VPI versus Traditional Dipping & Baking

[shayne31]

We have recently experienced a motor failure which resulted in a full re-wind (bar stock) of the armature of a 400 hp DC motor. The motor manufacturer recommended the VPI process, however, that would have extended the repair by 48 hours (according to the service provider). We instead went with the traditional dip and bake, and the motor has since failed again, requiring the armature to be re-wound a second time. I guess my question is two-fold: First, regardless of the type of winding protection, would the application lead to a catastrophic failure after 11 hours of run time if correctly applied? Second, despite the superiority of the VPI process, is traditional dipping and baking frowned upon in the industry? Any thoughts or feelings would be greatly appreciated.

Cheers

 

[aolalde]

DC Armatures are subject to centrifugal forces proportional to the rotor diameter and speed. The impregnation process plays a role, but more important is the mechanical support provided by the banding. The tension on the tapes or steel wire while forming the banding is critical. Tension in excess of 400 Lb is required depending on the banding tape width. When the banding is properly applied the coils do not move (vibrate) otherwise severe abrasion of the insulation will lead to the development of short circuits.

The VPI impregnation is superior because it deeply penetrates into all the insulation materials and fills all the air spaces with 100% solids resin (no thinners), improving the insulation and providing solid heat conductive means; however, it provides only a small part of the mechanical support. A poorly banded armature will fail in short time in spite of being VPI or dip and bake.

 

[Dnicejr]

Have you thought about MCE testing of your repaired motors when they come back from the repair shop? I have per seen where a motor was turned around on a rush basis and when it came back it had a nice new yellow paint job. However upon performing the MCE testing we found that the wires inside the junction box had been burned and had not been fixed. Needless to say it didn't look good on the MCE results, so the repair vendor was called in and the wires were repaird onsite and then it tested good. Testing as the motors come back can keep your motor shops on their best behavior.

 

[petronila]

Hello Shayne 31,

I Agree with aolalde about the VPI and Banding Tape Concepts, but I Think a second failure in only 11 Hours
needs to be research in detail. I Think your Repair shop most test properly al stator windings and make a review of the second failure cause, the burned armature winding most be analysed looking for some material or brazing failure. A winding data review will help too, looking for Winding Mistakes in Equalizers pich, winding progression or Commutator pitch. Other think to review is varnish cure time and the mechanical balancing. The shop most conduce a good no load and load test ask they for the test reports and all results.

Best Regards

Petronila

 

[powerjunx]

I think petronilla has a point. Shayne, you must investigate the root-cause failure, aside from putting a biased to your self on the differences between VPI and Traditional varnishing method. Take a look the detail of control failures, like; loss of field that leads to overspeeding, overloading etc.

Good thing to start is to review the Evaluation Test Report/ Findings of probable cause(History) prior to failure. Secondly, furnished the Final Test report if provided.

We used traditional varnishing but never had problems that turns out it never exceeds to the machine repair warranty. There might be other cause that you may never take into account.

By the way, how the commutator looks like upon its failure? Can you describe?

 

[shayne31]

First, I would like to thank all you for your valuable insight and advice. Regarding the condition of the commutator, it apparently had a burnt appearance. A vibro analysis has since shown that there was excess vibration on the output end present at start-up. Maintenance records have shown that the motor bearing temperatures were in the 140F range.

 

[petronila]

Shayne,
140 F motor bearing temperature is OK, and the Start-UP vibration could be considered normal if you are performing a full load motor´s start. If this motor only runs 11 Hours after the second repair this is very abnormal ask to the repair shop for the failure report, this could help to give you the righ direction to find the failure.
Another think could help: ask to the maintenance people if in the failure´s moment somebody report something abnormal.What happeneed with the drive´s protection??(Shut or not) What happened with the motor´s fan?? (WAS SPINNING WITH RIGHT DIRECTION??)the maintenance people most have the information.I think is better to ask and research more about the failure´s moment.

Regards

Petronila

 

[edison123]

I have been using dip & bake in my shop for all kinds of AC & DC machines for over 30 years without any problem. As others say, I would look at root causes for such a short-term failure.

Since you mentioned commutator burn out, I would look at possible wrong equalizer winding connection, improper neutral axis, wrong armature winding (???) etc.


* Basically, I would like a full-time job on part-time basis *

 

[powerjunx]

shayne,

As you say it's burn out, if it is totally burn out that appears to be black coloration on the surface usually cause by overload. Hint tha machine failed within in 11 hours. The Vibration reading at start up and bearing temperature is normal, thus it will be disregarded.
Possible hint to look after on the commutator surfaces is traceable commutator bar burn-intervals that is it forms a burn out pattern on the surface. If you take this to account, perhaps i can lead you to most probable cause.
Furthermore, it would be better to investigate for the failure since rewinding of 400 hp is quite expensive, exclusive of downtime.

 

[fflinders]

I would agree with everyone here. The decision to use VPI or not is very application dependent. If it is a demanding application in terms of thermal dissipation due to overload then VPI would be reccommended. Having said this 11 hours is a very very short time. I have seen 750kW mining haul truck motors (very demanding application)run for several thousand hours before failure when the impregnation of resin was been very poor. The reason for the failure at this relativly short period was poor VPI. The normal failure mode is turn to turn fault due to vibration induced abrasion. In your case (due to the very very short time) the impregnation of resin (dipped or VPI) is unlikely to have any bearing on the failure.

As suggested by others you need to analyse this failure very carefully. I beleive the reason can only be one of two things. First the rewinder has made some kind of gross error or there is a fault in the drive/control system that feeds the motor.

You need to get to the bottom of this otherwise you could end up with a third failure!

Some questions.

What is the application and drive system?
Has the reason for the first failure been identified?
Did the aramture bird nest?
Has there been a turn to turn fault behind the riser which has caused the commutator end band to burn and fail?

 

[powerjunx]

fflinders,

Maybe shayne is indirectly handle this machine. Hence, she/he might not give further details.

 

[machmech]

Hello fbcybil

In the U.S. we have our Independence day (Forth of July)weekend, many folks are away with Family and may
not return until Tuesday, or Wednesday.

During this time Work is the furthest thought we have.

Chuck