Electrical Engineering ⇒ Topic : Dynamometer Type Instruments (Ammeters and Voltmeters)

Dynamometer Type Instruments (Ammeters and Voltmeters)

These instruments are the modified form of permanent-magnet type. Here magnetic field is not produced by a permanent magnet but by two air-cored fixed coils placed on either side of the moving coil. Such instruments can be used as ammeters or as voltmeters but are generally used as wattmeters. They are suitable for d.c.as well as a.c. work.

Principle. These instruments are based on the principle that mechanical force exists between the current carrying conductors

                                                                               figure (a)

Construction. It essentially consists of a fixed coil and a moving coil. The fixed coil is split into two equal parts (F, F) which are placed close together and parallel to each other. The moving coil (1//) is pivoted in between the two fixed coils and carries a pointer as shown in Fig. (a) The current is led into and out of the moving coil by means of two spiral hair-springs which also provide the controlling torque. *Air friction damping is provided by means of the aluminium vanes that move in the sector shaped champer at the bottom of the instrument.

Working. For use as an ammeter or voltmeter, the fixed coils FF and the moving coil M are so connected that the **same current flows through the two coils. Due to these currents, mechanical force exists between the coils. The result is that moving coil M moves the pointer over the scale. The pointer comes to rest at a position where deflecting torque is equal to the controlling torque. Since the polarity of the fields produced by both fixed and moving coils is reversed by the reversal of current,the deflection of the moving system is always in the same direction regardless of the direction of current through the coils. For this reason, dynamometer instruments can be used for both d.c. and a. c . measurements.

(1) As ammeter. When the instrument is used as an ammeter, the fixed coils and the moving coil are connected in series so that the same cun-ent flows through the two coils as shown in Fig. (a)

In that case, I_f=I_m = I so that

                                   (a)                                                                                                                       (b)

                                                                        figure 

For measuring large currents, moving coil is shunted; the shunt being in series with the fixed coils as shown in Fig. (b). The fixed coils carry the main current while the moving coil carries a current proportional to the main current

(2) As voltmeter. When the instrument is used as a voltmeter, both fixed coils and the moving coil are connected in series together with a high resistance R_s, (called multiplier) having a negligible temperature co-efficient as shown in Fig.(a). Therefore, current in both the coils is the same and is proportional to the voltage V being measured

                                                      (a)                                                                                      (b)

                                                                                     figure

It may be seen that whether the instrument is used as an ammeter or voltmeter, the deflection is directly proportional to the square of quantity (current or voltage) being measured. Doubling the current will make the deflection four times as large. Hence, the scale of dynamometer type instruments is *not uniform; being crowded at the beginning and open at the upper end of the scale as shown in Fig. (b). The obvious disadvantage of such a scale is that the divisions near the start of the scale are small and cannot be read accurately