If a motor was installed and the motor was ran to verify the magnetic center. Then the proper coupling space was provided. Then why would the motor Hunt Electrical magnetic center and why would we not see any major vibration changes when this takes place?

When is this motor trying to find the magnetic center? Is it just during start-up or is it "hunting" all the time.
I think the reason that you do not see much vibration assiciated with the magnetic center hunting depends on how much movement there is. The hunting is mainly visible in motors with sleeve bearings. At start-up the shaft moves back and forth until the center is found. As long as the rotor or the bearing components do not hit another part of the motor, then there is really no force there to cause a vibration. The shaft is sort of moving freely in space. I think that maybe you would see something going on at 2XLF but I am not sure if it would stay out of center long enough to get good data.
I have run across a motor that exhibited extremely high 2xLF with no soft foot or other reason apparent. The motor was sent out for repair and it was found that some spacer washers in the bearing were in the wrong place. This was a motor with rolling element bearings. After the rebuild the 2XLF was pretty much back to normal. It was my reasoning that the spacers in the wrong place was causing the motor to run off of magnetic center and that is where the 2XLF came from. Never was able to prove it though.

Vibeguy2004, This motor is sleeve bearing, 2 pole. Orginally the analyst atributed the shaft movement at the coupling to the pump thrust bearing having to much clearence. The pumps thrust bearing was then replaced and the end plate was machined to correct clearences. After the unit was re-started the pump shaft was holding steady but the motor shaft is hunting which can be seen on the shaft and the coupling.
Thanks Alan

Alan,

Does the motor hunt continually or only at startup? Is the motor properly leveled. Not a lot of experience with sleeve bearing units but I saw this with an 800 HP motor that was not level. It would hunt and then center when up to speed.

Ken Culverson

Dai Wei, The motor is hunting all the time and it can be seen in the flexing of the shim packs on the coupling. I beleive the base is level.

THANKS
Alan

How confident are you that the coupling spacing is correct? If you're pulling, or pushing, the shaft off mag center with improper spacing, it will constantly try to get to center.

I know the Maintenance person that installed it so I am pretty sure it is set at the correct spacing.
Alan

Was the motor overhauled and assembled incorectly I came across an instance where the spacers were internal and had been fitted in reverse

At our plant, we have axial shaft movement of the motor-side of a shim pack coupling on a family of large sleeve bearing motors driving centrifugal pumps.

The axial movement is estimated up to 1/8" peak/peak. The movement occurs only at low flow, and goes away when the pump goes to high flow.

Our belief is that the driving force for the vibration comes from small movement of the pump within the clearances provided at it's thrust bearing (10 mils clearance). The pump may have either broad-band flow-induced movement or might slam back and forth within this clearance. Now imagine that this small pump shaft movement within clearances forms the excitation for another mass spring system consisting of the coupling/motor: The motor has no axial support except through the springy coupling to the pump. When you have have impacting or broadband excitation at the pump, you excite the natural frequency of the motor mass on the coupling spring. With a fairly large motor rorot mass and fairly flexible coupling for a spring, you can imagine the resonant frequency would be fairly low. And that is what we see...maybe 1 - 2 hz movement of the motor shaft.

The fact that it goes away at high flow tends to confirm that it originates in the pumpm in our case.

In some 2 pole motors the axial mOvement of rotor acn be due to (i) motor not levelled properly (ii) large aero dynamic forces of fan acting axially.
Due to above rotor can either shift one side or hunt contineously. Of course this will be in uncoupled condition.

To avoid this problem, use the coupling of limited-end-float type. In which axial gap is predominantly less than axial gap in DE side bearing of motor.

DKSONI
INDIA

Dinesh hit it on the head. Would see this on pipeline motors all the time.

Use a simple level and check it on the motor shaft and foot. The level should be as close to 'perfect' as possible. If the motor is hunting due to the fan, check to make sure any filters or openings are not blocked (normal cause for turbulence).

Howard

Going a little bit deeper, the force causing the hunting cannot be just any force. A rotor of the friction bearing motor running uncoupled can be made to hunt by pushing slightly on the shaft in axial direction. What follows is a damped oscillation in axial direction with frequency given by the “stiffness” of the electromagnetic spring (strength of the field) and mass of the rotor. Normally after few cycles the oscillation subsides.
Some motors hunt for the magnetic center on their own. In other words there must be some kind of forcing function on the right side of the equation. This forcing function has to be periodic one, with period close to the natural frequency of the system: Mass of the rotor plus the “stiffness” of the field (or the multiple of it). The rotating speed is always way too high. What can be this mysterious periodic slow force?
jank

This message has been edited. Last edited by: jank,

Jank,

Could it be related to pole pass slip fequency?

Walt

I haven't heard of axial forces in connection with rotor assymetries which are the cause of pole pass oscillations (but you never know). And for the discussion of uncoupled oscillation, you can rule out pole pass frequency because that will be many minutes long.

EASA Technical Note 15 has a lot of good info on magnetic centering force. They explain why 2-pole centering force is lower than 4/6 etc pole motors:
1 – less circumference per horsepower (lower magnetizing current for the same horsepower)
2 – no radial rotor vents (which when present may try to align with stator vents).

They also suggest that air flow patterns cause oscillation during uncoupled run:
"Another effect sometimes noted on two pole motors is that their magnetic center seems to float or oscillate around the shaft scribe mark. This is not due to a change in absolute magnetic centering force but occurs due to airflow forces on either end of the rotor that are not perfectly balanced. This can be easily verified by running the motor uncoupled at no load. If the motor is double end ventilated, and one air inlet is partially or totally blocked, the imbalance in air moving forces between ends of the motor will cause an axial movement of the rotor. When the opposite air inlet is now blocked, the rotor will move in the reverse direction. The magnetic center is determined by both the true magnetic centering force and the airflow forces acting at each end of the rotor."

Of course the system changes fairly dramatically when you couple it up to a machine with a thrust bearing through a shim pack coupling. Even though it's not a rigid coupling, I believe it's a lot stiffer than the small magnetic centering force. And there certainly can be axial movement induced by the driven machine as demonstrated by our experience described above - 4 pole sleeve bearing motor driving centrifugla pump through shim pack coupling - the motor shaft axial movement is present only at low flows (not at high flows and not uncoupled).

The airflow theory is a plausible one, no doubt about it. But somehow I cannot understand why should this aerodynamic force be periodic. Actually it certainly is periodic, but at much higher frequency than the no-coupled oscillation usually is.
Let me propose this theory:
Due to manufacturing tolerances, the ideal magnetic center is not constant during one revolution of the rotor. Many times I have seen an axial wobble of the rotor lamination when in the lathe. Similarly the stator iron is not perfectly axially symmetrical. The interaction of the rotating field with those irregularities may multiply the slip frequency to bring it close to the natural frequency of the axial rotor oscillation.
If this is any close to the reality, this oscillation should depend on the voltage (“stiffness of the spring”). I will watch for it next time I have an oscillating 2 pole on the test bed.
jank

As Pete, and Jank indicated above, axial movement of a rotor in axial direction for a sleeved motor could be modeled by a weak spring and mass.

Let me propose an idea as far as mysterious slow force is concerned.

There is no slow force! Instead, there is a very low stiffness system with low resonance frequency and RANDOM FORCE NOISE ( flow, coupling, loading, etc) causing relatively large movement of the rotor at resonance frequency. If type of force, such as impacting, is periodic then modulation of resonance frequency will occur.

David

I agree with David for some situations. I have noticed this effect with boiler feed water pumps that use disk-pack shaft couplings. The motor mass and coupling's soft axial spring form a simple spring-mass system. Excitation is from pump flow turbulence, but not at a single excitation frequency.

Walt

I agree with Dave and Walt. The magnetic centering force of a motor rotor is very weak. I have preached to customers that if you have to make a choice between locking down a rotor axially off magnetic center, or moving the rotor where the shoulder is very close to the bearing, then choose the former. Many times the motors are not on level or have soft axial stiffness style couplings.
The forces don't have to be high to start the oscillation.
This same phenomena occurs in motors larger than NEMA frame size with ball bearings. In those cases, the axial spring is slightly larger than a sleeve bearing (the cage vs. nothing!), yet there can be axial vibration that can be seen as multiple sidebands around 1x, 2x,3x etc. at the natural frequency of the spring/mass system. This usually is in the 200 to 300 cpm range. Too high to be pole pass and too low to be anything else. To eliminate this vibration many motor designs incorporate Smalley washers to preload the floating bearing. These washers accomplish two fixes. The above mentioned one and they are also utilized when the load is insufficient to provide good ball tracking (skidding balls).