Rotary converters
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A rotary converter is a type of
electrical machine In electrical engineering, electric machine is a general term for machines using electromagnetic forces, such as electric motors, electric generators, and others. They are electromechanical energy converters: an electric motor converts electricity ...
which acts as a mechanical
rectifier A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an Power ...
,
inverter A power inverter, inverter or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the opp ...
or
frequency converter A frequency changer or frequency converter is an electronic or electromechanical device that converts alternating current ( AC) of one frequency to alternating current of another frequency. The device may also change the voltage, but if it does, ...
. Rotary converters were used to convert
alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...
(AC) to
direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even ...
(DC), or DC to AC power, before the advent of chemical or solid state power rectification and inverting. They were commonly used to provide DC power for commercial, industrial and railway electrification from an AC power source.


Principles of operation

The rotary converter can be thought of as a motor-generator, where the two machines share a single rotating armature and set of
field coil A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. It consists of a coil of wire through which a current flows. In a rotating ...
s. The basic construction of the rotary converter consists of a DC generator (dynamo) with a set of
slip ring A slip ring is an electromechanical device that allows the transmission of power and electrical signals from a stationary to a rotating structure. A slip ring can be used in any electromechanical system that requires rotation while transmitting p ...
s tapped into its rotor windings at evenly spaced intervals. When a dynamo is spun the electric currents in its rotor windings alternate as it rotates in the magnetic field of the stationary field windings. This alternating current is rectified by means of a
commutator In mathematics, the commutator gives an indication of the extent to which a certain binary operation fails to be commutative. There are different definitions used in group theory and ring theory. Group theory The commutator of two elements, a ...
, which allows
direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even ...
to be extracted from the rotor. This principle is taken advantage of by energizing the same rotor windings with AC power, which causes the machine to act as a synchronous AC motor. The rotation of the energized coils excites the stationary field windings producing part of the direct current. The other part is alternating current from the
slip ring A slip ring is an electromechanical device that allows the transmission of power and electrical signals from a stationary to a rotating structure. A slip ring can be used in any electromechanical system that requires rotation while transmitting p ...
s, which is directly rectified into DC by the
commutator In mathematics, the commutator gives an indication of the extent to which a certain binary operation fails to be commutative. There are different definitions used in group theory and ring theory. Group theory The commutator of two elements, a ...
. This makes the rotary converter a hybrid dynamo and mechanical rectifier. When used in this way it is referred to as a synchronous rotary converter or simply a synchronous converter. The AC slip rings also allow the machine to act as an alternator. The device can be reversed and DC applied to the field and commutator windings to spin the machine and produce AC power. When operated as a DC to AC machine it is referred to as an inverted rotary converter. One way to envision what is happening in an AC-to-DC rotary converter is to imagine a rotary reversing switch that is being driven at a speed that is synchronous with the power line. Such a switch could
rectify ''Rectify'' is an American television drama series exploring the life of a man after he is released from prison after nearly 20 years on death row following a wrongful conviction. It was created by Ray McKinnon and is the first original series f ...
the AC input waveform with no magnetic components at all save those driving the switch. The rotary converter is somewhat more complex than this trivial case because it delivers near-DC rather than the pulsating DC that would result from just the reversing switch, but the analogy may be helpful in understanding how the rotary converter avoids transforming all of the energy from electrical to mechanical and back to electrical. The advantage of the rotary converter over the discrete motor-generator set is that the rotary converter avoids converting all of the power flow into mechanical energy and then back into electrical energy; some of the electrical energy instead flows directly from input to output, allowing the rotary converter to be much smaller and lighter than a motor-generator set of an equivalent power-handling capability. The advantages of a motor-generator set include adjustable
voltage regulation In electrical engineering, particularly power engineering, voltage regulation is a measure of change in the voltage magnitude between the sending and receiving end of a component, such as a transmission or distribution line. Voltage regulation desc ...
, which can compensate for
voltage drop Voltage drop is the decrease of electrical potential along the path of a current flowing in an electrical circuit. Voltage drops in the internal resistance of the source, across conductors, across contacts, and across connectors are undesirable ...
in the supply network; it also provided complete
power isolation Galvanic isolation is a principle of isolating functional sections of electrical systems to prevent current flow; no direct conduction path is permitted.John Huntington ''Show Networks and Control Systems: Formerly Control Systems for Live E ...
, harmonics isolation, greater surge and transient protection, and sag (brownout) protection through increased momentum. In this first illustration of a single-phase to direct-current rotary converter, it may be used five different ways: * If the coil is rotated, alternating currents can be taken from the collector rings, and it is called an
alternator An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature.Go ...
. * if the coil is rotated, direct current can be taken from the commutator, and it is called a
dynamo file:DynamoElectricMachinesEndViewPartlySection USP284110.png, "Dynamo Electric Machine" (end view, partly section, ) A dynamo is an electrical generator that creates direct current using a commutator (electric), commutator. Dynamos were the f ...
. * If the coil is rotated, two separate currents can be taken from the armature, one providing direct current and the other providing alternating current. Such a machine is called a ''double current generator''. * If a direct current is applied to the commutator, the coil will begin to rotate as a commutated electric motor and an alternating current can be taken out of the collector rings. This is called an ''inverted rotary converter'' (see
inverter A power inverter, inverter or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the opp ...
). * If the machine is brought up to synchronous speed by external means and if the direction of the current through the armature has the correct relationship to the field coils, then the coil will continue to rotate in synchronism with the alternating current as a
synchronous motor A synchronous electric motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles. Sync ...
. A direct current can be taken from the commutator. When used this way, it is called a ''rotary converter''.


Self-balancing dynamo

The self-balancing dynamo is of similar construction to the single- and two-phase rotary converter. It was commonly used to create a completely balanced three-wire 120/240-volt AC electrical supply. The AC extracted from the slip rings was fed into a transformer with a single center-tapped winding. The center-tapped winding forms the DC neutral wire. It needed to be driven by a mechanical power source, such as a steam engine, diesel engine, or electric motor. It could be considered a rotary converter used as a double current generator; the alternating current was used to balance the DC neutral wire.


History

The rotary converter was invented by Charles S. Bradley in 1888. A typical use for this type of AC/DC converter was for
railway electrification A railway electrification system supplies electric power to railway trains and trams without an on-board prime mover or local fuel supply. Electric railways use either electric locomotives (hauling passengers or freight in separate cars), el ...
, where utility power was supplied as alternating current but the trains were designed to work on direct current. Before the invention of
mercury arc rectifier Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Mercury ...
s and high-power
semiconductor A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
rectifier A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an Power ...
s, this conversion could only be accomplished using motor-generators or rotary converters. Rotary converters soon filled the need to combine all the competing
electric power Electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second. Standard prefixes apply to watts as with other SI units: thousands, millions and billions o ...
delivery systems that cropped up in the 1880s and early 1890s. These included single phase AC systems, poly-phase AC systems, low voltage incandescent lighting, high voltage arc lighting, and existing DC motors in factories and street cars.Hughes, Thomas Parke. Networks of Power: Electrification in Western Society, 1880–1930. Baltimore, Maryland: Johns Hopkins University Press - 1993, pp=120–121 Most machinery and appliances at that time were operated by DC power, which was provided by rotary converter substations for residential, commercial and industrial consumption. Rotary converters provided high current DC power for industrial
electrochemical Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...
processes such as
electroplating Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the reduction of cations of that metal by means of a direct electric current. The part to be ...
.
Steel mill A steel mill or steelworks is an industrial plant for the manufacture of steel. It may be an integrated steel works carrying out all steps of steelmaking from smelting iron ore to rolled product, but may also be a plant where steel semi-finish ...
s needed large amounts of on-site DC power for their main roll drive motors. Similarly,
paper mill A paper mill is a factory devoted to making paper from vegetable fibres such as wood pulp, old rags, and other ingredients. Prior to the invention and adoption of the Fourdrinier machine and other types of paper machine that use an endless belt, ...
s and
printing press A printing press is a mechanical device for applying pressure to an inked surface resting upon a printing, print medium (such as paper or cloth), thereby transferring the ink. It marked a dramatic improvement on earlier printing methods in wh ...
es required direct current to start and stop their motors in perfect synchronization to prevent tearing the sheet.


Obsolescence

The stopgap of needing to use rotary converters was slowly overcome as older systems were retired or upgraded to match the newer AC universal system. AC to DC synchronous rotary converters were made obsolete by
mercury arc rectifier Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Mercury ...
s in the 1930s and later on by semiconductor rectifiers in the 1960s. Some of the original
New York City Subway The New York City Subway is a rapid transit system owned by the government of New York City and leased to the New York City Transit Authority, an affiliate agency of the state-run Metropolitan Transportation Authority (MTA). Opened on October 2 ...
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.


See also

*
Cascade converter A cascade converter is a type of motor-generator which was patented in 1902 by J. L. la Cour and O. S. Bragstad. It consists of an induction motor driving a dynamo through a shaft. In addition, the rotor of the induction motor is electrically c ...
*
Frequency converter A frequency changer or frequency converter is an electronic or electromechanical device that converts alternating current ( AC) of one frequency to alternating current of another frequency. The device may also change the voltage, but if it does, ...
*
Motor–generator A motor–generator (an M–G set) is a device for converting electrical power to another form. Motor–generator sets are used to convert frequency, voltage, or phase of power. They may also be used to isolate electrical loads from the electr ...
*
Rotary converter plant A rotary converter plant is a facility at which rotary converters convert one form of electricity to another form of electricity. The installed combinations of motors and generators at a plant determine the possible type(s) of conversion. Such ...
*
Rotary phase converter A rotary phase converter, abbreviated RPC, is an electrical machine that converts power from one polyphase system to another, converting through rotary motion. Typically, single-phase electric power is used to produce three-phase electric power ...
*
Three-phase electric power Three-phase electric power (abbreviated 3φ) is a common type of alternating current used in electricity generation, transmission, and distribution. It is a type of polyphase system employing three wires (or four including an optional neutral ...
*
Traction current converter plant A traction substation, traction current converter plant, rectifier station or traction power substation (TPSS) is an electrical substation that converts electric power from the form provided by the electrical power industry for public utility se ...


References

* * Greenberg, Bernard S. (1999)
"Rotary Converter Power Technology: AC, DC, and Subway Power."
nycsubway.org. {{Commons category, Rotary converters Electric power systems components AC power Inverters Rectifiers