Monday, January 12, 2015

Transformers: Magnetic domains , Multiple tapped windings and Cooling transformers.

MAGNETIC DOMAINS

Magnetic materials contain tiny magnetic structures in their molecular material, known as domains. These domains can be affected by outside sources of magnetism. Figure 13–22 illustrates a magnetic domain that has not been polarized by an outside magnetic source.

Now assume that the north pole of a magnet is placed toward the top of the material that contains the magnetic domains (Figure 13–23). Notice the structure of the domain has changed to realign the molecules in the direction of the outside magnetic field.

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If the polarity of the magnetic pole is changed (Figure 13–24), the molecular structure of the domain will change to realign itself with the new magnetic lines of flux. This external influence can be produced by an electromagnet as well as a permanent magnet.

In certain types of cores, the molecular structure of the domain will snap back to its neutral position when the magnetizing force is removed. This type of core is used in the construction of reactors or chokes (Figure 13–25). A core of this type is constructed by

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separated sections of the steel laminations with an airgap. This airgap breaks the path of the magnet through the core material and is responsible for the domains returning to their neutral position once the magnetizing force is removed.

The core construction of a transformer, however, does not contain an airgap. The steel laminations are connected together in such a manner as to produce a very low reluctance path for the magnetic lines of flux. In this type of core, the domains remain in their set posi- tion once the magnetizing force has been removed. This type of core “remembers” where it was last set. This was the principle of operation of the core memory of early computers. It is also the reason why transformers can have extremely high inrush currents when they are first connected to the power line.

The amount of inrush current in the primary of a transformer is limited by the following three factors:

1. The amount of applied voltage.

2. The resistance of the wire in the primary winding.

3. The flux change of the magnetic field in the core. The amount of flux change deter- mines the amount of inductive reactance produced in the primary winding when power is applied.

Figure 13–26 illustrates a simple isolation-type transformer. The alternating cur- rent applied to the primary winding produces a magnetic field around the winding. As the current changes in magnitude and direction, the magnetic lines of flux change also. Because the lines of flux in the core are continually changing polarity, the magnetic domains in the core material are changing also. As stated previously, the magnetic domains in the core of a transformer remember their last set position. For this reason, the point on the waveform where current is disconnected from the primary winding can have a great bearing on the amount of inrush current when the transformer is reconnected to power. For example, assume that the power supplying the primary winding is disconnected at the zero crossing point (Figure 13–27). In this instance, the magnetic domains would be set at the neutral point. When power is restored to the primary winding, the core material can be magnetized by either magnetic polarity. This permits a change of

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flux, which is the dominant current-limiting factor. In this instance, the amount of inrush current would be relatively low.

If the power supplying current to the primary winding is interrupted at the peak point of the positive or negative half-cycle, however, the domains in the core material will be set at that position. Figure 13–28 illustrates this condition. It is assumed that the current was stopped as it reached its peak positive point. If the power should be reconnected to the primary winding during the positive half-cycle, only a very small amount of flux change can take place. Because the core material is saturated in the positive direction, the primary winding of the transformer is essentially an air core inductor, which greatly decreases the inductive characteristics of the winding. The inrush current in this situation would be limited by the resistance of the winding and a very small amount of inductive reactance.

This characteristic of transformers can be demonstrated with a clamp-on ammeter that has a “peak hold” capability. If the ammeter is connected to one of the primary leads, and power is switched on and off several times, it can be seen that the amount of inrush current will vary over a wide range.

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MULTIPLE TAPPED WINDINGS

It is not uncommon for transformers to be designed with windings that have more than one set of lead wires connected to the primary or secondary. The trans- former shown in Figure 13–29 contains a secondary winding rated at 24 V. The primary winding contains several taps, however. One of the primary lead wires is labeled C and is the common for the other leads. The other leads are labeled 120, 208, and 240, respectively. This transformer is designed in such a manner that it can be connected to different primary voltages without changing the value of the secondary voltage. In this example, it is assumed that the secondary winding has a total of 120 turns of wire. To maintain the proper turns ratio, the primary would have 600 turns of wire between C and 120, 1040 turns between C and 208, and 1200 turns between C and 240.

The transformer shown in Figure 13–30 contains a single primary winding. The secondary winding, however, has been tapped at several points. One of the secondary lead wires is labeled C and is common to the other lead wires. When rated voltage is applied to

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the primary, voltages of 12, 24, and 48 V can be obtained at the secondary. It should also be noted that this arrangement of taps permits the transformer to be used as a center-tapped transformer for two of the voltages. If a load is placed across the lead wires labeled C and 24, the lead wire labeled 12 becomes a center tap. If a load is placed across the C and 48 lead wires, the 24 lead wire becomes a center tap.

In this example, it is assumed the primary winding has 300 turns of wire. To produce the proper turns ratio, it would require 30 turns of wire between C and 12, 60 turns of wire between C and 24, and 120 turns of wire between C and 48.

The transformer shown in Figure 13–31 is similar to the transformer in Figure 13–30. The transformer in Figure 13–31, however, has multiple secondary windings instead of a single secondary winding with multiple taps. The advantage of the transformer in Figure 13–31 is that the secondary windings are electrically isolated from each other. These secondary windings can be either stepup or stepdown depending on the application of the transformer.

COOLING TRANSFORMERS

The core-and-coil assembly is placed in a pressed steel tank and is completely covered with an insulating oil. This insulating oil removes heat from the core and the coil windings and insulates the windings from the core and the transformer case. The core-and-coil assembly for a typical transformer contains channels or ducts. These ducts permit the oil to circulate and remove the heat. As the oil gains heat, its density decreases and it rises. As it rises and circulates in the transformer, it contacts the tank walls and the heat is transferred from the oil to the tank walls. The walls are cooled by air circulation. The specific gravity of the oil increases as it loses heat. As a result, the oil flows down to the bottom of the tank and again circulates up through the coil ducts to repeat the cooling process. Transformers having a very large

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kVA capacity require more than the available surface area of the transformer case for cooling.

The surface area of a transformer can be increased by the use of tubes or fins added to the steel tank assembly (Figure 13–32). This increased surface area will take more heat from the oil and will radiate it faster to the surrounding air in a given time.

Transformer Oil

The insulating oil used in transformers is a high-grade oil that must be kept clean and moisture-free. This type of oil should be checked periodically to determine whether there is any change in its insulating ability. If traces of moisture or foreign materials are found, the oil must be filtered or replaced. The insulating fluid in some transformers is a nonflammable, nonexplosive liquid. One fluid of this type is Pyranol, manufactured by the General Electric Company. This liquid is a synthetic dielectric that is an effective cooling and insulating agent.

Oil-cooled transformers are considered a fire hazard in some locations. Air-cooled transformers are used under these conditions. Such transformers permit the natural circulation of air to remove the heat from the coils and the core. A perforated metal enclosure pro- vides mechanical protection to the coil and windings and permits air to circulate through the windings.

In addition to the method described, other ways of cooling transformers include forced air circulation, natural air circulation, natural circulation of oil with water cooling, and forced oil circulation (for large transformers).

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