A pair of carbon brushes
Several types of
generators or electric motors to function, the coils of the rotor must be
connected to complete an electrical circuit. Formerly this was proficient by
affixing a copper or brass commutator or 'slip ring to the shaft, with springs
pressing braided copper wire 'brushes' onto the rings which demeanour the
current. Such brushes offer poor commutation as they moved from one commutator
section to the next. The cure was the prologue of 'high resistance brushes'
made from graphite (sometimes with added copper). Although the conflict was of the
order of tens of milliohms, they were high conflict enough to provide a gradual
shift of current from one commutator section to the next. The term brush is
relics in use.
How the brushes are used?
Accurate work of
the brush depends on the application. Graphite/carbon powder is commonly used. Einhell Carbon
Brushes is used for better conductance (rare for AC applications
and not on automotive fuel pumps that run on carbon commutators). In order to
maximize electrical conductivity and green strength, highly dendritic
(electrolytic) copper powder is used. Binders, mostly phenol or other resins or
pitches, are mixed in so the powder holds its shape when compacted. Other additives
comprise metal powders, and solid lubricants like MoS2, WS2. Much expertise and
research is needed in order to define a brush grade mixture for each
application or motor.
The brush compound is compressed in a tool consisting of upper and
lower punch and die, on mechanical or hydraulic presses. In this step,
depending on later processing, the copper-wire (called shunt wire) can be
inserted repeatedly through a hole in the upper punch also fixed into the
pressed brush block by the powder pressed around. This operation, called
"tamping", is usually performed using electrolytic copper powder,
perhaps with silver coating for some high performance applications. After this
progression, the brush is still very fragile and in professional jargon called
a 'green brush'.
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