• When a three-phase voltage is applied to the stator winding, a rotating magnetic field of constant magnitude is produced.
• This rotating field is produced by the contributions of space-displaced phase windings carrying appropriate time displaced currents.
• These currents, which are time displaced by 120 electrical degrees, are shown below-
• The principle of a 3-phase, two-pole stator having three identical windings placed 120 space degrees apart is shown in above figure.
• The flux (assumed sinusoidal) due to three-phase windings is shown in above figure.
• The assumed positive directions of the fluxes are shown in figure. Let the maximum value of flux due to any one of the three phases be m.
• The resultant flux r, at any instant is given by the vector sum of the individual fluxes. 1, 2, and. 3 due to three phases.
The constant magnetic flux is rotating at a synchronous speed passes through the air-gap, sweeps past the rotor surface and so cuts the rotor conductors which as yet stationary
• Due to the relative speed between the rotating flux and stationary conductor, an emf is induced in the conductors.
• The frequency of the induced emf is same as the supply frequency, magnitude is proportional to the relative velocity between the flux and the conductors and its direction is given by Fleming’s right hand rule
• Since the rotor conductors form a closed circuit, rotor current is produced whose direction, as given by Lenz’s law, is such as to oppose the cause producing it.
• In this case the cause, which produces the rotor current, is relative speed between the rotating flux and stationary rotor conductors.
• Hence, the rotor starts running in the same direction as that of the rotating flux and tries to catch up with the rotating flux.
• This operation is explained in the diagram clearly.
Slip:
• The rotor never catches up the stator’s rotating flux. Always there is a difference between the rotor and rotating flux.
• The difference between the synchronous speed Ns and the actual speed of the rotor N is known as Slip.
• When the rotor is stationary, the frequency of the rotor current is equal to the supply current.
• When the rotor starts revolving, then the frequency depends upon the slip speed.
• Let at any slip speed, the frequency of the rotor current is f ’. Then
• This rotating field is produced by the contributions of space-displaced phase windings carrying appropriate time displaced currents.
• These currents, which are time displaced by 120 electrical degrees, are shown below-
• The principle of a 3-phase, two-pole stator having three identical windings placed 120 space degrees apart is shown in above figure.
• The flux (assumed sinusoidal) due to three-phase windings is shown in above figure.
• The assumed positive directions of the fluxes are shown in figure. Let the maximum value of flux due to any one of the three phases be m.
• The resultant flux r, at any instant is given by the vector sum of the individual fluxes. 1, 2, and. 3 due to three phases.
The constant magnetic flux is rotating at a synchronous speed passes through the air-gap, sweeps past the rotor surface and so cuts the rotor conductors which as yet stationary
• Due to the relative speed between the rotating flux and stationary conductor, an emf is induced in the conductors.
• The frequency of the induced emf is same as the supply frequency, magnitude is proportional to the relative velocity between the flux and the conductors and its direction is given by Fleming’s right hand rule
• Since the rotor conductors form a closed circuit, rotor current is produced whose direction, as given by Lenz’s law, is such as to oppose the cause producing it.
• In this case the cause, which produces the rotor current, is relative speed between the rotating flux and stationary rotor conductors.
• Hence, the rotor starts running in the same direction as that of the rotating flux and tries to catch up with the rotating flux.
• This operation is explained in the diagram clearly.
• The rotor never catches up the stator’s rotating flux. Always there is a difference between the rotor and rotating flux.
• The difference between the synchronous speed Ns and the actual speed of the rotor N is known as Slip.
• When the rotor is stationary, the frequency of the rotor current is equal to the supply current.
• When the rotor starts revolving, then the frequency depends upon the slip speed.
• Let at any slip speed, the frequency of the rotor current is f ’. Then
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