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.

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