Part 1: Contamination
One of the most common issue that fortunately can be addressed during maintenance is contamination. Power Electronical presents to you a thorough description expressing this issue – cause, effects, its detection and solution through a series of articles. This is first part that talks about different type of contamination, how machines get contaminated and what are its effects.
There have been several surveys into industrial motor failures. The EPRI survey in 1983, IEEE survey in 1985 and one again in 1995 indicated that most failing component was bearings (nearly around 40 – 60 %) followed by stator windings (16 – 30 %).
Bearing failures is a different study altogether and besides issues of contamination of grease and abrasion/spall on the parts in contact (inner/outer race and balls/rollers) which are direct in nature, any other mechanical issue with motor such as unbalance, bad foundation, alignment problems with load etc., ultimately manifests on the bearing causing it to fail. However, such bearing defects can be detected at incipient stage, ahead in time using various techniques like vibration monitoring, motor line current spectrums, acoustic emissions, spike energy detection. Also, such bearings can be replaced easily, and downtimes associated are not high. Activity can be quickly undertaken during a planned outage.
Stator windings – second most failing component requires elaborate techniques to predict or detect signs of failures. Also, the cost of repair is high and so are downtime requirements. If it is a top bar failure, it could take up a few days, but if it is failure of bottom bar, repair times could take several weeks. Also, this largely depends on availability of spare stator bars and its preservation in good condition.
Based on this, there have been further study on the contributors of failures and the famous acronym of T.E.A.M stresses (Thermal, Electric, Ambient, Mechanical). It may also be noted that the stresses are seldom acting by itself but working together giving more weightage to meaning of the word – “Team”. Let me explain with an example – Partial discharges are small arcs in localized zones and contributes to “Electric Stress”, typically seen in 6.6 kV and above rated motors/generators. Such PD may occur in small air gaps between coil and core in straight portion of the slots where the coils/bars are inserted. It is called as “Slot Discharges”. This causes erosion of anti-corona paint or tape on coil sides and resin content of epoxy-mica tapes. The coil thickness is marginally reduced, and coil vibrates in slots causing “Mechanical Stress”. Now, both stresses act together – Electro-Mechanical Stress. Similarly, there are Thermo-Mechanical Stress – vibration causing friction of parts and increasing its temperatures.
In this article, we focus on the A = Ambient Stress, which means the environmental factors or simply put – contamination.
Motors and generators are often installed at locations that could be broadly classified as “indoor” or “outdoor”. While it is easy to perceive that outdoor locations are exposed to dirt, dust, rain which can find its way inside the machine, indoor locations are generally in good condition as they could be clean generator floors or generally has good housekeeping around the application. Is it so? Also, what if the application mandates installation of machine in corrosive atmospheres e.g., coal mining application, saw-mills.
What are contaminants?
So generally external particulate matter that enters inside the machine are “contaminants”. They spread mainly along the stator overhang areas as it has relatively larger surface area and closer to the points of entry of these contaminants. In case these particles are in excess in the atmosphere around the motor, it may spread into the slot areas, stator core, rotor core, inside the rotor through open areas around the shaft and through air gaps between stator and rotor.