Deflecting torque and controlling torque

Deflecting system: 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. 

The deflecting torque overcomes 1) The inertia of the moving system 2) controlling torque provided by controlling system 3) Damping torque provided by a damping system. 

As most of the meters are generally current sensing meters so Td is directly proportional to the current I.

The deflecting system uses one of the following effects to produce deflecting torque. 

1) Magnetic effect: When a current carrying conductor is placed in a uniform magnetic field it experiences a force which causes to move conductor. 

2) Thermal effect: The current to be measured is passed through the element which heats up and causes a rise in its temperature which is converted to an emf by a thermocouple. 

3) Electrostatic effect: When two plates are charged electrostatically then there is a force exerted between them which moves one of the plates. 

4) Induction effect: When a nonmagnetic conducting disc is placed in a magnetic field produced by electromagnets excited by AC then the disc will produce torque 

5) Hall effect: if a bar of semiconducting material is placed in the uniform magnetic field and if the bar carries current, then an emf is produced between the two edges of the conductor. the effect is mainly used in flux meters. 

Thus the deflecting system provides the deflecting torque or operating torque for movement of the pointer from its zero position. it acts as the prime mover for the deflection of the pointer.

Controlling torque:

1) it produces a force equal and opposite to the deflecting force in order to make the deflection of a pointer at a definite magnitude. If this system is absent then the pointer will swing beyond its final position for the given magnitude and deflection will become indefinite.

2) It brings the moving system back to zero position when the force which causes the movement of the moving system is removed. It will never come back to its zero position in the absence of the controlling system.

There are 2 types of control systems.

1) Gravity control:

This type of control consists of a small weight attached to the moving system whose position is adjustable. This weight produces a controlling torque due to gravity. This weight is called control weight. 

As shown in fig at the zero position of the pointer The controlling torque is zero. if the system deflects, the weight position also changes. The system deflects through an angle Theta, The control weight acts at a distance l from the center. The sine component is responsible to bring back the pointer to the initial position. this is called controlling torque.

2) Spring control:

Difference between Gravity control and spring control:

Electrical Measurements Theory of Indicating instruments

Indicating instruments: The principle, different types of control and damping arrangements in indicating instruments, Permanent Magnet Moving Coil (PMMC), Moving Iron (MI), electrostatic and dynamometer type meters, errors in indicating instruments, the extension of instrument range for ammeters and voltmeters. 

Introduction: 

Measurement: The measurement of a given quantity is the result of comparison between the quantity to be measured and a definite standard. Th instruments which are used for such measurements are called measuring instruments. 

The 3 basic quantities in the electrical measurement are 

Current (Measured by Ammeter) 

Voltage (Measured by Voltmeter 

Power (Measured by wattmeter (Power meter)) 

The necessary requirement of a measuring instrument: 1) With the introduction of the instrument in the circuit, the circuit conditions should not get altered and thus the quantity to be measured should not get affected due to the instrument used. 2) The power consumed by the instruments for their operation should be as small as possible. 

Classification of measuring instruments:

Primary instruments: These are the instruments mainly used for standardization of the secondary instruments and examples are like tangent galvanometer, bourdon tube, rayleighs galvanometer etc. 

Secondary instruments: 

These are mainly classified as 

1. Indicating instruments: These instruments make use of dial and pointer for showing or indicating the quantity for ex: Ammeters and Voltmeters. 

2. Recording instruments: These instruments give a continuous record of electrical quantity over a period of time ex: ECG, EEG, X-Y PLOTTER 

3. Integrating instruments: These instruments measure the total quantity of electricity delivered over a period of time. like a household, energy meter registers a number of revolutions made by the disc to give the total energy delivered with the help of counting mechanism. 

An essential requirement of an instrument: In case of measuring instruments the effect of an unknown quantity is converted into a mechanical force which is transmitted to the pointer which moves over a calibrated scale. for satisfactory operation of any indicating instruments following system must be present in an instrument. 

1) The deflecting system producing deflecting torque: Td 

2) The controlling system producing controlling torque: Tc 

3) The damping system producing damping torque: TD 

Flemings Left-hand rule: When current flows through a conducting wire, and an external magnetic field is applied across that flow, the conducting wire experiences a force perpendicular both to that field and to the direction of the current flow (i.e they are mutually perpendicular).

   

Flemings Right-hand rule: Fleming's Right-hand Rule (for generators) shows the direction of induced current when a conductor attached to a circuit moves in a magnetic field. The right hand is held with the thumb, index finger and middle finger mutually perpendicular to each other (at right angles), as shown in the diagram.

So basically for motoring action and to produce thrust on the conductor Flemings left-hand rule is used.