1000 hp reciprocating compressor synchronous motor alignment 2

[dithomas]

The foundation of a 50 year old 1000 HP reciprocating compressor was recently rebuild. The synchronous motor is of the type where the rotor is attached to the compressor shaft. It is now time to get the stator back in place and aligned.

How does one account for the magnetic center of the stator with the position of the rotor fixed by the compressor.

Thank you in advance for your assistance.

 

[waross]

One method is to take careful measurements before disassembling the old installation.
Align the center of the rotor iron with the center of the stator iron as a starting point.

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

 

[electricpete]

I assume this is sleeve bearing motor with flexible coupling between motor and compressor shaft and thrust bearing in compressor.

There are several things to coordinate . When all is done:
• the coupling gap between shaft ends should be correct
• the running position of the rotor should be reasonably near it's mechanical center, i.e. not close to either limit of mechanical travel (limits of travel are established by interaction of shaft shoulder and bearing axial faces).
• ideally the rotor is near magnetic center

The adjustments at your disposal.
• Rotor can be slid axially within the mechanical limts of travel established by the bearings mounted to the frame.
• Entire motor frame can be moved axially. (often compressor is much more difficult to move).
• The motors I am familiar with have the core bolted to the frame with slotted holes in the core "feet" to facilitate moving the stator core axially relative to the frame and bearings. I understand this is not universal and other machines don't have ability to move core relative to frame but have instead means to move both bearings relative to the frame.

Going with the familiar configuration I described, here's what we do.
• Move the motor rotor into the position which gives the desired coupling gap and scribe the shaft.
• Perform uncoupled run and scribe the shaft (this gives estimate magnetic center).
• Move the core with respect to stator frame by a distance equal to the distance between above two scribe marks.
• Check mechanical centering. For motor with 1/2" endplay I'd prefer to end up coupled at least 1/8" away from the limit of mechanical travel and certainly more than 1/16" away. Exactly how far away you need to be depends in part on any play or stretch allowed by coupling (want to be at least that far away) as well as providing a margin for error.
• If you end up not quite on magnetic center as a result of keeping distance from mechanical limits of travel, that's usually not a problem. The magnetic centering force is weak. Folklore tells us that holding the motor off magnetic center can result in hunting ..I haven't seen that.
• Re-check airgap (can be disturbed when core is moved).


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[electricpete]

• If you end up not quite on magnetic center as a result of keeping distance from mechanical limits of travel, that's usually not a problem. The magnetic centering force is weak. Folklore tells us that holding the motor off magnetic center can result in hunting ..I haven't seen that.

I forgot to say: if you are unhappy with where you end up (can' satisfy mechanical centering and magnetic centering to your satisfaction), another option at that point in time would be to make two more moves: once move of the entire stator frame so mechanical center is centered on where mag center was, another move of core with respect to frame to put mag center back where it was (core moves with frame). At the end of all this we have moved the stator frame once and the core twice... could have limited it to no more than one core move if did careful measurements up front.

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[waross]

The synchronous motor is of the type where the rotor is attached to the compressor shaft

I visualize a motor with no bearings. Generally the only option is to move the stator.
I would center as best as possible mechanically, check for thrust against the bearings when running and make whatever small adjustments are needed.

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

 

[dithomas]

Thanks for the replies,

This is an over hung motor on a recip compressor and there are no motor bearings. The bearings and thrusts shoulders are in the compressor.

I was at the site today and learned that the rotor pole face is .05 inches is off of mechanical center of the stator pole face to the out board end. The rotor pole face is slightly smaller than the stator pole face. The motors and compressor have been running this way for 5 to 10 years and there is no obvious damage to the thrust shoulder on the compressor bearing and the bearing temps have been within range.

I am wondering if there is not much thrust from the rotor on this type of motor.

it is E&M 1964 vintage brush type salient pole 1000 HP 2300 V, 327 rpm motor. The pole faces are not very long at approx. 18".

I also found out that this and the other motors regularly had the rotors removed for cleaning at a motor shop. There is evidence that this rotor may not have been installed back far enough onto the shaft. The rotor has a split bore that clamps to the compressor shaft and there is no locating shoulder on the shaft.


Dan

 

[electricpete]

the rotor pole face is .05 inches is off of mechanical center of the stator pole face to the out board end. The rotor pole face is slightly smaller than the stator pole face

I assume "stator pole face" is a typo... do you mean the stator core?
The axial magnetic force arises from difference in fringing flux pattern at the two ends. The rotor pulls toward the higher flux density. The fact that stator core is longer than rotor pole face works in your favor to reduce magnetic centering force. You didn't say how much longer. I don't work with this type machine but certainly for induction motor 0.05" would not cause the slightest concern to me. You have a shorter core which hurts a little but a larger airgap which helps… maybe offset each other. My guess fwiw the axial magnetic centering force will be insignificant and certainly seems supported by your experience.


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[electricpete]

I don't work with this type machine but certainly for induction motor 0.05" would not cause the slightest concern to me. You have a shorter core which hurts a little but a larger airgap which helps… maybe offset each other.

Correction: shorter doesn't really hurt. But Slow speed and high circumference is a factor. Anyway my gut feel it's not a problem but you may want to check with someone that works with these.

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[dithomas]

I meant to say 0.5 inches rather than 0.05. The rotor pole face is maybe 0.5 shorter so the rotor pole is just outside of the stator on the outboard end. I do not have exact measurements.

We have E&M going into their vault to see what info they still have on this motor.

Thanks.

 

[waross]

Is this a salient pole machine?

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

 

[dArsonval]

Dan;

Reading in-between the lines here...

12 Jan 14 14:58 Post: "...How does one account for the magnetic center of the stator with the position of the rotor fixed by the compressor. (?)"

13 Jan 14 14:42 Post: "...The rotor has a split bore that clamps to the compressor shaft and there is no locating shoulder on the shaft."

If the rotor is not really "fixed" in (a) position as originally posted? The reader assumes it's adjustable (?)

What about the stator?

Is it mounted directly to the compressor with a register 'fixed' fit?
Or, does it have a foundation mounting as like the compressor?

As waross Bill denotes; "...careful measurements before dismantling...".

In an ideal situation, a multitude of measurements should/would have been recorded before removing,
disturbing, or re-building any portion of this apparatus.

It reads like you've got merely the repositioning of the rotor on the shaft to contend with.

If the stator position is shifted/moved in relation to the compressors foundation as well... it gets complicated.

Hence, highlighting your last post of... "We have E&M going into their vault to see what info they still have on this motor."

The whole piece of equipment can be mechanically, and electrically realigned. It can be done.

The year 1964 is hardly "vintage" for E&M.
They go back to when ... ships were made of wood, and Men were made of steel.

Keep us posted on what you find out.

Always enjoying this forum,

John

 

[electricpete]

Salient pole - yes per earlier post.

there are no motor bearings

It boggles my mind to think of 1000 hp 327 rpm (22 pole in 60hz land) salient pole rotor completely overhung. I have seen engine-driven generators where the generator has no inboard bearing (they rely on the engine), but never missing an outboard bearing. Short rotor helps mechanically in some ways, but that makes it even larger diameter. What is the approximate airgap diameter? What is the approximate airgap depth. Out of curiosity.. is it air cooled?

I doubt I'll be able to help but I'd like to visualize the machine.

Aside from magnetic centering force, I think offcenter rotor increases the magnetizing path reluctance slightly (similar effect to increasing airgap). For a given exiting voltage you'd get slightly higher exciting current. For a given exciting current you'd get slightly lower flux per pole and slightly lower peak torque (less margin to pole slip)






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[electricpete]

Also: horizontal or vertical motor shaft?
(I thought horizontal but better not assume).

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[electricpete]

hermetically sealed?

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[dithomas]

Everyone,

Horizontal shaft and stator is mounted on it's own base plates.

John, I will try and address your questions.
The rotor can be repositioned on the shaft. This will be the final solution is E&M says to center the rotor.

The Stator is seperate from the compressor frame and sits on it's own pads as part of the compressor foundation.

They took very careful dimensions of the compressor and the stator in relation to the compressor but no one bothered to look at the rotor to stator alignment. Go figure!!

Moving the stator required plugging and tapping new holes 1/2" out board of the existing hole in the base plates. Not really an option at this point.

These motors and compressor were referred to small units by the compressor guy that is on site.

Electricpete: Envision a big flat pulley on the compressor shaft that would be connected to a steam engine with a belt but rather than a pulley the motor rotor is on the shaft. I will take a few photos tomorrow.

I also like this forum. many years ago in a land far away in eastern Ohio, I grew up in a steel mill hot strip mill and spend many hours with large DC motors but never got into the rewinding or too much of the engineering. I was involved with alignment and balancing but only as an observer or helper. I spend my time with the drives and controls. 1000's and 1000's of control relays!!!

They will start the motor and monitor thrust bear temp with the existing alignment.

Dan

 

[dithomas]

I have attached photos of the overhung 1000 HP Salient pole 2300V motor.

I hope I inserted the photos correctly.

Dan

 

[dArsonval]

Dan, you are the Man!

As soon as I saw your photos I thought "Duh!" I totally get your scenario now.

And for electricpete; I found photos in my archives to help you, myself,
and everyone following this thread... an additional option at better understanding what we're all talkin' about.

Pete wrote earlier,
"... It boggles my mind to think of 1000 hp 327 rpm (22 pole in 60hz land) salient pole rotor completely overhung.
I have seen engine-driven generators where the generator has no inboard bearing (they rely on the engine),
but never missing an outboard bearing."

Please note, the URL of photos below is not an attempt to derail this thread/conversation.

I'm posting it here, to augment an additional "visual reference" in understanding the topic being discussed.

http://www.pbase.com/visual_first/motor_driven_compressor

Always enjoying this forum,

John

 

[electricpete]

Thanks for the pictures John and Dan. I wouldn't have completely believed it unless I had "seen" it.

Axial pull toward center center (when rotor off-center axially) tends to be small because majority of flux lines remain radial, only very small axial component near the end. Small fractional unbalance (between ends) of an axial component that was small to begin with tends to give forces that still remains relatively small. On the other hand, radial pull on rotor is large when rotor is off-center (and tends to pull it farther off-center) because the vast majority of the flux is radial… small imbalance in the large radial component can give comparitively large radial force. Slow speed machines tend to have larger force because they're bigger (for one thing). Perhaps sync machine has advantage over induction because the force only acts on portions of the circumference where poles are. Sync machine is missing the "damping" of unbalanced magnetic pull by rotor currents which act to reduce u.m.p. Multiple parallel stator circuits would help provide "damping" on the stator side (single circuit wye would not).

Rotor which can flex under that force is troublesome, and bearing on one side generally isn't nearly as effective in resisting movement as bearing on both sides. These machines would seem to me particularly susceptible to rubs from this cause since they are so large and missing the outboard bearing. Maybe gyroscopic stiffening from that large diameter rotor helps even at this slow speed. If you told me they are D.O.L. start, I would be even more surprised.

Sorry for the detour. Let us know how it comes out.


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[waross]

Actually it does Pete.
Back in the day it was common to start synchronous motors DOL on the damper winding. The motors started and accelerated as induction motors. When the motor was near synchronous speed and the phase of the back EMF as seen in the field windings was low, the polarized field frequency relay would apply the field and the motors would transition from induction operation to synchronous operation and pull into step.
Many ears ago one of my daily duties was to go out in the mill to the synchronous motor, charge the spring of the power breaker that was used as a motor starter and push the start button to start the synchronous motor DOL. The field was manually adjusted. When the field warmed up the resistance would increase and the field current would drop. Part of the starting sequence was to return 20 or 30 minutes later and readjust the field current with the big rheostat. Labour was sure cheap back then.

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

 

[electricpete]

Thanks Bill. Actually I thought better of my last post right after I posted it. I thought about posting again to clarify my clarification, but flagged it instead. What I don't know - what fraction FLA would sync motor draw during dol start?

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