We recently had a customer sustain significant damage on their 2500 HP Allen Bradley Powerflex 7000 medium voltage VFD due 
to degraded rubber O-rings designed for reducing noise between the SGCT and the heat sink it sits on. After some digging, we found this to be a known problem in this Rockwell medium voltage VFD. In November of 2016 an engineering note was released in the Rockwell Knowledge base regarding the Powerflex 7000 medium voltage VFD: note 969771 states:
“In certain applications, the silicone O-Ring degraded prematurely and resulted in cracks or the O-Ring breaking apart.
In most of the few known cases, when the cracked O-Ring breaks, it usually pops off the power cage without causing any damage.
However, in a few rare cases when the O-Ring bre
aks and touches the live parts, based on the tests in RA lab, there is a few mA electric current flow through the O-Ring, which can over heat the material, and if this condition persists, it could cause short circuit between the live parts.
PowerFlex 7000 MV Drives acoustic noise specification exceeds the industrial standards, such as, OSHA.
The sole purpose of the O-Ring is audible noise dampening and it has minimal impact on the overall audible noise level of the drive and can be removed.
Rockwell Allen Bradley Powerflex 7000 PF7000 MV Drives have 1500A and 1750A SGCT’s installed between Feb 2011 and Oct 2016.
The manufacturer’s recommendation is to completely remove the O-rings from all SGCTs during the next maintenance cycle for the Poweflex 7000 medium voltage VFD. We found this to be fairly straight forward if you follow the instructions attached here: How to remove the O-Ring
In the particular case mentioned above, within the Powerflex 7000 medium voltage VFD two separate SGCTs on the rectifier section were damaged, causing them to short and damaging several surface mount components on the SGCT driver card as well.
Given how expensive new Poweflex 7000 medium voltage VFD SGCT cards are, coupled with the additional cost of downtime, EMA recommends removing these O-rings immediately. A good, consistent preventative maintenance program on the Rockwell Allen Bradley Powerflex 7000 medium voltage VFD would solve many of these problems because visual indications like this would be noticed and remedied BEFORE damage occurs. What does a good PM entail? See our blog here for more information.
EMA would love to help you with preventative maintenance on all of your medium voltage VFDs, including the Allen Bradley Powerflex 7000; we service all makes and models, for more information or some examples of preventative maintenance reports we’ve done for our customers, contact us 1-800-848-2504 or info@emainc.net. You can also talk to us via our Live Web Support during normal working hours. No One, Anywhere Is Better At Drives Than We Are!