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.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
Now that is a Sherlock Ohms tale I can relate to. How many times does the root cause of the problem end up being something so basic, but likely often overlooked because you're searching for the complicated problem.
Ah, the old crossed wiring problem.While you might think this would be easy to catch, it obviously isn't.It is good that they were able to diagnose and fix the problem without any long term consequences.
When I was working on satellites we were warned about the reversed sign problem in attitude control systems.Surprisingly one got by the extensive testing and the satellite was sent into deep space instead of into near earth orbit (oops!).
Wow, Naperlou, how often does that happen? I would imagine the satellite mistake was a multi-million dollar goof. Can a mistake like that be corrected? Could the satellite be brought into near orbit?
This incident sounds a lot like the one I had contributed where one of three pumps was running without power. There was a bad check valve and the other two were driving it backwards. In the case of the Reversed Power Switches also, the one blower was driving the other backwards. Just because a motor is spinning does not mean there is power on it.
Maybe this would be worth a case by itself, but since it is similar in nature, I add it here.
Today I discovered the hard way that you can reverse a split capacitor motor by forcing it to turn bbackwards while the motor is running. If the connected equipment can overpower and stall the motor and cause it to turn backwards it will happily reverse direction and continue running in reverse until the next time the power is turned off and back on. I had seen this before (not knowing exactly what i was seeing) on larger motors of several Hp with failing capacitors driving pumps against back pressure, but this present incident is with nearly a new fractional Hp motor. Thanks to the folks at Brother tech support for educating me on this.
Some time back I checked into a motel while on a road trip. The weather was mild, but the room became very warm after a few hours. I went to the air conditioning control and turned on the cooling. The room became warmer. The more I turned down the temperature the warmer the room became.
I called the desk and asked for another room. When the night manager came to help me move rooms he explained that they had been having similar problems since the air conditioning system was upgraded a few weeks earlier. The wiring for some of the room controls had interchanged so that my control was setting the temperature for another room and visa versa. Someone checked into the other room after I arrived and felt that the room was cool, so turned on the heat in my room. I started getting warm, so I turned on the cooling in the other room. One thing led toanother, and I was roasting while the other person was freezing!
In 1985, when Cahners Publishing Company, then the owner of Design News, moved into a new building the staff had a difficult time regulating the temperature. The HVAC people had mixed up the thermostats so the editorial groups regulated the temperature on the executive side of the building and vice versa. The execs got cooler and thus turned up the heat on the editors (literally), who then adjusted their thermostat to further cool the execs. The initial solution was to place locked plastic enclosures around the thermostats, but the editors--many of whom were engineers--quickly subverted the "lock out" and the cycle repeated itself until someone got the HVAC guys in to check out the system. They uncrossed the controls.
I love stories of first real work experiences. We all start our jobs with book learning and abstract theories only to run slam into the real world. My first concerned a bandsaw with a spring loaded tensioner. Every time I broke a blade, the spring unloaded with a loud bang and was quickly followed by catcalls and ridicule from all of my new coworkers. Finally a veteran tool & die maker told me the problem was the blade welder on that particular saw and that I needed to weld the blades on another machine. But I was not told until I was properly humbled. Sort of an initiation I guess.
From Dell / Intel® New Paradigms in Design Work Scott Hamilton, vertical market strategist for Dell Precision workstations, 5/2/2013 3
Early in my career, I worked as a draftsman and remember the days of drawing on vellum with numbered pencils and Mylar with plastic lead. This was a fun experience in the sense that I ...
I've been using workstations for more than 10 years and love finding ways to get more performance from my system. With demanding professional applications that require more power each ...
A lasting memory from my first job as an engineer in an auto assembly plant is standing on hard concrete at six in the morning, vending-machine coffee clutched in hand, listening to ...
A quick look into the merger of two powerhouse 3D printing OEMs and the new leader in rapid prototyping solutions, Stratasys. The industrial revolution is now led by 3D printing and engineers are given the opportunity to fully maximize their design capabilities, reduce their time-to-market and functionally test prototypes cheaper, faster and easier. Bruce Bradshaw, Director of Marketing in North America, will explore the large product offering and variety of materials that will help CAD designers articulate their product design with actual, physical prototypes. This broadcast will dive deep into technical information including application specific stories from real world customers and their experiences with 3D printing. 3D Printing is
To save this item to your list of favorite Design News content so you can find it later in your Profile page, click the "Save It" button next to the item.
If you found this interesting or useful, please use the links to the services below to share it with other readers. You will need a free account with each service to share an item via that service.
Tweet This
4 
