# Rotary converter > Turning alternating current into direct current with one machine - Date: 1887 - Era: Electric age - Domain: [Electricity & Electronics](https://lastcodex.org/en/domains/electricity/) - Rebuild stage: V (Electric age) - Difficulty: 4/5 - URL: https://lastcodex.org/en/inventions/rotary-converter/ A rotary converter is a single rotating machine that turns alternating current into direct current, or the reverse. Its armature winding is connected both to slip rings and to a commutator, so it runs as a motor on the alternating side and acts as a generator on the direct side. Charles Bradley described the idea in a patent application of 1887, and converters fed the direct-current tramways and railways of the early 20th century. ![Rotary converter · Turning alternating current into direct current with one machine](https://lastcodex.org/_astro/plate.DKODnW-E_2vKApz.webp) ## Legend 1. Slip rings, where alternating current enters 2. Commutator, where direct current leaves 3. Armature with a closed winding 4. Field poles and their coils 5. Leads from the winding to the slip rings ## What it is A rotary converter changes alternating current into direct current, or direct into alternating, with one machine. It looks like a direct-current [generator](/en/inventions/generator/): a field magnet, a rotating armature and a commutator with brushes. At the other end of the shaft it also carries slip rings, connected to points of the same armature winding. At the turn of the 20th century, power stations generated alternating current, which [transformers](/en/inventions/transformer/) could carry far at high voltage, while tramways, railways, electroplating and many city networks used direct current. Rotary converters in substations joined the two systems. A motor driving a separate generator, called a motor-generator, did the same work with two machines. ## How it works In a direct-current generator, the voltage induced in each coil of the rotating armature alternates as the coil passes from one field pole to the next. The commutator, a ring of copper bars connected to the winding, and the brushes that rub on it reverse each coil's connection at those moments, every half turn in a two-pole machine, so the current leaving the brushes always flows the same way. If leads are also taken from the winding at equally spaced points to slip rings, alternating current can be drawn from the rings while direct current leaves the commutator. Charles Bradley showed this with a ring armature tapped at four points 90 degrees apart, each connected to its own contact ring. With three points 120 degrees apart, the rings carry three-phase current. ![Close view of a commutator: copper bars separated by thin insulating strips around a shaft, with carbon brushes held in metal holders pressing on it](https://lastcodex.org/_astro/v2.CZCUmpyI_Z1jTlGJ.webp) _The brushes rub on the commutator bars and pick up direct current from the same winding that the slip rings feed._ The machine works in both directions. Fed with alternating current through the rings, it runs as a synchronous motor, turning in step with the supply, and the commutator delivers direct current. Run the other way, with direct current in and single-phase current taken from two rings, the armature is wholly a motor at the instants when the alternating output is zero and acts partly as a generator near its peaks, so the power passes through it in pulses. The ratio between the two voltages is fixed by the connections and depends on the number of rings. To change the direct voltage, the alternating supply was adjusted, usually with transformers. The field magnet was excited with direct current taken from the machine's own commutator. ## How to do it ![A wooden dowel on a stand carrying two full copper bands and a third band split into two halves, with brass strip brushes pressing on them, a battery wired to the split band and a two-colour LED wired to the full bands](https://lastcodex.org/_astro/v3.DTVkyPXL_1kehqi.webp) _Turning the split band under its brushes reverses the current in the LED every half turn: direct current becomes alternating._ A hand-turned model shows the part that does the conversion, the commutator joined to slip rings, with a small battery. You need a wooden dowel about 2 cm thick and 15 cm long, two wooden uprights to carry it, adhesive copper tape, thin insulated wire, four strips of springy brass or copper cut from a sheet, a battery of a few volts, a small resistor to protect it, and a two-lead two-colour LED with the series resistor its maker gives for that battery voltage. Wrap three bands of copper tape round the dowel, about 1 cm wide and 1 cm apart. Leave the first two whole: they are the slip rings. Cut the third band into two halves with a gap of about 5 mm on each side: it is the commutator. Drill two small holes and run a wire under the tape from one half of the commutator to the first ring, and from the other half to the second ring. Rest the dowel in notches in the uprights so it turns freely. Screw the four brass strips to the base so that each presses on the copper: one on each ring, and two on the commutator on opposite sides of the dowel. Make each brush tip narrower than the gaps. Connect the battery, through its resistor, to the two commutator brushes: the resistor limits the current whenever a brush bridges the gap between the two halves. Connect the LED and its resistor in series between the two ring brushes. Turn the dowel slowly. The LED shows one colour for half a turn and the other colour for the next half: the current in it reverses twice per turn, as it does in a supply of alternating current. Turn faster and the colours blend. A real converter works the other way as well, taking alternating current in at the rings and giving direct current out at the commutator. ## History and evolution ![A ring armature on a shaft, its continuous winding tapped at four points by wires running to four contact rings beside a commutator](https://lastcodex.org/_astro/v1.o3qmW34d_Z9YzJB.webp) _Bradley's machine of 1887: one ring winding, tapped at four points for four contact rings, with a commutator on the same shaft._ On 9 May 1887 Charles S. Bradley of Yonkers, New York, applied for a patent on a dynamo whose ring winding was connected both to a commutator and to four contact rings. He wrote that the machine could serve "for changing a continuous current to an alternating current, or vice versa". The patent, US 390,439, was granted in 1888. In a later application he reduced the taps to three and described a motor whose commutator rectified the whole current for direct-current motors, electroplating baths or arc lamps. At Westinghouse, Benjamin Lamme designed a rotary converter as a direct-current generator combined with a synchronous motor, and only later learned of Bradley's patent. Polyphase systems made the converter a standard machine. Power was sent at high voltage to substations, where rotary converters turned it into direct current for tram and railway lines. They needed attendants in the substations. Mercury-arc rectifiers, glass bulbs containing mercury with no moving parts, also converted alternating current into direct current, for the New York subway among others, and were in turn replaced by thyristors, semiconductor switches. ## Key facts - Patent application: 9 May 1887, Charles S. Bradley; US patent 390,439 granted in 1888 - Main use: Substations turning polyphase AC into DC for tramways and railways - Later converters with no moving parts: Mercury-arc rectifiers, then thyristors ## Requires - [Electric Generator](https://lastcodex.org/en/inventions/generator/): Motion turned into current (1831) - [Electric Motor](https://lastcodex.org/en/inventions/electric-motor/): Current turned into rotation (1834) ## Sources - [C. S. Bradley, Dynamo-electric machine, US patent 390,439, application filed 9 May 1887, granted 2 October 1888](https://patents.google.com/patent/US390439A/en), United States Patent Office, via Google Patents - [Decision in favor of the Tesla rotating magnetic field patents (Westinghouse v. New England Granite Co.; prior art includes 'the Bradley application of May 9, 1887'), Electrical World and Engineer, 8 September 1900](https://teslauniverse.com/nikola-tesla/articles/decision-favor-tesla-rotating-magnetic-field-patents), Electrical World and Engineer, transcribed by Tesla Universe - [C. S. Bradley, System of electrical distribution, US patent 409,450, application filed 20 October 1888, granted 20 August 1889](https://patents.google.com/patent/US409450A/en), United States Patent Office, via Google Patents - [AC Power History](https://edisontechcenter.org/ac-power-history-page/), Edison Tech Center - [HVDC: High Voltage Direct Current](https://edisontechcenter.org/hvdc/), Edison Tech Center - [Transformers (1911 Encyclopædia Britannica), section on rotatory transformers, citing S. P. Thompson, Proc. Inst. Elec. Eng., November 1898](https://en.wikisource.org/wiki/1911_Encyclop%C3%A6dia_Britannica/Transformers), Encyclopædia Britannica, 11th edition, transcribed on Wikisource - [Traction (1911 Encyclopædia Britannica)](https://en.wikisource.org/wiki/1911_Encyclop%C3%A6dia_Britannica/Traction), Encyclopædia Britannica, 11th edition, transcribed on Wikisource - [Dynamo (1911 Encyclopædia Britannica)](https://en.wikisource.org/wiki/1911_Encyclop%C3%A6dia_Britannica/Dynamo), Encyclopædia Britannica, 11th edition, transcribed on Wikisource - [T. J. Blalock, History: The Rotary Era, Part 1, IEEE Power & Energy Magazine, September/October 2013 (Bradley credited with the rotary converter as early as 1888; Lamme's claim)](https://magazine.ieee-pes.org/septemberoctober-2013/history-9/), IEEE Power & Energy Society Fact-checked on 2026-10-22. --- The Last Codex · https://lastcodex.org/en/inventions/rotary-converter/ · © The Last Codex. Texts, plates and figures: CC BY-NC-ND 4.0.