Synchronous machine

A synchronous machine spins in exact lockstep with the AC line frequency — never lagging, never leading. Unlike an induction motor, its rotor carries its own magnetic field (from a winding fed DC, or from permanent magnets), so the rotor magnet simply 'locks on' to the stator's rotating field and is dragged around at precisely the same speed, like a dancer following a partner's lead with no drift. On a 50 Hz grid a two-pole machine turns at exactly 3000 rpm, every cycle, all day.

Almost all of the world's electricity is made by giant synchronous machines acting as generators (alternators) in power stations, where their fixed speed sets the grid's frequency. As motors, synchronous machines shine where constant speed matters — and a special version, the synchronous condenser, is run with no load purely to supply or absorb reactive power and prop up grid voltage. Their one quirk is that they can't start themselves from a standstill on raw mains, since the field whips past too fast for the heavy rotor to grab on; they need a starting scheme or a drive.