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DC motor:
Maintenance intervals: The direct current motor must be maintained at regular intervals. The
maintenance intervals and extent are stated in the operating instructions of the motor.
Dirt and contamination: The cooling-air passages to the motor ventilation must be cleaned
regularly depending on the extent of contamination. When replacing the brushes, free the
inside of the motor from abraded brush material (brush dust), the brush equipment and
insulating parts of the commutator must be cleaned. After a protracted operating time, the
motor must be dismantled and the winding also freed from brush dust.
Condensation: Condensation inside the motor is not permissible. Possible remedial
measures: Use an air-condensation heater, or as an alternative, supply the motor field with
approx. 30% of the nominal exciter current without operating motor fan in order to heat up
and dry the inside of the motor.
Brushes: The current is fed to the commutator of the motor via carbon brushes. The brush
life depends on such plant conditions as diameter of the commutator, motor speed, current
level, and environmental conditions. The continuous current of the motor should be greater
than 50% of the rated current of the motor, a lower armature current, so-called operation at
partial load, results in higher brush wear, which can be partially compensated for with special
brush materials. The carbon brushes must rest well on the commutator and the brushes must
be able to move freely in the brush-holder. Brushes must not be worn down below the
permissible residual brush height (see marking on brushes). When replacing carbon brushes
ensure that the original type is inserted according to the printing on the brush.
Brush rocker: The brush rocker must be set to the line mark on the end shield and brush
rocker, if positioned incorrectly, brush sparking will result and could damage the brush and
the commutator.
Commutator: Keep the slots between the commutator segment clean by cleaning them.
Contaminated slots could cause flashovers and arcing on the commutator. Ribbing and burn
marks on the commutator must be removed by grinding.
If necessary, remove the rotor and skim the commutator on the lathe, then the slots, for
example, must be reworked with a special saw. The minimum permissible commutator
diameter is stated in the operating instructions.
Insulation test: Before being installed or after long storage or shutdown periods, the
insulation resistance of the windings must be measured to ground with direct voltage.
Measurement should be performed when the winding is cooled (20 – 30 degrees room
temperature). Please observe the operating instructions of the insulation resistance meter, if
the converter is connected, first check that its input is safely isolated from the supply. 500 V
is the maximum permissible measuring voltage. When making the measurement, you must
wait approx. one minute until the final resistance value is reached.
Measuring circuit voltage: 100 V to 500 V direct voltage, minimum insulation resistance of
new or repaired windings: 10 M . Critical insulation resistance: 0.5 M / kV rated voltage of
the motor.
Example of 500 V rated voltage: 0.5 kV * 0.5 M / kV = 0.25 M = 250 k
Remedy if insulation resistance of winding is too low: Clean and dry winding.
Note: As the voltage measurement in the SINAMICS DCM is high-resistance, the motor
cables at the SINAMICS DCM can remain connected during the measurement of the
insulation resistance. --
Motor bearing: The motor bearing is subject to wear, which is dependent on system
conditions such as the motor speed and the magnitude of the resulting lateral forces on the
bearing. The bearing life is typically between 30,000 to 40,000 operating hours.
Smaller motors are equipped with bearings with permanent lubrication, bearings must be
replaced as soon as their useful life has elapsed. Larger motors are equipped with a
relubricating device. A lubrication instruction plate is located next to the rating plate of the
motor. It provides details of lubrication type, lubrication quantity and relubrication intervals. It
also gives details of the bearing type installed in the motor. After the bearing useful life has
elapsed, these bearings, too, must be replaced. The bearing cover contains a used
lubrication compartment, which must be freed from lubricant after relubrication has been
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