AC Induction Motors
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Diagnosing the Unusual in AC Induction Motors
by Harlow C. Hall
Abstract
This is a well written paper about 5 case histories on electric motors with unusual/a typical problems and diagnosing those problems. This paper would be good reference material for beginner and intermediate level analysts and engineers along with what I am sure will be at least one new situation for the experienced analyst. The author uses a large number of graphs and plots to illustrate the known and relevant information for the various conditions. This is a very practical paper with enough technical detail to properly describe the situation, but is not theoretical on any of the applications. This is a very descriptive paper that focuses on the relevant information for each condition. The five case histories include an improperly labeled motor, verification of proper sizing of a motor, an improperly connected motor and more.
PREVIEW
“case History #1:
This first case history involves a two-pole AC induction motor that was running at half speed. It points up the fact that you should never take anything for granted. Make the machine prove itself to you.
A molten metal receiving furnace is gas fired. The main combustion air fan, for this burner, is an overhung rotor fan direct coupled to a two pole 50 hp IEC AC induction motor. The fan rotor is constructed of aluminum sheet metal parts pop-riveted together. This is not what you would call a precision rotor.
The motor on this fan failed. It was replaced by a brand-new, out of the box, motor. The new motor was in service for several weeks when I was asked to see the maintenance supervisor, for that area. I was told they where having trouble maintaining the desired furnace temperature. They had replaced valves, controllers, etc., and could not maintain the proper temperature. The production supervisor thought the fan was running slow. I asked what he called, “slow”? He replied half speed. I said that was physically impossible. No motor is going to run with a 50% slip. It would have reached locked-rotor conditions before that. I asked if anyone had a contact tachometer. No one did. I asked, could I shut the fan down so I could install a piece of reflective tape that would let me use an optical tachometer.
They said no to this as well. At that point I had two options, use the residual unbalance (1 times run speed spike) displayed in the frequency spectrum or use a strobe light and solve the ambiguity problems that are common to this technique. I decided to use both methods to see what they would independently indicate.
Using a strobe light I found I could stop the shaft at 3596 RPM, 1797 RPM, 1572 RPM, 1348 RPM, and 898 RPM.
Point of fact, a strobe light will stop action at every whole integer multiple of the frequency of the turning speed from 1 times the turning speed up to N times the turning speed. Below 1 x the primary frequency it will freeze the action at every fraction of 1 x that ends in a zero. Example: 0.8750, 0.750, 0.6250, and 0.50. These fractional values correspond to 1572RPM, 1348 RPM, 1123 RPM, and 898 RPM. From this and the vibration data I concluded this motor must have been running at 1797 RPM.”
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