LearnTronics
History
Making DC — From Faraday's Disk to the Dynamo
Mechanical motion becomes a continuous source of electrical power
Faraday had shown that changing magnetic conditions can induce an EMF. The next step was to make the change happen continuously with a rotating machine.
motor: electrical → mechanical
generator: mechanical → electrical
Faraday's disk — 1831
Soon after discovering electromagnetic induction, Michael Faraday built a rotating generator now known as the Faraday disk. A conducting copper disk rotated in a magnetic field, producing a continuous EMF between its center and rim.
The machine was historically important because it showed that continuous mechanical motion could directly produce electrical output. It produced relatively low voltage and was not the form that later power generators would normally take, but the principle was unmistakable.
Pixii's magneto — 1832
The French instrument maker Hippolyte Pixii soon built a different type of generator. A permanent magnet rotated relative to coils of wire.
As one magnetic pole approached and then the opposite pole approached, the induced polarity naturally reversed. The raw electrical output therefore alternated in direction.
south pole passes coil → opposite induced polarity
Pixii's basic generator naturally produced an alternating voltage. The commutator was added only when a one-direction external output was desired. Later engineers would deliberately keep the alternating output and build transformers and AC motors around it.
If the machine naturally alternates, how do we get DC?
A commutator can perform a mechanical form of rectification. As the rotating system passes through the point where the generated polarity reverses, the commutator also reverses which internal conductor is connected to each external brush.
The internal EMF may alternate, but the external terminals can keep the same polarity.
Magneto versus dynamo
Early magneto-electric generators used permanent magnets to provide the magnetic field. That limited how strong the field could conveniently become.
A dynamo uses electromagnets for its field. Once a machine can use some of its own output to strengthen those field magnets, much larger electrical outputs become practical.
The self-excited dynamo
During 1866–1867, several inventors — including Charles Wheatstone, Werner Siemens, and Samuel Alfred Varley — independently developed versions of the self-excited dynamo principle.
A small amount of residual magnetism in the iron can produce a small initial voltage as the machine begins to turn. That current strengthens the field magnet, which produces more voltage, which further strengthens the field, until the machine builds up to its operating condition.
The Gramme dynamo
In 1871, Zénobe Gramme introduced a ring-armature dynamo that produced a smoother and more useful DC output than many earlier machines. Gramme machines became commercially important and helped move electrical generation from demonstration apparatus toward practical power equipment.
The same general machine could also operate in reverse as a motor — a striking demonstration that motors and generators are fundamentally reversible electromagnetic machines.
Measurement trail 13 — measuring generator output
We now have all the familiar quantities needed to describe the electrical output of a DC generator: voltage, current, and power.
A simple generator example
If a generator supplies 100 V while delivering 10 A:
That is 1 kilowatt of electrical power. If it maintained that output for one second, it would deliver:
The generator does not create energy. Mechanical input power must be supplied by some prime mover: a hand crank, steam engine, water turbine, or other source. Because of losses, useful electrical output power is always less than mechanical input power.
Lenz's law becomes something you can feel
A generator becomes harder to turn when it supplies more electrical load. That counter-torque is the mechanical consequence of Lenz's law: the induced electrical effects oppose the change producing them.
So when a generator delivers more electrical power, the prime mover must deliver more mechanical power. The resistance you feel at the shaft is part of the energy accounting.
DC generation opens another door
By the later nineteenth century, electricity no longer had to come from a chemical battery. A steam engine, waterwheel, or other mechanical source could drive a dynamo and produce useful electrical power continuously.
That made entirely new systems possible:
- electric lighting;
- larger motors;
- factories powered through wires;
- central generating stations;
- and eventually competition between different ways of generating and distributing power.