Electromagnetic generators fall into one of two broad categories, dynamos and alternators. Both dynamos and alternators required a source of power to drive their field coils. Alternators were fitted instead of dynamos (generators) from 1973 onwards. Alternators have several advantages over direct-current generators (dynamos). Like all dynamos, the Faraday disc converts kinetic energy to electrical energy. They had six turbo-driven dynamos and two diesel generators. Each set of four boilers drives two steam turbines, which turn three dynamos. He invented paralleling of dynamos and series multiple motor control. In simulations of planetary dynamos, reversals often emerge spontaneously from the underlying dynamics. This is similar in design to the commutators used in motors and dynamos. The earliest dynamos used permanent magnets to create the magnetic field. They were fitted with three electric dynamos, each rated at . Virtually all dynamos in astrophysics and geophysics are hydromagnetic dynamos. The station had six 200 horsepower Edison dynamos, each powered by a separate steam engine. Electricity was provided by six steam-driven dynamos.