Steel shrink ring commutators are named for the steel rings that are used to retain the copper bar and segment mica pack. They are sometimes called Dynamometer or Exciter commutators which refer to how the commutator is used. On some units (external shrink ring style) the steel rings can be seen on the outside diameter of the copper bars. Others (internal shrink ring style) have the steel rings in grooves on the front and rear of the copper bars. Both styles rely on the interference fit on the inner diameter of the steel rings to maintain the stability of the copper bars and mica segment pack.
External Shrink Ring Commutator
External shrink ring commutators are used for applications that require extremely high rpm’s. This results in stress values that would be too high for other commutator designs. The high strength forged rings used for external shrink rings along with the unique design of this style commutator allows them to maintain stability even at extremely high rpm’s. This style is more expensive than v-bound or glass band commutators due to the higher material costs and the additional labor required to assemble.
Internal Shrink Ring Commutator
Internal shrink ring commutators also utilize high strength forged ring material to retain the segment pack. Internal shrink ring commutators and are more expensive than v-bound or glass band units as well. Once an internal shrink ring commutator is assembled it is very difficult if not impossible to take it apart to replace insulation or clear any internal shorts. Due to lower cost and ease of repair, many internal shrink ring commutators are converted to the v-bound style.