Which factors determine the synchronous speed of the rotating magnetic field in a three-phase induction motor?
Supply frequency and number of poles
In a three-phase induction motor the stator windings, fed from the three-phase supply, set up a magnetic field that rotates around the stator bore at a fixed speed called the synchronous speed.
This speed is determined solely by how fast the supply alternates and by how the windings are arranged into magnetic poles, expressed by \(N_s=\dfrac{120f}{P}\), where f is the supply frequency in hertz and P is the number of poles.
So raising the frequency speeds the field up, and using more poles slows it down; only these two quantities appear in the formula.
Rotor resistance and load torque affect the slip and the actual rotor speed, but they do not alter the speed of the stator field itself.
Stator voltage and power factor influence torque, current and efficiency, yet the field still rotates at the same synchronous speed regardless of voltage.
Rotor current and slip are consequences of loading; they change with load but leave the synchronous speed untouched.
Hence, the answer is Supply frequency and number of poles.
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