Power Transformer Characteristics

Power transformers, Figure 1, typically have the following features:

  1. They are heavy (because of their iron core) and can be very large, like the transformers on telephone poles. However, power transformers in electronic devices are much smaller and vary considerably in size.
  2. They have a laminated iron core to reduce eddy-current losses.

Power transformers

Figure 1: Power transformers

  1. The flux path is as short as possible to reduce leakage flux and minimize magnetizing energy needed.
  2. They use one of two core shapes-core type (primary and secondary windings on separate legs of the core) and shell type (both primary and secondary windings on the center leg), again see Figure 1.
  3. They have one or more primary and secondary windings.
  4. They may have one tapped winding, such as the autotransformer, Figure 2. The autotransformer is a special single-winding transformer.

 When the input source is connected from one end to the tap while the output is across the whole coil, the voltage is stepped up.

If the input voltage is applied across the whole coil and the output taken from the tap to one end, the voltage is stepped down. Variacs or Powerstats are trade names for autotransformers having a movable wiper-arm tap that allows varying voltage from 0 V to the maximum level. They are often used as a variable ac source in experimental setups.

  1. They isolate the circuitry connected to its secondary from the primary ac source (e.g., the ac power line). A 1:1 turns ratio transformer is specifically designed for isolation applications, Figure 3. (NOTE: All standard transformers isolate secondary from primary source regardless of turns ratios, except for autotransformers.)


Figure 2: Autotransformer

Figure 3: Using an isolation transformer for safety when using a powerstat or variac-type variable voltage autotransformer

When using this special 1:1 turns-ratio isolation transformer, the load on the secondary has no direct connection to the primary source. This provides a safety measure for people working with the circuitry connected to the isolation transformer secondary.

  1. They cannot operate at high frequency because of excessive losses that would be created.
  2. Just two examples of the many color codes in use are shown in Figures 4a and b.

Figure 4: Typical power and audio transformer color-coding systems

Safety Hints Caution

Bad shock can occur if no isolation transformer is used on the autotransformer input side, because there is no isolation from the raw line voltage and ground connections in the autotransformer circuit. For this reason, do not use autotransformers with a movable wiper-arm tap unless you plug the autotransformer into an isolation transformer output, which isolates it from the raw ac power source.

Practical Notes

Some power transformers are used when it is necessary to be able to select either a 120- or 240-V primary input source for the transformer. Therefore, there are transformers with two primary windings. These primary windings may be connected in series, when 240 V is the source (i.e., the end of one winding is connected to the beginning of the other winding, being careful to pay attention to phasing of the two windings).

Alternatively, by connecting the two primaries in parallel (again being careful of phasing), the transformer primary can be fed from a 120-V source. By using these techniques, the secondary voltage output will be the same with 120- or 240-V inputs, as appropriate.

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