In this lecture, we are going to learn about deflecting torque in measuring instruments. We will discuss the effects used to produce the deflecting torque in measuring instruments.
For satisfactory operation of an indicating instrument, three forces/ Torques are essentially required, these are as follows.
Deflecting System or Deflecting Torque
In most of the indicating instruments, the mechanical force proportional to the quantity to be measured is generated. This force or torque deflects the pointer. The system which produces such a deflecting torque is called deflecting system and the torque is denoted as Td.
The deflecting torque overcomes,
- The inertia of the moving system
- The controlling torque provided by the controlling system
- The damping torque is provided by the damping system.
The deflecting system uses one of the following effects produced by current or voltage, to produce deflecting torque.
Effects Used to Produce Deflecting Torque
The various effects used in different types of electrical measuring instruments to produce deflecting torque are:
1. magnetic Effect:
- When a current-carrying conductor is placed in a uniform magnetic field, it experiences a force that causes to move it. This effect is mostly used in many instruments like moving iron attraction and repulsion type, the permanent magnet moving coil instrument, etc.
2. Thermal Effect:
- The current to be measured is passed through a small element which heats it to cause a rise in temperature which is converted to an e.m.f by a thermocouple attached to it.
- When two dissimilar metals are connected end to end to form a closed loop and the two junctions formed are maintained at different temperatures, then e.m.f is induced which causes the flow of current through the closed circuit which is called a thermocouple.
3. Electrostatic Effects:
- when two plates are charged, there is a force exerted between them, which moves one of the plates. This effect is used in electrostatic instruments which are normally voltmeters.
4. Induction Effects:
- When a non-magnetic conducting disc is placed in a magnetic field produced by electromagnets that are excited by alternating currents, an e.m.f. is induced in it.
- If a closed path is provided, there is a flow of current in the disc. The interaction between induced currents and the alternating magnetic fields exerts a force on the disc which causes to move it. This interaction is called an induction effect. This principle is mainly used in energy meters.
5. Hall Effect:
- If a bar of semiconducting material is placed in a uniform magnetic field and if the bar carries current, then an e.m.f. is produced between two edges of the conductor. The magnitude of this e.m.f. depends on the flux density of the magnetic field, the current passing through the conducting bar, and the hall effect coefficient which is consent for a given semiconductor. This effect is mainly used in flux meters.
- Thus the deflecting system provides the deflecting torque or operating torque for the movement of the pointer from its zero position. It acts as the prime mover for the deflection of the pointer.
