What determines the frequency of alternating current?

I am certain this question has been asked before but I search and cannot see anything just yet. I read most AC current is 60 cycles per second in the United States and some say 50 but that does not matter to the question.

I am thinking a typical electric generator belonging to a hydroelectric plant generating power. The generator has some value for the revolutions per second as the water produces mechanical motion to spin its rotor. As the rotor spins the polarity switches from positive negative and visa versa. Is it the case that this value is the same as the 60 cycles per second that feeds into my home?

The transformer I believe does not affect frequency so must the frequency produced at the generator match the value at the electric plug. It is such a precise value I have difficulty thinking it is the speed of the generator but I do not know so I ask.

• You are correct. Number of poles matters too. It is possible to produce multiple electrical cycles from a single mechanical revolution. That way, the turbine can spin more slowly so the water can be traveling slower, but pushing harder. They continuously adjust the speed to keep it so precise. They do this by adjusting the strength of the magnets (field) which changes the mechanical resistance/torque but also changes the sine wave amplitude. I don't know how or if they control RPM and amplitude independently. Aug 4, 2020 at 14:22
• @DKNguyen I guess we lust built the speed control sytem over generator by control water flowrate to fix frequency after that voltage can use magnetic field strength to adjust output power. Aug 4, 2020 at 14:30
• @Mlab Ah that makes sense. Throttle back the water or steam. Two degrees of freedom. Aug 4, 2020 at 14:38
• Across the whole grid, the frequency actually drops when load is added, because those large spinning machines slow down slightly. Not much ... maybe 0.01 or 0.1Hz. Network operators watch this frequency drop and start another generator if necessary. (Slightly over-simplified). In the UK you can watch the grid frequency in real time, at gridwatch.templar.co.uk Mayeb you can find a similar site for the US.
– user16324
Aug 4, 2020 at 14:56
• Rotor speed is controlled by the governor (input power), not by the field. Aug 4, 2020 at 16:31