Explaining Different Types of Transformers
Transformers are used to meet a wide variety of needs. Some transformers can be several stories high, like the type found at a generating station or small enough to hold in your hand, which might be used with the charging cradle for a video camera. No matter what the shape or size, a transformers purpose remains the same: transforming electrical power from one type to another.
There are many different types of transformers in use today. This resource will take a closer look at Power Transformers, Auto Transformers, Distribution Transformers, Instrument Transformers, Isolation Transformers, Potential Transformers and Current Transformers.
How Transformers Work
There are four primary parts to the basic transformer. The parts include the Input Connection, the Output Connection, the Windings or Coils and the Core.
- Input Connections - The input side of a transformer is called the primary side because the main electrical power to be changed is connected at this point.
- Output Connections - The output side or secondary side of the transformer is where the electrical power is sent to the load. Depending on the requirement of the load, the incoming electric power is either increased or decreased.
- Winding - Transformers have two windings, being the primary winding and the secondary winding. The primary winding is the coil that draws power from the source. The secondary winding is the coil that delivers the energy at the transformed or changed voltage to the load. Usually, these two coils are subdivided into several coils in order to reduce the creation of flux.
- Core - The transformer core is used to provide a controlled path for the magnetic flux generated in the transformer.
The core is generally not a solid bar of steel, rather a construction of many thin laminated steel sheets or layers. This
construction is used to help eliminate and reduce heating.
Transformers generally have one of two types of cores: Core Type and Shell Type. These two types are distinguished from each other by the manner in which the primary and secondary coils are place around the steel core.
- Core type - With this type, the windings surround the laminated core.
- Shell type - With this type, the windings are surrounded by the laminated core.
The ratio between the number of actual turns of wire in each coil is the key in determining the type of transformer and what the output voltage will be. The ratio between output voltage and input voltage is the same as the ratio of the number of turns between the two windings.
A transformers output voltage is greater than the input voltage if the secondary winding has more turns of wire than the primary winding. The output voltage is stepped up, and considered to be a "step-up transformer". If the secondary winding has fewer turns than the primary winding, the output voltage is lower. This is a "step-down transformer".
- Single-phase Power - Single-phase transformers are often used to supply power for residential lighting, receptacle, air-conditioning, and heating needs. Single phase transformers can be made even more versatile by having both the primary winding and secondary winding made in two equal parts. The two parts of either winding can then be reconnected in series or parallel configurations.
- Three-phase Power - Power may be supplied through a three-phase circuit containing transformers in which a set of three single-phase transformers is used, or on three-phase transformer is used. When a considerable amount of power is involved in the transformation of three-phase power, it is more economical to use a three-phase transformer. The unique arrangement of the windings and core saves a lot of iron.
- Delta and Wye Defined - There are two connection configurations for three-phase power: Delta and Wye. Delta and Wye are Greek letters that represent the way the conductors on the transformers are configured. In a delta connection, the three conductors are connected end to end in a triangle or delta shape. For a wye, all the conductors radiate from the center, meaning they are connected at one common point.
- Three-phase Transformers - Three-phase transformers have six windings; three primary and three secondary. The six windings are connected by the manufacturer as either delta or wye. As previously stated, the primary windings and secondary windings may each be connected in a delta or wye configuration. They do not have to be connected in the same configuration in the same transformer. The actual connection configurations used depend upon the application.
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 two0winding transformer. The autotransformer is considered unsafe for use on ordinary distribution circuits. This is because the high-voltage primary circuits are connected directly to the low-voltage secondary circuit.
Technically, any true transformer, whether used to transfer signals or power, is isolating, as the primary and secondary are not connected by conductors but only by induction. However, only transformers whose primary purpose is to isolate circuits (opposed to the more common transformer function of voltage conversion), are routinely described as isolation transformers.
There are two types of instrument transformers:
- Current - Used with an ammeter to measure current in AC voltages
- Potential - Used with a voltmeter to measure voltage (potential difference) in AC.
Current Transformers are a type of instrument transformers. They are used for the measurement of electric currents.A current transformer has a primary coil of one or more turns of heavy wire. It is always connected in series in the circuit in which current is to be measured. The secondary coil is made up of many turns of fine wire, which must always be connected across the ammeter terminals. The secondary of a current transformer must never be open-circuited. This is because the primary is not connected to a constant source. There is a wide range of possible primary voltages, because the device can be connected to many types of conductors. The secondary must always be available (closed-circuited) to react with the primary, to prevent the core from becoming completely magnetized. If this happens, the instruments will no longer read accurately.
A clamp-on ammeter works in a similar way. By opening the clamp and placing it around a current carrying conductor, the conductor itself acts as a single turn primary. The secondary and the ammeter are conveniently mounted in the handle of the device. The dial allows a number of current ranges to be gauged accurately.
In general, a potential transformer is very similar to a standard two-winding transformer, except that it handles a very small amount of power. Transformers for this service are always the shell type, because this construction has been proven to provide better accuracy.
Potential Transformers (like the one pictured above) are designed for monitoring single-phase and three-phase power line voltages in power metering applications.
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