Electrical Systems - Husqvarna DMS 240 Workshop Manual

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ELECTRICAL SYSTEM
Electrical system
The DMS 240 is fitted with a traditional electric motor,
i.e. both stator and rotor have windings and the current
is fed to the rotor via carbon brushes to the rotor's
collector.
The machine's logical and basic electrical design
means that checks and servicing can be carried out
quickly and easily. Electronic components and
connection points are well positioned and grouped
in a housing that is easy to remove from the machine.
Carbon brushes
Inspection/replacement
Checks and replacement of carbon brushes is very
simple on this machine, simply unscrew the covers with
a screwdriver and pull out the carbon brushes.
The carbon brushes are wear parts that are checked
regularly; every week if the machine is used daily. When
checking the carbon brushes, it is important that these
are reinstalled in the same direction in order to retain
the worn surface to the collector, see text below about
running in.
Replace the carbon brushes when at least 6 mm of
carbon brushes remain. Note that the carbon brushes
have a mark for this minimum dimension.
Running in
After replacing the carbon brushes, the machine shall
run without load around 10 minutes to allow the car-
bon brushes to abut to the collector.
Worn carbon brushes – checking
The importance of replacing the brushes in good time
is demonstrated by the adjoining illustrations, the
example is taken from another type of machine.
The illustrations to the left show a brush that is worn
but still operable. Both the brush and the collector have
minor scratches in the direction of rotation which is
entirely normal.
The illustrations to the right show the results when
the brush is so worn that the spring, which presses the
collector down, has reached its end position and no
longer gives the brush the correct pressure to the collec-
tor. Sparking occurs which damages both brushes and,
even worse, the collector in a very short time.
Pay attention to sparking near the collector
If the brushes are not worn, sparking could be due to
the brushes being unable to move in their holders as a
result of, for example, dirt. This must be rectified.
Sparking at the collector also occurs if the stator or
rotor has short-circuited windings that cause power
surges.