Basic_Electrical_Engineering_4th_edition
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ELECTRICAL ENGINEERING |
3.4.The power input to a load was measured by two wattmeter method. The readings were 800 W and 400 W, the latter reading being obtained after reversing the potential coil connections. Find
3.5. |
power and p.f. of the load. |
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[Ans. 400 W, 0.189] |
Prove that the power in a balanced 3-phase circuit can be measured from the readings of two |
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wattmeters. Draw the relevant connection and phasor diagrams. If the readings on two wattme |
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ters so connected are 5 KW and 0.5 KW, the latter reading being obtained after reversal of the |
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current coil connections. Calculate the power and p.f. of the load. Show how the wattmeters are |
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3.6. |
connected to measure the reactive power in a circuit. |
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[Ans. 4.5 kW, 0.426] |
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A load of impedances each 9 )21 is supplied through a line to which a voltage of 415 V is |
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applied. The impedance of each line is 2 |
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j4Q. Determine the power input and output when the |
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load is (a) star connected (b) |
mesh connected. |
[Ans. |
(a) 2556, 2091W (b) 5925, 3555.] |
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CHAPTER
4Basic Instruments
4.1INTRODUCTION
Ammeters and voltmeters are classifiedtogether as there is no difference intheprinciple
for torque production. A voltmeter carries a current proportional to the voltage which produces operating torque whereas in case of an ammeter, the torque is produced by the current to be measured or by a definite fraction ofit.
The basic philosophy ofmeasurement is thatwhenever we insert aninstrumentfor meas urement ofcertain quantity, its insertionshouldnot disturb the corresponding already existing quantity inthe network.
An ammeteris used to measure currentin a circuit andhence it is to be connected in series with the circuit. Now if there is certain current in the circuit, the insertion of ammeter in the circuit should not disturb the current value. This requires that the ammeter must have low resistance. Similarly, a voltmeter is connected across the voltage to be measured and must, therefore, have a high resistance so that the current taken by it should he small. A low range ammeter i.e. the one which can measure small values of current can be used as a voltmeter by connecting a high resistance in series with it. Of course, the current through the converted voltmeter should not exceed it's range when used as an ammeter.
It is to benotedthattorquein an a.c. instrument canbe producedonlywhenthere are two fluxes displaced in time and space. If an ammeter or a voltmeter whose operating torque is proportional to the current passing through it, will produce a pulsating torque if used in ac circuits and hence the inertia ofthe moving system ofthe instrument will not be able to respond to thepulsatingtorque and hence no deflectionwouldbe observed.
It is, therefore, essentialthatwhile measuring a.c. quantities, thetorque shouldbe propor tional to the square ofthe current passing through the instrument. The deflection would thenbe proportional to the mean value ofthe square ofcurrent (assuming springcontrol is used) andthe instrument can thus be used for the measurement ofrms values of current or voltage.
4.2TYPES OF INSTRUMENTS
The following types ofammeters andvoltmeters are in common use :
(a) Moving coil
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