What is the effect on the synchronous speed of a three-phase induction motor when the supply frequency is increased while keeping the number of poles constant?
The synchronous speed increases
The synchronous speed of a three-phase induction motor is the speed of the rotating magnetic field, and it is fixed by only two factors: the supply frequency and the number of poles.
These are related by \(N_{s}=\dfrac{120f}{P}\), where f is the supply frequency and P the number of poles.
Here the number of poles P is held constant, so the equation reduces to a direct proportionality between Ns and f.
Therefore raising the supply frequency must raise the synchronous speed in the same ratio.
The speed cannot stay unchanged or fall, because f sits in the numerator and is being increased.
The field cannot reverse either, since direction depends on the phase sequence of the supply, not on its frequency.
Hence, the answer is The synchronous speed increases.
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