Dynamos and Electric Generators

Every electric motor is an electric generator and every electric generator is an electric motor. It just depends upon whether electric energy is converted to mechanical or mechanical energy is converted to electrical, or which is driver and which is driven.

In German "dynamo" is synonymous with electric generator, but in English, "dynamo" is archaic for all but astrophysical electric generators. The dynamo was invented in Germany by Siemans, 18??. This dynamo as well as all electric motors/generators depend upon insulated wire where the topology of the current can be altered or arranged to make any direction or topology of magnetic field desired.

The problem in astrophysical bodies is how can a simple sphere of conducting fluid, a star, (or an accretion disk forming a black hole) manage to create the complicated topology of currents or fields necessary to create a dynamo? The answer lies in the surprising coherence of the fluid flows created by convection in a rotating, stratified fluid, i.e., a star or accretion disk: See The α ω Dynamo and The Pulsed Jet Rotation Experiment. The New Mexico Liquid Sodium α ω Dynamo Experiment was designed to demonstrate just this naturally occurring coherence that leads to such a dynamo.

The range in size of natural and man-made dynamos or electric generator- motors is awesome, 46 orders of magnitude, from the smallest (common) motor, and electric watch, to the awesome galactic black hole accretion disk dynamo.

motor/generator field, 〈B Power Power decay time Total Energy
  gauss watts erg/s years erg
Watch Motor 100 10-7 1 2 2
Large Power Gen 104 109 1016 60Hz/ 20y 6 × 1024
Earth's dynamo 0.5 107 1014 3 × 104 5 × 1033
Sun's dynamo 5000 1023 1030 11 3 × 1038
Magnetic A stars 5 × 108 1026 1033 10? 3 × 1048
Neutron stars 1012 to 1013 1014 to 1016 1021 to 1023 107 1041 to 1043
The Galaxy 5 × 10-6 6 × 1031 6 × 1038 108 2 × 1054
AGN, Radio Lobes 3 × 104 to 10-6 2 × 1039 2 × 1046 10to 10 2 × 1061 to 62

Table 1: A list of dynamos on and off the earth and some properties (approximate). Surprisingly the earth's magnetic field is sustained by a dynamo only ~ 1% of the power of the largest man-made electric power plant.

Figure 1: Shows the schematic wiring diagram of a standard DC (direct current) electric generator (or motor). Figure 1(a) is where the exciting field is maintained by a battery, the same as if field were created by a permanent magnetic. The field of such a generator cannot runaway or exponentiate with a load. Figure 1(b) shows the same generator, but with the output fed to the exciting field coil. Such a "series" generator will runaway or exponentiate its field strength until either it halts the driving motor or melts down, i.e., burns up. Either event is called back reaction or saturation of the generator.

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Copyright 1993, 1994, 1995, 1996, 1997, Nikos Drakos, Computer Based Learning Unit, University of Leeds.

This document was last updated by Kate Weatherall on 5 December 2000