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In three-phase the 2nd and 3rd phase are offset by about 120 degrees(\$\frac{2\pi}{3}\$ radians). But how is this achieved?

The concept that just each phase is offset by 120 degrees sounds easy but designing a circuit for this looks more difficult.

Does it go back to when the electricity is generated at the power station or done later on in the transmission?

For example each generator is configured to generate one phase slightly later than another?

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There are 3 phase alternators that produce a 3 phase supply which is then transmitted over 3 or 4 wire systems.One first needs to understand how a single phase voltage is generated based on Faraday's laws of electromagnetic induction.The generated voltage is, among other things, also dependent on the (sine of the) angle between the magnetic field and the coil in which the voltage is induced.Now in a 3 phase generator, the the magnets and the coils are aligned (mechanically) in such a way that the angles are 120 degrees apart.Consequently, the instantaneous voltage of each phase offsets the other phases by 2π/3 radians.

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The phases are 120 degrees apart because the generators have 3 poles which are mechanically 120 degrees apart. When you spin the generator armature at any constant speed, in a single north-south magnetic field, three phases separated by 120 degrees is the only result possible.

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