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Rotary Converter Power Technology

From nycsubway.org

A peregrination through yesterday's technology, by Bernard S. Greenberg.

AC, DC, and Subway Power

wimg_8312.jpg A rotary converter (click here for enlargement of photo), or synchronous converter, is a large, rotating electromechanical device, like a motor or a generator, formerly used to convert alternating current (AC) to direct current (DC). Not merely like a motor or a generator, or even comprising a pairing of each, the rotary converter is actually both at once, a diabolically ingenious hybrid of an AC motor and a DC generator (dynamo). Unlike a motor, it turns no load, and unlike a generator, it is turned by no engine -- it just sits there and turns, its own load and engine, producing noise, heat, dust, expense, and DC. (The photo to the left, as well as the other photos on this page, were taken at IRT Substation 21 by, and included with the kind permission of, David M. Rosenthal. Click on that link to see the whole set.)

DC was preferred for electric traction for most of this century because the speed of direct current motors can be controlled by varying the current and voltage applied to them, which is a key desideratum for vehicle propulsion. AC motors, invented by the Serbian-American genius Nikola Tesla (1856-1943), operate on a fundamentally different principle, and must rotate in sympathetic synchrony with the generator producing the current (or a fixed multiple or submultiple of its speed, depending upon the design of each). While in recent years ingenious solid-state "cycle chopping" and other advanced techniques have afforded significant additional flexibility in the control of AC motors, they were not available when the three divisions of the New York Subway, and most other classic transit systems, were built.

AC is preferable to DC for power distribution and generation because power loss in transmission lines (which is fuel and money lost) decreases significantly with higher voltages, and (until very recently) only AC could be transformed between voltages efficiently. Thus, electric traction power in New York (at least for the IRT and BMT) for most of this century was generated at the three "subway" AC power stations (Kent Avenue, Brooklyn, for the BMT, and W. 59th St and E. 74th St for the IRT), at 25 Hz, 11,000 volts, and distributed at that voltage over the city via oil-filled cables to several dozen widely-scattered substations near the subway. In the substations, the power would be reduced by transformer to (approximately) 400 volts AC, and thence converted to (nominally) 600 volts DC for the third rail (as well as the now-vanished trolleys and el's). This last step was accomplished with rotary converters.

The substations, or converter stations, were mysterious-looking buildings, frequently adorned with elaborate architectural frills designed to help them fit into neighborhoods, but vexingly bereft of windows, floors, or other signs of human habitation. If you could stand on your toes and look into the front grating-door, on summer days when it might be open, you would see what looks for all the world like a power station, with railed galleries, overhead hanging "travelling" cranes, huge Titanic-era "mad scientist lab" bare-metal "touch-this-you-die" knife switches, meters, and lamp-clusters on vertical black panelboards ringing a vast, dimly-lit open space enclosing huge rotating "generators" (but oddly no turbines nor boilers to turn them). The "generators" were not generators at all, but rotary converters, and there to convert AC to DC.

From the converter station, propulsion DC would travel over short distances (a few blocks, at most) to the right-of-way, appearing at a tiny trackside brick shack (when outdoors), the breaker house. The therein-housed circuit breakers were not "fuses", but huge remote-control (from the substation) switches, capable of interrupting a DC current of several hundred amperes, for disconnecting the substation from the third rail when needed. (Overcurrent protection was provided at the substation, with another array of circuit breakers controlled by that, as well as other, criteria.)


Being a large, complex, and expensive rotating machine, a rotary converter is worn by use, and best preserved by not running it at all when not needed. Each substation thus required a small staff to start and stop individual converters around rush-hours, a nontrivial operation, as well as round-the-clock keep their moving parts in top operating condition, and otherwise supervise, care for, and maintain them and their elaborate array of auxiliary apparatus: backup batteries, motor-generator to charge them, starting and lubrication equipment, etc. All of this was quite costly to construct and operate.

Rotary converters were gradually replaced in the mid-century by the new technology of the mercury arc rectifier, in wholly new, unmanned, totally windowless "monolith"-looking substations. In these devices, an electric spark (arc) vaporizes mercury contained in a steel tank into a vapor that only lets electric current pass in one direction. Unlike rotary converters, mercury arc rectifiers had virtually no moving parts, and required no personnel, realizing a great savings in operating cost. In even later years, solid-state rectifiers of sufficient capacity for the traction power industry were developed, and substations became no more exciting than giant computer chips. Unlike rotary converters, mercury arc and solid-state rectifiers benefit from higher frequency, and thus are supplied from the city's conventional commercial 60 Hz power mains, obviating the need for 25 Hz power (and special power stations). Today, solid state is the only technology being used for this application.

As of this writing (March 1999) there are no "25-cycle" converters left in service in New York City, although a rapidly dwindling number of 60-cycle ones still spin.

For those with some knowledge of electrical engineering ...

Slip rings (3 phase)


A synchronous converter is a fixed-field rotating machine with (AC) slip rings on one side and a (DC) commutator on the other (click on these images of each for enlarged views). It can be viewed as a synchronous motor (three- or six-phase in the New York system) whose armature is additionally equipped with a commutator and brushes from which DC is obtained, or, equally validly, as a DC generator additionally equipped with slip rings to which AC is applied. Conceived of a remarkable conceptual symmetry, provided any of the three commodities AC, DC, or rotation (and a field source), a rotary converter will produce the other two.

Here is a conceptual diagram of a single-phase, two-pole (one pair) converter:


Polyphase operation provides converters additional smoothness, stability, and efficiency. As with a polyphase synchronous motor, additional phase taps are equally spaced on the armature winding. Three-phase converters (see photo) had three slip-rings, and six-phase converters had six. (True six-phase power can be obtained from dual, phase-opposite secondaries of the three transformers of a three-phase system.) Multiple poles and ingenious armature-winding techniques rooted in the motor and generator technology of the time allowed multiple, alternating DC brushes to be employed, reducing the current through each brush (see commutator photo) and the rotational speed of the armature. In general, the line frequency (Hz) of a synchronous motor or converter (or AC generator) is its rotational speed (RPS) multiplied by the number of its pole pairs.

That such a strange device should work at all is not in the least obvious, and almost something of a miracle. When properly designed, the AC and DC currents in the armature tend to cancel each other, and most of the power fed into it goes out as DC (which is its purpose). Of course, some power is lost as heat, both in friction and in "core losses" (induced currents heating up the iron parts), but to a significantly lesser degree than would be incurred with a motor-generator combination, which is in every way twice as much machine, with expectably twice as much mechanical and core loss.

All principles relevant to the design, use, and maintenance of large AC synchronous motors were relevant to the synchronous converter, as were all to the design, use, and maintenance of DC generators, for the rotary converter was both at once. The switching action of the commutator on the AC supply voltage, however, produced bizarre, sharply-spiked waveforms in the armature atypical of either AC motors or DC generators, whose resultant transients presented unique design challenges.

Synchronous converters could be started and brought up to speed (synchrony) as AC induction motors (with the help of additional "starting" windings), as DC motors back-fed from siblings or the third rail, or with the aid of a smaller "auxiliary" motor attached, which latter was standard in the IRT substations. Converters ranged in capacity from 500 to 4,000 (10,000? I think?) kilowatts. Kings Highway, the smallest BMT substation (obit c. 1964), boasted a 500-500-2000 configuration.

The BMT and IRT systems employed 25-cycle (25 Hz) power (generation and distribution) for the sole benefit of the rotary converters. Running proportionately slower than 60-cycle converters of the same number of poles, 25-cycle converters are subject to quadratically smaller mechanical stresses. 60-cycle converters did (and do!) exist, but the additional poles required to bring their speed down create proportionately higher voltage gradients between adjacent commutator bars, which, in turn, precipitate dangerous and expensive arcing and insulation breakdown. Although 60-cycle converters were used on the later, and thus more technologically advantaged, IND, the already-established advantages of 25-cycle converters led to the use of 25-cycle power on the earlier BMT and IRT systems. The use of this 25-cycle power for lighting led to memorable visibly flickering incandescent lights in many subway stations for much of the century now ending.

Rotary converter technology, now completely obsolete, was first deployed in 1894 at Niagara Falls. I do not know who was responsible for its invention, but I would not be in the least surprised, above all on account of its remarkable ingenuity, were it the man who at that time single-handedly invented all the rest of AC motor and power distribution technology, Nikola Tesla.

Anyone interested in this technology or the remarkable buildings which housed it should inspect or obtain a copy of New York's Forgotten Substations -- the Power Behind the Subway, by Christopher Payne (Princeton Architectural Press, 2002). This remarkable and inexpensive volume features dozens of high-quality, artistic and evocative monochrome photos of substation interiors, exteriors, and equipment in their last days (all were decommissioned by 1999), as well as explanations, history, diagrams, and contemporary maps of feeder cables, substation locations, and the like, as well as exploring the romance and fascination of this then centenarian antique technology. This is now the definitive volume on the Rotary Converter in New York City.

Humor note: Boston used synchronous converters, too. There was a small substation at Kendall Square, which used to be a traffic circle (Traffic circles are called "rotaries" in Boston). During the reconstruction of Kendall Square in the early 80's, the ancient substation was replaced by an unobtrusive solid-state model, and the traffic circle was removed. "Well, I guess it indeed converted the rotary", remarked a friend of mine, the late Dan Weinreb.


Text and diagrams on this page Copyright © 1999 Bernard S. Greenberg. First written 22 March 1999. Photos of IRT Substation 21 © 1996 by David M. Rosenthal.

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