View source for Third-brush dynamo

{{Short description|A third-brush dynamo and Evolution of Dynamo's}}
[[File:Three-brush dynamo, circuit (Autocar Handbook, 13th ed, 1935).jpg|thumb|300px|Three-brush dynamo charging circuit]]
A '''third-brush dynamo''' was a type of [[dynamo]], an [[electrical generator]], formerly used for [[car battery|battery]] charging on [[motor vehicle]]s.  It was superseded, first by a two-brush dynamo equipped with an external [[voltage regulator]], and later by an [[alternator (automotive)|alternator]].

==Construction==
As the name implies, the machine had three [[Brush (electric)|brushes]] in contact with the [[Commutator (electric)|commutator]].  One was [[Ground (electricity)|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 coil|field winding]] 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]]
* [[Metadyne]]

==References==
* [http://www.ytmag.com/articles/artint4.htm Third Brush Generators] - Yesterday's Tractors website

[[Category:Electrical generators]]
[[Category:Automotive charging circuits]]
[[Category:Automotive electrics]]


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