A transformer is a device working on mutual inductance principle to convert electrical energy from one voltage/ current level to another. There are different types of transformers used for different purposes.
(1) POWER TRANSFORMER
- Laminated core Transformer

- Toroidal Transformer

- Auto transformer
- Variable auto transformer

- Induction regulator Transformer(high voltage transformer)

- Polyphase transformer

- Grounding transformer
- Phase-shifting transformer
- Variable-frequency transformer(three-phase transformer)

- Leakage or stray field transformers

- Resonant transformer
- Constant voltage transformer
- Ferrite core transformer
- Planar transformer

- Oil cooled transformer

- Cast resin transformer
- Isolating transformer
(2) Instrument transformer

- Current transformer

- Voltage transformer or potential transformer
- Combined instrument transformer

(3) Pulse transformer

(4) RF transformer
- Air-core transformer
- Ferrite-core transformer
- Transmission-line transformer
- Balun transformer
(5) IF transformer
(6) Audio transformer
- Two speaker level audio transformer

- Five speaker level audio transformer

- Loudspeaker transformer

Output transformer
- Small signal transformer
- Inter stage and coupling transformers
Other types of transformers are
- Hedgehog transformer
- Rotary transformer
- Variable differential transformer