Electrical Engineering ⇒ Topic : Range extension of dynamometer wattmeters

Range extension. Since a dynamometer wattmeter is principally used for a.c. power measurement, range extension shall be discussed with reference to a.c. power measurement. The usual ranges of dynamometer wattmeters are from 0-25 A to 0-50 A for current coil circuit and from 0-15 V to 0-750 V for potential coil circuit. The cun-ent coil (i . e . , fixed coils) carries the whole of load current and for this reason it is made of thick copper wire of comparatively few turns. The current through the potential coil (i.e., movable coil) is limited to about 30 mA by means of a high series resistance (i . e . , multiplier).

                                                                    figure (a)

For higher current ranges, a 0-5 A instrument is used with a current transformer (C.T.) as shown in Fig. a (i) . On high voltage a.c. circuits, besides the current transformer, a potential transformer is used with a 0-110 V instrument as shown in Fig.a (ii). When instrument transformers (i . e . , C.T. and P.T.) are used in this way, indications of the pointer must be multiplied by the "transformation ratios" (N 'N) to get the true power, just as the reading of a shunted ammeter must be multiplied by the multiplying power of the shunt to get the equivalent unshunted deflection