Q.Why is carbon used in the brushes in a DC generator?
The inductances of the energized windings,
and the mechanics of a brush type motor or generator make arcing and
sparking an unavoidable characteristic of the machine at the brush
interface. Electrical arcs produce very high temperatures. High enough
to melt metals.
Voltage drop across brushes is 1-2 volts
Carbon has a very high melting point (~3500degC)
compared with other conductors. Brushes made with copper or steel would
wear out faster because of factors including:
1) The melted metal will break away from, or even be vaporized, leaving the brush with less material to do its job.
2)
If the melted material doesn't vaporize or otherwise move away from the
interface, it creates a mess for the brush. It can weld together
bristles of the brush. It can build up material that interferes with the
motion of the rotor.
3) As the metal melts in the presence of an
arc, conductive material can be liberated, therby prolonging the arc,
exasperating the wearing action.
Carbon is less prone to those high temperature effects and associated impacts to the generator operation.
Also, Graphite is conductive and auto-lubricant
(low friction coefficient) so, it provides a good dynamic contact to
rings with minimum erosion.
Carbon Graphite - Carbon
Graphite brushes have been used in motor applications since the use of
actual copper wire brushes that wore away the commutator. This material
is mainly used as a base for impregnating metal graphite brushes today.
Carbon graphite can be used in lower current density and lower speed applications because of its high resistivity.
Electrographitic – Electrographitic brushes are
baked in excess of 2000°C to increase the strength of the material.
These can stand up to very high current densities and high speed
applications. With a lower resistance, electrographitic brush grades can
also tolerate higher temperatures. There are many different grades in
this material and it is the most widely used brush on the market today.
Graphite – Graphite brushes are available in several
grades with different resistances and strengths needed for specific
applications, such as high speed turbo generators and slip ring motors.
Generally low to medium current density and lower speed applications use
this grade as a cheaper alternative to electrographitic or metal
graphite brushes.
Metal Graphite – Metal graphite brushes offer very
low resistance and high current density which means a much lower voltage
drop across the brush. These are used commonly in low voltage machines
that require these specifications such as DC motors,
low voltage generators, and slip ring inductions motors. The low
resistance also offers less EMI (electromagnetic interference) if that
may be a problem for a particular industrial application around
sensitive equipment.
