This is a AC induction ,750 rpm, 250 KW motor driving a fan.This overhauled motor was recently replaced and since then high overall vib upto 5 mm/s was observed.Pl find some high resolution spectrum.Clear 2 x LF(6000 cpm )is observed(Line Fr in India is 50 Hz), with multiple running rpm harmonics.I have not seen 2 x LF with running rpm harmonics earlier(though 2 x LF with PPF or 1/3 LF sidebands is seen sometimes).Stator and Slot no is not known.

1) Has anyone observed this.What does this running rpm sidebands of 2 x LF indicate.(besides looking for soft foot)
2) Is it necessary to make a call

csp

2LF-running_sidebands.doc (234 Kb, 72 downloads)

Eccentric rotor is possible. Soft foot can create an eccentric rotor condition. But, since this motor just got overhauled, sounds like a reassembly problem. Maybe some of the "motor dudes" on here can help you out better.

Don't forget the possibility of a loosness condition creating the 2xLF. There have been many changes since the motor was removed for the overhaul.

Have you tried watching the spectrum in live mode and then shut the motor off to see if the 2Xlf goes away? just trying to confirm motor problem.
It sounds like something within the motor itself is loose. Just my novice opinion.

As mentioned, the problem could stem from a soft foot condition, either angular or short. One way to check for soft foot is to perform a running soft foot check. Use your data collector to monitor the 2xLF peak and loosen a motor foot. Look for a change in the peak. Tighten it back and continue to another foot. Do this for each foot being certain to tighten the bolt before moving to the next. A soft foot condition will usually cause a change in peak. If seen, the motor should be realigned paying close attention to soft foot. Not only should a short leg be checked, but angular soft foot needs correcting also.

Gary B.

Yes I agree that soft foot and alighnment are to be reviewed on next s/d.This fan cantv be stopped untill the next monthly s/d.This soft checking by loosening a bolt at a time wont be allowed in work permit.

My interest is has any one seen 2 x LF with multiple sidebands of running rpm. what was the observation

csp

CSP

Is this a belted application?

Howard

Motor Doc

This is direct drive,Gear coupling,centre hung Fan.

csp

CSP

During the overhaul, was the motor rewound? If so, did the repair shop check for foot flatness? Did the repair shop bolt the motor down and check airgap (probably not if the motor is 230/380 Vac)?

I would agree with the other respondents to this string. It does look like the frame may have been twisted. If the alignment assumed parallel foot flatness then when the motor feet are tightened, it will cause the frame to twist, changing the airgap.

We just had this very situation at one of my clients, a large steel mill, and yes, there were multiple RPM sidebands. We confirmed eccentricity with Current Signature Analysis and a more in-depth analysis. The end result on this larger motor was frame damage that resulted in a foot problem.

Howard

Mr Howard,

Please explain methods to check Air Gap in small and medium size motors

Mr Howard,

As i mentioned earlier soft foot is the prime suspect and will check on oppertunity.We dont have a well documentation of motor repair shop .This motor was overhauled 3 yrs back and kept as spare and details are not documented.We dont have a proper system for recording air gap.
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We confirmed eccentricity with Current Signature Analysis and a more in-depth analysis.
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1)can yoy briefly explain/or attatch any literature for detecting air gap problem by MCSA.I felt MCSA generally gave idea about rotor bar crack/end ring and loose connections.

2)I will also like to know methods to check Air Gap in small and medium size motors

csp

Gentlemen:

Setting Airgap:

In setting the airgap in a medium voltage motor, there are normally access plates or openings in both ends of the motor. Feeler guages are used at 12, 3, 6 and 9 o'clock positions at both ends. There should be no more than 5% difference between readings. It is always a good idea to rotate the shaft 90 degrees and re-check. The end-shield is then pinned in that location.

In smaller motors where there is no access, the endshields have to be checked in a lathe and the bore and rabbet fits checked to ensure that everything is in line.

Foot flatness can be checked, before assembly, on a Bullard lathe.

On Motor Current Signature Analysis:

You are correct. The correct term is Current Signature Analysis when checking other than the rotor bars and Electrical Signature Analysis when checking the power supply (Voltage Signature) and motor (Current Signature).

With ESA you can check electrical and mechanical condition including the condition of the bearings.

The signature for static eccentricity is the number of rotor bars times the rpm +/- 1 and 2 times the line frequency. If it is dynamic eccentricity each of these peaks will have running speed sidebands.

Sincerely,
Howard

Checked this motor and found one foot of DE side has angular softfoot.Around 3 mm gap was there on one end.Tried to insert some small shims at this end as at a temporary measure due to lack of time .Minor decrease in vibration.

Q How this type on angular soft foot is rectified.Does some taper kind of shims help or we have to grind and machine the foot/base

CSP

Did the readings drop when the foot was loose and then went back up when it was retightened with the shims? Also did you try loosening the other foot diagonal from this one?

quote:

Q How this type on angular soft foot is rectified.Does some taper kind of shims help or we have to grind and machine the foot/base

The most ideal solution is to machine the foot/base. Machining the motor foot can be tough (have to pull it and send it to the shop). Machining the base in-place if necessary would be many times harder (we tried and never succeeded).

Step shim technique is used by some. Our mechanics find that difficult.

We have had reasonable success using plastic shims (Sof' Shoe see www.precisionbrand.com). These are not considered to be a permanent solution by most people: For one thing, every time you untorque the machine you're supposed to replace those plastic shims. For another thing, I understand the original vibration pattern may reappear after some period of years (we haven't seen that yet after 7 years for some machines). I'll agree it may be considered a short term fix, but in our less-than ideal world, it has turned into a permanent fix for us (we will probably just replace those shims if problems reappear).

CSP

We have run into a similar problem and opted to step shim our motor. I have not had much luck using plastic shims in our plant. Use your handy dandy shim kit and step shim it and tourque the fastners when done. It will work.

Regards, Dave