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Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
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The frequency of rotor is obtained by
f
sf
s
sfтАЛ
sf
The rotor frequency (frтАЛ) of an induction motor is defined as the product of the supply frequency (f) and the slip (s). It represents the frequency of the induced EMF in the rotor conductors when the rotor is running at a speed N relative to the rotating magnetic field.
The rotor frequency (frтАЛ) of an induction motor is defined as the product of the supply frequency (f) and the slip (s). It represents the frequency of the induced EMF in the rotor conductors when the rotor is running at a speed N relative to the rotating magnetic field.
s=NsтАЛNsтАЛтИТNтАЛ тАФ Definition of slip
frтАЛ=sтЛЕf тАФ Calculation of rotor frequency
In an induction motor, the stator magnetic field rotates at synchronous speed NsтАЛ. The rotor rotates at speed N. The relative speed between the stator flux and the rotor is (NsтАЛтИТN). The slip s is defined as the ratio NsтАЛNsтАЛтИТNтАЛ. The frequency of the induced voltage in the rotor is proportional to this relative speed, leading to the relation frтАЛ=s├Чf.
At standstill (N=0), slip s=1, so frтАЛ=f.
At synchronous speed (N=NsтАЛ), slip s=0, so frтАЛ=0 (DC induced in rotor).
The rotor frequency is typically very low in normal operating conditions as s is usually small (2-5%).
Allows for easy determination of rotor speed if frequency is known.
Essential for calculating rotor copper losses (PcuтАЛ=sтЛЕPagтАЛ).
Speed control of induction motors.
Analysis of torque-slip characteristics.
Slip is dimensionless, usually expressed as a decimal or percentage.
Option D (f/s) is incorrect as it represents an inverse relationship that contradicts the physical principle of magnetic field interaction.
B is correct тАФ The rotor frequency frтАЛ is directly proportional to the slip s and the stator supply frequency f, given by frтАЛ=sтЛЕf.
Remember that frтАЛ is the frequency seen by the rotor conductors; if you ever get confused, just recall that when the motor is blocked (s=1), the rotor sees the full stator frequency f.