Ac machines: Rotating Magnetic Field and its production

This is a presentation that mainly covers knowledge about the rotating magnetic field in induction motors and its production. It also covers the main conditions for the production of RMF. I hope you find it useful. :)

1. Topic : Rotating Magnetic Field and its production

2. Rotating M agnetic Field (R.M.F) has constant amplitude but keeps on rotating continuously in a plane with a certain speed. Is produced with the help of three phase stationary windings Current winding produces the magnetic field. Due to the interaction of the three fluxes produced in the windings, RMF is produced without physically rotating the rotor.

3. Circuit diagram for three phase induction motor

4. Production Of RMF: The three phase windings are displaced from each other by 120° e. The windings are supplied by a balanced three phase ac supply. The three phase currents flow simultaneously through the windings and are displaced from each other by 120° electrical. Each alternating phase current produces its own flux which is sinusoidal. So all three fluxes are sinusoidal and are separated from each other by 120°. If the phase sequence of the windings is R-Y-B, then mathematical equations for the instantaneous values of the three fluxes ΦR , ΦY ,ΦB can be written as, ΦR = Φmsin ( ωt ) ΦY = Φmsin ( ωt - 120) ΦB = Φmsin ( ωt - 240)

5. As windings are identical and supply is balanced, the magnitude of each flux is  Φm  .  

6. Case 1 : ω t = 0 Φ R = Φ msin (0) = 0 Φ Y = Φ msin (-120) = -0.866 Φ m Φ B = Φ msin (120) = +0.866 Φ m Case 2 : ω t = 60 Φ R = Φ msin (60) = +0.866 Φ m Φ Y = Φ msin (- 60) = -0.866 Φ m Φ B = Φ msin (- 180) = 0 Case 3 : ω t = 120 Φ R = Φ msin (120) = +0.866 Φ m Φ Y = Φ msin (180) = 0 Φ B = Φ msin (-120) = -0.866 Φ m Case 4 : ω t = 180 Φ R = Φ msin (180) = 0 Φ Y = Φ msin (60) = +.866 Φ m Φ B = Φ msin (-60) = -0.866 Φ m By comparing the electrical and phasor diagrams we can find the flux rotates one complete 360 degree on the 180 degree displacement of flux.

7. Conditions for Production of RMF The stator 3- phase winding should be placed at 120 degrees is space The current supply to these winding should be balanced. The direction of rotation of the magnetic field can be varied according to the phase sequence. A three-phase winding displaced in space by 120 is fed by a three-phase current displaced in time by 120 : It produces a resultant magnetic flux which rotates in space as if actual magnetic poles were being rotated mechanically.

8. Conclusions: The magnitude of the Rotating magnetic field is always constant i.e. its value remains the same at any instant of time. The direction of RMF is decided according to the phase sequence of the winding The speed of rotation of the RMF is equal to the angular frequency of the supply voltage which in a way depends on the synchronous speed of the machine.

9. THANK Y OU