In an alternator, frequency per revolution is equal to

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Option D: number of pole-pairs

Explanation & Concepts:
In an alternator, the frequency of the generated voltage is determined by the number of pole pairs passing a conductor in a given time. Since each full revolution contains P/2 pole-pairs, the frequency per revolution is exactly equal to the number of pole-pairs.

Working Principle: An alternator generates AC voltage through electromagnetic induction. As the rotor turns, each pair of N-S poles induces one complete cycle (positive and negative half-cycle) in the stator windings. Therefore, if there are 'P' poles, there are 'P/2' pole-pairs, resulting in 'P/2' cycles per mechanical revolution.

Key Formulas: $f = \frac{P \cdot N}{120}$ — where $f$ is frequency (Hz), $P$ is number of poles, and $N$ is speed (rpm); $f_{rev} = \frac{P}{2} = p$ — where $p$ is the number of pole-pairs

Key Points: One cycle of voltage is generated when a pair of North and South poles passes a stator conductor.; A machine with $P$ poles has $P/2$ pairs of poles.; Mechanical speed and electrical frequency are directly proportional in synchronous machines.; The SI unit of frequency is Hertz (Hz), representing cycles per second.

Advantages: Predictable frequency control via speed regulation; High efficiency in power generation

Disadvantages: Requires constant speed (synchronous speed) to maintain fixed frequency; Frequency depends on construction (number of poles)

Applications: Hydro-electric power plants; Thermal power plants; Industrial synchronous generators

Additional Info: The term 'pole-pair' is denoted by the letter 'p', where $p = P/2$.. Option A is incorrect because the number of poles gives the number of half-cycles per revolution.. Option C refers to rotational speed, not the electromagnetic frequency conversion factor.

Answer: D is correct — The frequency of an alternator per mechanical revolution is defined by the number of pole-pairs ($p = P/2$).

Exam Tip: Always remember that 1 revolution = $P/2$ cycles, so to find frequency in Hz, multiply the pole-pairs by the rotational speed in revolutions per second (rps).

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Option A: number of poles

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Option B: twice the number of poles

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Option C: speed in rps

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