Dismantling a 180 Kw permanent magnet rotor motor 4

[petronila]

Hi All

Just wondering if some one has a safe procedure for dismantling large AC machines with permanent magnet rotor.

Thanks

Petronila

 

[rhatcher]

Hi Petronila,

The procedure for a vertical machine having rolling element bearings is to remove the lower bearing cap bolts and, with the motor in the vertical orientation, lift the rotor straight up. The magnetic forces should balance to keep the rotor in the center of the stator bore without contact with the stator. However, be careful not to place your hands between the rotor and the stator because an uneven lift can pull the rotor to the side where it will be forcefully drawn to the stator. It might be a good idea to place a sheet of Nomex inside the stator so that it will be protected in the event the rotor becomes 'stuck' to the stator.

Let me know if you have a horizontal motor and I will ask what the correct procedure is. In either case, once the rotor is removed it should be wrapped in a sheet of Nomex to protect it from contamination, otherwise any metallic particles in the vicinity will adhere to it and be difficult to remove.


I hope this helps.

 

[petronila]

It is very helpful rhatcher, Thanks

It is a horizontal motor. then my idea bolt the frame firmly, screw an eye bolt in D.E and pull the rotor axially with a hoist or crane. However, I would like to know your inputs about the correct procedure.

Thanks again

Petronila

 

[zlatkodo]

Hi, Petronila,
Can you illustrate this procedure with a few photos of motor, rotor etc?

 

[petronila]

Hi Zlatkodo

Unfortunately the motor was delivered to the customer and we don´t have it any more with us. So it is not possible.
Next time when we receive some similar motor I will do that.

Petronila

 

[LionelHutz]

I was at a motor repair shop talking about this a couple of weeks ago. Put the motor on end and pull the rotor out was the basic process. The Nmes wrap is a great idea.

 

[edison123]

Hi Ray
Agree vertical is the way to go for such PM rotors. Just curious how you can place Nomex between stator and rotor with the end bells (though their bolts are removed) are still on the bearings thus preventing access to the air gap.

Muthu

http://www.edison.co.in

 

[waross]

Just curious how you can remove a rotor without removing at least one end bell.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

Bill - Once you unbolt both the end caps from the stator and remove the bottom bearing housing bolts, then you can lift the rotor along with the top end cap and both bearings.

Muthu

http://www.edison.co.in

 

[waross]

Got it. Thanks.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rhatcher]

The responses are correct, I left out a step.

- Remove the bearing cap bolts from both ends of the motor.
- Place the motor in a vertical position with the drive end up.
- Remove the DE bracket.
- Lift the rotor straight up.

 

[electricpete]

I imagine immediately after you pull the upper endbell but before you get a chance to insert any separator paper, the rotor would inevitably pull over to contact at the top?

Although I’m sure it’s easier to separate it when it’s only touching at the top (with bottom still held away from stator by lower bearing) than it would be with full contact between rotor and stator top to bottom (as it would be if you continued and pulled rotor up out of bottom bearing housing without any separator).


=====================================
(2B)+(2B)' ?

 

[rhatcher]

Pete,

The rotor will be constrained from touching the stator by the ODE, lower, bearing until it is lifted clear from the ODE bearing housing. At that point, the magnetic forces will keep it centered unless the lift is off centered. Having said this, I don't know what degree of 'off center lift' it would take to overcome the centering force and stick the rotor to the stator because I've never seen it happen, the rotor magnets are very strong. My suggestion for Nomex inserted before the lift is purely a cautionary suggestion on my part, it is not in our manual.

I could not find the link to our PM motor manual but here is the excerpt relating to motor disassembly. For the record, this is Baldor-Reliance MN433, Motor Repair Guidelines - RPM AC Synchronous PM Motors.

2.0 Disassembly and Reassembly

Motor Disassembly

When removing the PM rotating assembly from the motor care must be taken to overcome the inherent magnetic forces
that will try to hold the rotating assembly (rotor and shaft) in the stator winding. It is recommended that the motor be
disassembled and reassembled in a vertical drive end shaft up position using a hoist to remove the rotating assembly.
In the horizontal position first remove any accessory items (fans, blower, feedback devices, etc.) Also remove the bearing
inner cap bolts (if provided). Mount the motor in a vertical drive end shaft up position and remove the drive end bracket.
The opposite drive end bracket can remain installed. The thread in the end of the shaft can be used with an eye bolt to lift
the rotating assembly with the hoist out of the frame/winding stator.
Warning: Pacemaker danger – Magnetic and electromagnetic fields in the vicinity of current carrying conductors and
permanent magnet motors can result in a serious health hazard to persons with cardiac pacemakers, metal
implants and hearing aids. To avoid risk, stay away from the area surrounding a permanent magnet motor

Rotor Protection After Removal

After the rotating assembly is removed from the frame take care to not allow the rotor to come into contact or near other
magnetic materials especially small metal shavings. The rotor should be protected with a non-magnetic cover (cloth,
Styrofoam etc.) to prevent any metallic contamination from adhering to the rotor outside surface. Keep the rotating
assembly clean and free from dirt, oil and grease.
Replacement of bearings can be done as with any standard induction motor using generally accepted standard
procedures.

Reassembly

After the motor has been properly repaired, the rotating assembly can be carefully reinserted back into the frame using the
same hoist method. Using the hoist slowly lower the rotating assembly back into the motor frame. A solenoid effect will
want to pull the rotating assembly into the motor frame.
Warning: Keep hands and fingers away from the rotor and frame during the insertion process to prevent potential injury as
the rotating assembly is being lowered and magnetically pulled into the motor frame.
The symmetric magnetic forces will help keep the rotating assembly centered such that you do not need to help guide
or keep the rotating assembly centered while lowering into position. Keep hands clear of the air gap in case the rotating
assembly comes loose and falls into position.
Bracket Assembly
Once the rotating assembly is properly located in the frame and into the opposite drive end bracket the motor can
be moved back into a horizontal position. As with all RPM AC motors the bracket and frame assembly needs to be
assembled on a smooth level machined flat surface in order to proper align the brackets and to ensure that the feet are
aligned on in the same plane. If present, insure that all inner cap retaining bolts and bracket bolts are fastened securely.
Reference 7.0 Bolt Torque Specifications for the proper torque ratings.

 

[electricpete]

Thanks Ray. I agree the lower bearing might prevent the rotor from tilting to contact the top of the stator.

But for me, without knowing the specifics of the motor (height, bearing configuration, airgap), based on typical geometries for squirrel cage motors (I’m not familiar with permanent magnet), it’s not obvious that the lower bearing will guarantee there will be no contact.

I’ll show an example calculation.
Pick a random example bearing 6313 C3.
Look up the internal radial clearance using table 8.3 (and bore diameter 65mm) here

https://www.ntnglobal.com/en/products/catalog/pdf/2202E_a08.pdf

It gives a range 23-43 microns. More internal clearance causes more tilting. Let’s pick the middle of the road 33 microns and assume that when mounted with an interference fit to the shaft, the bearing internal radial clearance is reduced to 30 microns.

How does 30 microns radial internal clearance translate to an ability to tilt the bearing? That is shown in Section 4.7, Figure 1 here:

https://www.nsk.com/common/data/ctrgPdf/split/e728/NSK_CAT_E728g_4.pdf

Select the curve for 6313 bearing and reading up from 30 micron internal clearance, we see the angular clearance is 20 minutes. Since there are 60 minutes per degree, this corresponds to 0.33 degrees. Since there are pi radians per 180 degrees, this corresponds to 0.33*pi/180 = 0.0057 radians, also known as 0.0057 inch per inch.

What happens when we apply a slope 0.0057 inch per inch from bottom bearing up to top of rotor/stator iron. Let’s say the vertical distance is 20”. Then the horizontal offset from center becomes 0.0057 * 20 = 0.11”. That is bigger than many airgaps (for squirrel cage motors at least), there could easily be contact.

Maybe the airgap is larger for these motors. Or maybe the OEM’s will take that into account and select something like a double bearing. Or maybe contact at the top only can be tolerated during assembly/disassembly. It beats me. It’s not all that important to me, I’m just thinking out loud, and after I went through to trouble to do an example calc I decided to capture it here for posterity.

Edit - there may be some factor of 2 funny business with that angle that doesn't make sense to me. They define the angular clearance as "The angular clearance... is the maximum angular displacement of the two ring axes when one of the bearing rings is fixed and the other is free and unloaded", which seems very straightforward and is the basis for my calculations. But in two places they interject a factor of 2 that doesn't make sense to me. First they say "In such cases, if the inner and outer ring misalignment angle is greater than a half of the bearing’s angular clearance, it will create an unusual amount of stress" (why half?). Near the end they say "The deflection angle of the inner and outer rings is +/- theta/2".


=====================================
(2B)+(2B)' ?

 

[rhatcher]

Pete,

What is posted is an excerpt from a manufacturer's manual that, per the OP's question, applies to large PM motors. In my opinion it is a good, non-specific, procedure that should apply to most large PM motors.

 

[electricpete]

it's just a point of curiosity, no concern.


=====================================
(2B)+(2B)' ?

 

[edison123]

The fundamental requirement of assembling vertical machines is that the rotor be plumb and be as much radially centered as possible.

Though I have no experience in assembling PM magnet rotor motors, I think given the equal radial magnetic pull in vertical position, the PM rotor would center itself inside the stator almost perfectly making it easier to dismantle and assemble than a non-PM rotor, which is likely to swing a bit due to crane/hoist movements.

Muthu

http://www.edison.co.in

 

[rhatcher]

Pete,
My apologies, I obviously need to take a break. I’ll leave the forum in your capable hands.

 

[electricpete]

Ray - I'm not sure exactly where younger coming from. Either way, no worries. I appreciate your insight on this thread and I voted you a star for your reply dated 18 Jul 19 02:02. I apologize if I dragged the thread too far off course with my own questions / comments.

=====================================
(2B)+(2B)' ?

 

[waross]

Given that magnetic force is inversely proportional to the air gap and given the small air gaps in a motor, won't any deflection tend to produce more deflection until the rotor is hard against the stator at one point?
In small permanent brushed motors it is extremely difficult to keep the rotor centered.
I would expect the rotor to be against one side of the stator.
I would insert the separator paper in the other side.
Then pull the rotor across and finish installing the separator paper.
What don't I know about larger PM motors?
Do some shops disregard the contact and just drag the rotor out?
Please keep us posted petronila.

Bill
--------------------
"Why not the best?"
Jimmy Carter