The Expanding sphere generator was a weapon used by the Lyrans and by some Orion pirates. The ESG was used as a defensive tool to block incoming projectiles like missiles. It puts a sphere of energy around the ship; it is charged like a phaser capacitor. Its size is controlled by a setting: the smaller the size, the more powerful it is. Its offensive purpose is to ram enemy starships. Only the ship using the ESG knows where it is taking the enemy by surprise. It is also effective against Hydran Stinger-class fighters. However, it couldn't be used in atmospheres and nebulae. (TOS games: Starfleet Command (game), Star Trek: Starfleet Command II - Empires at War)
The Lyrans seemed to field ESGs on ships in batteries of two or three, if they didn't field single hardpoints. Some special purpose-built ships were built around the ESG and they took different forms, the most common being the ESG Global Operations (EGO) Cruisers and Battlecruisers. (RIS Bouteina)