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I'm struggling to get my head around induction motors. Specifically, from what I can see the rotating magnetic field created by the 3 phases creates flux lines which sweep across the rotor bars which induces current in the rotor. Right? So why does this need to be AC? I've also read that the alternating current induces current in a rotor.

Basically, what causes the induction? The alternating current or the rotating magnetic field?

Apologies for the noon question but my research suggests that both create induction

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  • \$\begingroup\$ if it wasn't DC, how would the magnetic fields move? \$\endgroup\$ – Marcus Müller Dec 30 '19 at 18:16
  • \$\begingroup\$ also, induction is pretty well-defined: it's current caused by magnetic field change. \$\endgroup\$ – Marcus Müller Dec 30 '19 at 18:16
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    \$\begingroup\$ Induction induces a voltage not a current. Current flows due to the induced voltage and the shorted nature of the rotor. \$\endgroup\$ – Andy aka Dec 30 '19 at 19:10
  • \$\begingroup\$ Sorry mate I'm doing my best here. Current causes by magnetic field change. Change being the flux lines caused by the rotation or the alternating current? A brushless dc motor has a rotating magnetic field but has magnets on the rotor so no induction from the rotating magnetic field? \$\endgroup\$ – Barryd Dec 30 '19 at 19:14
  • \$\begingroup\$ @Barryd There is still is. It's just called armature reaction in that case because it's not intended to have a significant role in operation for that kind of design. \$\endgroup\$ – DKNguyen Dec 30 '19 at 19:36
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So why does this need to be AC?

Three-phase AC current makes the magnetic field rotate.

Basically, what causes the induction?

The rotating stator field induces AC voltage across the rotor conductors resulting in AC rotor currents. Those currents produce a magnetic field that rotates with respect to the rotor.

The above is the intuitive explanation. The motor can also be viewed as a device that has an equivalent circuit that is a transformer

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In 1820, Oersted discovered that current flowing in a wire produces a magnetic field around the wire. In 1831, Faraday discovered that if you move a wire through a magnetic field, it "induces" electricity to flow in that wire. In basic AC induction motors there are no permanent magnets, the magnets are electromagnets. If you applied DC, the magnetic fields would be expand around the wires (coils) when you first energize them, then stay that way, so nothing would move. With AC power, the current going into the electromagnets is constantly changing from zero, to full positive, to zero, then full negative, then repeat. All the while, the magnetic fields in those coils are then expanding, then collapsing, then expanding again, over and over. So by virtue of that relative "motion" of the magnetic fields moving across the rotor bars as they expand and contract across them, they induce current to flow in them. The current flowing in the rotor bars then creates its own magnetic fields, which are in opposition to those in the stator, repelling from one another and making the rotor spin.

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