Monday, January 12, 2015

Transformers: Transformer characteristics , Transformer construction and Transformer inrush current .

TRANSFORMER CHARACTERISTICS

The characteristic curves of a 50-kVA transformer are shown in Figure 13–17. The efficiency of this transformer is very high even when the load is as low as 10% of the rated load. Note that the efficiency curve is nearly flat between 20% of the rated load to about 20% overload. The load–voltage characteristics show that there is little change in

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the terminal voltage from a full-load condition to a no-load condition. This condition was shown in the previous voltage regulation calculations. With a lagging power factor load, the secondary terminal voltage change is slightly greater than it is for a unity power factor load. However, the percentage of voltage regulation is satisfactory. With the leading power factor load, the terminal voltage increases with the load, giving a negative voltage regulation.

TRANSFORMER CONSTRUCTION

The single-phase transformer is a simple alternating-current device. The core consists of thin annealed sheets (laminations) of silicon steel. These laminations may be in the form of rectangular strips or L-shaped stampings. When separate sheets are used to make the core, the sheets are placed with butt joints in each layer. The core can also be made by winding a long strip of silicon steel to the desired size. In the laminated core, the sheets are stacked so that the butt joints are staggered in successive layers to ensure a low reluctance. Because the spiral-wound core has no joints, it has a very low reluctance.

Three basic designs of cores are available for use in transformers. These cores are known as the core type, the shell type, and the combined core-and-shell type. Figure 13–18 shows the core-type structure for a transformer.

In the core-type structure, the windings surround the laminated silicon steel core. The low-voltage winding generally is placed next to the core with the high-voltage winding wound over the low-voltage winding. These windings are carefully insulated from each other. The advantage of placing the low-voltage coils next to the core is that a reduction can be made in the material required to insulate the high-voltage windings. Many simplified transformer drawings show the primary winding on one leg of the core and the secondary winding on the other leg. However, such an arrangement gives rise to excessive leakage flux. In practice, both the primary and secondary windings are placed on the same leg of the core to minimize the leakage.

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Shell-Type Core

A shell-type core is shown in Figure 13–19. In this structure, the silicon steel core surrounds the windings. The entire flux passes through the center leg of the core and then divides. One-half of the flux passes through each of the two outside legs.

The coil arrangement and the flux path for a shell-type core are shown in Figure 13–20. The low-voltage coil windings are placed next to the laminated core. The high- voltage windings are placed between the low-voltage windings. This coil arrangement provides adequate insulation between the coils. Because the high-voltage coils are not adjacent to the core structure, less insulation is required. Pancake coils wound with rectangular copper wire may be used with this type of core. The fact that the coil windings surround the core and the core surrounds the coils means that the leakage flux is minimized.

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Type H Core

A type H core with coil windings is shown in Figure 13–21. The core is shaped like a cross when viewed from above. The coils are constructed so that the high-voltage wind- ings are located between the low-voltage windings. This arrangement of coils minimizes the insulation required. As a result, the only high-voltage insulation required is placed between the high-voltage windings and the low-voltage windings. The structure of this core-and-coil assembly keeps the leakage flux to a minimum because the coil wind- ings are placed on a center leg and are surrounded by the four outside legs of the core structure. The H core is often used for distribution transformers with two high-voltage windings, each rated at 2400 V, and two low-voltage windings, each rated at 120 V. Such a transformer can be used to step down either 2400 V or 4800 V to 120 V, 240 V, or 120/240 V.

The GE 220 Class Transformer

A 220 class transformer developed by the General Electric Company has a wound core. That is, a long strip of silicon steel is wound in a tight spiral around the insulated windings. This type of core has the following advantages:

1. It can be manufactured more easily than the conventional core, consisting of lamina- tions stacked and clamped together.

2. The magnetic circuit path is relatively short and has a large cross section.

3. The construction of the core helps reduce the flux leakage.

4. The flux path direction is always along the grain of the silicon steel, thus reducing the iron losses.

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TRANSFORMER INRUSH CURRENT

Although transformers and reactors are both inductive devices, there is a great difference in their operating characteristics. Reactors are often used to prevent inrush current from becoming excessive when a circuit is first turned on. Transformers, how- ever, can produce extremely high inrush currents when power is first applied to the primary winding. The type of core used is primarily responsible for this difference in characteristics.