Routine Maintenance of an AC Motor:

• Thoroughly clean the motor's interior and exterior surfaces, removing dirt, grease, and oil using dry air and cleaning solvents.
• Maintain a uniform air gap between the stator and rotor as specified by the manufacturer. An uneven air gap can lead to vibration and premature wear.
• Lubricate bearings according to the manufacturer's recommended schedule and using the specified lubricant type and quantity.

Regularly inspect the following components:

(i) Stator :

• Inspect windings for damage, discoloration, or signs of rubbing.
• Check insulation resistance between windings and earth.
• Inspect winding heaters, if fitted, for functionality.

(ii) Rotor :

• Check for damage, overheating, or signs of rubbing.
• Ensure proper fitting during reassembly.

(iii) Bearings :

• Inspect for signs of damage or wear.
• Renew bearings as per Planned Maintenance Schedule (PMS).
• Ensure proper lubrication using oil or grease.

(iv) Enclosure :

• Clean the cooling fan and air filter.
• Inspect for corrosion and repaint, if necessary.

(v) Terminal Box :

• Check connections for tightness.
• Inspect for overheating or physical damage.

Single phasing is a fault condition in a three-phase AC system where the supply to one phase is interrupted or becomes disconnected. This creates an imbalance in the system, with the remaining two phases carrying a disproportionately higher current. As illustrated in the provided figure, one phase is effectively "open," resulting in unequal phase currents and voltages. This imbalance leads to increased current in the remaining phases.

• If single phasing occurs while the motor is running, it will continue to operate but with significantly reduced torque and increased noise levels.
• The increased current in the remaining phases generates excessive heat, leading to overheating of the motor windings. Prolonged operation under these conditions can cause motor burn-out.
• In severe cases, the excessive heat generated by single phasing can ignite surrounding materials, resulting in an electrical fire.
• The unbalanced currents and potential for overheating can create hazardous conditions, increasing the risk of electric shock and injury to personnel.

Protection Against Single Phasing

• Single phasing is accurately detected by sensing the negative sequence component of voltage or current.
• Two CTs sense the line currents of the motor and feed the signal to a negative sequence filter.
• The filter processes the input from CTs and generates an output proportional to the negative sequence component of the current.
• The filter output energizes a coil that trips the motor circuit breaker when a negative sequence current is detected, preventing further damage.