Constant-current transformer
Constant-current transformer two-coil transformer with a moveable secondary coil used to provide constant output current to a variable load. Constant current is maintained by mounting both the primary and secondary coils on the center element of a shelltype core and allowing the secondary coil to move up and down with changes in demand for load current. Increasing current demand due to a reduction in load impedance causes the secondary coil to move away from the primary coil. Increasing the coil separation increases flux leakage and reduces the secondary output voltage. The reduced output voltage counteracts the demand for more current. Increases in load impedance reverse the process. Movement of the secondary coil is controlled automatically by attaching the secondary coil to a counterweight and pulley assembly and orienting the coil windings such that their flux directions oppose. Increases in secondary current increase the magnetic repulsion between the coils, which, aided by the counterweight, moves the secondary coil away from the primary. Reductions in secondary current produce the opposite effect.
Source: http://ee.sharif.edu/~lcsl/useful/Electrical%20Engineering%20Dictionary.pdf
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Author of the text: Ed. P.A. Laplante Boca Raton: CRC Press LLC, 2000
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