C21 Controller problem

[najafabadi]

Hi everybody,

One of our UX120FC21 robots that works in an important production line encountered a serious problem. Suddenly smokes  come out from its controller and it was stopped. We changed the controller and replaced it with a complete spare controller and now production line is working. I want to find the reason of the problem in the burned controller.
First look inside to the controller circuits show that two of 3-phase input wires have become over-current and burned? Why did this situation happen?
Does anyone have similar experience that can help us to find the origin of this problem?
Please notice that our controller is standard Japanese model and we haven't access to the inside of it easily. Also the controller is clean of dust because we changed its fans about 6 months ago and we cleaned it at that time. Also it may be useful for you to know that the robot works in 3 shifts (24/6) for past 8 years.
Many thanks in advance for your help.

Najafabadi

[Luke]

Hi Najafabadi
One thing that comes to mind is upstream power supply protection, as you mention over-current as a cause.
Perhaps consider thermographic report of the controller internals?

[najafabadi]

Hi
We dismantle the burned controller and find that IPM (Intelligent Power Module) of joint 6 has burned.
Now the question is what is the main reason of such defect?

[Luke]

Hi Najafabadi
This is from the C Controller Electrical Maint Manual and might help:

1.4.11 1GC/1GD/1JK POWER BLOCK
The function of the power block is to supply precisely controlled current to the AC servo
motors, one motor per robot joint, which produces motor rotation and mechanical unit
motion.
The power block receives 3-phase 60/210 VAC, for teach/repeat modes respectively.
The 3-phase 60/210 VAC is rectified to 86/300 VDC by a diode module. This rectified
86/300 VDC is routed back to the relay chassis, where inrush current protection and
discharge control are performed. Also located on the relay chassis is filter capacitor C1.
Capacitor C1 filters AC ripple from the 86/300 VDC output from the power block diode
module. The filtered 86/300 VDC is routed back to the power block and is supplied to
the insulated gate bipolar transistors (IGBTs) located in the intelligent power modules
(IPMs), one per axis, via the P and N bus bars. Hybrid integrated circuits (HICs) receive
PWM signals from the 1GB board and route them to control ICs that gate the IGBTs.
When gated, the IPMs output the current that drives the AC servo motors.

Luke

[najafabadi]

Hi
Thank you for the guidance. I attached a photo of the power module card that you can see the defect part highlighted with a yellow circle around it.
Why did this defect happen?
Why was only JT-6 IPM's defected?
How can we repair this board?
I hope someone with similar experience guide me.
Many thanks in advance for your help.