Autotransformers
(74)A transformer is a device used to increase or decrease AC voltage as well as transfer alternating current from one circuit to another. Its operation is based on the phenomenon of electromagnetic induction. Autotransformers are a special type of transformers.
Autotransformer – what is it?
An autotransformer is a type of transformer with a single winding. In an autotransformer, a part of the winding is shared by both the primary and secondary voltage. Both the supplying and the receiving side of the autotransformer are electrically connected. Autotransformers can have a fixed or variable number of turns in the secondary winding, which means that they can be adjusted as needed. It can be done smoothly (using a sliding brush) or by means of stepped regulation (done with a tap changer), which allows operators to control the process of electrical energy transformation.
In the case of autotransformers, the concept of the rated accuracy power, i.e. the apparent power transferred from the primary to the secondary part, is of key importance. Rated power, on the other hand, applies to the apparent power at rated operating conditions (at a rated current and voltage).
Pros and cons of autotransformers
Autotransformers have a number of advantages, such as small weight, reduced power loss and higher efficiency compared to conventional transformers, which is why they are sometimes called “energy efficient transformers”. However, there are also some drawbacks. First of all, all interference which appears at the secondary part is transferred directly to the primary one (such interference is of galvanic, not magnetic nature, i.e. caused by direct electrical contact) and the other way around. This may prove to be very dangerous for users, operators and maintenance technicians. What is more, the short-circuit current is much higher than the one which occurs in standard transformers.
Despite this, autotransformers can be employed in a wide range of applications where their advantages outweigh the disadvantages. It is important to remember, however, that we should use the component in line with its intended purpose and take the necessary safety precautions.
Single-phase and three-phase autotransformers
Single-phase autotransformers are often utilized in technical laboratories in order to regulate wide ranges of voltage. They enable the operator to adjust voltage with great precision, which is crucial for conducting experiments and tests on electrical devices. Three-phase autotransformers, on the other hand, can be used in a number of industrial applications, such as:
- Start-up of induction or synchronous motors – it decreases the load on the power grid during the motor start-up, which ensures smooth start and a lower risk of overload.
- Voltage regulation in power lines – it ensures smooth adjustment of voltage in power lines, which enables better load management and lowers the risk of interference.
- Connecting power grids which transmit different voltages for more efficient management of electric power transmission.
In industrial applications, three-phase autotransformers translate into greater flexibility and efficiency of electric power transmission management.
Variable autotransformers
Apart from fixed autotransformers, variable autotransformers are also available. Variable autotransformers enable smooth or gradual adjustment of voltage at the output.
The winding of a variable autotransformer is fitted with tap changers, which make it possible to change the output voltage in a stepwise manner. Thanks to this, the user can modify output voltage according to the requirements of a given device or system.
Variable autotransformers in which voltage can be adjusted smoothly feature a sliding brush used for output voltage regulation. Such autotransformers come in handy whenever we need to alter voltage at a slower pace.
Variable autotransformers provide the user with more flexibility and control over voltage, compared to standard transformers, which may prove useful in a number of industrial, laboratorial and home applications.
Transformer rating plates
A nameplate attached to the transformer informs the user of the year of manufacture, manufacturer name or symbol, transformer type according to standard markings, permissible ambient temperature range, standards (standard number) according to which the device was manufactured, its serial number, rated power (apparent power measured in VA or kVA), number of phases, short-circuit voltage, rated voltages, rated currents, no-load loss (iron loss), frequency, load loss (copper loss) at a rated load, IP rating, type of cooling, total weight, isolation class and a vector group for multi-phase transformers.
If you plan to purchase and use an autotransformer, it is crucial that you choose a device which offers optimum parameters for its intended application.
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