In _______ instrument the deflecting torque depends on the frequency.
Right Answer is:
Induction type instrument
In the Induction type instrument, the deflecting torque depends on the frequency.
Induction type Instrument:-
Induction-type instruments are suitable only for a.c. measurements. These instruments may be used either as an ammeter or voltmeter or wattmeter or energy meter. Perhaps the widest application of the induction principle is in watt-hour or energy meter.
The basic principle of induction type instruments is that when two alternating fluxes φ1 and φ2 (whose magnitudes depend upon the current or voltage to be measured having a phase difference θ pass through a metallic disc, usually of copper or aluminum. These alternating fluxes induce currents in the disc. The current produced by one flux reacts with the other flux, and vice-versa, to produce the deflecting torque that acts on the disc. It can be proved that net deflecting torque on the disc is given by;
Td ∝ φ1m φ2m sinθ
Since the magnitude of eddy currents also depends on the flux and frequency producing them, the instantaneous value of torque is proportional to the square of current or voltage under measurement and the value of mean torque is proportional to the mean square value of this current or voltage.
Td ∝ I2
Advantages of Induction Instruments
The following are the advantages of induction instruments:
- A full-scale deflection of about 300° is obtainable, giving a long and open scale.
- Stray magnetic fields having a limited effect on their readings.
- Good and very efficient damping.
- Disadvantages of Induction Instruments
The following are the disadvantages of induction instruments:
- Their power consumption being fairly high, they are more expansible.
- They can be used for a.c. measurement only.
- The scale is cramped initially because the deflecting torque is proportional to the square of the current or voltage to be measured.
Errors In Induction Instruments
Following are the errors in induction instruments:
Error Due to Frequency Variation:- Variation of frequency directly affects the deflecting torque for a given current because the deflecting torque is directly proportional to the frequency. The value of impedance also varies with the change in frequency. Some compensation for frequency error can be provided by shunting the induction ammeter by the non-inductive shunt.
Error Due to Temperature Variation: Since the resistance of the eddy current paths in the disc is dependent upon the temperature, it may produce serious errors.
Compensation for temperature error is obtained by shunting the instrument in case of an ammeter with a shunt of the material having a higher temperature coefficient than that of aluminum of which the disc is made.
In voltmeters, a combination of the shunt and swamping resistance is often used in series with the instrument.