A frictionless dynamo is an electrical generator, coupled with a device that reduces friction, that produces electrical current with the use of a commutator. The commutator is a moving part of a rotary electrical switch that periodically reverses the current direction between the rotor and the external circuit.
When Emmett Brown found himself stuck in the 19th century, he had difficulty generating the 1.21 jigowatts that were necessary to power the flux capacitor, allowing the ability to break the time barrier and travel through time.
However, in the 1890s, he examined the circuitry of the hoverboard that had been left behind when Marty McFly traveled from 1885 to 1985. Utilizing this techology, he was able to create a frictionless dynamo that was able to store multiples of the 1.21 jigowatts that were needed for time travel.
He used this frictionless dynamo to power the Jules Verne Train and the steam time car.