Siemens 680 - Manual (Page 18)

- 17 -
Maintenance
When this method is used on the stator, the stator
phases may be connected in series or in parallel to
suit the available power supply if both ends of all
phases are accessible. If only three leads are
brought out of the motor, the current may be
circulated between one terminal and the other two
connected together. If this is done, the temperature
of the single lead connection must be checked
frequently, and it is desirable to shift the leads
occasionally. Usually 50 to 100% of full load current
will produce the required temperature. The dc voltage
required for this current will be 0.25 - 5.0% of the
normal voltage per phase, and the corresponding
power will be 0.50 - 3.25% of the rating.
Alternating current can be used on the stators of
squirrel cage induction motors if the rotors are
removed. Alternating current is usually not as easy
to control as required voltage control, and a.c.
requires a higher voltage source, approximately 10 to
30% of the rated winding voltage. In addition, care
must be taken that miscellaneous parts adjacent to
the windings, such as lead studs, core supporting
member, etc., do no overheat due to induced
currents and the lack of normal ventilation.
CAUTION
High temperatures.
May cause damage to insulation.
Avoid hot spots and radiant type heat
Bearings
Long life of bearings is assured by maintaining proper
alignment, belt tension, and lubrication at all times.
Incorrect alignment of solid and flexible couplings can
cause excessive load on bearings, and excessive
vibration and thrust. Misalignment of belt drives can
cause thrust or harmful shaft oscillation. Improper
alignment of gear drives will produce shock loads and
may bend the shaft.
Excessive belt tension often causes overheating and
failure of bearings. Bearings tend to overheat when
pulley centers are too close, pulley diameters are too
small, or belt speed is too high.
Bearing Construction
Two types of bearings are employed in induction
motors. These are;
1. Antifriction
bearings.
2. Sleeve bearings.
The type of bearing mounting and supporting
structure will depend upon the type of bearing.
Bearing housing construction will also vary with the
type of bearing. Antifriction bearing motors have one
piece bearing bracket construction. Sleeve bearing
motors have split bearing housings.
Insulated Bearings
One or both bearings may be insulated to prevent
shaft currents from pitting bearing surfaces. The
insulation is located at the joint between the bearing
housing or bracket and the bearing. Insulated
bearings are designated by an instruction plate on
the bearing housing.
Check periodically to be sure the insulation has not
been weakened or destroyed.
The bearing insulation can be checked using an
ohmmeter or circuit test light. For sleeve bearing
motors with one bearing insulated, the shaft must be
raised a few mils at the non-drive end of the motor so
that it is not in contact with the bearing and the shaft
coupling must be parted so that the shaft is not
grounded through the driven equipment. For motors
with both bearings insulated, disconnect bearing
grounding strap before testing for insulation integrity.
Bearing temperature devices must be disconnected
and oil rings must not be in contact with both the
shaft and the bearing.