three sine waves, called phases, each separated by 120 , and each appearing at one of the three slots in the outlet (Fig. 18-9A). This voltage is commonly employed with heavy appliances, such as the kitchen oven/stove (if they are electric), heating (if it is electric), and the laundry washer and dryer. A 117-V rms outlet supplies just one phase, appearing between two of the three slots in the outlet. The third opening in the outlet leads to an earth ground (Fig. 18-9B). In Electronic Devices The smallest power transformers are found in electronic equipment such as television sets, ham radios, and home computers. Most solid-state devices use low voltages, ranging from about 5 V up to perhaps 50 V. This equipment needs step-down power transformers in its power supplies. Solid-state equipment usually (but not always) consumes relatively little power, so the transformers are usually not very bulky. The exception is high-powered AF or RF amplifiers, whose tran-
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