Magnetic center and alignment

[zipped]

Friends i have a little confusion over motors having magnetic center. When a motor is at rest, the shaft is on the magnetic center. We carry out alignment at this rest condition. Now when the motor starts it runs on magnetic center.
How does the coupling accommodates this axial movement?
or is there any precautions to be taken during shaft alignment?

 

[electricpete]

Magnetic center is a consideration only for horizontal sleeve bearing motors. For those, you can mark magnetic center during uncoupled run (need someone that knows what they're doing and won't hurt themselves in the process). Ideally you leave shaft in that position while coupling (move shafts on hub if necessary and observe coupling gap requirements also). But only if it is sufficient distance from mechanical limits of travel of motor shaft. How far that is is not well defined, but certainly 1/16" at least, preferably 1/8". In some cases you can make adjustment so you can meet both mechanical and magnetic center (we have a motor where stator core can be slide axially to accommodate this). If you don't have that capability, it is imo much more important to maintain minimum clearance from mechanical limits of travel than to be on magnetic center.

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(2B)+(2B)' ?

 

[zipped]

correction in qstn:: When a motor is at rest, the shaft is on the "mechanical" center

 

[electricpete]

When coupled, the motor is generally in the same position that it was when you coupled it up (held in place by small-clearance driven machine, through coupling). If that is mechanical center, that's where it will stay.

If your question is not answered, can you please re-state the question?

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(2B)+(2B)' ?

 

[oldfieldguy]

A motor at rest may or may NOT be at magnetic center for various reasons.

Typically, there is a means of showing the motor's magnetic center as determined by a factory or shop test run. The prudent man will do a test run with the motor in its final location to verify the REAL magnetic center.

The magnetic center is the driving parameter for alignment and coupling. Failure to consider magnetic center results in axial loads in places they should not be.

In explaining these ideas to some mechanical people on a recent project, I pointed out that all that magnetism that wants to spin that motor to produce 22,000 horsepower ALSO wants that motor at magnetic enter, and did they REALLY want their coupling to fight 22,000 horsepower pushing endways on that coupling and the compressor up the shaft from it.

old field guy

 

[zipped]

Dear, my question is how does a coupling accommodate the axial play exhibited by motors having magnetic center?

 

[byrdj]

It depends on how it was design.

does your motor have a thrust bearing, if so, whoever built the motor and set the thrust set how close to electrical center it will be.

If your driven also has a thrust, then the coupling must allow for some float and that float is determined by the thermal expansions of the the two rotors and the thrust positions.

if your driven has no thrust, it will require the coupling to be solid to set its running axial position. this is the case with most centrifical oil pumps

if the motor has no thrust, then the driven will. if the thermal expainsion of the driven end of the coupling is great, then flaot in the coupling could be used or the coupling made solid but with a cold offset so the motor starts off magnetic, but is near at running temp.

Being dead on magnetic center can cause axail vibration and a lot of applications preferrr to have the motor "push" the driven

 

[zipped]

ok, now for example, thermal expansion is very negligible. In this case how do we take care of axial float. Because even if there is no thermal expansion there will be some float due to movement of motor shaft during start up.

 

[byrdj]

does the motor have a thrust?
does the driven have a thrust?

 

[zipped]

Dear byrdj, could you pls elaborate on role of thrust bearings to accommodate axial float?

 

[byrdj]

the thrust will only allow a few thousands of an inch of rotor float.

 

[zipped]

I am nor sure, but you may consider four cases.
1. Both driver and driven have thrust
2. only driver has thrust
3. only driven has thrust
4. no thrust at all.
what would be the scenario in the above four cases?

 

[byrdj]

dosen't my first addres 1-3
1) need float
2) no float needed, but coupling must set driven axial position
3) no float needed
4) the device will destroy itself of coast down or starup

 

[zipped]

pls explain your second statement, it is not clear - "but coupling must set driven axial position"
Also how much float ?? how do we give float?

 

[byrdj]

If the driven has no thrust, then the thrust of the motor must position driven's rotor in running position. if the coupling has excessive float, then the driven's rotor would destroy itself.

the desired coupling float is determined by the design.

the mechanism for float is in the design of the coupling. all flexable couplings will have float, but the selection of components will establish the amount desired, ie grid spring, gear hub lenght. a true solid coupling will have no float.

 

[zipped]

Apart from the above, do we need to do anything with DBSE (distance between shaft ends) to accommodate the float?

 

[byrdj]

If the DBSE is not within the specifications of the coupling, then the coupling must be modified or there will be no float (or the coupling can not be made). OR the distance between motor and driven must be adjusted (if possible)

is this for class room discussion or home work since the questions are so non specific

 

[zipped]

It may seem non specific to you. But I am asking this because i have read somewhere that float in motors with magnetic center is taken care by adjusting DBSE.
E.g. if mag center is lying X mm from mech center (outside motor). Then X mm is subtracted from DBSE and accordingly DBSE is set before start up of an equipment.

 

[Tmoose]

Often a "limited end float coupling" is specified. The coupling endplay is reduced, but not necessarily completely eliminated. Could be rubber disks in between the shaft ends, etc. During the alignment process things are set up so the driven component's thrust bearing will prevent the motor rotor from ever touching the motor's internal bumpers/stops, which are usually not capable of handling rotation for very long at all.

 

[rmw]

ByrdJ,

I think you have hit on the real answer in your last question.

rmw