Misalignment is a functional condition: it is not just a matter of completing a perfect cold alignment, as we all know. Offsets need to be factored in to accommodate thermal growth or rather, thermal growth differential between two machine train components. If that last factor affects alignment precision, there are others.
Dynamic reaction, often neglected, can play an important role: a machine “sits” differently when it delivers (motor) a 500 HP (for example) couple or receives it (driven rotor). Working stresses change the equation: a contracting or expanding pipe will push/pull on one component, or twist it out of shape. And finally, a significant unbalance will create a dynamic, temporary, acting misalignment: the center of a shaft moving by 4 Mils p-p can hardly line up with its counterpart. Obviously, any condition resulting in driver or driven overreaction (in terms of vibration) also resolves itself into misalignment.
The short catalog of causes (which we will likely explain at length in future articles) often explain why the millwright staring at alignment correction with work order in hand may feel a sudden wrath when thinking about his colleague the analyst: with the best of intentions and the machine still warm, no alignment fault seems present, so our poor millwright feels like someone is not performing properly. This can lead to credibility issues within maintenance.
Another item tied to misalignment: the case of the misalignment-induced energy-loss. One paper reported there were no such mechanical losses from misalignment. We will protect the innocent, and prove them wrong: whenever an asynchronous AC electric-motor vibrates, two things occur, in varying degrees: the RPM DROPS slightly, and the amperage draw rises slightly. This is common knowledge, but not necessarily common to just anyone as extensive experience with balancing and other field procedures should likely be common trait of those who know this for a fact.
In the test for misalignment, the professor(s) who controlled the experiment did not propose to simultaneously monitor the vibration to the amount of misalignment. No one ever suggested that misalignment was the loss by itself: if misalignment fails to produce mechanical energy losses through vibration, the misalignment is resolved as an applied force without any movement. Instead, they merely monitored the output through a dynamometer, without measuring the input.
Whosoever says “force applied without any movement” quickly realizes that no mechanical work (our old friend W = F d) is being performed (since d = 0, or close enough) and thus, no great energy losses occur. Simple! But when will we have a misalignment incapable of producing vibration? Given a relatively stiff structure for a small test bench, the dynamic stiffness could be important enough (low mobility, or very small responses per force unit) as to prevent any such vibration and losses. As a note, yes, the author is aware of rotational equations. Just consider Work along the horizontal axis for the sake of simplicity.
It should thus become clear that as a protocol to confirm or invalidate any misalignment-borne losses, the chosen approach lacked scientific rigor. The previous did not consider the possible pernicious losses within the electro-magnetic system should any air-gap variation effect come into play. In fact, for a very small motor, the probability of such losses becomes negligible, and their cumulative effect would be trifling should they be manifest.
© 2007 by François Gagnon
Off the “Maintenance Forum”, “ElectricPete” had a question: “I have never heard this before and at first glance I’m skeptical, but always interested to learn. Do you have any additional information on this? (For example: Any example of how much change you have seen? Any reference to discuss this? Any possible explanation for the physical mechanism?)”
FG replies:
The observation is purely empirical, over the balancing of hundreds of rotors, and really ties in best with the reverse of my statement…
Let me explain: while balancing, the slight RPM variation is typically known in high-precision work as a source of error. Not a huge one by any account, but still present. If using a balancer with a photocell or lasertach, the increase in RPM can be notable as one alleviates the 1X vibration amplitude. In lighter cases, it may only be beyond the decimal point. Worst case I’ve ever noted: abnormally vibrating (0.7 ips pk or more)light rotor (boiler fan) mounted directly on 2-pole motor shaft, 45 RPM variation from start to finish. Usual variation will usually be between 2 and 10 RPM (4-5 is a good target).
Obviously, VFDs will obliterate signs of this if the control is sophisticated and the RPM fixed to a preset: you’ll go right back to “nominal” RPM = last setting (within control precision).
The mechanical energy wasted by unwanted motion must come from somewhere… For the motor, there are only two possible variations to consume energy away from the main task and dedicate it to “shaking”: RPM and power consumption.
For RPM, the “hit” is also power consumption. For purposes of this example, I’ll use “neverland” RPMs (reality never has 1800 RPM unless synchro or VFD, and this neglects losses): take a 200HP, 4-pole motor at 1800 RPM, reduced (gears) to 900 RPM. What is the power output? 400HP. A drop of 9 RPM on that motor considered in a different application would account for (close to) 10 HP, if the relationship were linear… (9 for 1800, or 5% of speed applied to 200HP). Loose terms. This is not a thesis… The previous merely demonstrates the link between total power and RPM x HP. And we’re still constructing the example.
Consider the total power delivered by the motor, and the total amount of slip: if the motor delivers 200 HP while slipping by 20 RPM (1780 RPM rotor in relation to 1800 RPM magnetic field), the amount of slip can serve as a quasi linear power scale: 2 RPM of slip corresponds (in this case, for this motor) to 20 HP. Thus, for this motor, an extra 9 RPM of slip would represent a considerable amount or 45% of total motor power: the motor running at 1771 RPM would obviously be under overloaded strain as it would be delivering 290 HP. No need to mention the presumption of all other factors being constant and adequate (stable 60Hz line, normal voltage fed to motor at lugs, no insanely inappropriate environment temperature, etc). In this case, unbalance responsible for a 2 RPM drop means that 20 HP are used up in shaking the structure.
Do take note that my observation predates the wide adoption of high-efficiency motors. Namely because no one has wanted to pay our fees to cover balancing tasks in quite some time 
Thus, I can not positively affirm same behavior, but one could suppose same principles apply.
Being aware of the clauses of industrial power supply contracts (expected levels, penalties for spikes above steady consumption, and the like) and given our care to often seek better documentation of exact costs, not to mention the sensitive units, we had monitored many motors during balancing procedures.
Observations often placed Amp draw difference at fractional to 5 A, depending on motor. One of these days, on a large synchro motor, I’d like to get the exact power consumption before and after a balance job, but it’ll likely be as an observer busy on other items.
© 2007 by François Gagnon
