A transformer may be step up or step down. If the output voltage is greater than the input voltage, the transformer is said to be a step-up transformer. A transformer is said to be a step-down transformer if the output voltage is less than the input voltage. This classification is carried out on the basis of the ratio of input and output voltages. From application point of view, the following transformers are most important:
- Power and distribution transformer: These transformers are used for transmission and distribution of power.
- Autotransformer: These transformers are used to change the voltage within relatively small limits and are used for starting AC motors, and so on.
- Transformers for feed installations with static converters: These are used for converting AC to DC and also DC to AC. The first one is used for rectification purposes and the second one for inversion purposes.
- Testing transformers: These are used to conduct tests at high and ultra-high voltages.
- Power transformers for special applications: These are used in furnaces, welding and so on.
- Radio transformers: These are used in radio engineering and similar purposes.
From frequency range point of view, transformers can be divided as (50–400 Hz) audio transformer, wide band and narrow band transformers and pulse transformers. Transformers can also be divided depending on the number of windings such as two winding (conventional) and single winding known as autotransformer.