The various types of torques that are essential for the satisfactory operation of an analog indicating instrument are as follows,- Deflecting torque,
- Controlling torque, and
- Damping torque
Now let us see about deflecting torque in measuring instruments and how it is produced,
What is Deflecting Torque?
The torque (force) required for the deflection of the pointer is called deflecting torque. The system which provides the deflecting torque, when the current is passed through it is called a deflecting system. Every instrument will have a deflecting system that converts the electric energy into mechanical energy and thus providing the necessary and sufficient deflecting torque.
In general deflecting torque is nothing but the movement of the pointer in a measuring instrument. There are different types of electrical instruments to measure different types of electrical quantities like current, voltage, power, energy, etc.
When we apply the quantity to be measured to the instrument. There will be a pointer that moves on the scale to a value so that the amount of quantity applied will be indicated by the instrument.
This mechanical movement of the pointer will be proportional to the applied electrical quantity to be measured. The conversion of electrical quantity into mechanical force will be done by the deflecting system. Also, the amount of deflecting torque produced must overcome,
- The inertia of moving system (like pointer, moving coil).
- It should overcome the controlling torque produced by springs.
- The damping torque produced by the damping system.
Effects Used to Produce Deflecting Torque :
The various effects used in different types of electrical measuring instruments to produce deflecting torque are,
Magnetic Effect :
Most of the ammeters, voltmeters, wattmeters, integrality meters utilize the magnetic field for the production of deflecting torques. These types of instruments use permanent magnets or electromagnets to get the required magnetic field. The magnetic effect is used in moving coil and moving iron instruments.
In PMMC instruments the deflecting torque is produced on the current carrying moving coil when it is placed in the magnetic field produced permanent magnets. The coil tends to move and deflects the pointer attached to it. Whereas in moving iron instruments, there will be a movable soft iron piece that gets attracted or repelled when placed in a magnetic field produced by the stationary electromagnets. Depending upon these there are attraction and repulsion type moving iron instruments.
Electromagnetic Induction Effect :
Voltmeters, ammeters, wattmeters, and energy meters utilize the effect of electromagnetic induction. In these types of instruments, the deflecting torque required is produced by the eddy currents induced in the aluminum disc due to the flux caused by the electromagnet. Since these type of instruments works on the induction phenomenon, they can be used for measuring ac quantities only.
The principle of induction effect is mostly used in energy meters. It consists of a driving system in which two magnets shunt and series are placed facing each other and magnetized proportionally to the supply voltage and current respectively as shown above. A movable aluminum disc is placed in between the two magnets. An eddy current flows in the disc due to flux produced by the load current and hence torque is produced.
Heating Effect :
Ammeters and voltmeters also work on the effect of heating. In technical terminology, they are also called thermal instruments. In these instruments, the action depends upon the heat caused by the current under measurement. This effect can be used to measure both the ac and dc quantities.
When a current to be measured is passed through a coil causes a rise in the temperature of the coil. The heat produced can be converted to emf with the help of a thermocouple (a transducer). Hotwire instrument is also based on the principle of heating effect for producing deflecting torque.
Chemical Effect :
Integrating instruments such as watt-hour meters make use of the chemical effect. These instruments measure the total energy supplied in a given interval of time. This effect is mainly used for the measurement of dc ampere-hours.
Electrostatic Effect :
Only voltmeters make use of the electrostatic effect. The principle of operation of electrostatic instruments is the force of attraction or repulsion between two charged bodies, which leads to the production of deflecting torque.
This effect is used only for measuring the voltages on both ac and dc. The principle of electrostatic effect is well suited for measuring very high voltages. It consists of two plates, fixed and movable. When potential is applied across the plates there exists an electrostatic field and causes a force to exert between the plates.
