IEEE Power & Energy Society


The Rotary Era, Part 2

Early AC-to-DC Power Conversion, Continued

The first part of guest history author Thomas J. Blalock’s article on rotary converters appeared in the September/October 2013 issue of IEEE Power & Energy Magazine. In that part of his article, Tom covered the need for a practical and economical means to convert ac power to dc power, the invention of the rotary or synchronous converter in the late 1880s, and early use of the rotary converter for such applications as supplying dc power to existing Edison dc distribution systems and supplying dc motive power to street railways.

In this second and concluding part of his article, Tom discusses the further technological development and use of rotary converters in the 20th century and the eventual decline and virtual extinction of rotaries as they were supplanted by newer technologies.

As this two-part article is Tom Blalock’s 15th contribution to these pages, he is well known to our readers and needs no further introduction here. Suffice it to say that we welcome him back as our guest history author for part two of his fascinating article on the life and times of rotary converters.

—Carl Sulzberger
Associate Editor, History

The author’s examination of the development and use of rotary converters to provide direct current (dc) power began in the September/October 2013 issue of this magazine. He now completes this discussion with an account of further significant advances in the technology of early “rotaries” and their eventual decline.

60-Hz Rotary Converters

In 1915, an article was published in the General Electric Review describing a very large installation of rotary converters for the Aluminum Company of America (later, ALCOA) at Massena Springs in upstate New York. That article contained the following comment:

Not many years ago the 60-cycle rotary converter attained a reputation for sensitiveness in operation which was usually manifested by flashing at the commutator. Pulsation and alternating or direct current line disturbances frequently caused flash-overs and shut-downs.

The article then goes on to say that improvements in rotary converter design had since caused such problems to be “practically eliminated.”

This was the largest installation of rotary converters at that time, and it utilized 60-Hz (“60-cycle”) machines. The Hall-Heroult electrolytic process for the production of molten aluminum had been simultaneously discovered in 1886 by Charles Martin Hall of the United States and Paul Heroult of France. The process involves the passage of huge dc currents, at very low voltage, through a mixture containing aluminum oxide in a carbon-lined steel box with carbon electrodes.

figure 1. Rotary converter installation for the Aluminum Company of America (from the September 1915 issue of General Electric Review, vol. XVIII, no. 9, cover).

figure 1. Rotary converter installation for the Aluminum Company of America (from the September 1915 issue of General Electric Review, vol. XVIII, no. 9, cover).

This particular aluminum production installation included a total of 18 rotary converters of 2,500-kW capacity, for a total of 45,000 kW. The step-down transformers provided 377-V, six-phase alternating current (ac) to the diametrically connected rotaries. The transformers received power at 110 kV from a hydroelectric development on the St. Lawrence River, about 50 mi (80.5 km) distant (see Figure 1).

The rotary converters provided dc at about 500 V, and the total available current with all rotaries operating was as much as 90,000 A. Years ago, the author recalls an “industrial legend” type of tale regarding the fact that such huge dc currents in these installations magnetized even the structural steel of the building and that the maintenance personnel kept track of their tools merely by sticking them to building columns.

The above comment regarding flashover problems with 60-Hz rotary converters relates to the fact that virtually all of the early rotaries had been designed for operation on 25-Hz. That fact, in turn, relates to the very extensive use of 25-Hz ac power during the early 20th century. The proliferation of that frequency was a direct consequence of its use by Westinghouse in the first hydroelectric installation at Niagara Falls in 1895.

While 60-Hz power installations were developed for general purpose lighting use, 25-Hz power was employed in industry because it was more suitable for slow-speed, high-horse-power electric motors (fewer magnetic poles were required in motor windings designed for a lower frequency).

The Bethlehem Steel plant in Bethlehem, Pennsylvania, generated 25-Hz power, as did most other early 20th century steel plants, for use with large, slow-speed rolling mill motors. In addition, 25-Hz rotary converters often were used in such plants to provide dc power for machine tool motors and other uses that required simple speed control.

Eventually, the Bethlehem Steel plant had a total of 14 rotary converters in operation, with a total capacity of 12,000 kW, to supply 250-V dc at various locations. In addition, there were 11 motor-generator sets (totaling about 8,000 kW) for similar purposes. The reasons for choosing motor-generator sets in some locations are not known. However, the 25-Hz power system operated at 6,600 V, which meant that it was possible to use synchronous motors for motor-generator sets with no need for step-down transformers.

The design of early rotary converters was such that the maximum allowable peripheral speeds of the armature and commutator were limited so the machines had to be designed with a sufficient number of winding poles to limit the revolutions per minute to acceptable values.

figure 2. The dc end of a rotary converter at the New York City Transit Authority’s West 53rd Street (Manhattan) substation showing brush numbering and polarities (photo courtesy of Thomas J. Blalock).

figure 2. The dc end of a rotary converter at the New York City Transit Authority’s West 53rd Street (Manhattan) substation showing brush numbering and polarities (photo courtesy of Thomas J. Blalock).

A commutator brush assembly was required to correspond with each winding pole, with the brushes alternating positive and negative around the commutator. This meant that the brushes of opposite polarity became rather close together with greater numbers of poles in the armature winding (see Figure 2).

This was not a great problem for 25-Hz converters, but it did turn out to be so for 60-Hz machines because of the correspondingly greater number of poles required for a given revolutions per minute. Consequently, the early design of 60-Hz rotaries was difficult, especially for traction machines operating at 500–600 V between brushes.

It was learned that such rotaries were very prone to flashovers between positive and negative brushes in the event of any sort of unusual disturbance on the dc distribution system. For example, a sudden inadvertent loss of trolley pole contact on a streetcar line, or a derailment on a third-rail subway line, would lead to a rapid rise in voltage that could cause a commutator flashover on a rotary converter having close brush spacings.

The New York Edison Company

The early entrenchment of dc power distribution in New York City began with Thomas Edison’s pioneering Pearl Street Generating Station in 1882, which was named an IEEE Milestone in Electrical Engineering in May 2011. Unfortunately, however, the former site of that station is now a parking lot.

By 1901, the original Edison Electric Illuminating Company of New York had become the New York Edison Company. The original Pearl Street Station was gone and had been replaced by seven other dc generating stations in Manhattan. These stations all used reciprocating steam engines, totaling 26,500 hp, to drive the dc generators.

Four of these stations contained large battery banks that served as back-up sources of power in the event of generator failure. Each of these batteries was capable of supplying 6,000 A for a period of three hours. In later years, as the use of ac power rapidly expanded, the ability to use batteries for back-up power was an argument used to justify the continued use of dc distribution. These battery banks were also used to establish the neutral connection for the Edison three-wire, 110/220-V dc distribution network so that three-wire generators were not required. The conventional two-wire generators supplied 250 V to the outer conductors of the network, and the neutral was obtained from the center of the series-connected battery cells.

In 1901, the New York Edison Company placed into operation a large ac generating station called Waterside. The station was later expanded and, eventually, it occupied the east side of First Avenue in Manhattan from 38th Street to 41st Street. This imposing complex was, however, demolished a few years ago.

Waterside was constructed as a means of eliminating the need for dc generating stations scattered throughout Manhattan. High voltage ac was generated and then distributed underground to rotary converter substations that supplied 110/220-V dc power to the existing underground Edison networks.

figure 3. A 2,000-kW General Electric rotary converter being installed at the 27th Street substation of the New York Edison Company (photo courtesy of the Museum of Innovation and Science (miSci) Archives, Schenectady, New York).

figure 3. A 2,000-kW General Electric rotary converter being installed at the 27th Street substation of the New York Edison Company (photo courtesy of the Museum of Innovation and Science (miSci) Archives, Schenectady, New York).

By 1902, there were a total of 14 such rotary converter substations in Manhattan, 11 of which contained back-up battery banks as described above. These substations provided a total of 24,300 kW of dc power. Seven of them were actually added to the existing dc generating stations. However, shortly thereafter, four of those were retired, leaving only three dc generating stations in Manhattan (Duane, East 12th, and West 26th Streets), and these continued to provide a total of 9,500 kW of dc power for some time. By the 1930s, there were a total of 40 rotary converter substations in operation (see Figure 3).

In 1963, a total of 23 rotary converter substations were still in operation but, by 1976, only two remained: West 26th Street and West 39th Street. These two were finally retired the following year, bringing an end to the rotary converter era for the supply of dc power in Manhattan.

An interesting aspect of the longevity of the West 39th Street rotary converter substation is that it prolonged the use of an archaic means of controlling Broadway show lighting in the theaters of the Times Square area. These were known as “resistance dimmers,” and their use continued long into the age of computer lighting control because they were the only type of dimmers that could operate with the dc power services still in use in the old Broadway theaters.

The year 1977 was not the end of the use of dc power in New York City, however. As the old rotary converter substations were retired, still existing dc power loads were picked up by new solid-state rectifier units installed by the Consolidated Edison Company (Con Edison), the descendant of the former New York Edison Company.

According to a former Con Edison engineer, about 6,000 dc customers still remained in Manhattan in 1996. Practically all of this load consisted of 220-V dc motors and was being supplied by somewhere between 300 and 400 rectifier units of 100–250 kW capacity.

In 2001, Con Edison began a program to force the conversion of all remaining dc loads to ac power. It was not until 2005, however, that a deadline for this effort was imposed for the end of that year. Ultimately, the last dc service cable in Manhattan was not cut until November 2007, as was reported in an article in the May/June 2008 issue of this magazine.

Rotary converters were capable of operating in what was termed an “inverted” mode; that is, a rotary supplied with dc could produce ac at its slip rings. In fact, a rotary could also function as a so-called “double-current generator” by driving it with some form of prime mover and simultaneously drawing ac current from the slip rings and dc current from the commutator.

In 1898, prior to the construction of Waterside Station, a need arose for the transfer of electric power from the downtown area of Manhattan to midtown and vice-versa. This was a result of the fact that the daily peak load on the downtown dc network occurred at a different time of day than the peak load on the midtown network.

Accordingly, a 6,600-V ac connection was established between the Duane Street dc generating station (downtown) and the dc generating station on West 39th Street. Rotary converters were installed at both locations and, at different times of day, either converter could be operated inverted to transfer excess power at that station to the other station with its rotary operating in the normal converter mode.

This, however, was not the first use of an inverted rotary in New York City. In 1896, the need arose for additional dc power in the Coney Island area of Brooklyn due to the rapid growth of the amusement parks there. It so happens that there is also a Pearl Street in Brooklyn, and, ironically, an Edison dc generating station had been established at that location. A rotary converter was installed there to energize a 6,600-V ac feeder to a rotary converter substation at Coney Island. Unlike the Duane Street to West 39th Street connection, this was a one-way power transfer link.

After Waterside went into operation, an interesting procedure was developed to restart the rotary converters at the various substations in the event that all ac power from Waterside was lost (apparently, this did happen on occasion). First, the dc loads were transferred to the substation batteries. Then, these same batteries were used to energize the fields of the inoperative rotary converters. As soon as possible, a generator at Waterside was started up with its field energized as well. This generator, along with the rotaries in the substations, would then be operating as a gigantic Selsyn system, and all of the rotary converters would be restarted simultaneously.

Obviously, there was great opportunity for disaster during such a procedure if just one operator made a mistake. Accordingly, this start-up procedure was actually practiced as a drill once a month to minimize the chance of errors during a real occurrence.

The New York City Subway System

Before subways, there were surface and elevated traction companies in New York City. In 1899, the Metropolitan Street Railway began electrified operation using a new power station at 96th Street in Manhattan. Eleven steam engine-driven alternators provided 6,600-V, three-phase, 25-Hz power to seven substations in which rotary converters were used to supply 500-V dc power to the surface streetcars.

Similarly, in the same year, construction began on a power station at 74th Street to supply 11,000-V, three-phase, 25-Hz power to eight rotary converter substations for the operation of elevated railway lines that previously had been powered by small steam locomotives. Eight large steam engines drove the alternators in the power station, which was operated by the Manhattan Elevated Railroad Company.

The first subway operation in New York City was the Interborough Rapid Transit line, which opened in 1904 (the IRT). A massive power station was constructed at 59th Street in which ten steam engine-driven alternators provided 11,000-V, three-phase, 25-Hz power to eight rotary converter substations, just as at the 74th Street power station. This imposing structure was designed by noted architect Stanford White and still stands today.

The 74th Street power station also still stands, and it and the 59th Street station are used to supply steam for heating throughout Manhattan. The 96th Street power station is long gone, and its site is now occupied by a highway exit ramp.

The original Allis-Chalmers engines at 59th Street were said to have been the largest stationary steam engines ever built. When the last engines were retired in the 1950s, one of them was offered to the Smithsonian Institution. It was far too massive for them to accept, but the Smithsonian did acquire one of the huge piston connecting rods and constructed a scale model of the 59th Street station.

In 1909, the Brooklyn Rapid Transit Company built a new power station on Kent Avenue, in the Williamsburg section of Brooklyn, to augment smaller existing power stations used for their extensive elevated and surface traction operation. By this time, the era of huge power station steam engines had ended, and Kent Avenue was equipped with nine turbine-driven alternators supplying 6,600-V, three-phase, 25-Hz power to 26 rotary converter substations scattered throughout the Borough of Brooklyn.

In 1918, as a consequence of a lengthy labor strike against the Brooklyn Rapid Transit Company, an inexperienced substitute motorman was responsible for a horrific wreck at Malbone Street in Brooklyn. Subsequent to this incident, the company went into receivership but was then reorganized to become the Brooklyn Manhattan Transit subway (the BMT). This new entity extended operations into Manhattan, and, to obtain 600-V dc power for the third rails of this additional trackage, the BMT purchased surplus power from the IRT.

In 1976, the author visited an IRT substation on East 57th Street in Manhattan. At the end of the switchboard were panels containing watthour meters that once were used to meter power sold to a nearby BMT subway line.

Eventually, the Kent Avenue power station was replaced by newer power sources and was retired from service. The building was not demolished until 2008, however.

figure 4. An operator observing a large rotary converter at Hammel’s substation in Queens, New York City (photo courtesy of Robert W. Lobenstein).

figure 4. An operator observing a large rotary converter at Hammel’s substation in Queens, New York City (photo courtesy of Robert W. Lobenstein).

It should be noted that all of the above transit operations utilized tried-and-true 25-Hz rotary converters in their substations (see Figure 4).

Both the IRT and the BMT were privately owned companies. During the 1930s, the City of New York began the construction of a municipally owned subway that was to become known as the Independent subway (the IND).

One of the more interesting IND stations is at 6th Avenue (Avenue of the Americas) and 50th Street, serving the Rockefeller Center area. That massive complex was under construction at the same time as the 6th Avenue IND subway. Consequently, this subway station is an integral part of the basement level of the Rockefeller Center building, originally called the RCA Building. Today, this structure has become the GE Building but is more popularly known as “30 Rock.”

By the 1930s, the development of 60-Hz rotary converters had progressed considerably. Also, the use of a device known as a mercury arc rectifier had become rather commonplace. Both were used to power the new IND subway and, therefore, there was no longer any need for 25-Hz power since mercury arc rectifiers could be operated on 60-Hz power. The necessary power, then, was purchased directly from the New York Edison Company.

Early mercury arc rectifiers consisted of a large steel tank having a pool of mercury at the bottom, which served as a cathode element. The ac power supplied to the device was most often in the form of 12 phase, obtained from three-phase transformers having multiple secondary windings in each phase interconnected so as to obtain 12-phase power. Thus, there were 12 anode elements in a circular arrangement above the mercury pool.

The mercury arc rectifiers did away with the need to maintain bearings, commutators, and brushes as was necessary with rotary converters. Ironically, however, these rectifiers were prone to a serious operating malfunction known as an “arc-back” that often was precipitated by some abnormal electrical event on the dc load side, just as was true for commutator flashovers on 60-Hz rotaries. In any event, the original IND subway installation included five 60-Hz rotary converter substations and a total of 82 mercury arc rectifier substations located in Manhattan, Brooklyn, Queens, and the Bronx.

figure 5. A 1932-vintage IND subway mercury arc rectifier installation (photo courtesy of Robert W. Lobenstein).

figure 5. A 1932-vintage IND subway mercury arc rectifier installation (photo courtesy of Robert W. Lobenstein).

Early IND rectifier substations utilized mercury arc rectifiers as described above. However, later installations were made up of “Ignitrons,” which consisted of individual steel cylinders for each anode. This effectively eliminated the arc-back problem, and they were more efficient than the old mercury arc rectifiers. Generally, both types of rectifier installation were rated at 3,000 kW (see Figure 5).

A major IND rotary converter substation was located a few blocks from Rockefeller Center, and it served, in addition, as the main power control center for the IND subway. Eventually, it came to serve as a control center for all three subways when, in 1940, both the IRT and the BMT came under city ownership.

In 1976, the author was privileged to arrange for a tour of this substation with the New York City Transit Authority. In spite of the design progress that had been made with 60-Hz rotary converters by the 1930s, the author was told by operators there that they were still very wary of being too close to the converters when they were in operation, for fear of the possibility of a commutator flashover.

An early article describing the original IND power installations mentioned that a 15-hp gear-reduction motor was permanently mounted on the end of the shaft of each rotary converter to facilitate the periodic turning down or smoothing of the commutator surface made necessary by sparking and flashing during operation. The 15-hp motor allowed the armature of the rotary to act as a lathe. The shaving off of the scarred commutator surface was done by cutting tools temporarily bolted to the frame of the rotary.

There were four converters of 4,000 kW rating each. Also, interestingly, these were 12-phase rotaries requiring 12 slip rings on the ac end. An article appearing in General Electric Review, which described the IND installations, mentioned that a 4,000-kW, 12-phase, 60-Hz rotary occupied no more space than a 3,000-kW, six-phase, 25-Hz rotary as was commonly used in earlier installations (4,000-kW, 25-Hz rotaries were also installed in some IRT substations). In 1976, there had already been an installation in this substation of a 4,000-kW solid-state rectifier unit to supplement the four existing rotary converters.

The elaborate procedure used by the New York Edison Company during the early 20th century to cope with the total loss of ac power to their rotary converter substations was described previously in this article. The following excerpt from a 1930s IRT substation operating manual certainly indicates that they, as well, prepared for such an event:

figure 6. An operator closing feeder switch at the New York City Transit Authority’s West 53rd Street substation (photo courtesy of Thomas J. Blalock).

figure 6. An operator closing feeder switch at the New York City Transit Authority’s West 53rd Street substation (photo courtesy of Thomas J. Blalock).

Please see to it that in each substation kerosene lanterns are kept lighted during the hours from sunset to sunrise. One lantern should be kept on the switchboard floor, one on the rotary floor, and one in the basement. These lanterns are to be kept clean and in good condition at all times.

Back-up lighting was required to be able to perform manual switching operations necessary before normal power was restored (see Figure 6).


The development of high-power, solid-state electronic rectifier equipment during the latter years of the 20th century led to the retirement of both rotary converter and mercury arc rectifier installations.

As previously mentioned, Con Edison shut down its last New York City rotary converter substations in 1977. All of the early Niagara Falls hydroelectric stations and, presumably, their connected rotary converters have been long retired. The Bethlehem Steel plant is completely closed down, so all of its dc conversion equipment is long gone as well.

According to Robert Lobenstein, former general superintendent of power operations (now retired) for the New York City Metropolitan Transit Authority, the last 25-Hz rotary converters were shut down at the end of 1999.

In at least three of the old rotary converter substations, only some of the converters were removed to make space for new solid-state rectifier units. The remaining machines were left in place at the rear of the substations thanks to Lobenstein’s interest in preserving electric power history artifacts.

The author was given a tour of three substations (on West 53rd Street in Manhattan, and on Nostrand Avenue and at Prospect Park in Brooklyn) in 2008, and photos taken at that time have been used in this article with Lobenstein’s permission.

The last IND 60-Hz rotary was shut down in 2005. Some of the Ignitrons are still in place and could be used but generally are not.

As far as the author knows, there are now no rotary converters operating anywhere in the United States, but he would be thrilled to learn that is not actually true.

For Further Reading

J. L. Burnham, “The 45,000-kW synchronous converter substation of the Aluminum Company of America at Massena Springs, NY,” Gen. Electric Rev., vol. 18, no. 9, pp. 873–878, Sept. 1915.

T. J. Blalock, “Edison’s direct current influenced ‘Broadway’ show lighting,” IEEE Power Eng. Rev., vol. 22, no. 10, pp. 36–37, Oct. 2002.

J. Romano. (2001, Mar. 18). A push to unplug D.C. power, NY Times [Online]. Available:

J. Rasenberger. (2005, Jan. 2). Fade to black, NY Times [Online]. Available:

R. W. Lobenstein and C. L. Sulzberger, “Eyewitness to dc history: The first and last days of dc service in New York City,” IEEE Power Energy Mag., vol. 6, no. 3, pp. 84–90, May–June 2008.

S. D. Sprong, “The system of the New York Edison Company—Part II,” Gen. Electric Rev., vol. 9, no. 1, pp. 14–21, June 1907.

H. Zimmer and E. M. Bill, “The direct-current supply for the independent system of city subways, New York City,” Gen. Electric Rev., vol. 35, no. 10, pp. 503–509, Oct. 1932.

T. J. Blalock. (2012, Mar. 1). The railway power stations of New York City. [Online]. Available:

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