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Rotor current frequency = Fractional slip ├Ч ________
Rotor speed
Supply frequency
No. of poles
EMF
Supply frequency
The rotor current frequency, denoted by frтАЛ, is directly proportional to the slip s of the induction motor and the stator supply frequency fsтАЛ. It is mathematically defined as the product of the fractional slip and the supply frequency.
The rotor current frequency, denoted by frтАЛ, is directly proportional to the slip s of the induction motor and the stator supply frequency fsтАЛ. It is mathematically defined as the product of the fractional slip and the supply frequency.
frтАЛ=s├ЧfsтАЛ тАФ where frтАЛ is rotor frequency, s is fractional slip, and fsтАЛ is stator supply frequency
In an induction motor, the rotor conductors are cut by the rotating magnetic field produced by the stator. The speed at which the magnetic field cuts the rotor conductors is the slip speed, NsтАЛтИТNrтАЛ. When the rotor is stationary, the frequency of the induced EMF is equal to the supply frequency fsтАЛ. As the rotor rotates at speed NrтАЛ, the relative speed decreases, causing the frequency of the induced rotor EMF and current to decrease proportionately by a factor of s.
At standstill, the slip s=1, so the rotor frequency is equal to the supply frequency fsтАЛ.
At synchronous speed, the slip s=0, resulting in zero rotor frequency (DC).
The rotor frequency is always much lower than the supply frequency in normal operating conditions.
Rotor frequency is responsible for the rotor reactance, XrтАЛ=2╧АfrтАЛLrтАЛ.
Explains the shift in torque characteristics as motor speed changes.
Provides a basis for analyzing rotor copper losses.
Determining the rotor copper loss calculation (Pcu,rotorтАЛ=s├ЧPagтАЛ).
Analysis of wound rotor induction motor performance.
The fractional slip s is defined as s=NsтАЛNsтАЛтИТNrтАЛтАЛ.
Option A is incorrect because rotor speed NrтАЛ affects frequency only indirectly via slip.
Option C is incorrect as the number of poles P is used to determine synchronous speed, not rotor frequency directly.
Option D is incorrect because EMF magnitude depends on flux and frequency, but is not the multiplier for frequency itself.
B is correct тАФ Rotor current frequency is given by the product of fractional slip and supply frequency.
Remember that at standstill, the induction motor acts like a transformer, hence rotor frequency equals supply frequency.
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