POLARITY CHECK OF OHMMETER TERMINAL

TROUBLESHOOTING - TM-55-1930-209-14P-9-4_92

TROUBLESHOOTING PROCEDURES FOR GENERATOR/REGULATOR

TM-55-1930-209-14P-9-4 Water Purification Barges (NSN 1930-01-234-2165) Electric Power System Manual
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TROUBLESHOOTING TESTING AND ADJUSTING A circuit continuity tester, similar to the 8S4627 (circuit continuity tester) can be used for checking low resistance continuity. This tool can also be used to check rectifiers and controlled rectifiers. It uses a 3-volt battery source. This is all that is needed to activate the gate of a controlled rectifier. A Kelvin or Wheatstone bridge can be used to measure the resistance of the revolving field and stator winding of the generator or the exciter armature. A 500 to 1000 volts megohmeter is needed to measure the insulation resistance of the generator stator, revolving field, exciter armature, and exciter field. A DC ammeter with the correct current shunt is used to measure the current of the exiter field. A clamp-on volt-ammeter with a 0 to 600 volts and 0 to 600 amperes scales is used to measure line voltage and line current. Some generators are rated higher than 600 amperes, but these units are normally connected with two or more conductors in parallel per phase. To measure line current for these generators, measure the current in each conductor per phase and add the currents together. NOTE: See WARNING below, if generator is rated over 600 volts. A direct-reading tachometer is used to measure rpm. When exact frequency control is needed, a direct comparison can be made with outside line power. On generators with higher than 600 volts rating do hot use direct-reading test equipment to measure line voltage or line current, (even though the instrument has higher voltage capacity). Voltage and current transformers with a high voltage rating must be used. On power circuits higher than 600 volts, direct- reading meters CAN have failure of insulation. Where the same adjustments are necessary on two or more units, (as in parallel operation) use the same instrument for measuring all voltages. In many of the following resistance test procedures, the ohmmeter connecting polarity (positive or negative) is given for testing the respective component. NOTE: The identification of lead polarity on ohmmeter instruments is not necessarily the same. POLARITY CHECK OF OHMMETER TERMINAL 1. Selector switch. 2. Selector switch. 3. Mark to show polarity of ohmmeter cables. Volt-ohm-milliameter cable polarity must be known. The red, positive “+”, color on the cable of an ohmmeter can be either positive (+) or negative (-) and identification of each cable is needed. A second volt-ohm-milliameter (multimeter) can be used with the test ohmmeter to find the correct polarity of the connecting cables. Turn one instrument selector switch (1) to DC volts, and turn the test ohmmeter selector switch (2) to ohms (RX1). Connect the cables of both instruments together until both instruments have a meter indication at the same time. (Change the cables if necessary). The red, positive “+”, lead from the instrument with selector switch (1) on DC volts is connected to the positive lead of the test ohmmeter. Put a mark on the ohmmeter to show the polarity of the ohmmeter cables (3). the correct positive and negative identification of the cables has been made. On the digital multimeter, the above procedure is not necessary. Polarity is indicated on the readout. For further information on using the digital multimeter, see Special Instruction, Form SEHS7734. 53

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