Synchronous Generators

Every large coal, gas, nuclear, and hydro plant on the grid uses a synchronous generator whose rotation is locked to the grid's electrical frequency. At 60 Hz, a two-pole generator spins at exactly 3,600 RPM. If it slows, the frequency drops. If it speeds up, the frequency rises. The grid's frequency is, quite literally, the heartbeat of every spinning generator connected to it.

A synchronous generator converts mechanical rotation into AC electricity at a frequency determined by f = (RPM x poles) / 120. The rotor's magnetic field, spinning inside stationary coils, induces alternating voltage. Once connected to the grid, the generator locks to the grid's frequency: any mismatch between mechanical input and electrical output causes corrective torques that keep it synchronized.

This synchronization provides three critical grid services for free. Inertia: the spinning rotor stores kinetic energy that resists frequency changes, buying seconds for backup generators to respond. Reactive power: the generator's excitation system supplies the magnetizing current that AC motors and transformers need. Fault current: during a short circuit, the generator delivers a surge of current that triggers protective breakers.

Worked Example

A synchronous generator has 4 poles and must produce 60 Hz electricity.

• Calculate required speed. RPM = (120 x f) / poles = (120 x 60) / 4 = 1,800 RPM.

What happens if the steam turbine driving this generator suddenly loses 10% of its fuel supply?

The generator cannot slow down independently; it is locked to the grid. Instead, it draws kinetic energy from its spinning rotor, converting rotational energy to electricity. The rotor slows slightly, and since all generators are synchronized, the entire grid's frequency dips. Frequency monitoring systems detect the dip within milliseconds and dispatch backup generation.

Synchronous generators are the physical mechanism that keeps every outlet on the grid running at the same frequency.

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