Electrical Engineering ⇒ Topic : Range Extension of Dynamometer Type Instruments
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DJ
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Range Extension of Dynamometer Type Instruments The chief application of dynamometer type instruments is as wattmeters. However, they can be used as ammeters and voltmeters. (1) Ammeter. The usual range of an unshunted dynamometer ammeter is 0-50 mA because the moving coil of the instrument cannot carry current more than 50 mA. However, the range can be increased by connecting a suitable shunt across the moving coil of the instrument. In an a.c. ammeter, shunts are used only for a range upto 0-5 A as the division of *current depends upon the inductance and resistance of the instrument. For the measurement of higher alternating currents, a transformer is used in conjunction with 0-5 A a.c. ammeter. Suppose we wish to measure a very large alternating current. By means of a transformer (known as **current transformer), we step this current down to say 50 ties. Assume that, as the output of the transformer is measured by then a.c. ammeter, a reading of 4A is indicated. The original or line cun-ent is then = 4 x 50 = 200 A.By putting current transformer of different ratios, different current ranges of the instrument can be obtained. (2) Voltmeter. The principal use of dynamometer voltmeter is for the measurement of alternating voltages. The range of the a.c. voltmeter can be increased from 0-750 V by the use of multipliers i.e., by connecting a suitable high resistance R, in series with the instrument. For ranges higher than 0-750 V, where the power wasted in the multiplier would be excessive, a transformer is used in conjunction with a 0-110 V a.c. voltmeter. Suppose we wish to measure a very large alternating voltage. By means of a transformer (known as potential transformer), we step this voltage down to say 20 times. Assume that, as the output of the transformer is measured by the 0-110 V a.c. voltmeter, a reading of 100 V is indicated. The original voltage is then = 20 x 100 = 2000 V.By putting voltage transformer of different ratios, different voltage ranges of the instrument can be obtained. | |
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