Monday, November 9, 2009

Repair of KS94 Temperature Controller manufactured by PMA

I recently repaired a KS94 Temperature Controller manufactured by PMA. The power supply in the KS94 had failed. The Switch-Mode-Power-Supply's Switching Transistor was shorted. Because the transistor was shorted, I removed the Switching Transformer to make sure it was still good. With my DMM set to resistance mode, I found that the primary winding was shorted to another winding. I drew a schematic of the SMPS to determine if this was a correct function of the Switching Transformer. It was not. The primary winding shorted to the secondary winding. This Switching Transformer was custom manufactured for this application and was not available for purchase. The Switching Transformer was a sealed epoxy unit, so rewinding it would be near impossible without destroying it in the process of disassembly.

The owner of the KS94 Temperature Controller needed it back quickly for an urgent production run, and a new KS94 unit was weeks away if purchased. Thus, another solution had to be worked out.

Ron Abts had me work out the necessary voltage requirements for the KS94 Temperature Controller. Then he went to our parts department and pulled two power supplies off the shelf. The one power supply would produce the proper voltages for the CPU Logic Circuits and the Analog Circuits. The second power supply would make the required isolated voltages for the 4-20mA output. I installed the power supplies, and tested the KS94 Temperature Controller good. The PT-100 Temperature Sensor Circuit worked and the 4-20mA output tracked the Temperature Sensor's changes good.

There are three output relays in this KS94 Temperature Controller. I tested them as part of the KS94 tests. I found them to have high contact resistance, so I replaced them as well.

The KS94 Temperature Controller is in the customer's machine and working.

Repair of AF-300E$ General Electric / Fuji Electric Inverter Drive

A company in New Mexico sent us a Switch-Mode-Power-Supply/IGBT Firing Board from a General Electric / Fuji Electric AF-300E$ Inverter Drive. This AF-300E$ runs a 200 Horse Power Motor. The company only sent the boards in for repair because the drive was to too large to ship to us.

The part number of the Switch-Mode-Power-Supply / IGBT Firing Board is EP-3603A-C2-Z4. The Switch-Mode-Power-Supply Board had a blown trace and external smoke residue covering the board's conformal coating. I cleaned the board and drew a schematic of the blown trace circuit and found that it was the ground path for the High Voltage Bus Monitoring Circuit.

I then tested all the components on both the Power Supply Board and the IGBT Firing Board with my DMM. All the IGBT Pre-driver Transistors on the IGBT Firing Board were shorted.

Judging from the smoke residue and the shorted IGBT Pre-driver stages, we concluded that most likely the IGBT's in the drive were also shorted. I made the repairs to the SMPS Board and the IGBT Firing Board. I drew a schematic of the IGBT Firing Circuit to find out how they were being fired. Then powered up the board with 460VAC. I then tested the IGBT Firing Circuit and the Bus Monitoring Circuit with input signals. Each circuit functioned correctly. In addition to testing these two circuits, I tested good the voltages of the SMPS Circuit. These tests had to be done before mating the boards to the drive. Otherwise the drive would be blown again.

Because we knew that there had to be damage to the drive itself, Ron Abts flew out to the company in New Mexico to check out the drive. He found that the IGBT's were in fact shorted and a 600 Amp Bus Fuse was open. He also found that there was a problem with the Three-Phase-Bridge-Rectifier Circuit. This circuit was made up of four each parallel per line phase of Dual-Diode-Power Modules. Some of the diodes did not measure correctly with his DMM. He also found that the Main Contactor Pre-charge Resistor was open. He partially disassembled the drive to get the IGBT and Dual-Diode-Power Module part numbers. He phoned these part numbers back to the shop so that we could get an early jump on getting them in. He then recommended that the drive be shipped to us, so that the repairs to the drive could be completed. The company agreed and shipped us the drive.

Upon receiving the drive, I started the disassembly process to get down to the IGBT's so that I could replace them. This drive is huge. The current carrying members are large flat metal bars. And so I had to be very methodical in the disassembly of the drive, so that I could re-assemble it correctly. One part not re-installed or installed in error would mean the destruction of the drive again. And wiping out all the effort placed into the repair of the drive.

Once I got down to the IGBT's and Three Phase Bridge Power Modules, I was able to test them. The drive was using three parallel Dual-IGBT's per motor phase. The U phase IGBT's were not shorted, but failed to conduct on my IGBT Tester. The V and W phases were shorted through and through.

I tested the Main Contactor and found the contact resistance to be too high. I disassembled the Main Contactor and cleaned the contacts. I then replaced all the damaged power modules while Ron Abts and Charles Green worked the new pre-charge resistor into the drive. Once the drive was completely re-assembled, we ran the drive unloaded and viewed the IGBT Firing Signals and U,V,W Outputs with a Fluke Oscilloscope. All looked good. We then ran a motor.

Ron went back with the repaired drive to install it at the company site in New Mexico. The repaired drive is working in the machine.