MadSci Network: Engineering

Re: about the three phase motors

Date: Sun Dec 30 15:50:03 2012
Posted By: Martin Smith, Engineering, B.E., M.EngSc., Uni of Qld / airline pilot
Area of science: Engineering
ID: 1356877656.Eg

All electric motors and generators work by having some sort of rotating magnetic field.  In a 
motor a rotating magnetic field is induced around a rotor and the rotor is "pulled" around by 
that rotating magnetic field.  We call this a torque, or a turning force.

In a two phase motor the phases are 180 degrees apart and the field simply flips from one 
side to the other.  This means if the rotor is lined up with one of the poles it won't be pulled 
around, or if slightly to the "wrong side" of one of the poles could be pulled backwards and 
rotate in the opposite to desired direction.

In a three phase motor the windings are 120 degrees apart so no matter where the rotor is 
resting the magnetic field will always be pulling the rotor around and in the right direction.

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