Not in all cases, but in most turbomacinery The answer is yes. This weekend we started a 12,000 HP refrigeration compressor at our Alky unit and now I know why they call it T-rex. The motor starts as an induction motor then synchs at speed. The sounds this machine makes while ramping up is something else. I attached a little case history on the torsional vibration from yesterdays start-up

C-5700_torsional_vibration.doc (84 Kb, 40 downloads)

Eric, not sure what you mean by "These torsion modes show up in the radial vibration probes due to radial cross coupling from radial displacement changing the mass moment of inertia."

What is typical, in virtually any machine train with a gearbox...when torsional activity occurs, there is a simple conversion of torsional motion to radial or linear motion at mesh. If you have torsional motion, you have to have variable torque, and since this occurs at a radius (mesh) it gets translated to radial motion that is detectable by radial probes.

Without a gearbox you will rarely see the effects of torsional activity since the mechanism to convert torsional motion to radial motion isn't typically present.

John from PA

Thanks for sharing that case history. It is interesting.

It checks out that 42hz resonance would be excited around (30-42/4)*60=1170rpm and the 18hz resonance would be excited around (30-18/4)*60=1530 rpm.

I have read a little bit about sync motor torsional oscillations during startup. It is apparently much more common than induction motor torsional oscillations during startup, even though the sync motor is starting by induction motor action of the armitisour (sp?) windings... so both sync and induction motors have slip frequency go from 120 hz to 0 or near zero as you described. Apparently the reason is that the salient pole pieces of the sync motor create much higher torque oscillations (even though there is no field current on those poles during startup) due to reluctance effects. With all that said, I was under the impression that salient pole rotors were reserved for lower speed and smooth rotors were used at higher speeds like 1800 and 3600 rpm. Do you happen to know if it is a saleint pole or smooth rotor sync motor?

It makes sense that a gearbox will provide a mechanism for translating torque oscillations into radial vibration. Also in the case of horizontal motors with torque applied to the stator (equal/opposite of that on the rotor), I believe the assymmetric H/V stiffness of the stator support provides another possible mechanism to couple the torsional oscillation to radial vibration of the stator frame which might possibly be picked up on a motor bearing housing vibration measurement.

Once again, great case study. Thanks for sharing.

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John,
The cross coupling and mass moment of inertia are in reference to a rotating mass about it's centerline. Any eccentricity from the centerline will add additional inertia to the rotating system, which will show up radially. As you mentioned a gearbox will create high torsional vibration from gearmesh oscillations.

Pete,
Thanks, I figured this would change things up a liitle.
I will find out more information on this motor. I know just enough electrical to be dangerous .
I do know this motor starts up with different amplitudes and constant phase every time. I figured this motor had a thermal bow from the large inrush of current. I asked the electrical engineer for that area and he said this was the only synchronous motor that start as an induction motor then synchs later. I will find out about the salient poles or whatever this motor has tomorrow.

Yeah, I thought it was neat how the torsional vibration was exactly at 4 X slip frequency!

As John said a gear acts to convert or couple torsional and lateral vibration. I have tested a number of synchronous motors with gears. The torsional is quite evident.

In the following article radial vibration resulted at a torsional resonance (as well as another minor issue). It turned out to be a broken fuel pump on an Allison engine driving a generator through a three element gear. Most likely, this caused fuel delivery issues that resulted in combustion irregularities, because I also saw torsional vibration on another pipeline generator (different pipeline) due to dirty fuel filters with radial vibration at the 1st torsional (perhaps a little at the second).

W. C. FOILES, Technical article: A tale of two symptoms, Vibrations, 11 (1995), pp. 7–11.

I would see increased torsional vibration reported at the 1st torsional on large synchronous motors when the rubber blocks in Holset couplings would deteriorate.

Bill
Last year we changed out the coupling rubber inserts on a large compressor due to 1X vibration on the coupling side. On start-up I also noticed the 1st torsional was much lower after maintenance.

Pete,
I checked this morning and the motor is a typical synchronous design but it starts under a load. This explains the high torque oscillations.

Erik, I came across similar problem. In this case problem was created by Variable Frequency Drive. Finally vibration was reduced by modifing VFD software. But it is the gear box which traslates torsional vibration in to radial vibration. But main draw back is we have to put strain gauges and torsional vibration measuring bands to exactly know severity of the problem.

All,

I wonder if the mechanism of vibration sensed in radial direction as a result of torque variation is the same in a gearbox ( where variable torque applied to an inertia rotor in a certain point and certain direction ) as in motor (torque is being developed by each bar even by a zero inertia rotor and also acting upon the stator torsionally )?
Likely, the mechanisms are different.

Also, will torque variation due to variable load(for instance, in a pump due to a fluctuating flow ) result in radial vibration on this pump? I guess the answer is 'no'.

David

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