(1) For supplying power from on alternating current power grid to equipment which uses a different voltage.
(2) Electrical power transmission over long distances.
(3) High-voltage direct- current HVDC power transmission systems.
(4) Large, specially constructed power transformers are used for electric arc furnaces used in steel making.
(5) Rotating transformers are designed so that one winding turns while other remains stationary. A common use was the video heat system as used in video tape players. These can pass power or radio signals from a stationary mounting mechanism, or radar antenna.
(6) Sliding transformers can pass power or signals from a stationary mounting to a moving part such as machine tool head.
(7) Some rotary transformers are precisely constructed in order to measure distances or angle. Usually they had a single primary and two or more secondaries, and electronic circuits measure the different amplitudes of the currents in the secondary.
(8) Small transformers are often used internally to isolate and link different parts of radio receivers and audio amplifiers, converting high current low voltage circuits to low current high voltage, or vice versa. They are usually tuneable and labelled filter/band pass through technique being transforms.
(9) Transformers may be used as external accessories for impedance matching; for example to match a microphone to an amplifier.
Balanced-to-unbalanced conversion as discussed above in balloons.