Large AC Induction Motor - Groan at Startup
Large AC Induction Motor - Groan at Startup
(OP)
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.
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.





RE: Large AC Induction Motor - Groan at Startup
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RE: Large AC Induction Motor - Groan at Startup
RE: Large AC Induction Motor - Groan at Startup
RE: Large AC Induction Motor - Groan at Startup
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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RE: Large AC Induction Motor - Groan at Startup
Gunnar Englund
www.gke.org
RE: Large AC Induction Motor - Groan at Startup
I took that to mean the tone normally emitted by 60 Hz. equipment such as transformers (and some large motors).
Respectfully
RE: Large AC Induction Motor - Groan at Startup
Winding movement can only lead to premature failure.
RE: Large AC Induction Motor - Groan at Startup
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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RE: Large AC Induction Motor - Groan at Startup
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
RE: Large AC Induction Motor - Groan at Startup
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
RE: Large AC Induction Motor - Groan at Startup
RE: Large AC Induction Motor - Groan at Startup
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.
RE: Large AC Induction Motor - Groan at Startup
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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RE: Large AC Induction Motor - Groan at Startup
RE: Large AC Induction Motor - Groan at Startup
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.
RE: Large AC Induction Motor - Groan at Startup
Please let us all know how this works out for you! and don't forget FAQ731-376
RE: Large AC Induction Motor - Groan at Startup
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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