Maintenance
Electric Strip Heater Drying
1.Remove bearing housings.
2.Remove rotor.
3.Direct a fan on stator to carry away the moisture.
4.Attach temperature indicators to winding and apply heat as specified in the Insulation Drying Temperature table and follow procedures described for drying insulation.
5.Radiant type heaters are not recommended because some parts may become scorched before remote parts reach desired temperature.
Circulating Current Drying
1.Remove bearing housings.
2.Center the rotor in the stator core.
3.Wedge fiber strips into the lower part of the air gap so rotor does not touch stator core, or remove rotor.
4.Direct fan on unit to blow away excessive moisture.
5.Attach temperature indicators to windings. Do not exceed the drying temperatures in the Insulation Drying Temperature table.
6.An external source of current can be used to circulate direct current through the winding of any type of alternating current motor. A portable low voltage
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.
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