whether synchronous or induction three-phase electric machines use rotating magnetic field for its operation to understand concept of rotating magnetic field for three-phase machines consider this simplified winding it has got just three coils they are arranged 120° apart magnetic field produced by a single wire carrying current will be as shown for this special Arrangement magnetic field produced by 3phase AC current will be aown at a particular instant now have a close look at how magnetic field varies as current in coil varies you can notice that orientation of magnetic field changes with current but its magnitude Remains
the Same from these three positions it's clear that it is like a magnetic field of uniform strength rotating speed of rotation of magnetic field is known as synchronous speed the big question how to quantify synchronous speed this simplified winding has got just two poles you can notice that magnetic flux is able to rotate through two poles when current take one complete Revolution so for two pole case synchronous speed is same as frequency qu of AC current here is a case of winding with four poles for the four pole system magnetic field will again rotate through
two poles for full revolution of current so you can note that speed of magnetic field has become half this time so for General P pole system synchronous speed is given by this equation in an actual motor there can be two to 14 poles this motor is having 10 poles so for a 60 HZ power supply magnetic field will rotate at this much angular velocity such a rotating magnetic field can make the rotor rotate for induction as well as synchronous motor
