A generator is a device that converts mechanical energy to electrical energy for use in an external circuit. The source of mechanical energy may vary widely from a hand crank to an internal combustion engine. Generators provide nearly all of the power for electric power grids.
(source : https://en.wikipedia.org/wiki/Electric_generator#Induction_generator)

A.C. generators operate on the same fundamental principles of electromagnetic induction as D.C. generators. Alternating voltage may be generated by rotating a coil in the magnetic field or by rotating a magnetic field within a stationary coil. The value of the voltage generated depends on- the number of turns in the coil. strength of the field. the speed at which the coil or magnetic field rotates.

(Source : https://www.youtube.com/channel/UCoXkSUe6C7y5FzZySgzm3Xg)

An induction generator (or asynchronous generator) is a type of alternating current (AC) electrical generator. Induction generators are often used in wind turbines and some micro hydro installations due to their ability to produce useful power at varying rotor speeds. Induction generators are mechanically and electrically simpler than other generator types. They are also more rugged, requiring no brushes or commutators.

Induction generators are particularly suitable for wind generating stations as in this case speed is always a variable factor. Unlike synchronous motors, induction generators are load-dependent and cannot be used alone for grid frequency control.
(Source : https://en.wikipedia.org/wiki/Induction_generator#Use_of_induction_generators)

Recommended courses :

MIT OpenCourseWare - Electromagnetic Theory
"Electromagnetic Theory covers the basic principles of electromagnetism: experimental basis, electrostatics, magnetic fields of steady currents, motional e.m.f. and electromagnetic induction, Maxwell's equations, propagation and radiation of electromagnetic waves, electric and magnetic properties of matter, and conservation laws. This is a graduate level subject which uses appropriate mathematics but whose emphasis is on physical phenomena and principles."

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Electropaedia - Electrical Machines - Generators
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