Metal Finishing Guide Book


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DVM should have "true rms reading" capabilities. Make sure that the test leads are equipped with heavy plastic leads and rated for 5,000 V DC service. The DVM should have at least the following ranges: voltage of 10 mV to 1,000 V AC and DC, current of 1 to 10 mA AC and DC, and resistance of 0.1 ohm to 10 megohm. Some additional features to look for are autoranging and/or a diode testing range, which measures the forward voltage of a diode rectifier. An alarm on some DVM instruments is a convenient means to measure continuity in cables and wire harnesses. The oscilloscope should be a high-quality, battery-operated portable instrument. 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, 747

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