Generator Frequency Calculator

Synchronous generator frequency: f = (P · n) / 120

In an AC synchronous generator, output frequency is locked to the rotor speed by f = (P · n) / 120, where P is the number of magnetic poles and n is the shaft speed in RPM. For Brazil's 60 Hz grid: 2 poles need 3600 RPM, 4 poles need 1800 RPM, 6 poles need 1200 RPM. Europe and much of Asia use 50 Hz; Itaipu generates both — half of its 20 turbines run at 50 Hz for Paraguay, the other half at 60 Hz for Brazil, decoupled by a DC link. Example: a 4-pole machine at 1800 RPM gives f = (4 · 1800) / 120 = 60 Hz. Hydro units use 64–100 poles at low speed; wind turbines run variable speed and synthesize 60 Hz through a back-to-back converter (rectify, then invert).

Applications: diesel gensets, hydro, wind

This relation drives the design of diesel gensets, hydroelectric units (Itaipu's 20 × 700 MW machines), turbo-generators in thermal plants, and selection of pole count for wind generators with full-power converters. It also dictates RPM targets for synchronizing a new unit to the grid.

FAQ

Why the constant 120? It combines 60 s/min with the factor of 2 between pole pairs and poles: f [Hz] = (pole pairs · RPM) / 60, and poles = 2 · pole pairs.

Can I run a 60 Hz machine at 50 Hz? Only if you reduce speed proportionally and accept lower output; flux and cooling must still be checked.

Why so many poles in hydro units? Water turbines spin slowly (90–200 RPM); more poles produce 60 Hz without a gearbox.

Does the load change frequency? No — frequency tracks shaft speed. The governor adjusts torque to keep RPM constant as load varies.