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I quote from my book-

When a synchronous generator's rotor is spun, a voltage EA is induced in the generator's stator windings. If a load is attached to the terminals ofthe generator, a current flows. But a three-phase stator current flow will produce a magnetic field of its own in the machine. This stator magnetic field distorts the original rotor magnetic field, changing the resulting phase vo ltage. This effect is called armature reaction because the armature (stator) current affects the magnetic field which produced it in the first place."

When the rotor magnetic filed BR crosses the coil EA is at its maximum value. So, BR and EA coincides. A lagging load is attached with the generator. So the current will lag. Assuming IA where it is in the picture. My question is, why is BS is perpendicular to IA. It should be perpendicular to the plane the coil is on, in other words, it should be perpendicular to EA applying the right hand rule. Align all your fingers to the direction of current flow, the thumb will show the magnetic field direction.

  • \$\begingroup\$ Coil EA is shown where in your diagrams? What is it about the quote that you feel is either relevant or needs explaining to you? What do diagrams (a) to (d) represent? What precisely is current IA - i.e. in what is it flowing? \$\endgroup\$ – Andy aka Nov 27 '14 at 10:03

When the synchoronous generator loaded, an angle δ occurs between stator magnetic field and rotor magnetic field. It is called "load angle". Hence the IA represents the stator magnetic fied and the BS is now perpendicular to this surface. Now the terminal voltage VΦ is related to Bnet, Bnet=BS+Br. You can observe exact load angle δ between VΦ and EA in your figure (d). Also the load angle δ is shown on voltage vector diagram between Ea and VΦ. Sorry my poor English. I hope i can explain it.

  • \$\begingroup\$ How does I_A_MAX represent the stator magnetic field. It represents the current through the stator conductor. Guessing it as a writing mistake, my question is actually, a line of your answers. WHY B_S is perpendicular to I_A_MAX????? The current is coming out of the page above and going into the page under. So, by the right hand rule, should not the resulting magnetic flux be perpendicular to the surface the coil is? Please explain. \$\endgroup\$ – Quazi Mishkatul Alam Nov 27 '14 at 12:10
  • \$\begingroup\$ when the generator loading with lagging load, the maximum stator current IAmax obtained in the stator coil which is behind the φ angle according to the rotation (in figure (b) the angle φ is between EAmax and IAmax). Estat is the armature reaction voltage and Estat=-jXIA. The dotted line represents the IAmax lie on coil that lagging as φ from EAmax line. Hence the B_S is perpendicular to this coil surface. \$\endgroup\$ – ferahfeza Nov 27 '14 at 12:43

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