Fresh from college, I joined a major electrical manufacturing company in India. As a part of my training during the first year, I was assigned to assist a team that was installing and commissioning a control room plenum ventilation system. The system consisted of a huge ventilation room, which was fed by two large ventilation blowers on the east and west sides of the control room.
My task was to hold the multimeter and megger, and I started my work with great alacrity.
After checking the connections, I checked the power supply to the three-phase induction motor driving the east side ventilation blower. It was switched on. The motor started rotating, but in the reverse direction. I concluded that the connections to the two phases needed to be interchanged to fix the rotation. We did it at the motor terminals and switched it back on. There was no change in the direction of rotation. I was truly stumped. Were all my fundamentals wrong?
My senior colleague noticed that the motor was also rotating at much slower speed, about one-fifth of the synchronous speed. This was another assault on my “fundamentals,” since I was taught that, under normal conditions, an induction always runs at slightly below the synchronous speed and cannot run at such low speeds.
We spent a great deal of time trying to find out what the problem was –- with no success. The electrical team was under tremendous pressure from the client, as the next day was the deadline. And it was almost closing time.
All of a sudden, one of our colleagues came running up to us, asking why the ventilation motor on the west side was running. This was totally unexpected.
Then it dawned on me. When we were switching on what we thought was the east side motor, actually the west side motor was getting switched on. There was a problem with the cabling. At once, we knew the cause of the problem, and it was easy to set things right. We went ahead with a successful commissioning the next day.
This entry was submitted by Nagraj Rao and edited by Rob Spiegel.
Nagraj Rao is an electrical engineer with a master's degree in power systems engineering from the Indian Institute of Science, Bangalore. He currently lives in Bangalore, India, and teaches courses on renewable energy, control systems, and automotive electronics to students of electrical/electronics engineering. He has more than 30 years of experience working in the domain of drives, controls, automation, and project and systems engineering for metallurgical and other industries at Bharat Heavy Electricals Ltd.
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