Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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is no more than 5% of the full load rated line current, which is usually indicated on the electrical schematic. If this reading is correct, the next step is to measure the line current with a load of parts in the process tank that will require full out- put of the power supply. Measure all three incoming lines and verify that the cur- rents are balanced to within 10% from one phase to the next. If an imbalance is detected, there could be a fuse blown or a thyristor shorted, or the gate signal to some of the thyristors may be improper. To determine which of the above is the problem, use the DVM on a high AC voltage range and measure the line-to-line AC voltages. Extreme care should be exercised when making line voltage measurements to prevent any metal parts from coming in contact with the live conductors. At the same time, protective eye wear should be used. Measure the line-to-line voltages at each of the thyristors, after the thyristor fuses. If all voltages are okay, no fuses are blown, and all contactors and safety switches are working, next measure the line-to-line voltage at the output of the thyristors near the connection to the primary of the main power transformer. If these voltages are relatively balanced but reduced in value, the thyristor regulator is in proper working condition. If after testing both the electrical controls and the AC power sections you find that everything is normal (i.e., no defective fuses or thyristors, all electrical con- trols functioning) except for unbalanced line currents, there may be a problem with the main power transformer or the diode section on the low-voltage sec- ondary side of the transformer. The DC power section typically consists of diodes, output bus connections, and metering for output voltage and current (in a secondary thyristor configuration, you would find thyristors in place of diodes). Testing in this section of the pow- er supply consists of locating shorted or open diodes and verifying metering cal- ibrations. Because of the high currents that flow in the low-voltage diode busing, a loose connection will cause a great deal of heat to be generated, which will cause a discoloration of the copper bus bars. By physically inspecting the DC power sec- tion in detail, some of these connection problems may be located and repaired simply by cleaning. The clamp-on ammeter may be useful for moderate-sized diodes that are supplied with a flexible cable connection from one side of the case. Diodes that are supplied with a flexible connection at one end of the case can be checked with the clamp-on ammeter. Measure the current at each diode by plac- ing the clamp-on ammeter around the flexible lead. A diode that is open will draw no current, whereas a diode that is shorted will draw excessive current. In either case, the diode should be replaced. As these diodes are removed, the DVM may be used on the diode range to verify that the diode being removed is, in fact, bad. A defective diode will read either open or shorted in both directions. The DVM may be also used to determine possible metering circuit defects. To check the power-supply voltmeter, measure the voltage across the output terminals of the rectifier and the terminals at the back of the panel voltmeter. Compare these readings with that of the panel voltmeter. They should all agree. Current is typically determined by measuring the voltage drop across a precision resistor placed at the output terminals known as a shunt. This voltage drop at full output will typically be 50 mV. This low-level voltage signal has to be multiplied by a factor before comparing it to the actual meter reading. The oscilloscope is useful in locating problems where complete diode cir- cuit branches have burned open and left a missing section in the wave shape; how- ever, this may also be a symptom of thyristor problems on the primary of the main 645

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