An elementary book on electricity and magnetism and their applications . ator or motormeans the resistance of the insulation like that between thecommutator and the frame. It is necessary that this resistance should exceed a certain mini-mum number of ohms. Forinstance, a 100-kw. generatorat 110 volts should have aninsulation resistance of at leastone tenth of a megohm or 100000 °hmS- T° measure test of insulation resistance. this insulation resistance, con- nect a voltmeter across a110-volt or, better, a 500-volt d-c. line and call this line voltageE and the r
An elementary book on electricity and magnetism and their applications . ator or motormeans the resistance of the insulation like that between thecommutator and the frame. It is necessary that this resistance should exceed a certain mini-mum number of ohms. Forinstance, a 100-kw. generatorat 110 volts should have aninsulation resistance of at leastone tenth of a megohm or 100000 °hmS- T° measure test of insulation resistance. this insulation resistance, con- nect a voltmeter across a110-volt or, better, a 500-volt d-c. line and call this line voltageE and the resistance of the instrument Rv. Then connectthe frame of the machine with one side of the line and thecommutator in series with the voltmeter to the other side ofthe line, as shown in figure 128. Call e the reading of thevoltmeter in this position, which shows the voltage across theresistance of the instrument itself. It is evident, then, that(E-e) is the voltage drop across the insulation of the these two resistances are in series, it follows that , s. Generator ELECTRIC MEASUREMENTS 195 the resistances are proportional to the voltages across each. That is, Insulation resistance _ Voltage across insulationVoltmeter resistance Voltage across voltmeter Let X = insulation resistance, then we have X E-e Rv eand For example, a voltmeter has a resistance of 15,000 ohms and isused on a 113-volt line. When connected as in figure 128, the volt-meter reads 4 volts. Then the insulation resistance between the frameand the armature coils is X = 15,000(4^-1) =405,000 ohms (about). 138. Testing the insulation of covered wire. The insulationresistance of a covered wire is usually expressed in megohmsfor a given length, such as a kilometer or a mile or one thousandfeet. To determine this resistance, a known length of the wireis soaked in salt water and then connected through a sensitivegalvanometer with some source of constant voltage, as shownin figure 129. First the galvanome
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectmagnetism, bookyear19
