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Nikola Tesla: Research in High Frequency Phenomena, 1894

Two-Phase Induction Motor

In the two-phase motor, two sets of coils are set perpendicular to each other surrounding the core. When alternating current is sent to the coils, they become electromagnets where polarity rapidly changes with each reversal of current flow. As the first coils are supplied with current, they create a magnetic field which starts the core turning. When the first coils' current supply reverses, the second coil set is at its maximum supply point and creates its own magnetic field; the core spins on. In effect the "magnetization" amount never varies and a rotating magnetic field is created. The result is a smooth-running, commutator-free motor with the rotor as its only moving part.