Types of Transformers

If we look at an entire electrical utility system, we find that transformers are used to meet a wide variety of needs. Some transformers can be several stories high. This type might be found at a generating station. Other transformers are small enough to hold in your hand. This type might be used with the charging cradle for a video camera. No matter what the shape or size, the purpose remains the same: transforming electrical power from one type to another, such as stepping up or stepping down the power.

Let's take a closer look at several transformer types in use today.

Power Transformer

The Power Transformer is used primarily to couple electrical energy from a power supply line to a circuit system, or to one or more components of the system. In one type of power transformer, there are three separate secondary windings, each designed for a different current and voltage output. All of the windings are identified by color-coded insulation on the wire leads.

A power transformer used with solid state circuits is called a Rectifier Transformer. A power transformer's rating is given in terms of the secondaries' maximum voltage and current-delivering capacity.

For example, a power transformer that is to be operated from a 60-hertz, 120-volt power line may read:

600 V. CT @ 90 ma, 6.3 V @ 3 amp, 5 V. @ 2 amp

The efficiency of a power transformer with a power rating between 50 and 400 watts can range from 82 to 94 percent.

Transformers with higher power ratings are usually more efficient because of the larger wire gauge used in the windings, and the larger cross-sectional area of the core.

Distribution Transformer

A pole-type Distribution Transformer is used to supply relatively small amounts of power to residences. It is used at the end of the electrical utility's delivery system. Let's trace the path of this power from the generating station to a house.

Power transformers are used at generating stations to step up the generated voltage to high levels (115 to 765 kV) for transmission. Transmission voltages are stepped down (34 or 69 kV) by transformers at substations for local distribution.

From this point, the electrical power is fed to a distribution substation or even directly to a factory. At the factory, transformers once again step the voltage down to usable levels. For home use, the voltage is stepped down in a number of steps, with the last step handled by the local pole-type or pad-mounted (mounted on the ground) distribution transformer.

After all of these steps, the homeowner has access to 240 and 120 volts for running appliances and lighting.

Autotransformer
The Autotransformer is a special type of power transformer. It consists of a single, continuous winding that is tapped on one side to provide either a step-up or step-down function.

This is different from a conventional two-winding transformer, which has the primary and secondary completely insulted from each other, but magnetically linked by a common core. The autotransformer's windings are both electrically and magnetically interconnected. An autotransformer is initially cheaper than a similarly-rated two-winding transformer. It also has better regulation (smaller voltage drops), and greater efficiency. Furthermore, it can be used to obtain the neutral wire of a three-wire 240/120-volt service, just like the secondary of a two-winding transformer.

It is commonly used to transform between two high-voltage circuits, say one at 22,000 volts and the other at 13,800 volts. But, the autotransformer is considered unsafe for use on ordinary distribution circuits. This is because the high-voltage primary circuit is connected directly to the low-voltage secondary circuit.

Isolation Transformer
An Isolation Transformer is a very unique transformer. It has a 1:1 turns ratio. Therefore, it does not step voltage up or down. Instead, it serves as a safety device. It is used to isolate the grounded conductor of a power line from a chassis or any portion of a circuit load.

Using an isolation transformer does not reduce the danger of shock if contact is made across the transformer's secondary winding.