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The TSLW dual-current generators are developed to realize both ac and dc outputs in one machine. There are very few suppliers in the industry for this kind of products and we are one of the earliest ones. Importers and Wholesalers of Electronic Accessories, Seven Star® is located in the heart of New York City. The early years of the development and expansion of electric power for residential, commercial, and industrial use was marked by intense and well-documented competition between the proponents of dc power and those supporting ac power.
One important early means to convert ac to dc power was a complex machine known as a synchronous or rotary converter or simply a rotary. This is the 15th “History” article authored by Tom Blalock for publication in IEEE Power & Energy Magazine since the magazine’s inception in January 2003. We are honored to have Tom back as our guest history author for this and the next issue of IEEE Power & Energy Magazine. During the early 20th century, machines known own as synchronous converters, or rotary converters, were mainly responsible for the successful transition from early Edison direct current (dc) power distribution to alternating current (ac) distribution. Eventually, however, these unique machines were replaced, first by devices known as mercury arc rectifiers and, later, by solid-state rectifying equipment such as is still in use today for ac-to-dc power conversion.
Regardless, these “rotaries” were essential for several decades as coupling devices between the old dc power installations and the new ac power generating stations. Thomas Alva Edison used dc for his early electric power installations because, at that time, it was the only viable alternative. Consequently, virtually all electrical development during the early and mid-19th century concentrated on dc equipment. During the late 19th century, the inefficiency of dc power distribution became apparent, and the early erroneous opinions regarding ac had been dispelled. Bradley had worked with Edison in Menlo Park, New Jersey, but subsequently formed his own Bradley Electric Power Company in Yonkers, New York.
It was (and still is) possible to convert ac into dc by means of a straightforward motor-generator set.
Regarding its function as a motor, a rotary converter was essentially an inside-out synchronous motor. Since the slip rings and the commutator connected to the same armature winding, there was a direct electrical connection through the machine.
Because these are decidedly nonstandard voltages, a bank of step-down transformers was always required for use with a rotary converter. The 1893 World’s Columbian Exposition in Chicago, Illinois, was a very important world’s fair. The Exposition was to be entirely illuminated by electric lights, and the electrical industry was rather startled when Westinghouse managed to wrestle the contract for this endeavor away from General Electric, which had in its possession all of Thomas Edison’s electric light patents. Westinghouse created an immense display in the Electricity Building of the Exposition, which included steam engine-driven ac generators that provided the power to light the fair. It was noticed that one of these machines exhibited a peculiar “beating” sound with accompanying sparking at the commutator under certain load conditions. Then, in 1895, Westinghouse provided two rotary converters to supply the excitation for the ac generators that the company had constructed for the first significant power station at Niagara Falls in New York. It turned out that other sources of excitation would have had to be provided anyway because the rotary converters operated from the ac output of the generators that they were to excite. As a result of the problems with the Niagara exciters as well as with rotary converters that Westinghouse had built for chemical factories at Niagara Falls, Benjamin Lamme and Charles Scott (both Westinghouse engineers) embarked on an extensive study of the hunting problem. They determined that the cause of the phenomenon was a repetitive shifting of the magnetic flux distribution in the salient (projecting) magnetic poles of the field winding on the stator of the machine. At this same time, General Electric built (using Bradley’s patents) several large rotary converters for the Pittsburgh Reduction Company at Niagara Falls for the production of aluminum. The solution to the hunting problem, pioneered by Lamme and Scott, turned out to be the installation of copper damping coils on the salient field poles. The concept of such a damper was patented in 1892 by two French engineers, Maurice Hutin and Maurice LeBlanc, both of Paris. During early investigations using a standard dc generator modified by the addition of slip rings to become a rotary converter, Lamme and his associates noticed another peculiar result.
Lamme claimed that a detailed mathematical analysis of this phenomenon was carried out by one of his associates but that they never received credit for this effort. Barton’s presentation included an elaborate schematic drawing showing in detail all of the various electrical loads supplied by power generated at Niagara Falls. In that era, it was common practice to define the capacities of electrical machinery in terms of horsepower (hp), rather than kilowatts (kW), with 1 hp being equivalent to 0.746 kW. Also, locally in Niagara Falls, three chemical companies used a total of 3,000 hp in rotary converters to supply dc for electrolysis processes that produced chemicals such as potash (potassium hydroxide), sodium, and caustic soda (sodium hydroxide).
The Natural Food Company used a total of 2,500 hp in rotary converters in its factory that produced its famous shredded wheat breakfast cereal. The ac from the Westinghouse generators at Niagara Falls was stepped up to 22,000 V and sent to the city of Buffalo, a distance of about 25 mi (40.2 km).
In addition, the Lockport Gas and Electric Company used a rotary converter to supply the Edison dc distribution for lighting. Eventually, the rotary converter exciters at the Niagara Falls power station were fitted with Lamme’s amortisseur windings and, so, became usable. During Lamme’s investigation into the fact that a rotary converter could handle more current without overheating than the same machine used as a conventional dc generator, it was learned that the greatest armature heating occurred near the points where the three-phase slip rings connected to the armature winding. This naturally led to the idea of supplying current to the armature at more than three points. The simplest means of accomplishing this was to connect the six transformer secondary leads, two from each phase, to six slip rings on the converter in the proper sequence. Each phase would connect to points diametrically opposite each other on the armature winding.
If the step-down transformer primaries needed to be wye-connected, then a so-called “double-delta” secondary configuration could be used to provide closed paths to circulate third-harmonic currents. The complication and expense of providing six rings and six brush assemblies meant that six-phase operation usually was resorted to only on the largest capacity machines where every means of reducing their size and weight was of value.
For three-phase rotaries, simple wye-connected secondaries could not be used to provide a neutral connection because the resultant flow of dc neutral current in the transformer windings would tend to saturate the magnetic cores of the transformers.
For six-phase diametrical rotaries, the center points of each transformer secondary could be tapped and connected to provide a neutral connection.
In the early 20th century, electric power systems often were not yet sufficiently robust to allow for the full voltage starting of large machines such as rotary converters.

However, reduced voltage starting had to be used to minimize the disruption to the ac power system supplying the machine. Huge three-pole, double-throw knife switches, mounted on vertical slate panels, were usually used to provide the switching from reduced to full running voltage. One problem encountered during the ac starting of a rotary converter was that a high voltage would be induced into the unenergized field winding due to its many turns.
In large substations housing several rotary converters, any rotary could be started from the dc end using the power available on the station’s dc bus bars, supplied by the rotaries that were already running. When close to running speed, the rotary would be generating an ac voltage at its slip rings, and it would then have to be connected to the ac power system with the aid of a conventional synchroscope. A complication with the dc starting of rotary converters supplying streetcar or subway lines was that significant dc voltage fluctuations often appeared on the station bus bars due to the constant starting and stopping of cars or trains. Accordingly, in such substations, a small (that is, relative to the size of the rotary converters) motor-generator set, called a starting set, was sometimes provided. Other small motor-generator sets often were required in such substations to charge the station battery that provided 125-V dc for the operation of circuit breakers and similar control functions. A large rotary converter could also be brought up to speed by means of a dedicated starting motor on the end of its shaft.
About PESThe mission of the IEEE Power & Energy Society is to be the leading provider of scientific and engineering information on electric power and energy for the betterment of society, and the preferred professional development source for its members. Rotary units convert incoming AC Power into rotary mechanical power (spinning motor), which transmits its rotary power to a generator, which converts its mechanical power into AC Power. Solid State units convert incoming AC Power into DC power (rectifier stage), and then convert the DC into the required AC Voltage and Frequency (inverter stage).
For over 60 years Georator has designed and manufactured a complete line of power frequency converter products, both Rotary type (Motor-Generator sets) and Solid State (Electronic) units. Comparing to other existing products, our dual-current generators have many obvious technical advantages, including the quality of output waveform, the quality of dc power, the reliability, the operation temperature rise, etc. In both commercial markets and military markets, our dual-current generators have enjoyed the good reputation in complete quality, competitive pricing and comprehensive technical services. Since it's Inception in 1987, Seven Star® has been a leader in 220 Volt accessories, comprising of Transformers, Voltage Converters, Voltage Regulators, Power Inverters, Adapters, Adapter Plugs. As ac emerged by the early 20th century as the preferred system and dc power began a long, gradual decline, it remained necessary to continue to supply existing dc distribution systems and provide power to such uses as dc streetcar and subway systems. Rotary converters were the most efficient means of converting ac power into dc power so as to be able to continue supplying established dc motor loads (see Figure 1). A typical traction rotary converter substation of Old Colony Street Railway Company, Brockton, Massachusetts, with three 750-kW rotaries at left (from Cyclopedia of Applied Electricity, 1913, vol.
In fact, even the respected scientists of that era labored under the mistaken belief that the ac power discovered by Michael Faraday during his early induction experiments would be useless for electric power distribution because any power flow during one half cycle would be negated during the subsequent half cycle. Edison’s early success in the realm of electric power distribution meant that the use of dc became firmly entrenched, particularly in the downtown areas of large cities.
The work of, primarily, William Stanley and Nikola Tesla had led to the availability of both a practical transformer and a functional ac motor so that the use of ac distribution became superior to dc. However, in 1893, the recently incorporated General Electric Company bought Bradley’s small company to acquire his rotary converter patent. An operator lubricating slip rings on a large rotary converter (photo courtesy of Robert W. However, the overall efficiency of such a set is the product of the efficiencies of the individual machines. That is, its rotor (armature) contained a multiphase winding that was fed via a set of slip rings on the ac end of the machine, and the stator then contained a dc field winding. This meant that there was a fixed relationship between the ac voltage applied to the slip rings and the dc output voltage, with the latter being slightly less than the peak value of the former (on a sine-wave basis). A rotary converter installation with air-blast step-down transformers and a blower for cooling at left (from Cyclopedia of Applied Electricity, 1913, vol.
Even at the turn of the 20th century, however, transformer efficiencies were in the high 90% range.
It was conceived as a celebration of the achievement of Christopher Columbus 400 years earlier, but it opened one year late due to construction delays. In fact, Westinghouse had to develop a short-lived type of electric lamp (the so-called “stopper lamp”) to avoid infringing on the patents held by General Electric. As a part of this exhibit, Westinghouse included rotary converters to demonstrate the ability to change ac into dc for use with conventional dc motors. The operators managed to deal with this matter so not much attention was paid to it at the time. The hunting problem manifested itself rather severely in these machines, and, consequently, the machines proved to be unusable. In effect, this was causing the machine to cycle back and forth between generator and motor operation with consequent small variations in speed that manifested themselves as the audible hunting behavior. These coils effectively short-circuited the periodic magnetic field variations causing the hunting behavior. They also held several patents in the fields of telephony and electrical resonance (there was another Maurice LeBlanc, who was a noted French novelist). The same machine could handle a much larger current when operating as a rotary converter than it could when operating as a dc generator; in one case, 400 A versus 280 A.
He claimed that exactly the same analysis was carried out years later by others, most probably by General Electric engineers, and that this later effort received “very considerable” credit. By that time, the electric power development of Niagara Falls had become quite extensive, and a presentation on this topic was given by Philip Barton, a representative of the Niagara Falls Power Company. Barton’s schematic indicates that a total of 5,000 hp in rotary converters were installed at the Pittsburgh Reduction Company in Niagara Falls for the production of aluminum (this company later became Alcoa).
Perhaps adjustable speed dc motors were needed for mixers or for conveyors through continuously fed ovens. Also, a 500-hp rotary converter from the 1893 Chicago Exposition was installed in this power station, presumably to supply dc for the local streetcar line. Since step-down transformers were always needed to operate rotary converters, the secondary connections of these transformers could be used to provide six-phase power from the available threephase power. The more even distribution of heating in the armature meant that less core material was required to dissipate that heat. Such a connection could be provided via the step-down transformer secondaries so as to provide a path for neutral current to flow into or out of the armature winding.
Accordingly, a zig-zag, or interconnected, type of wye winding had to be used to prevent this.

The dc current flow in one winding half was opposite that in the other, so the same effect was obtained as with a three-phase zig-zag connection. Since rotary converters were equipped with amortisseur windings on their stators to prevent hunting, as discussed previously, they were capable of being started as induction motors just as was done with large synchronous motors.
Chicago City Railway Company rotary converter substation with pedestal at right for two three-pole, double-throw starting switches (from Cyclopedia of Applied Electricity, 1913, vol. Since step-down transformers were always required with rotaries, taps on the secondaries of these transformers were used to provide the reduced starting voltage. These switches were located between the adjacent transformers and the rotary to minimize the lengths of high current connections. Accordingly, the field winding was temporarily broken into several sections during starting to minimize this induced voltage and avoid puncturing the field winding insulation.
Three rotary converters at the New York City Transit Authority’s Nostrand Avenue, Brooklyn, substation with a starting motor-generator set and two battery-charging motor-generator sets in the foreground (photo courtesy of Thomas J.
A multistep starting resistance was provided on the station switchboard, which could be connected to any rotary converter to enable it to be gradually brought up to speed while operating as a dc shunt motor.
When synchronism was indicated, the remotely controlled circuit breaker in the circuit supplying the step-down transformer primaries would be closed. This fluctuation would make it difficult to synchronize a rotary to the ac line when starting. An induction starting motor on the ac end of a 2,000-kW rotary converter at the New York City Transit Authority’s Prospect Park, Brooklyn, substation (photo courtesy of Thomas J. Usually, this would be an ac induction motor having windings with two fewer poles than the rotary converter to assure that the latter could be brought up to, or slightly above, its synchronous running speed.
Ellison, “Hail and farewell to two historic stations,” Around the System (Consolidated Edison Company newsletter), Nov. It is made available to members of the Society at no additional cost, and to non-members by subscription. Our products are used in testing electrical products to be exported to 50 Hz countries (Europe, Asia), operating 50 Hz foreign equipment in the United States and other 60 Hz countries, and testing aircraft and shipboard 400 Hz equipment.
If you are new to specifying frequency conversion and power supplies, send a quick quote as a guide for Georator to recommend the right choice for your application. Blalock, a frequent contributor to these pages, investigates these now long-gone machines that allowed ac power systems to actually support their old rival systems during the decline and eventual extinction of dc power systems.
By then, Westinghouse was manufacturing rotary converters, and, in 1896, the inevitable squabbling between the two companies regarding patent rights was resolved as part of an extensive cross-licensing agreement that covered all forms of electrical equipment.
The rotary converter was a single machine that combined the functions of motor and generator and, so, inherently exhibited a significantly higher efficiency than a motor-generator set.
The generator function was provided by a commutator and brushes on the dc end of the armature. The two major (but by no means the only) uses for rotary converters were to supply Edison dc distribution networks at 250 V and streetcar trolley wires at 500 V. Thus, the need for transformers did not adversely affect the overall efficiency of a rotary converter installation.
That was, however, the first indication of the problem of what is termed “hunting” in rotary converters that eventually proved to be very problematic in larger machines. When this basic fact was first realized, provision was made for the temporary use of a dc generator in a nearby construction shack to provide excitation needed to start up the station for the first time. The only major difference between the design of the General Electric and Westinghouse machines, as far as their field structures were concerned, was that Westinghouse used laminated field poles while General Electric used solid poles (following standard dc generator practice). It so happened that the solid field poles used by General Electric in their converters provided this same damping action. As a result of this French origin, the damper winding used in rotating electrical machines was known as an “amortisseur” winding throughout the 20th century. These limits were referred to by Lamme as the “smoking currents,” reflecting an apparent standard practice of the time wherein the maximum current a machine could handle was a current slightly less than what caused noticeable smoke to issue from the windings. The same was true in the nearby towns of Lockport and Tonawanda, as well as on the Canadian side of the Niagara River. It effectively supplied six voltages to the armature, which differed from each other by 60°. A 1,000-kW rotary converter, for example, that supplied a streetcar line at 500-V, 2,000-A dc would require about 1,500-A, threephase, ac for its operation. A small multipole knife switch, called a field break-up switch, was mounted on the frame of the machine to provide this function. Also offered are Uninterruptible Power Sources (UPS) for backup power with optional frequency and voltage conversion.
Lamme, of the Westinghouse Electric & Manufacturing Company in Pittsburgh, Pennsylvania, also laid claim to its invention. For 500-V dc, the ac voltage applied to the slip rings had to be about 370 V and, similarly, for 250-V dc, the ac voltage had to be about 185 V.
Most often, the transformers were located inside the converter substation so as to minimize the lengths of the low-voltage, high-current connections between the transformers and the converter. In later years, however, when General Electric shifted to laminated field poles to reduce the eddy-current losses in larger machines, they then had to resort to the use of copper dampers on the field poles as well. Amortisseur translates roughly as “to deaden,” which is in keeping with its function to damp out or deaden various forms of magnetic oscillations.
The total streetcar load fed by rotary converters at all locations was nearly 10,000 hp (about 7.5 MW), which included a streetcar line in the Niagara River gorge itself. His duties as a development engineer at the former General Electric High-Voltage Engineering Laboratory and later as a test engineer in the Transformer Test Department, both in Pittsfield, Massachusetts, included a broad range of duties, including lightning protection and high-voltage switching surge studies. It is certainly likely that the Westinghouse Company was responsible for improvements to the basic design of these machines during this period. In fact, the earliest experiments at Westinghouse involved the addition of slip rings to a standard dc railway generator (see Figure 2). It was not good practice to use oil-filled transformers inside due to the fire hazard, so dry air-blast transformers were normally used, with the cooling air supplied from an electric motor-driven blower (see Figure 3). All of the reasons for choosing AC or DC have now been changed due to further advances in technology. Since retiring from General Electric, Tom has actively pursued his hobby of “industrial archaeology,” with particular emphasis on the exploration, preservation, and careful documentation of historically important and interesting electric power projects and equipment.

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