Strange Bands Welded to Rotor Circumference

[Laplacian]

One of our facilities recently shopped a 1500HP, 3600rpm motor experiencing vibration problems. When the motor was torn down, two rings were discovered spot welded to the outer circumference of the rotor; one on each end about 1/4 distance (rotor length) from the end rings.

A 1" wide groove was machined out of the entire circumference of the rotor for each band about 3/8" deep (even through the rotor bars). The ring is made of a magnetic steel, unknown type though and tack welded to random points of the rotor laminations. The band is continuous around the rotor's circumference.

Has anyone seen this before or want to guess what the purpose is? It looks like it is significantly affecting the flux distribution through the rotor and some people say it was for balancing. I'm seeking input from anyone with an idea.

 

[aolalde]

To me, the function of those steel straps is mechanical support for the rotor bars. Probably those bars are made of copper or copper alloy which spinning at 3600 rpm impose a large tensile stress to the end of the rotor lamination teeth.

 

[edison123]

The rotor core laminations are assembled and press locked with thick end pressure plates whose OD often stops just below the start of the rotor slots, leaving the rotor teeth as a cantilever. At high speeds, the unsupported and unpressed (?) rotor teeth at the core ends often break off due to fatigue etc. and damage a healthy stator winding and some times even the stator core.

A good engineering practice is to axially compress the rotor teeth also with core lock end plates having fingers in them. This would of course involve tedious indexing and machining out the slots from thick round end lock plates whose OD stops just below the rotor OD. Given such high speeds, merely welding the fingers to the end pressure plates is a no-no.

In a classic example of curing the symptom but not the disease, the OEM/repair shop have taken a short-cut by providing these rings around the rotor core ends to prevent the broken teeth from flying into the stator winding and damaging it. These magnetic rings would definitely affect the rotor performance by acting as shunts. God knows what sort of currents flow through these rings during starting.

 

[electricpete]

I have to agree. I would think it was not designed from scratch that way but an attempt to fix a problem.

What is the OEM and vintage... Siemens 1970's ?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[mwimbush]

Could this band be a way of controlling chattering of the laminates due to hysterisis, such as is done with coils on contactors

 

[jbartos]

Suggestion: Normally, the rotor is without the mentioned steel rings. Visit

http://www.reliance.com/mtr/mtrthrmn.htm

for a photo.
The added steel rings may have been added to support structural soundness of the cage, if manufactured from softer copper or aluminum and not being held in the rotor slots by its proper geometrical design. This is more on mechanical side rather than on electrical side. The current will flow in the copper bars approximately the same way since the contacts to the steel rings and the steel ring will have higher resistance.
Questions remain: How those rings were installed on the rotor? Were they opened and welded at the ends? This aspect is not clearly seen in the original posting.

 

[Mendit]

Many methods have been tried to prevent the rotor bars moving axially during operation, some OEMs use resibanding at the ends (Teco Westinghouse). Reliance have drilled and inserted 6 pins around the rotor, and tack welded to laminations. Some have brazed axial blocking pieces into the rotor-bar connections. Some as you state have put in a radial locator ring.