In this article we will discuss about the connection of voltmeters for the measurement of high voltages.
If the supply voltage is high, the resistance of the coil alone may not be sufficient enough to prevent flow of heavy current through the voltmeter. In order that a short-circuit may not occur between the lines at the point of connection, the resistance of the instrument must be increased further.
In case of d.c. instruments the resistance is increased by connecting an additional resistance in series with the coil. This series resistance is called swamp resistance or swamping resistance. It is considerably greater than the resistance of the coil. Therefore greater portion of the supply voltage is dropped across the swamp resistance, the voltage across the coil of the instrument being considerably low.
But the scale of the instrument is graduated in such a way that the pointer indicates total line voltage directly on the scale. The swamp resistance and the coil of the meter are enclosed in the same cover forming a complete instrument. The connections of the meter and the swamp resistance are shown in fig. 59(a).
In case of a.c. supply the range of a voltmeter can be extended either by connecting a swamp resistance or by using a potential transformer with the instrument.
The potential transformer has the following advantages over a swamp resistance:
(i) The loss of power in a potential transformer is negligible in comparison to that in a swamp resistance.
(ii) Potential transformer isolates the instruments from the supply system. This ensures safety to the operator.
The connections of a voltmeter with a potential transformer are shown in fig. 59(b). The primary coil of the transformer has large number of turns and is made of wire having small cross-sectional area. The secondary coil has a few turns with comparatively large cross-sectional area. Both the primary and the secondary coils are wound on a laminated iron core. Voltmeter is connected across the secondary coil.
The ratio of the primary to secondary turns are so adjusted that when the rated supply voltage is applied across the primary, the voltage across the secondary is 110 volts. But the scale of the instrument is graduated in such a way that the pointer reads directly the supply voltage on the scale.