Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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For many smaller and middle-sized operations, computer automation is becoming financially feasible. Benefits include reductions in rework and reject rates, in downtime, and in chemical costs. Additional savings could be realized by the reduced power usage of a computer-controlled operation. In the near future computer automation may very well be the key factor in whether certain metal- finishing operations are profitable. RECOMMENDED TEST EQUIPMENT Aside from the usual hand tools usually found in a well-equipped industrial tool box, the following are recommended tools for power supply troubleshooting: 1. A clamp-on AC ammeter 2. A digital volt-ohm meter (DVM) 3. A battery-operated oscilloscope There are several options to consider when purchasing these instruments for testing in an industrial environment. The clamp-on ammeter should be an AC device, as it will be used at currents up to 1,000 A AC. All exposed metal parts must be sufficiently insulated to ensure safe use around 600 V AC equipment. An ana- log-type clamp-on ammeter is preferred over most digital ammeter types, unless the digital unit is sufficiently filtered to prevent display jitter when measuring incoming line AC. When buying a digital ammeter, one should test the instrument on an operating power supply before making the final purchase decision. The digital volt-ohm meter best suited for power-supply testing is battery operated and durably packaged so that it will stand up in an industrial envi- ronment. A heavy-duty rubber-covered case is best. To be the most useful, the 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 continu- ity in cables and wire harnesses. The oscilloscope should be a high-quality, battery-operated portable instru- 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 ter- minals. This measurement can be performed at no load to determine the mag- netizing 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 bal- ance 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 643

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