Aggregates Manager

August 2014

Aggregates Manager Digital Magazine

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Page 26 of 43

EQUIPMENT MANAGEMENT 25 AGGREGATES MANAGER August 2014 Note the numbered lines to the le of each section on the page. ese reference other sections in the drawing set. You will occasionally fi nd a wire or device that continues to another area in the drawing set. Using a multi-meter for troubleshooting Multi-meters come in a variety of styles, but o en carry similar features. Most will allow AC and DC voltage readings up to 300 volts, resistance, and continuity checking, as well as current and milliamp readings up to 10 amps. Some of the higher end units off er auto scaling and even source milliamps, voltage, and RTD for simulations. It is important for the technician to recognize the selection of lead plug-ins. is is necessary to change for reading amps or transmi er signal milliamps (ma). Since amperage is read in series of a circuit and voltage is read in parallel, the leads must be plugged into the proper ports for operation. If leads are le in the amperage ports while testing for voltage, you will most likely blow the internal fuse. Make sure you keep spares because this is a common mistake. When testing voltage or current, always start at the highest se ing on the meter if signal values are unknown. You can then select lower ranges to improve the reading resolution. When measuring resistance, always verify that power is off and locked out. It is usually a good idea to li fi eld wires from terminals when measuring resistance to isolate your components and avoid reading paralleled resistance elsewhere in the circuit. When reading fuses for continuity, pull them out fi rst. You could read a low resis- tance through motor windings or a transformer and think the fuse is good! When troubleshooting power circuits such as 480vac or 600vac motor circuits, it is o en necessary to own a clamp current meter. is will allow for independent phase current testing without li ing any wires on terminals. Motor circuit troubleshooting Recognizing issues in the motor control circuit should be a straightforward task. Motor issues typically present them- selves as blown fuses, tripped feeder breakers, open windings in a phase, bad contactor, defective overload, seized motor, or mechanical jam. First, check the fuses or feeder breaker above the starter contactor. With power off , each of the three-phase fuses can be pulled to check for continuity. Analyzing the motor circuit requires a clamp-on amp meter. Watch the motor control circuit while an operator engages the run command. If the contactor energizes and trips a breaker and immediately blows a fuse, this is a sign of a short in either the motor wind- ings or the wiring. With power off , read resistance between each phase on the starter load side outgoing terminals. e larger the motor, the lower the resistance, but the resistance should be the same across each phase. If a phase is shorted in the motor or wiring, you would read 0 resistance. Also measure resistance between each phase and ground. is resistance should be in the millions of ohms. If a short exists in the motor or wiring to ground, you will read very low resistance. Now, isolate the motor at the motor terminal junction box and see if the short is in the wiring back to the starter or in the motor windings. On dredges, old corroded wiring or water in fi eld terminal boxes is o en the culprit. If a short is not discovered, the issue may be an open wire or winding on the motor. A bad contact in the starter contactor will cause the same open eff ect. When one of the three phases is open, the motor will hum loudly while vibrating, and the two phases with connectivity to the motor will read high locked ro- tor current with a clamp-on amp meter. e phase that reads no current is the open circuit. With power off and locked out, you can test the starter con- tactor contacts and overloads with a continuity test across the individual phase of the contactor. You should read open circuit until you physically press in the contactor. While pressed in, you would read 0 ohms if the contactor and overloads are good. If this proves okay, the next step is to check your wiring and isolate the motor windings with resistance checks along the way. PLC I/O troubleshooting It is common for laymen to blame PLC code or hardware for Clamp-on amp meter

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