ABB DC Machines DMI, Инструкция по эксплуатации и техническому обслуживанию

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Brushes and commutation
The most common reason for sparking is poor
contact between the brush surface and the
commutator surface. It is also possible that
certain brushes which have better contact than
others take more current and therefore spark
due to overloading.
Sparking may be progressive, i.e. start with
small sparks, seemingly harmless, and then
slowly or quickly progress to more serious
sparking, especially if the commutator surface
has been burnt. It is important that the
commutator surface is inspected regularly and
any sign of increased sparking at the brush
edges is noted.
Faint sparking may occur and can be accepted
if it does not go any further than that.
General
Good commutation depends on many
influences such as the air humidity, the gas
or oil content in the air, dust particles.
Other factors are: cooling air temperature too
low, low loads over extended periods of time or
vibrations. A critical factor for achieving good
commutation is the correct choice of brush
grade. ABB has many years of experience in the
selection of suitable types. Nevertheless, the
brush grade may have to be changed if unex-
pected operating conditions occur.
Please contact ABB if a change of brush grade
is planned.
To get the right advice, the following informa-
tion should be provided:
– Type and serial number of the machine
– Actual normal and overload current
– Description of the commutator contact
surface condition
– Brushes: grade, wear/1000h, condition
of the contact surfaces, wedging effect
(lateral wear)
– Environment: air humidity, oil mist, dust,
chemical vapours and surrounding air tem-
perature
Patina
A new commutator has a clean copper
surface. After some time of operation a patina
is formed, consisting of copper oxides mixed
with particles from the brushes which together
produce a hard, resistant coating which
protects the commutator.
A homogeneous patina, whose colour is not
relevant, produces ideal running conditions
and minimum wear.
It is important that the temperature of the con-
tact surface is sufficiently high to activate the
chemical process. The temperature is primarily
dependent on the current density in the brush,
the frictional losses and the cooling air tem-
perature.
Another important fact or is the humidity of the
surrounding air, since moisture is a necessary
element in the chemical process.
A favourable water content for a satisfactory
patina is about 10 gram/m
3 .
Sparking
So called patterning may also occur, which
means that certain evenly spaced laminations,
for instance every third, acquire a dark patina
or becomes slightly blackened in parts on the
edge of lamination. Usually this may be
considered merely as a blemish, but in some
cases it may lead to flattening and damage
on the commutator surface. In certain applica-
tions there are often rapid current and speed
fluctuations. The sparking occurring in this case
is usually quite harmless.
If a motor supplied by a converter begins to
spark without apparent reason, a fault in the
converter may be suspected. Possible reasons
might be a fault in the control equipment, a fuse
etc.
Caution
Do not touch a good patina.