Ener-D
Enerdy, known as Momentum in the Japanese anime and short for "duel energy", is a form of energy in Yu-Gi-Oh! 5D's. It is the ultimate energy generation system that keeps spinning in one direction to create an endless power supply. Enerdy is the source of power in New Domino City that keeps the city running while making the technology for Duel Runners, Turbo Duels, and Solid Vision(the latest in Kaiba Corb's line of holographic technology) possible.
Further info on the Enerdy System
It is known that Kaiba Corporation's M.I.D.S is in charge of this technology; the lead developers who had begun the research into it were Yusei Fudo's parents. In the English anime, its name comes from energy, but replacing the G with a D, which stands for dueling. The change may have been due to the fact that in English, the term momentum refers to the product of a body's mass times its velocity. Momentum is mathematically related to kinetic energy even though it is a different phenomena.
The Enerdy system is implied to be a Perpetual Motion Machine, and runs at full capacity in order to meet the power requirements of New Domino City. Its limit is in its ability to channel excess energy it produces. The appearance of the Crimson Dragon during the duels between Yusei Fudo and Jack Atlas caused the system to exceed its limits. During their first match when the Crimson Dragon appeared, it overpowered the system by dramatically overflowing the energy generation to it. As it was incapable of handling so much power, the system completely shut down, leaving the city in a total blackout. The overflow problem occurred once again during the Fortune Cup, though according to Zigzix it was running at full operation. Apparently some modifications had to have been made to the system, since the 2nd appearance of the Crimson Dragon only caused the temperature and pressure within the Enerdy system to reach critical point rather then cause the system to blackout as it had before.
Old Momentum
After the Fortune Cup, the existence of a first Enerdy system was revealed within Satellite's inner grounds. However, according to Rex Goodwin, as it was the first system, it was totally insecure and imperfect when it was activated seventeen years before by M.I.D.S. Consequently, the system's activation resulted in the major catastrophic event known as the "Zero Reverse Effect". This was the reason behind the destruction and splitting of Domino City into New Domino City and Satellite. The system was abandoned on Satellite, until it was "found" by the Dark Signers. According to Goodwin, the old system is being used as a door to the Underworld.
It's also known that this system possesses four controllers (safety switches) located in the B.A.D. area. These controllers are a fail-safe switches placed long ago after the system was shut down completely. Each of the controllers is code-named with the name of one of the Immortal Gods and is guarded by the Dark Signer who possess the corresponding Earthbound Immortal Duel monster card. The main system itself is represented by the Spider mark and is guarded by the Dark Signer boss Rudger.
