Problem
Totally Enclosed Fan Cooled (TEFC) motors rampant and frequent half to yearly winding burnouts against expected 20 years’ lives cut down NPK fertilizer production 10-15%.
The infructuous enormous maintenance expenses and repetitive tasks depressed the plant personnel especially operating and electrical morale.
Hence, all implored the author, transferred as electrical chief to end this 5‑yearlong sorry situation.
Note: This root cause analysis (RCA) is from real-time scenarios that happened in industries during the tenure of one or two decades ago. These articles will help you to improve your troubleshooting skills and knowledge.
TEFC motor Essential Information
The finned body motor end covers fitted antifriction bearings support the motor rotor (below figure).
The stator windings distribute around the motor body bore. Keyed to the rotor internal fan circulates housing bottled air within the motor interior.
The interior circulating air picks up the rotor and stator winding heat, conducts it to the motor finned casing. This way the ambient corrosives and dusts laden air does not enter inside the motor and affect the internals.
An external fan blown outside air over the motor casing integral fins absorbs the motor losses developed heat and rejects it into the environs.
Thus, the motor winding temps remain well within their temperature ratings; e.g., motor designs aim to hold the common 115 o C rated class B windings temperature.
around 80o C. Remember, the cooler the winding the longer their lives; as winding temp increase exponentially decreases the expected 20-25 years winding lives.
The Plant’s TEFC Motor Problem
The corrosive dust sticks to the fins and fills the inter-fin air paths; hence the motor outside fan blown air contacts a lot less heat transfer area than design.
In addition, the fills’ lower than metal conductivities reduce the heat dissipation to the environs.
All these led to high winding and bearing temperatures and consequent their within ½ to year burnt windings, reduced plant availabilities and production, and increased maintenance expenses
Constraints that perpetuated the problem
The electrical engineers and crews accepted it as electrical motors ’windings’ characteristics in corrosive dust areas and lived with it for over 5-years, but the author transferred to head the electrical section could not.
He realized the below-given constraints within few days:
- Cleaning the running motors’ fins is impractical and a safety hazard; hence none ever cleaned the motor outsides. Hence, plant dust filled the inter fin paths and reduced the motor cooling to near nothing.
- The corrosive environs corroded away the came with the motor thin sheet steel external fans.
- The corroded large dia fasteners defied unscrewing; and smaller dia fasteners broke while unscrewing. Crew drilled the body holes and tapped for the next larger dia fastener. These delayed even simple repair tasks.
- Hence, the motors failed too often and crews considered improving the dismal situation impossible and lived with it.
- The motor vendors not supplying essential replacement external fans and fan covers worsened the situation
The author’s solution to the problem
Several days thinking resulted solutions are:
- Replace the thin corrosion prone sheet steel external cooling fans and fan covers with 8 mm thick never corroding FRP (fiber-reinforced plastics) items. These last forever! A local FRP vendor made these using the damaged originals as models.
- House each motor’s field push button stations in stainless steel hinge self-closing door ribbed body thin wall FRP box bolted suitably near the motor. Push button failures and thus plant start delays vanished. The electrical crew selected one out the several 3 local FRP vendor shown designs and improved upon it.
- Use SS 316 up to 6 mm screws; use only hex head or Allen head screws; no slotted head screws; grease all fasteners before tightening. Crew could unscrew and screw in fasteners easily instead of the past hours and hours futile attempts.
- Put in onetime inter-fin fills clean out efforts during a plant turnaround (TA); immediately sand blast clean and epoxy paint the motor body; cover the paint dried motor with an 8 to 10 mm thick conveyor belt piece.
- Use the motor eye bolt (below figure) to secure the belt piece. Cut the belt width to overlap part fan cover and to cover the shaft coupling and length to cover the motor entirely until the base-plate. Secure the belt to the motor body using the lifting eye bolt (below figure)
- On line hosing effectively shines the cover without wetting the terminal box.
- Crew can dissemble and assemble the motors for maintenance with the cover in position; hence, the cover and motor never separate; hence, the crews nicknamed the motors ‘Snail Motors‘.
RCA solution Benefits
RCA solution Benefits are
- Near extinct motor burnouts achieved at negligible costs and efforts because of:
- Simplest, loss proof, most effective and near zero cost cover
- The cover effectively prevents dust fall on the motor; hence, no dust fills the inter fins paths. Thus, vanishes a near impossible recurring and never done inter fins cleaning tasks
- The belt prevents air scatter; rather confines the airflow along the entire inter fins paths. I.e. better than design motor cooling
- The 1 + 2 + 3 + 4 offered high fins passage air velocities prevent dust settling
- In addition, high velocity airflow over bare painted metal and consequent high cooling rates keep the windings cooler than design
- Hence, vanished winding burnouts, remarkably boosted on stream hours, production and product quality
- Formerly during each failed motor plant shutdown (SD) and restart reprocessing was necessary to deliver specification products
- Near zero rewinding and other maintenance costs amazed and cheered all, except the rewinding vendors!
Author: S. Raghava Chari
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