Magnetic center offset of powerplant generators.

Hi, I am an electrical engineer working as a Field service engineer for powerplants for the past two years. I have a query about how the magnetic center offset value i.e. the offset between center lines of stator and rotor during assembly of generators is arrived at. And what can be the associated problems with leaving this value on the higher side or beyond the design value. Thanks.

7 months ago Like Comment Follow Flag More

Rakesh A., Edgar V. and 2 others like this

9 comments

Rakesh

Rakesh A. • Hi,

Generally the generator is assembled with the stator and rotor magnetic centres aligned and after the initial assembly and testing, the shaft is marked in the drive end side just after the bearings or the endshield indicating the center alignment.....If there is a mis match in the magnetic centres, the rotor always tries to align itself while it rotates under the influence of magnetic forces which would constantly produce axial thrust on the bearings..... Larger the offset, more will be the pull and more will be the thrust.... In smaller machines where antifriction bearings are used, there will be excess temperature rise if the offset is greater than the bearing clearances.....

Hope this is helpful

7 months ago

Phani

Phani K. • Can a mismatch in the offset result in higher vibrations at the drive end journal of the generator rotor?

7 months ago

Rakesh

Rakesh A. • Yes..... It can result in axial vibrations due to imbalance

7 months ago

Max

Max L. • Hi,

to my knowledge manufacturers do not measure offset of magnetical centres during manufacturing, as it can only be determined at the factory acceptance test, and it is left to be "as is" (if it is "pretty close" to 0), because adjustment would require major changes in the construction. Therefore a slight magnetical AXIAL misalignment betweem the rotor and the stator must be tolerated (despite that customers tend to believe otherwise).

I gensets, the bearings often have some sort of axial play, because the diesel engine has an axially locating bearing in its drive end. Therefore it is much more important so make sure that the generator rotor is close to its axial BEARING center of play while running. This measure is often stamped on the endshield as to aid in aligning the coimplete genset.

In addition to the above, the RADIAL alignment of the rotor in respect to the stator has to be very precise (and is determined by measuring the stator/rotor airgap), as there will be radial magnetic pull if it is not. The radial pull might add to the load on the bearings.

As pointed out above, the axial force due to slight axial magnetic misalignment is very small, and does not normally harm any components, as long as the mechanical bearing alingment is performed correctly.

Axial vibrations normally come from the diesel engine, not from the generator. This can simply be verified by running the generator without the diesel engine at the generator manufacturers test bed; there are no vibrations at the FAT of a proper generator.

7 months ago

Rakesh

Rakesh A. • I agree with max on the fact that the offset is generally left as it is if it is within acceptable limits.....However, radial alignment is very important since any significant misalignment can induce Unbalanced magnetic pull which apart from adding radial load on the bearing can also cause rotordynamic instability in high speed generators used in steam and gas power plants. Which translates as high amplitude vibrations.

7 months ago

Steve

Steve S. • This is a very good discussion with good feedback. A couple more small things. Generators are often coupled to the engine through flexible plates or a flex. coupling. This is in part to address normal amounts of axial misalignment and to make sure there is little or no axial load on engine bearings, which they are not designed for. As for radial alignment some large generators actually have a specific amount of misalignment in a static condition to address gravity forces on the rotor. The rotor aligns itself when running. A reputable manufacture will hold tight tolerance on vibration when doing end of line testing. This most often picks up misalignment issues among other things.

7 months ago

Art

Art W. • I have a comment on a smaller motor. In hard disk drives, HDDs, the spindle is driven by a BDCM. HDDs, in many applications, must be quiet. When the spindle rotor and stator are magnetically misaligned axially, acoustic noise is produced. Hence, this is the reason for axial magnetic alignment in spindle motors in HDDs. Regards, Art Wagner

7 months ago

Prem Kumar

Prem Kumar C. • Non-uniform air gap resulting from misalignment of rotor axis w.r.t stator axis must be avoided whatever it takes to accomplish. The magnetic pull as Max Lundell has rightly pointed out, can cause a load on the bearing. Rakesh Athukattu has also mentioned the same. With hydro generators, the inaccurate assembly of the spider and sectorial stator at the hydel site are PRIME cause. In some of the rotating machines, the misalignment of the rotor axis w.r.t TRUE MACHINE AXIS varies over the length of the axis ! In smaller machines, this comes about because the bearing housing is machined inaccurately. In my opinion, the remedy for such ills lies in ACCURATE MANUFACTURING PRACTICES and STRICT QUALITY CONTROL practices.

7 months ago

Dantam

Dantam R. • There is some fine difference between magnetic offset and mechanical offset. Both create rotating unbalance loads that can damage bearings and cause rotordynamic issues. In large machines it can also trigger a problem known as thermal sensitivity. It is impractical to achieve zero offset. Acceptable offset varies from industry

to industry. It is not true that manufacturers do not recognize the problem. Reputed manufacturers comply with applicable balancing standards. eg ISO 1940, ISO 11342, API 610, API 617 etc. Special techinques are available for field balancing.

The Magnetic Center in Electric Machines. In Electric Machines such as Motors and Generators, the shaft tends to maintain a distinct axial position when running. This position may be different from the position of the shaft at rest and, in the case of motors, when the machine is running with a coupled load.

The Magnetic center is caused due to the magnetic forces between the rotor and stator attract each other. These magnetic forces tend to ensure that the gap between the stator and the rotor is as small as possible. Hence, if the axial position of the machine at rest(mechanical center) is different from the magnetic center, the rotor of the machine may slightly move axially to the magnetic center when running without load.

Causes of shift of the magnetic center from the mechanical center.

There are many causes for the shift of the rotor axially when running. Some of them are

Effect of the cooling fan when running (air flow)Different in the core stack length of the motor causes magnetic forces in the drive-end and the non-drive end to be unequal. These forces tend to balance each other by shifting the rotor axially.

In most machines, the magnetic center is indicated by an external indicator which is fitted on the stator and which points to a groove on the rotor. Correct positioning of this indicator ensures proper magnetic centering

Consequences of wrong Magnetic centering.

If the magnetic center is not set properly when the machine is reassembled after any maintenance work, the rotor may tend to shift beyond the axial limits permitted by the bearings. This is particularly true for sleeve bearings. This may cause the rotor to rub against the thrust collars of the bearing.

Adjusting the Magnetic Center

The Magnetic Center can be brought to the indicated position when the machine is at rest by either moving the bearings of by moving the stator depending on the provision of the manufacturer.

What is the function of the Conservator in a Transformer The Conservator is a cylindrical component of the transformer. The conservator is located at the top of the transformer. The Conservator is designed to act as a reservoir for the transformer oil. The level of the oil in the transformer can rise and fall due to

temperature. The increase of temperature can be caused either by a rise in ambient temperature or due to increased load on the transformer.

An increase in temperature causes the oil in the transformer to expand. The conservator provides space for this expansion of the oil. The oil level indicator in the conservator needs to be monitored to ensure that the level of oil does not fall below the alarm limit.

As the level of oil rises and falls inside the conservator, air enters and leaves the chamber. The air may carry moisture which may cause the oil to deteriorate. Breathers filled with silica gels are provided to separate moisture from the aspirated air.

The silica gel turns blue when it becomes saturated with moisture after which it needs to be replacedWe found a severe damage in the bearing pads and thrust collar in Generator Side of the Gear Box Wheel(Bull Gear)) Thrust Bearing. The thrust collar was worn out about 3mm maximum, also thrust bearing pads babbit on generator side. So total thrust float is about 5.5mm. Magnetic center of Generator was shifted 5~6mm. The normal vibration value has been a little high 2.0~2.4mils since last overhaul on October, 2001. We

overhauled the gear box, at the time we found it. This gear box type is single helical. I think it is impossible to be damaged by tubine thrust force because there is no damage in pinion thrust bearing. To transfer the thrust force to bull gear from turbine, at first the force could be transfered to thrust bearing of pinion gear. Also the pinion thrust bearing should have been damaged. I think the big thrust force could be produced in generator side. But I don't know why the big thrust force of generator could be produced. Please let me know possible causes of damaege of ger box thrust bearin. If you let me know your e-mail, I'll give

the photo of damaged parts. Generator: Brush 6.5 MWGear Box/Turbine : Peter Brotherhoods Ltd.Thrust Bering Type : MichellThank you.

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OLIMember

posted 09-05-2002 12:29 PM                

One way to create axial forces in a generator is to have the wrong magnetic center to start with or to have that changed/moved by missing grid phasing or electrical faults or by maintenance on the generator, changing windings or similar. I would look for that kind of problems. Do you have any axial vibration readings to compare before Oct 2001 and before this stop?

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Alex JMember

posted 09-05-2002 01:45 PM                

Just a comment. You shouldn't focuse only on high axial loads as it could put you on the wrong path. You should evaluate any other possibilities like electrostatic discharges etc.

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SasakiMember

posted 09-05-2002 11:55 PM                

It seems OLI is on the right track.The thrust collor of the generator was set to contact with the thrust bearing in October 2001 and the gererator rotor moved to the thrust bearing due to a magnetic force; to magnetic center. Then, both trust collor and bearing have been contacted each other for two years resulting the loss of the white metal on bearing.

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branden kimJunior Member

posted 09-06-2002 04:21 AM                

Unfortunately we don't have axial position indicator(axial vibration probe) on wheel and pinion of gear box. quote:

Originally posted by OLI:One way to create axial forces in a generator is to have the wrong magnetic center to start with or to have that changed/moved by missing grid phasing or electrical faults or by maintenance on the generator, changing windings or similar. I would look for that kind of problems. Do you have any axial vibration readings to compare before Oct 2001 and before this stop?

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NAQVIJunior Member

posted 09-21-2002 01:39 AM            

We have also experienced such problem of high axiall thrust but not excessive to this extent. we are facing high axiall thrust marks on the out board double row spherical bearing on a cooling water circulation pump run by a motor. we have five pumps and facing O/B bearing failures freuently on one pump as compared to simlilar other four pumps. there can be some preblem related to megnatic centre of the motor. What check can we make to solve the problem?

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OLIMember

posted 09-21-2002 03:39 AM                

Simplest way if possible, run motor with no coupling and try to mark some way where it have it´s natural axial position. Compare that to the maybe forced position when coupling is in place or running complete. Try to move things so running axial position is within the freedom of the normal axial play of the coupling compared to where the motor wan´t to run when running uncoupled. Good luck. Normally if no event has taken place it should be around the same place as the other similar pumps. So comparing position between the pumps may also give some help.

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NAQVIJunior Member

posted 10-01-2002 04:32 AM            

I understood the marking of the megnetic centre of the motor. I have checked from other persons at the plant they had already checked all that and moved the hub as required. We have also used high vicosity oil ISO 150 for these pumps. The pump speed is 490RPM and flow is 35000GPM double suction 1308HP.On both ends of the pump shaft we have double row spherical roller bearings. Coupling side bearing is a floating bearing and O/B side bearing is locating bearing. We also faced a very different problem that when a pump is stoped for preventive oil replacement and afterwards when it is run again temperature

increase is observed on the O/B bearing of that particulat pump. Constant oil leveler is working properly.Can anyone please suggest some rmedial action in this regard. High temperature bearing failures have been observed particularly on this pump in the last 20 years history of pump operation.

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OLIMember

posted 10-01-2002 10:51 AM                

Just an idea, have you verified that it is a suitable oil for that use. I am not a oil specialist but there is a connection between wrong oil and hi bearing temp due to inner friction if not suitable, and I may be wrong.

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galenJunior Member

posted 10-02-2002 11:32 PM            

Your problem may be positioning of the impellor. Double suction pumps realize axial forces when the impellor is not centered. Another possible cause is that the (heating) bearing is offset, cocked, or more likely loose due to the bearing housing being bored improperly.