Electrical Engineering ⇒ Topic : Deflecting Torque

There must be some agency which can detect the quantity to be measured.This agency is known as the deflecting torque which acts on the moving

                                                     FIGURE (a) The scale

system of the instrument so that the pointer is deflected on the scale, when an unknown quantity is being measured. For example, a coil carrying a current, when a placed suitably in a permanent magnetic field will experience a torque. This torque is used to drive the pointer in moving coil permanent magnet type instruments. Deflecting torque can be produced using one of the following effects:

(a) magnetic effect, (b) heating effect, (c) chemical effect, (d) electrostatic effect and (e) electromagnetic induction.

Consider a cyclist. The cycle moves only when the rider is pedalling. The rider produces the mechanical force necessary for the cycle to move. Here, cycle is the moving system and pedalling is the agency which produces the motion of the cycle and pedalling depicts the deflecting torque in an indicating instrument.

Deflecting Torque

One important requirement in indicating instruments is the arrangement for producing deflecting or operating torque (T_d) when the instrument is connected in the circuit to measure the given electrical quantity. This is achieved by utilising the various effects of electric current or voltage mentioned in Art. The deflecting torque causes the moving system (and hence the pointer attached to it) to move from zero position to indicate on a graduated scale the value of electrical quantity being measured. The actual method of producing the deflecting torque depends upon the type of instrument and shall be discussed while dealing with particular instrument.

Deflecting Torque

Making use of one of the following effects of current and voltage usually produces the deflecting torque: (i) Magnetic, (ii) Heating, (iii) Chemical, (iv) Electrostatic, and (v) Electromagnetic.

           When the instrument is connected to the electrical circuit to measure the electrical quantity,deflecting torque moves the moving system of the instrument from its zero position. The method of producing this torque depends on the type of instrument