Metal Finishing Guide Book

2013

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relays, timers, limit switches, flow switches, thermal switches, thermal overlay relays (heaters), and other 120 V AC protective devices. These items are typically drawn in the familiar ladder diagram format. Diagnostics in this area will usually require the DVM to measure continuity or the presence of control voltages at various components. To check for proper voltages at the low-power components, find the common on the ladder diagram and attach the voltmeter to it in the actual circuit. With the control power energized, you will be able to check the AC controls on the ladder diagram and measure for the presence of an AC voltage at the corresponding point in the actual circuit. This method is most useful when there is a loss of control circuit voltage that prevents a portion of the controls from working properly. When the missing voltage returns at a particular point in the circuit, this indicates you have just moved past the defective component, such as a contact, a terminal, an interlock, or a thermal switch. The faulty component can then be repaired or replaced. You may find there is more than one bad part; so be sure to test all of the low-power components. The AC power circuit is the portion of the power supply located between the AC input power terminals and the regulation thyristors at the primary of the three-phase power transformer (assuming a primary thyristor/secondary diode configuration). The components representing this AC power section are usually found near the center of the electrical schematic. The clamp-on ammeter is the diagnostic tool used in the AC power circuit. Place the ammeter around one of the incoming AC conductors. Operate the power supply with no load and check that the magnetizing current of the main transformer 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 output of the power supply. Measure all three incoming lines and verify that the currents 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 controls functioning) except for unbalanced line currents, there may be a problem with the main power transformer or the diode section on the low-voltage secondary 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 power supply consists of locating shorted or open diodes and verifying 748

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