33. Welder Starts but Fails to Generate Current
Generator or exciter brushes may be
Be sure that all brushes bear on the
loose or missing.
commutator and have proper spring
Field circuit of generator or exciter
Check for open circuits in rheostat, field
may be open.
leads, and field coils. Also check re-
sistors and rectifiers, if any. Some ma-
chines give less output when fields
34. Motor Trips off the Line
Unbalanced input voltage Ventilation
may be impaired.
35. Machine Fails to Start
Power circuit may be completely dead.
Look for open disconnect switch (par.
Section IV. MOTOR-GENERATOR
In normal use the motor-generator brushes wear
The commutators require practically no attention.
and the spring tension is lessened causing arcing of the
They should be cleaned from time to time with a clean
brushes and may cause pitting of the commutator bars.
rag, or while running with a piece of fine sandpaper.
The commutators require little attention, however, they
Never use emery cloth or paper for this purpose.
do need cleaning from time to time.
Warning: Care should be exercised in cleaning
a. Removal. The worn brushes are removed by
commutator to prevent physical injury to personnel.
disconnecting the pigtails and removing them from the
39. Brush Springs
Brushes should be replaced
It is desirable to check the brush springs (fig. 14)
before the pigtails are within 1/8 inch of the commutator
occasionally to make sure they are holding the brushes
or before the limit of spring travel is reached. New
on the commutator with a firm even pressure. The
brushes must be seated before they can be used on the
brush spring tensions for the brushes in the welding-
machine. This is accomplished by placing the new
brushes in position with a piece of medium sandpaper
readings are taken with a spring-scale gauge at the
placed under the brush toe. Hold the brush in its normal
instant the brush spring releases contact on the brush.
position by a slight pressure of the fingers. The
Correct the tension by bending the brush spring to
sandpaper then should be drawn back and forth under
provide the specified tension plus or minus two ounces.
the brush with the back of the sandpaper held closely in
contact with the commutator. This will wear the brush
40. Welding Machine
down to the curve of the commutator. When the end of
the brush has the proper curve, the operation is
The complete welding machine should be blown out with
complete. Care should be exercised to blow all of the
a low pressure air hose. Grease should be cleaned from
carbon dust away from the commutator.
the machine with an approved cleaning solvent.