Electrical motor with rotor issue 1

[Emad Shaaban]

Hi everybody
A 1800kw electrical motor running with normal temperature and current but with a humming noise.CBM team saying that no vibration has detected because of bearings or a mechanical issue, however the vibration specturm analysis shows frequency of 100hz beside the Ffund 50hz .We are bit sure that the problem is in rotor laminations because a similar motor with the same symposium was deconstructed and found a deformation in it’s lamination..

The question is how can I tell that the problem is rotor with out taking it out of service,we do not have a spectrum for current singular analysis but we are able to generate current instantaneous wave form using the Sepam relay recorder

 

[waross]

I am not a vibration expert, but from first principles:
The rotor frequency is the frequency corresponding to the synchronous speed (1500 RPM) minus the actual speed (1450 RPM ?) = 50 RPM or around 1.7 Hz.
I would expect the rotor magnetic vibration to be around 3.4 Hz.
100 Hz humming looks like stator laminations.

 

[waross]

And back to first principles;
In an induction motor;
Magnetic vibration is excited by the grid frequency, or 50 Hz x 2 = 100 Hz.
Mechanical vibration is excited by the speed at around 47 Hz x 2 = 94 Hz.

 

[EdStainless]

We would see this from time to time in motors that we built.
Sometimes it was very difficult to pick out the two frequencies.
We would set up to test and run the motor at no load to get some baseline data.
Then we would load to 110% of rating.
This not only increased the vibration amplitude but it also slowed the rotor enough that we could tell if it was stator or rotor related.

 

[waross]

Then we would load to 110% of rating.
This not only increased the vibration amplitude but it also slowed the rotor enough that we could tell if it was stator or rotor related.

This not only increased the vibration amplitude but it also slowed the rotor enough that we could tell if it was stator or rotor related

Good idea.
Would this show magnetic issues in the rotor or only mechanical issues, Ed?

 

[EdStainless]

Loose lams get a lot louder under these conditions.
But then so would bearings I would think.
It should be easier to pick out the source of the noise either way.

 

[Emad Shaaban]

Loose lams get a lot louder under these conditions.
But then so would bearings I would think.
It should be easier to pick out the source of the noise either way.

Thanks for every body,
Ed, Can you make it more clear,
If we loaded the motor at 110% and its speed get more slower ,does that mean the its rotor ,stater or mechnical ? same fore nosie ?

 

[spciesla]

Assuming this is an induction motor, the CBM team should be able to take high resolution spectrum data to separate the running speed (1X) peak from pole-passing frequency peaks and the 2X peak from the 2xLF (Line Frequency) peak, assuming that this is a 2 pole motor. The pole passing frequency is calculated from multiplying the number of poles by the slip frequency (synchronous speed - operating speed).

 

[panter]

Hi everybody
A 1800kw electrical motor running with normal temperature and current but with a humming noise.CBM team saying that no vibration has detected because of bearings or a mechanical issue, however the vibration specturm analysis shows frequency of 100hz beside the Ffund 50hz .We are bit sure that the problem is in rotor laminations because a similar motor with the same symposium was deconstructed and found a deformation in it’s lamination..

The question is how can I tell that the problem is rotor with out taking it out of service,we do not have a spectrum for current singular analysis but we are able to generate current instantaneous wave form using the Sepam relay recorder

Emad ,
What is motor rpm ? Is the Ammeter electronic or electromechanical. If it is an old one with a moving coil, you can see if there are current oscillations. If there are, that can be a good sign that there is a problem with rotor cage. 100 Hz oscillations can be a sign of not equal clearance between rotor and stator air gap. In some motor designs, a soft foot can be a good reason for this.

 

[edison123]

"We are bit sure that the problem is in rotor laminations because a similar motor with the same symposium was deconstructed and found a deformation in it’s lamination."

What kind of deformation? Post some pictures and complete motor nameplate.

 

[Emad Shaaban]

"We are bit sure that the problem is in rotor laminations because a similar motor with the same symposium was deconstructed and found a deformation in it’s lamination."

What kind of deformation? Post some pictures and complete motor nameplate.

 

[edison123]

Is that a stator - rotor rub? Aluminum die-cast rotor? Speed?

 

[Emad Shaaban]

Is that a stator - rotor rub? Aluminum die-cast rotor? Speed?

copper bar rotor