Contributing Writer Harshwardhan Gupta
I was once called in the late 90s to Mumbai (formerly Bombay) to solve a problem of a large motor of a hammer mill repeatedly burning out. The mill was used for grinding spices, and was originally supplied with a 15 horsepower, 2-pole DOL start 3-phase AC motor. The mill was regularly used to grind a variety of spice blends, and worked perfectly for about 12 odd weeks, after which, it just burned itself out after a long, grinding day (without ever tripping the overload relay, a fact that was missed all along by everyone). The mill only ran one 10-hour shift a day.
The attending engineers reasoned that the motor must have burned out due to overloading, so they replaced it with a 25 hp motor. This one worked fine till it too met the same fate after about 15 weeks. The same reasoning was applied again and the motor was replaced by a 35 hp one. This one lasted for about 18 weeks, and then a 50 hp one gave up after about 22 weeks.
The next was an incredible 75 hp giant, and that one was about to meet the same fate, when I was called in. I stepped into the mill room, which was hot and redolent with the fragrances of every exotic Indian spices like black peppercorn, cloves, cardamom, bay leaf, dried red chillies, mace, nutmeg, black cumin, saffron, ‘cheedphal’, ‘nagkesar’, coriander seeds, star anise, turmeric, black stone flower, allspice, sesame, etc.
The mill room was pretty clean and well-ventilated, and yet the smell of the spices was overwhelming and there was a barely detectable, fine particulate mist of spice powder everywhere in the air in the room. The mill had a large cyclone separator attached to the output end, and the separator had a huge fine cloth sock filter attached to its exhaust to catch all the fines escaping from the cyclone separator. With the mill running, the cloth sock has swelled up like a giant Casper ghost, reaching a height of 15 feet.
The giant motor smelt hot! I gingerly touched the running motor and almost got seared. It was as hot as an Indian griddle used to make bread. I then dangled a piece of fluff between the large cooling fins on the body of the motor to judge the air-flow from the cooling fan, but could not detect any air-flow at all. I re-checked by other methods and that only confirmed my observation.
Suspecting clogged air-passageways, I asked the operator to stop the mill and open the fan cowling of the motor. And lo and behold, the entire space below the edge of the cowl, where it overlapped the cooling fins, was completely blocked with very fine, very oily, caked-up spice dust. So this was the culprit, not the so-called overload.
I asked the operator to take a wire-brush and clean out the air passages thoroughly, and re-assemble the cowl; and further advised him to open out the cowl every week and clean out the air-passages, a fairly easy task.
After an hour of running at ‘full load’, the motor soon cooled down, and ran absolutely cool from then on. I further advised my client to replace that giant with the original 15 hp motor, and save a lot of Rupees in his power bill.
Essentially, the motor’s fan would suck-in the extremely fine, very oily spice dust floating around in the room, and the necessary cooling action would progressively reduce, and finally stop altogether - so the burnouts. Case dismissed.
About the Author:
Harshwardhan Gupta lives in Pune, India, has a B.Tech from IIT Bombay and runs his own machine design studio. He has designed many Word’s Firsts in various custom machines.