Problem was the Plant was built alongside where the motors where placed. The acidic gases from the batteries started to leak out and drifted over the electric motors and this affected the carbon brushes and sliprings.
When they moved the battery charging station elsewhere the problem went away.

There are many more cases like this that cause electric motors to fail.

Vibration, chemicals, hot atmospheres, poor maintenance, incorrect spring pressures.

One problem which is hard to detect is when the wrong information is being supplied by
operators to senior management 
i.e. Sometimes information passes through too many people before it gets to the right
person who unable to deal with the matter correctly.

For carbon brushes to perform satisfactorily on sliprings they require the correct 
environment to run in.

a. Clean air free of dust.
b. Correct amount of moisture in the air. ( Humidity ).
c. Avoid chemicals, gases and dust. ( fit filters if necessary ).
d. The correct current density for the grade of brushes.
e. Correct running temperature. ( A cold motor will not perform well )


For example a motor that run's under loaded will draw low current from the supply. 
It will never reach the correct working temperature.

Patina ( film forming ).

When carbon brushes are in use small particles of carbon dust mix with small
particles of metal removed from the slipring material forming an oxide mixture if the atmosphere around
the brushes is dry and hot this mixture just forms dust which collects in the brush
gear compartment of the motor. It may cause problems.

But if the atmosphere is of the correct humidity and contains enough moisture the 
carbon/metal oxides will mix together and form a very thin paste that coats over the
surface of the slipring and forms what we call a Patina.

This Patina ( film ) is the most important process that is required to assist the carbon
brushes to perform satisfactorily.

It works as follows, using the correct carbon brush grade with the correct atmosphere the
carbon brush will run on the surface of the collector and deposit an oxide paste 
continuously but as the Patina gets thicker the carbon brush will remove it and under ideal
conditions it will remove and or replace the Patina as required in order to maintain the 
correct film thickness.
Anything that Interferes with any of these properties will prevent the carbon brush from 
performing correctly.


Other causes that can interfere with this are,

a. Spring pressure incorrect
b. Sticky brushes.
c. Incorrectly mounted brush holders.
d. Incorrectly mounted brushes
e. Incorrect lengths of brush flexible leads.
f. Faulty windings in Rotor.
g. Loose connections
h. Worn brush boxes.
i. Lack of ventilation.

Over the years collectors wear and are often machined to maintain their roundness this 
operation makes the collector smaller in diameter.
If you fail to adjust the brush boxes and move them closer to the surface of the collector 
they will become unstable and vibrate. 
The distance between the bottom of the brush box and collector surface should not be more than 2.4 mm.

The spring pressure on each brush should be checked and recorded to ensure that all brushes have the same pressure on them and that it is the correct pressure set to the makers recommendations.
When carrying out this check ensure that the length of the brushes are approx. equal.

Check to ensure all brushes are positioned centrally on the collector. They must not 
overhang the sides of the collector.

Ensure that all brushes are able to slide freely in their boxes and that no dust is present 
in the boxes to cause any of the brushes to stick.

Check to ensure that the brush flexibles are positioned so as not to cause any brushes to hang up or touch other parts of the brush gear.

Also check that no brush flexibles have become hardened and stiff so as to cause hold up and reduce spring pressure on any particular brushes.

It is normal within reason for cathodic ( Kathodic ) brushes to wear at different rates compared to Anodic brushes this is due to electron flow from negative to positive as opposed to current flow from positive to negative. 

It has also been known in some cases for copper graphite grade positive brushes to wear more quickly than the negative brushes. Brush wear is a complicated business at most times.

To summarize on what has been said.

Check that the brushes are free to slide in their brush boxes.
Check that the spring pressures are correct and equal on all brushes.
Check that all brushes are mounted correctly.
Check for external influences or internal influences such as vibration, gases chemicals bad
maintenance.
Ensure that the brushes in any machine are all of the same grade.i.e. do not mix the brushes in any machine.
Ensure brush flexibles are of the correct length and do not cause any brushes to hang up 
after a few weeks of use.
Check to ensure motors are not running under loaded.
Check position of brush holders and distance from surface of slipring. 
(no more than 2.4 mm)
Check the running temperature of the motor whilst it is running.
Do the brushes have bevels on the running surface.
Do the slipring's have grooves in the surface
Are then any flats on the running surface of the collector.
Are the rings out of round

If you are using rocker type brush pressure fingers check to ensure the point of contact 
of rocker is central to top of the brush as if not this will apply a side thrust on the 
brushes causing high friction in the box and preventing the spring pressure from working 
correctly.
Bear in mind when checking that if rocker fingers are used they move in an arc across the top of the brush this means as the brush wears and gets lower in it's box so then the rocker will move across the top of the brush and cause side thrust and friction.

If any brush is carrying more than it's fair share of current it will wear more quickly.
Check the brush 'T' and 'A' dimensions and current densities.
Check to see if any brushes are wearing more quickly than the others if so investigate 
the reasons.
All brushes should wear at the same rates apart from the Anodic/Cathodic effect.

Check the bearings as worn bearings will cause the Armature or Rotor to vibrate.