I've recently encountered an interesting vibration problem on a 4000hp, 3600rpm (journal bearing)BFP motor. The motor was recently put in service after being repaired and put in storage for about 10-12 months. When started on site at full voltage and uncoupled the vibration is almost 1.0 in/sec horizontally at both ends. Most of the vibratory energy is seen at 1x or 3600rpm. Here's the interesting part. If they allow the motor to coast down to around 1600rpm then re-start it, sort of a "soft start" the vibration is less than 0.1 in/sec at all locations. Has anyone out there encountered a similar problem. I've never seen this type of problem before, but am wondering if there's some type of mechanical rotor problem that is being triggered by the high torque induced at a full voltage start from dead stop. Whatever the problem, it only occurs when started at full voltage from dead stop. On more than one occasion, the motor was allowed to coast down around 1600rpm, re-started, and vibration was less than 0.1 in/sec. While in the motor shop, the motor was started on low voltage using a soft starter and when brought up to full voltage, vibration levels were all less than 0.1 in/sec for a 1-hour test run. If you have any experience with a similar problem, I'd appreciate any input on what you may have found as the root cause.

Thanks,
Scott Tilley

We had a 1x motor vib problem that occurred during every normal full-voltage start at the plant and not in the shop during reduced voltage start (13.8kv motor - can't do a full voltage start in the shop). The motor was installed into the plant twice and removed twice for troubleshooting and the behavior was consistent in the plant during all that time and almost consistent in the shops (the high vib occurred during every start at the plant and did not occur at most starts in the shops... but did occur in a few starts at the shops).

Ours was a vertical 3500hp slow speed machine 324 rpm with a very flimsy rotor construction (the whole rotor spider attaches to shaft only at one point in center of shaft).

Also during coastdown test at the plant, the vib dropped dramatically by a factor of 10 the moment the power was cut (somewhat unusual for 1x vibration).

We spent a lot of effort look for base resonance and never found it.

Eventually, we determined that there was a rotor bow which was being assisted by the electromagnetic force. The electromagnetic radial force is much bigger during a full voltage start than a reduced voltage start. Once the rotor gets pulled into it's bowed shape by initial high electromagnetic force from starting (em force is definitely much higher during starting due to the high currents... that is when rub is most likely to occur during start), the em force will keep it there. Exactly why you might not see this during coastdown and restart? Not as obvious (we didn't try coastdown and restart). A hypothesis - the time that it takes the motor to get back up to speed during coastdown is less. There is less time with high currents at low speed during this coastdown/restart evolution. If you don't have enough time during the high side-pull starting condition, you may never have enough time to get the rotor far enough off center for the lower running electromagnetic force to keep it there. (the farther the rotor is offcenter of the airgap, the bigger the force).

The fix in our casewas to reduce the radial pad clearances to the bottom of their bands, to carefully machine the rotor to get TIR from 5 mils down to 2 mils (I'm sure some will disagree with this ... 5 mils TIR is very common on a motor rotor and usually doesn't cause any problems), and to spend a little extra time get the rotor cenetered in the airgap as tight as possible. That fix worked and the motor has operated for 8 months since then with no recurrence of the problem.

Your machine may be somewhat similar in that it is a large high-speed induction motor which means it may be operating somewhere near a flexible rotor critical. When the rotor is flexible, that bow reduces the airgap on one side and the electromagnetic force becomes more important. Machines with flexible rotors are more susceptible to electromagnetic effects and demand more attention to clearances, runout, and centering during repair.

I can see a vertical machine like ours being more susceptible to this behavior than a horizontal machine (I assume your is horizontal). In the vertical, it's easier to establish that orbiting motion (the only thing preventing it is the bearing, not gravity) and once it is established the em force gets bigger at the point of reduced clearance and tends to maintain the motion.

I would recommend to try coastdown test to see if you get behavior like we got. I would also expect that you might see pole pass frequency sidebands if you put the machine under load.

I also agree with your suggestion on another cause - possible some type of looseness on the rotor. Although I haven't seen it act the way you describe.

I agree with your suggestion that some kind of looseness might be a problem as well. Rotor bar loosenss, core loosenss (on a 3600 rpm machine I'll bet laminations are stacked onto shaft rather than onto a spider). I'm not sure how to check for that. I guess if the coastdown doesn't show dramatic change upon powerdown, this would be your prime suspect. Also in case of looseness you might see phase change between start and might see some harmonics.

This message has been edited. Last edited by: electricpete,

Thank you for your reply. I did view some coast-down data that was taken and the vibration drops fairly slow with speed. At 2500rpm there was still about 0.2 in/sec 1x vibration. This makes me wonder if there is something is moving/bending in the rotor due to high torque at start-up from dead stop. Do you think a crack somewhere in the rotor cause this?

Thanks again

I experienced something similar but can’t say it was identical. We had a 2 pole motor 4500 hp (boiler feed water pump) that had similar characteristics. Typical vibration was around 0.09 ips but occasionally when it was started the vibration would be in the neighborhood of 0.35 ips. We did a lot of work trying to figure out what the problem was. Even went as far as exchanging the rotor with one of its sister motors. The vibration followed the rotor. We never came to a solid conclusion on this rotor. We ended up replacing the rotor laminations and the problem went away for about 8 years. It then started to go back to its occasional high vibration starts. We could shut the machine down and restart and everything would be fine for several days, weeks or even months.

We had another problem with one of the other sister motors. Not the one we did the rotor exchange with. In this instance we did not have vibration issues until we had it out for a rewind and rebar modification, new laminations and the whole 9 yards. During the acceptance tests the motor would vibrate during the shop run using the soft start procedure. I can’t remember the amplitudes but the vibration started when we got up to speed. We would kill the power and before it coasted down it would wipe the bearings. We went through this about 3 times before the shop finally gave up and sent it to their headquarters shop 5 states away. What they finally told us was that the shaft had laminations in it due to the forging process that was used during manufacture. I am not talking about the rotor laminations but defects within the shaft itself. This was a 1950’s model motor. They built an entire new rotor for a small fee and the vibration problems went away.

This brings me to my thoughts on the first motor. It may have had shaft lamination issues also. When we had the rotor rebuilt the new laminations had enough of a tight fit on the shaft to provide it the stiffness it needed. As the motor was ran and started time after time the fit on the laminations may have loosened just a little bit and did not provide the extra bit of stiffness needed to keep things right. These are just my thoughts as there were no solid conclusions or proof otherwise

Scott: Is the high vibration of 1.0 inch/sec 1*Fline [hz] or is it 1*RPM [hz] (your ''1* 3600 rpm'' leaves both options open)? Of course this is hard to discriminate when you run the motor uncoupled so the question will change into: does vibration level decreases instantanously and significantly upon power removal?
Electricpete: I feel your posting leaves both options open as well: is your case 1*RPM or 1*Fline?
I like to verify whether we deal with a mechanical or an electrical-magnetical issue, therefore: can you additionally provide information on vibration immediately upon direct-on-line switching?
By the way: not all readers are USA readers: what is meant with ''the whole 9 yards'' and ''TIR''.
Regards, Arie Mol, NL

Thanks for all the replies,

In response to the last post, yes this is a mechanical vibration occurring at 1x rpm. Coastdown data was taken and the vibration dropped off slowly with speed. However, as I posted earlier, when they re-energized the rotor during the coastdown; (at around 2400rpm) when the motor came up to speed, the vibration was less than 0.1 in/sec. This was done a second time for repeatability verification. It seems this vibration only shows up when the motor is started at full voltage from a dead stop.

In reference to your grammatical questions, I'll try not to be too confusing. I'm just an old country boy from the deep south. I believe "TIR" refers to (Total Indicator Runout) measured with a dial indicator. The whole 9 yards usually refers to a very exhaustive procedure where no short-cuts are taken or procedures skipped. In this case, I think he's referring to a total motor re-build including a rewind, new rotor bars, re-stacked rotor and so on.

Thanks again,
Scott Tilley

quote:

Electricpete: I feel your posting leaves both options open as well: is your case 1*RPM or 1*Fline?

The vib in my example was 1x motor running speed.