A third-brush dynamo was a type of
dynamo
"Dynamo Electric Machine" (end view, partly section, )
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 ...
, an
electrical generator
In electricity generation, a generator, also called an ''electric generator'', ''electrical generator'', and ''electromagnetic generator'' is an electromechanical device that converts mechanical energy to electrical energy for use in an extern ...
, formerly used for
battery charging on
motor vehicle
A motor vehicle, also known as a motorized vehicle, automotive vehicle, automobile, or road vehicle, is a self-propelled land vehicle, commonly wheeled, that does not operate on railway track, rails (such as trains or trams), does not fly (such ...
s. It was superseded, first by a two-brush dynamo equipped with an external
voltage regulator
A voltage regulator is a system designed to automatically maintain a constant voltage. It may use a simple feed-forward design or may include negative feedback. It may use an electromechanical mechanism or electronic components. Depending on the ...
, and later by an
alternator
An alternator (or synchronous generator) 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 wit ...
.
Construction
As the name implies, the machine had three
brushes
A brush is a common tool with bristles, wire or other filaments. It generally consists of a handle or block to which filaments are affixed in either a parallel or perpendicular orientation, depending on the way the brush is to be gripped during u ...
in contact with 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, ...
. One was
earthed to the frame of the vehicle and another was connected (through a reverse-current cut-out) to the live terminal of the vehicle's battery. The third was connected to the
field winding
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 the field current flows.
In a ...
of the dynamo. The other end of the field winding was connected to a switch which could be adjusted (by inserting or removing resistance) to give "low" or "high" charge. This switch was sometimes combined with the vehicle's light switch so that switching on the headlights simultaneously put the dynamo in high charge mode.
Disadvantages
The third-brush dynamo had the advantage of simplicity but, by modern standards, it gave poor voltage regulation. This led to short battery life as a result of over-charging or under-charging.
See also
*
Amplidyne
An amplidyne is an obsolete electromechanical amplifier invented prior to World War II by Ernst Alexanderson. It consists of an electric motor driving a DC generator. The signal to be amplified is applied to the generator's field winding, and ...
*
Metadyne
References
Third Brush Generators- Yesterday's Tractors website
Electrical generators
Automotive charging circuits
Automotive electrics
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