To avoid unplanned motor stoppage and trouble motor should be maintained periodically. A planned preventive maintenance program can give motor long-life operation. Preventative maintenance programs provide care for motors before a major catastrophe can happen.
The goal of predictive maintenance programs is to reduce maintenance costs by detecting problems early which allow for better maintenance planning and less unex.pected failures. Predictive maintenance programs for motors observe the temperatures, vibrations, and other data to determine a time for an overhaul or replacement of the motor.
Inspect for bearing lubrication leakages. No leakage is allowed on the interior of the motor. If found, then replace the motor.
The motor cables must not touch or indicate broken insulation. If friction is found, replace the cable. If a broken insulation is found, replace the motor.
Inspect all mounting hardware for looseness. If any are found loose, then torque to OEM specification.
Motor mounts, resilient or steel for failure or breakage condition. Resilient mounts with ground straps between stator and truck frame must not show signs of cracking or damage.
The inlet filter, if applicable, for excessive dirt or damage. Clean or replace filter per OEM recommendation.
Motor temperature is a prime indicator of how wen Ii motor is operating. A hot motor greatly reduces the life of the unit. A 10°C (20°F) increase from the design motor temperature can reduce the life of the motor’s insulation in half.
Thermocouples, pyrometers (an electrical thermometer used to measure high temperatures), and infrared scanners are the types of devices used to measure the temperature of a motor.
There are four types of insulation test:
Insulation Resistance test:
This useful test is done using megohmmeter. The megohmmeter is attached to the motor whose windings are at ambient temperature. The megohmmeter measures insulation resistance between the windings and the frame of the motor.
Check the insulation 2-3 times to make a graph. If the insulation resistance remains constant again and again, the insulation system is in optimum condition. When the resistance decreases after two or three successive tests, remove the motor. A 500-volt megohmmeter should be used for motors with a voltage capacity of 2,400 volts or less and a 1000-volt megger if the rated voltage is greater than 2,400 volts.
Polarization Index Test:
The polarization index (P-I) is a method used to determine if any substance has contaminated the motor enough to cause serious damage to the windings which would shorten the motor’s life.
The megohmmeter is used to provide a constant voltage across the winding. The insulation resistance is measured after the first minute and after the tenth minute. A ratio is taken to compare the resistance at the tenth minute to the resistance at the first minute. If the ratio is above 2, then the motor winding must be cleaned, baked, and retested.
Surge test examines the turn-to-turn and phase-to-phase insulations. Phase-ta-phase insulation is the protection found between the winding and ground while the turn-to-turn insulation is a thin fl1m. applied to the surface of the copper wire.
In surge test generates a voltage through turn to turn and phase-to-phase insulations by discharging a capacitor into a winding to rapidly pulse the voltage to a specified level. Seeing the pattern on an oscilloscope reveals the findings of the overvoltage test through each phase of the motor. Since the three phases of the motor are identical, the test patterns must also be identical. Unequal patterns tell the tester that a motor short circuit has occurred.
DC High-Potential Test:
This test measures the insulation resistance compared to the ground but incorporates the dielectric strength of the insulation. The test applies a dc voltage in step increments up to an accepted voltage and measuring and plotting the leakage current. An abrupt upswing in the plot indicates an insulation flaw and the test should be aborted immediately to avoid failure under test.
All rotating machines, including motors, have a certain level of vibration that must remain stable even after long service. Some common causes of vibration are an imbalance, mechanical loosening, misalignment and a bent shaft. By observing the amplitude and frequency of the vibration with respect to the motor speed, the cause of the problem can be determined.
A wide array of tools exist to measure vibration. “Classical” vibration tools include the amplitude meter and the vibration analyzer and dynamic balancer.
Preventive maintenance checklist pdf for motor:
motors_check_list_aw.pdf (339.2 KB)
Motor checklist 2.pdf (142.4 KB)