Karl Metcalfe, technical support at the AEMT, offers some advice on how to ensure stored electric motors are properly maintained to make sure they are ready for operation when required.
Most commercial and industrial operations will rely on electric motors and for many organisations it is standard practice to store working spares so that, in the event of a motor failure, downtimes can be minimised by swapping out the failed motor with a working spare.
For this reason, it is essential that stored motors remain operation-ready, and this does require correct management and regular servicing.
Firstly, it is important to keep stored motors protected from vibration and extremes of temperature and weather, so outdoor storage is not advisable. Ideally, electric motors should be stored indoors in a clean and dry environment.
While a climate-controlled environment is the best option for storing electric motors, in some instances, this may not be possible. In such cases, the motor may need to be palletised outdoors. If this is the only option, the motor should be covered loosely with a waterproof covering, ensuring that air can circulate to help minimise condensation. It is also important to try and protect lower IP motors from pest intrusion.
Ensure that all openings – cable entries, air vents, grease pipes etc. – are suitably covered or sealed during storage.
Exposed surface areas could also be coated with a corrosion inhibitor which will need to be removed before the motor goes back into service.
To protect from vibration indoors, it is also a good idea to store motors in locations that are not near any ambient vibration areas.
Bearings in stored motors have been known to fail due to fretting or false brinelling as a result of environmental vibrations, which induces small scale motion of the bearings. Even minimal ambient vibration will cause motor bearings to wear over time. Storing motors on materials that can help absorb vibration is, therefore, good practice.
Regular shaft rotation also helps to reduce bearing wear on any one point of the bearings, helping to prolong their life. The AEMT advises that shafts are rotated at least once a month, with the shafts of larger motors turned more often.
While it is generally acknowledged that shaft turning is necessary on stored equipment, AEMT members have reported that, in practice, the task is rarely actioned often enough. So regular shaft rotation should be made someone’s responsibility, as part of a store’s maintenance schedule.
Ensuring adequate lubrication
For grease lubricated motors, the shelf life of the grease in the bearings should be a consideration. If the motor employs sealed bearings, it is very likely that if not in operation for an extended period – say two years – the grease inside the bearing will separate and will not work optimally when required to start turning again. Motors that utilise re-greaseable bearings should be regreased on average every year or so when in storage, to help prolong bearing life and ensure that the motor is fit for service when needed.
Any moisture or dampness in the air will eventually find its way into a motor and speed up the oil oxidation process. So, as well as minimising corrosion, a dry environment is advisable for storing motors to prolong the effective life of their lubrication.
The AEMT advises that before putting an oil-lubricated motor into storage, it is a good idea to first drain the oil, flush it through and replace it with fresh oil. In operational motors, the oil helps ensure that any debris or particles are held in suspension while the motor is running. But, when stationary, the debris held by the oil will sink to the bottom of the motor sump. Then, when the motor is finally started up again, all the debris will be quickly pulled through the motor.
Ensuring that stored motors are filled with the correct quantity of clean oil, and making sure that it is sealed correctly, will slow down the inevitable oil oxidisation process. In reality, however, all stored motors will be subject to some degree of temperature fluctuation, and this will cause the seals to expand and contract, allowing moisture to enter the motor. Because the oil in a stored motor will degrade over time, it is advisable, as part of the maintenance schedule, to regularly check the oil in stored motors, and it is good practice to change the oil before starting a motor after a long period of inactivity.
Testing stored motors
Before putting a motor into storage, it is also advisable to perform an insulation resistance (IR) test, which measures the total resistance between any two points separated by electrical insulation. This test determines how effective the insulation is in resisting the flow of electrical current. Before putting a motor back into service, repeat this test to ensure that any decreases in insulation effectiveness that may have occurred during storage can be addressed.
After installation, vibration levels should be recorded to evaluate the spectra on motors with rolling element bearings for any signs of bearing fault frequencies. A vibration analysis should be performed during start-up with uncoupled baseline vibration levels documented.
AEMT members have frequently reported stored motors being used as a source of unofficial spares for operational motors – most often, they are found to be missing parts such as terminal box lids and fan cowls. Obviously, this practise should be discouraged. While it is often only the intention to ‘borrow’ a part, too often, the part does not get replaced, which means that the spare motor will not be operation ready. If this absolutely has to happen, it should be recorded in a spares inventory or similar, and the implications of not having the donor motor available as a spare assessed.
Minimising downtime is critical in many sectors, with the cost of interrupted production or operations often having significant bottom-line implications. Therefore, being able to quickly and effectively respond to issues with plant and equipment is essential. If kept in the right environment and properly maintained, motors can be safely stored for many years without encountering any problems when they are put back into service. But to ensure this is the case, a planned approach to motor storage is crucial.