Hi all,

I observed many times the presence of 2FL in FFT of induction motor solo run test, even though vibration level is normal. 

Can anybody clarify the technical reason behind inducing 2 * line freq. in an induction motor?

What are the permissible level for that ?

If there is a fluctuating amplitude of 2FL then what its indicate? 

Original Post

Search for Magnetostriction

Here is one reference:

http://hyperphysics.phy-astr.g...olids/magstrict.html

I do not know of a "Severity Criteria", but I have encountered several instances of high vibrations and sound levels including:

2-pole motor with Beating between 2xSS and 2xLF

Resonant motor frame -- often end bell of large motors

Resonant motor support structure

Resonant stator bars -- in large generator

Excessive sound levels at 2xLF and harmonics -- motors, generators, and transformers -- typically caused by resonant structure and loose structure

Walt

Chetan Patil posted:

Hi all,

I observed many times the presence of 2FL in FFT of induction motor solo run test, even though vibration level is normal. 

Can anybody clarify the technical reason behind inducing 2 * line freq. in an induction motor?

What are the permissible level for that ?

If there is a fluctuating amplitude of 2FL then what its indicate? 

permissible level for 2 lf by API 541 3nd ED is 0.1ips and by  IEEE 814 STANDARD  recommended 0.05 ips 

aziz58 posted:
Chetan Patil posted:

Hi all,

I observed many times the presence of 2FL in FFT of induction motor solo run test, even though vibration level is normal. 

Can anybody clarify the technical reason behind inducing 2 * line freq. in an induction motor?

What are the permissible level for that ?

If there is a fluctuating amplitude of 2FL then what its indicate? 

permissible level for 2 lf by API 541 3nd ED is 0.1ips and by  IEEE 814 STANDARD  recommended 0.05 ips 

API 541 applies to motors in excess of 500 HP and sleeve bearings.  Since the OP didn’t post motor size, API may not apply.  A good summary of API 541 can be found at http://sites.ieee.org/northern...-IEEE-Edmonton-1.pdf

In these cases, I like to unscrew motor bolts one by one while it's running. If the vibration level decreases then its a soft foot issue. If not then electrical problem is suspected. It depends on the sort of side bands you got. Still, before you go that deep, its a good practice to take a minute checking the terminals tightening torque and phases current and voltage imbalance. 

"If there is a fluctuating amplitude of 2FL then what its indicate?"

It is generally beating of 2*LF with the pth harmonic of running speed, where p is the number of poles.  Although at no load the slip is typically a fraction of an rpm and it would be a verrryyy slooowww beat of several minutes.

 

 

 

 

 

 

 

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