Large AC Induction Motor - Groan at Startup

[vibegeek]

We have a 800HP ac induction motor with sleeve bearings that is experiencing a loud "groan" at initial start. The motor has an across the line starter, with no soft start capabilities. The frequency of the groan is 60Hz (line freq)and lasts until the rotor begins to roll. The motor has been sent to the rebuilder and no issues were found with the stator or rotor though the bearings were found oversized and replaced. The problem persists. This problem began after the motor had been in service for many years and two different shops have been through the motor and found no electrical anomallies.

Any ideas as to the cause of this condition?
Is it detremental to the life of the motor?
How do we fix it?

Any help appreciated.

 

[sms]

Sounds like a stator winding vibration. Have you checked all the stator supports, including any blocking or end ring supports are all ok, not cracked well attached?

-The future's so bright I gotta wear shades!

 

[aolalde]

I think one of the journal shoulders is rubbing against the bearing end during the accelerating process, after the motor reaches full speed the magnetic field centers the rotor-shaft and clears that axial rubbing. The possible solution is to space the bearing axial clearance so it does not reach the shaft journal shoulder while coasting to rest.

 

[TheBlacksmith]

I second the stator winding vibration. Prior to rotation, the stator current draw and resulting magnetic forces are likely highest and something that has come loose is being excited. Rotation increases the impedance enough to drop the current and stop the movement.

 

[jraef]

I agree with stator winding vibration as well, I have seen it may times on 500HP+ motors. It IS detrimental to the motor life. The sound is indicative of the winding materials moving, and movement means friction /wear, which means eventual failure, usually turn-to-turn in one or more windings. It may still take a long time to cause a catastrophic failure, but for sure you will not get maximum life out of your motor.

Is there a reason why you are not using a soft starter on that motor? That has relieved this issue every time I have come across it.

JRaef.com
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[Skogsgurra]

Puzzled by the 60 Hz observation. If it really is 60 Hz, then something unusual is going on. It should be 120 Hz if normal magnetic forces are the cause.

Gunnar Englund

http://www.gke.org

 

[waross]

Hi Gunnar;
I took that to mean the tone normally emitted by 60 Hz. equipment such as transformers (and some large motors).
Respectfully

 

[DickDV]

This might be a good place to try VPI, Vacuum Pressure Impregnation, with polyester or epoxy.

Winding movement can only lead to premature failure.

 

[electricpete]

It seems very unusual to me that there is any significant time period between energization and the time when the rotor begins to roll on a DOL-start motor. (you said the noise lasts until the rotor starts to roll)

Can you elaborate on that? How long is it between energization and rotor start to roll? Do you consider it normal (has it changed?). What kind of load are you starting?

One thing that comes to mind is binding of sleeve bearings which have been idle for awhile? (I have seen that lead to tripping of motor due to locked rotor upon start, but never delay in starting)

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[waross]

Years ago I worked on the installation of some MG sets. The 3000hp motor (Ball bearings) drove 5 DC generators, all with sleeve bearings. The crew rolled the sets manually before starting. They had an 8 foot long hook wrench on a coupling. The 200lb.+ electrician foreman would hang from the end of the wrench. The set would not start to turn until a second 200 lb. + electrician added his weight to the wrench. When the set had made a few revolutions and the bearings were being properly lubricated, 4 or 5 electricians could keep it turning by pushing on the rotors of the DC generators.
Apart from the noise, which should be addressed, you may wish to consider adding turning gear so as to get oil into the sleeve bearings before starting the motor.
respectfully

 

[mc5w]

You could also have a SLIGHTLY loose lamination say in the rotor and centrifugal force somehow "clamps" the loose lamination. There could also be something else loose as well too.

Back in 1987 I had a customer who had an old 175 watt mercury vapor barn light on the backside of his commercial building and he had me redo the wiring so that it would work. When running this thing was absolutely obnoxious because rust over the years had caused a limination to crack and become loose.

For a motor of this size anything that is loose can be just an obnoxious as a jet airplane engine!

Mike Cole

 

[wrsharper]

the only way they groan is if you don't pay her.

 

[vibegeek]

Sorry I have been gone so long.

Some clarifications>

Yes it is 60Hz. This is observable on most large motors, however on this motor the vibration is especially severe.

The condition is observable even on a hot start when there should be some residual oil film.

I would assume that if a shoulder were rubbing we should see some tell tail of this on the bearing?

Is there any further testing you can think of short of tearing the motor down again?

Any help appreciated.

 

[electricpete]

Yes it is 60Hz. This is observable on most large motors, however on this motor the vibration is especially severe.

As pointed out before by skogsgurra, 60hz is not a typical frequency for motor vibration or noise. 120hz is.

How did you determine the frequency of the vibration? Using vib instrument? It makes a difference. If it really is 60hz or 120hz, I would be inclined to discount a shoulder rub. If it's some other frequency perhaps a multiple of rnning speed, then that is still under consideration.

A groan during start which began after a few years of operation on a large motor...my gut would lean more towards rotor than stator. Have you monitored vibration after start to see how long it takes to stabilize? You might do a current signature analysis to check for pole pass sidebands around line frequency.

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[DaveScott]

I had a groan on start up on a 6000kW shredder induction DOL start motor. It turned out the the end ring on the rotor was damaged.

 

[vibegeek]

Yes we are certain of the 60Hz. Vibration was monitored beginning with the application of power up to full speed. The 60Hz vibration began at application of power and diminished in amplitude once the rotor began to roll, about .3 - .4 seconds after application of power.

I've read somewhere that this is most likely due to the motor basically being a transformer at start, with the core (rotor) vibrating at line frequency until the motor begins to roll.

A bad end ring may be worth looking into.

 

[HCBFlash]

My experience is with DickDV. If you think bearings could be a factor, then you obviously sense the sound of unsupported material moving about. Laminations doing this is not a rare problem, and the VPI treatment is very effective. If your stator and rotor had actually contacted from prior bearing (or shaft) problems/failures, you'll be noticing other strange problems occurring too.

Please let us all know how this works out for you! and don't forget faq731-376

 

[electricpete]

The 60Hz vibration began at application of power and diminished in amplitude once the rotor began to roll, about .3 - .4 seconds after application of power.

To revisit a prior point, 0.3-0.4 seconds between application of power and rotor beginning to roll would be very unusual in my experience.

I would treat this slow start as a part of the "problem" you are investigating. In fact I'm sure that I would be more concerned about it than I would be about a groan.

There are lot of possibilities. Just to throw out a few (many of which are already covered):
1 - You have a rotor bar problem. This can either be a cause of the slow starting (broken rotor bars = reduced torque capability) or a result of the slow starting (slow start increases mechanical stresses associated with thermal expansion on the rotor during start which are the primary contributors to rotor bar degradation).
1A - Did the shop perform rotor tests?
2 - Breaker closing abnormally with one phase 0.3-0.4 seconds behind the other two.
3 - Extremely high inertia load or breakaway torque

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