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kda3310:
The fundamental point you need to understand about AC motors that are not of the universal type is:
1. The stator coils produce a rotating magnetic field in physical space from a 3 phase source. In the simplest form of a 3 phase motor it has 6 coils and 6 poles, with 2 each for each phase. When you analyze the stator portion of the motor when excited with 3 phase power you will find that a magnetic vector of approximately constant magnitude is produced from the stator coils that makes one physical revolution in space per cycle. If you double the number poles and coils and appropriately wire them, then the vector will rotate at 1/2 the speed of the 6 pole version or once each two cycles.
If you insert a shaft in the middle of the stator poles, and on this shaft mount a permanent magnet with a N and S pole so positioned to forcefully interact with the rotating magnetic vector, then you have a synchronous motor that in the 6 pole 60 Hz version rotates at exactly 3600 RPM. In the 12 pole version it is 1800 RPM.
Instead of a permanent magnet on the rotor you can mount a coil of wire connected to 2 slip rings and a DC supply. This produces the equivalent of a permanent magnet on the rotor.
2. If instead of making the rotor a permanent magnet a shorted coil is mounted on the rotor, then a current is induced in the coil from the changing magnetic field. This induced current produces a magnetic field that interacts with the rotating magnetic vector and causes rotation of the rotor. The rotor will never run at synchronous speed because slip is needed to induce a current in the rotor coil.
3. Three phase motors are inherently self stating.
Go to the library and find a book on the theory of alternating-current machines. A book I can recommend is "Alternating-Current Machinery", by Bailey and Gault, 1951, McGraw-Hill. This may not be easy to find.
True single phase motors are a different animal. There is no rotating magnetic vector in space. There is simply a vector at one spatial angle that oscillates from a + magnetic value to a - value and back to a + value in one AC cycle in a 2 pole motor. Put a permanent magnet rotor in this motor and it will probably just vibrate or hum. Give the rotor a little kick in either direction you choose and the rotor will rotate in that direction at synchronous speed.
This is like a child on a swing and each cycle of the swing at the correct time point you add some energy to the swing.
Does this help provide you with some part of a basic understanding?
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