Dynamo
A dynamo is an electrical generator that creates direct current using a commutator. Dynamos employed electromagnets for self-starting by using residual magnetic field left in the iron cores of electromagnets. If a dynamo were never run before, it was usual to use a separate battery to excite or flash the field of the electromagnets to enable self-starting. Dynamos were the first practical electrical generators capable of delivering power for industry, and the foundation upon which many other later electric-power conversion devices were based, including the electric motor, the alternating-current alternator, and the rotary converter.
Today, the simpler and more reliable alternator dominates large scale power generation, for efficiency, reliability and cost reasons. A dynamo has the disadvantages of a mechanical commutator. Also, converting alternating to direct current using rectifiers is effective and usually economical.
History
Induction with permanent magnets
The operating principle of electromagnetic generators was discovered in the years 1831–1832 by Michael Faraday. The principle, later called Faraday's law, is that an electromotive force is generated in an electrical conductor which encircles a varying magnetic flux.He also built the first electromagnetic generator, called the Faraday disk, a type of homopolar generator, using a copper disc rotating between the poles of a horseshoe magnet. It produced a small DC voltage. This was not a dynamo in the current sense, because it did not use a commutator.
This design was inefficient, due to self-cancelling counterflows of current in regions of the disk that were not under the influence of the magnetic field. While current was induced directly underneath the magnet, the current would circulate backwards in regions that were outside the influence of the magnetic field. This counterflow limited the power output to the pickup wires, and induced waste heating of the copper disc. Later homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field effect in one current-flow direction.
Another disadvantage was that the output voltage was very low, due to the single current path through the magnetic flux. Faraday and others found that higher, more useful voltages could be produced by winding multiple turns of wire into a coil. Wire windings can conveniently produce any voltage desired by changing the number of turns, so they have been a feature of all subsequent generator designs, requiring the invention of the commutator to produce direct current.
First dynamos
The first commutated dynamo was built in 1832 by Hippolyte Pixii, a French instrument maker. It used a permanent magnet which was rotated by a crank. The spinning magnet was positioned so that its north and south poles passed by a piece of iron wrapped with insulated wire.Pixii found that the spinning magnet produced a pulse of current in the wire each time a pole passed the coil. However, the north and south poles of the magnet induced currents in opposite directions. To convert the alternating current to DC, Pixii invented a commutator, a split metal cylinder on the shaft, with two springy metal contacts that pressed against it.
file:Pacinotti dynamo.jpg|thumb|left|Pacinotti dynamo, 1860
This early design had a problem: the electric current it produced consisted of a series of "spikes" or pulses of current separated by none at all, resulting in a low average power output. As with electric motors of the period, the designers did not fully realize the seriously detrimental effects of large air gaps in the magnetic circuit.
Antonio Pacinotti, an Italian physics professor, solved this problem around 1860 by replacing the spinning two-pole axial coil with a multi-pole toroidal one, which he created by wrapping an iron ring with a continuous winding, connected to the commutator at many equally spaced points around the ring; the commutator being divided into many segments. This meant that some part of the coil was continually passing by the magnets, smoothing out the current.
The Woolrich Electrical Generator of 1844, now in Thinktank, Birmingham Science Museum, is the earliest electrical generator used in an industrial process. It was used by the firm of Elkingtons for commercial electroplating.
Self excitation
In 1827, independently of Faraday, Hungarian inventor Ányos Jedlik started experimenting with electromagnetic rotating devices which he called electromagnetic self-rotors. In the prototype of the single-pole electric starter, both the stationary and the revolving parts were electromagnetic.Around 1856, six years before Siemens and Wheatstone, Jedlik formulated the concept of the dynamo, but did not patent it as he thought he was not the first to realize the idea. Instead of permanent magnets, his dynamo used two electromagnets placed opposite to each other in order to induce a magnetic field around the rotor. This was also the discovery of the principle of dynamo self-excitation, which replaced permanent magnet designs.
Practical designs
The dynamo was the first electrical generator capable of delivering power for industry. The modern dynamo, fit for use in industrial applications, was invented by Henry Wilde with his paper presented to The Royal Society by Michael Faraday on 26th March 1866. It was independently invented by Sir Charles Wheatstone, Werner von Siemens and Samuel Alfred Varley. Varley took out a patent on 24 December 1866, while Siemens and Wheatstone both announced their discoveries on 17 January 1867, by delivering papers at the Royal Society.The "dynamo-electric machine" employed self-powering electromagnetic field coils rather than permanent magnets to create the stator field. Wheatstone's design was similar to Siemens', with the difference that in the Siemens design the stator electromagnets were in series with the rotor, but in Wheatstone's design they were in parallel. The use of electromagnets rather than permanent magnets greatly increased the power output of a dynamo and enabled high power generation for the first time. This invention led directly to the first major industrial uses of electricity. For example, in the 1870s Siemens used electromagnetic dynamos to power electric arc furnaces for the production of metals and other materials.
The dynamo machine that was developed consisted of a stationary structure, which provides the magnetic field, and a set of rotating windings which turn within that field. On larger machines the constant magnetic field is provided by one or more electromagnets, which are usually called field coils.
Zénobe Gramme reinvented Pacinotti's design in 1871 when designing the first commercial power plants operated in Paris. An advantage of Gramme's design was a better path for the magnetic flux, by filling the space occupied by the magnetic field with heavy iron cores and minimizing the air gaps between the stationary and rotating parts. The Gramme dynamo was one of the first machines to generate commercial quantities of power for industry. Further improvements were made on the Gramme ring, but the basic concept of a spinning endless loop of wire remains at the heart of all modern dynamos.
Charles F. Brush assembled his first dynamo in the summer of 1876 using a horse-drawn treadmill to power it. Brush's design modified the Gramme dynamo by shaping the ring armature like a disc rather than a cylinder shape. The field electromagnets were also positioned on the sides of the armature disc rather than around the circumference.
Rotary converters
After dynamos and motors were found to allow easy conversion back and forth between mechanical or electrical power, they were combined in devices called rotary converters, rotating machines whose purpose was not to provide mechanical power to loads but to convert one type of electric current into another, for example DC into AC. They were multi-field single-rotor devices with two or more sets of rotating contacts, one to provide power to one set of armature windings to turn the device, and one or more attached to other windings to produce the output current.The rotary converter can directly convert, internally, any type of electric power into any other. This includes converting between direct current and alternating current, three phase and single phase power, 25 Hz AC and 60 Hz AC, or many different output voltages at the same time. The size and mass of the rotor was made large so that the rotor would act as a flywheel to help smooth out any sudden surges or dropouts in the applied power.
The technology of rotary converters was replaced in the early 20th century by mercury-vapor rectifiers, which were smaller, did not produce vibration and noise, and required less maintenance. The same conversion tasks are now performed by solid state power semiconductor devices. Rotary converters remained in use in the West Side IRT subway in Manhattan into the late 1960s, and possibly some years later. They were powered by 25 Hz AC, and provided DC at 600 volts for the trains.
Limitations and decline
machines like dynamos and commutated DC motors have higher maintenance costs and power limitations than alternating current machines due to their use of the commutator. These disadvantages are:- The sliding friction between the brushes and commutator consumes power, which can be significant in a low power dynamo.
- Due to friction, the brushes and copper commutator segments wear down, creating dust. Large commutated machines require regular replacement of brushes and occasional resurfacing of the commutator. Commutated machines cannot be used in low particulate or sealed applications or in equipment that must operate for long periods without maintenance.
- The resistance of the sliding contact between brush and commutator causes a voltage drop called the "brush drop". This may be several volts, so it can cause large power losses in low voltage, high current machines. Alternating current motors, which do not use commutators, are much more efficient.
- There is a limit to the maximum current density and voltage which can be switched with a commutator. Very large direct current machines, say, with megawatt power ratings, cannot be built with commutators. The largest motors and generators are all alternating-current machines.
- The switching action of the commutator causes sparking at the contacts, posing a fire hazard in explosive atmospheres, and generating electromagnetic interference.