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Diagnosing Faults in Servo Drive
Systems Using an Infra Red Camera
Fletcher Moorland recently bought a Fluke Infra Red camera for site surveys and to assist in fault diagnosis and repairs throughout the company. Our R&D team always have their minds on how to use anything we have in new and innovative ways to improve our repair abilities.
One of our drives engineers was recently performing a repair on a power supply module for a Siemens 611 servo drive system. He wasn’t happy with what he was seeing, the module was not working correctly and one area of the unit seemed to be getting hotter than usual. A blown MOSFET transistor had been the original cause of this modules failure and had been replaced. He called on the R&D team to see if they could help, they decided the Infra-Red camera would tell them for sure if things were right or wrong, and where.
Within seconds the engineer had the answer, something was very wrong with this power supply module !
The photo above shows the infra red image of part of the power supplies chopper stage. The bright red area is one of the MOSFET transistors which was running excessively hot at just over 182 degrees C. The maximum junction temperature for this type of transistor is only 150 deg C. This would have blown in a very short space of time. The other transistor in the circuit (they are arranged as a pair) was also running hot at around 110 deg C.
To make sure, an Infra-Red photograph was taken of a known good 611 series power supply in our test panel.
This shows a more normal working temperature with both of the MOSFET transistors in the pair at the same temperature, a much cooler 75 deg C which is what we expected to see with the covers removed.
The engineer discussed the problem with the R&D team who suggested he looked at the gate drive circuit of the MOSFET transistor that was overheating. Surely enough he found a faulty resistor there that was causing the problem. The resistor was replaced, new transistor fitted and the power supply module was load tested again with the covers in place (to make sure the unit reached its correct operating temperature) before removing them checking again with the camera.
Now both transistors of the pair were at the same temperature and more importantly at the correct working temperature. The power supply module was re-assembled, load tested, soak tested and returned to the customer the next day. Without the camera the module would have taken longer to repair.
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