Obsolescence
AC to DC synchronous rotary converters were made obsolete by mercury arc rectifiers in the 1930s and later on by semiconductor rectifiers in the 1960s. Some of the original New York City Subway substations using synchronous rotary converters operated until 1999. Compared to the rotary converter, the mercury arc and semiconductor rectifiers did not need daily maintenance, manual synchronizing for parallel operation, nor skilled personnel, and they provided clean DC power. This enabled the new substations to be unmanned, only requiring periodic visits from a technician for inspection and maintenance. AC replaced DC in most applications and eventually the need for local DC substations diminished along with the need for rotary converters. Many DC customers converted to AC power, and on-site solid-state DC rectifiers were used to power the remaining DC equipment from the AC supply.
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