Question

In an a.c. generator the speed at which the coil rotates is doubled. How would this affect the frequency of output voltage?
(A) frequency is doubled.
(B) frequency is halved.
(C) frequency remains the same.
(D) can’t say.

Answer 1

An alternating current generator is an instrument that creates a potential difference. A simple alternating current generator is made up of a coil of wire rotating in a magnetic field. The AC generators operate on the principle of Faraday's law of electromagnetic induction which explains that the electromotive force EMF or voltage is produced in a current-carrying conductor that restricts a uniform magnetic field.

Complete step by step answer:
The AC generators are made up of one or more magnetic pole pairs in touch to their roles. The frequency of an AC generator is depending on the number of pairs of poles and the speed of the rotation of the coil. The frequency of the influenced AC current related to the revolution of the coil, the rate of revolution is expressed as Revolutions Per Minute, and frequency is expressed as cycles per second, the frequency of the induced AC current will be the rate of revolution of the coil divided by 60 seconds.
That is,
$\Rightarrow F = \dfrac{{RPM \times n}}{{60\,\,s}}$
Where $F$ denotes the frequency, $RPM$ denotes the revolution of the coil in one minute, $n$ denotes the number of pole pairs.
When the Revolutions per minute is doubled, then, obviously the frequency of the ac current will also be doubled as the $RPM$ is multiplied with the number of pole pairs. in an a.c generator when the speed at which the coil rotates is doubled the frequency is also doubled.

Therefore, in an a.c generator when the speed at which the coil rotates is doubled the frequency is also doubled. Hence the option (A) frequency is doubled is the correct answer.

Note:
An induction generator creates an electrical power when its rotor is rotated faster than the coincident speed. That is for a regular four-pole motor operating on a $60\,\,Hz$ electrical grid, the coincident speed is $1800$ rotations per minute.