Metal Finishing Guide Book

2012-2013

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ment. Some models incorporate a built-in digital display, which allows one to observe the power-supply output waveform while reading the DC operating point and the AC ripple content at the output bus. Although an oscilloscope is not always necessary, you will find it a convenient tool when making a quick check on an operating power supply to see if any further testing is necessary. Of these three electronic tools, the clamp-on ammeter is the first one you will most likely use to measure the three-phase line current. The measurement point should be just after the main contactor, near the transformer input terminals. This measurement can be performed at no load to determine the magnetizing current of the main transformer, which should be about 5% of full load rated line current. With a load on the DC output bus of the rectifier, the balance of the AC line current can be measured, and the three line currents should be within 10% of each other. The next instrument you may use is the DVM. It will allow you to verify the three-phase, line-to-line input voltages at the thyristor regulator section just ahead of the main transformer. If you then measure the line-to-line voltages on the transformer side of the thyristors, you can determine if the thyristor regulator part of the system is feeding balanced voltages to the main transformer. The oscilloscope is valuable when performing fast maintenance checks on a number of power supplies. The scope should be connected to the back of the output DC panel voltmeter. As the voltage control on the panel is increased, a waveform will appear that has six peaks and valleys for each cycle of the line frequency. Each period is 16.6 milliseconds long. If any of the six major peaks is missing or the valleys are too wide, there is a serious problem in the power circuit that must be investigated further. BASIC TROUBLESHOOTING This section briefly describes some basic diagnostics to determine why a power supply is not operating properly. Before starting any diagnostic test on a power supply, you should obtain a copy of the electrical schematic drawings for the particular equipment you are working on. On these drawings, you should be able to identify the basic functional areas that make up virtually any rectifier. The four basic building blocks of a power supply are the following: 1. Electrical controls 2. AC power circuits 3. DC power circuits 4. Electronic controls CAUTION: Only qualified personnel should attempt to service power supply equipment. Dangerous and lethal voltages may be present. The electrical controls provide simple low-power functions for the power supply. You will notice such items as push buttons (stop, start), pilot lights, 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 779

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