In this article we restrict ourselves to squirrel cage induction motors. There are only a few simple rules for the maintenance of these motors:
- Maintain high insulation resistance at all time and measure the insulation resistance by megger regularly.
- Keep the bearings lubricated properly and change the bearings at the recommended intervals.
- Maintain a uniform air gap.
- Keep the motor clean and dry always. Protect it from moisture.
If these simple rules are followed, your motor will run indefinitely and never give you any trouble. For the motors, cleaning is the most important maintenance that can be done. Regular cleaning must be done both on the outside and the inside of the motor. The winding should be cleaned with chemical agents like electro-solve and dried. In case insulation readings are low, then the windings can be dried by heating with an electric heater or by supplying an small intensity current through the windings, then followed by varnishing the windings.
In case compressed air is used for blowing the dust away, make sure that it is dry and has been passed through the dehumidifier. However prudent engineers do not use compressed air as the pressure of the air forced the dust and the contaminants inside the windings. It is better to use a long handle dog leg brush for the cleaning. Alternatively a vacuum cleaned can be used.
The motors become dirty from the outside in industries and then the overzealous staff paint them bright before the inspections. However it must be remembered that each additional layer of paint is reducing the heat transfer of the motor casing and fins. Years of painting lead to the overheating and the failure of the insulation and eventual burning of the motor. In case you find that the motor have become dirty it is a better option to clean them with a heavy duty cleaner than re painting.
In case you find that a motor is very hot when all the other parameters including the current drawn are normal, then a simple procedure of removing all the old layers of paint by a scraper will bring immediate relief.
Contamination of the windings due to the grease escaping from the ball bearings also contaminates the windings. In the bearings where a bearing covers are not available sealed ZZ type of bearings should be used. In the sealed bearings optimum quantity of grease is put and the same grease melts, lubricates and again gels.
Some engineers remove the shielding of the ZZ type of bearings to put additional grease. This action does uncountable damage to the bearings as well as the windings. In case there is evidence that the winding has been subjected to the onslaught of grease and oils, the same should be washed with cleaning agents like electrosolve and electraclean, etc.
When a motor is opened up for overhaul it should be thoroughly inspected so that the faults can be diagnosed before the breakdown takes place.
The stator can be damaged if the rotor is not properly inserted as the clearances are low. It is damaged on numerous cases and the motor left to run and no body informed. Look at the stator windings for any damages, overheating, rubbing signs, etc.
The rubbing marks can be caused by a worn bearing or due to misalignment. In case the bearing is due for replacement or if you find some fault it should be replaced right away. The insertion of bearing must be by heating in an oil bath as it is very gentle to the bearings.
In case the bearings are due and you find that the motor is running properly, the condition of the bearing can be done by condition monitoring equipment like Shock Pulse Measurement. The other method is to monitor the temperature around the bearing area by infra-red remote thermometer.
When replacing the bearings the correct amount of grease must be used. If you use less then you can damage the bearing as it can run dry. In case excess grease has been put it will cause churning and friction which results in overheating.
In case the bearings are to be reused they should be cleaned by inserting and washing in solvents like kerosene. After the bearings are washed they must be dried by compressed air. However care should be taken to not let the bearing rotate on air jet as it can damage the battery.
Flooding of Motors
In coastal areas and marine applications it is often that the motors get flooded with water. If it is fresh water the danger is not so great as the salt quantity to cause conduction and shorting of the motor and subsequent burn out is less. But if the motor has been flooded by salt water then they can be brought back to life by the following process:
- Wash the motor with distilled water to remove all the salt and the minerals.
- Any grease and oil deposits must be removed by a suitable electric grade solvent like electrosolve.
- Dry the stator windings with low power source or lamps.
- The repair must be done in an open space or with extractor fans to remove the water vapors.
- With the windings clean and dry re varnish the windings thoroughly.
- Dry the windings and check the insulation resistance.
Squirrel cage motors are very sturdy and hardly give trouble. They have to be monitored for performance and their maintenance done from time to time which is also simple. Few precautions as discussed above give them long life and ensure their trouble free operation.
- Squirrel Cage Motor: Wikipedia Commons
- Marine electrical equipment and practice by H.D. McGeorge
- Squirrel Cage Motor Construction: Wikipedia Commons
- Practical Marine Electrical Knowledge by Dennis T. Hall