Types of instruments

ELECTRICAL AND ELECTRONICS MEASUREMENT

1. Types of Instruments:- Permanent Magnet Moving Coil Instrument (PMMC ) Moving Iron Instrument Electrodynamic Instruments Hot Wire Instruments Thermocouple Instruments Induction Type Instruments Electrostatic Instruments Rectifier Type Instruments

2. Permanent Magnet Moving Coil (PMMC) Instruments  

3. Working Principle:- Working Principle : Refer to fig. (a) and (b), A torque is exerted on a current carrying coil placed in the field of a permanent magnet. The coil ‘C’ has a number of turns of thin insulated wire wound on a rectangular Aluminium former ‘F’. The former is carried on a spindle S mounted in jewel bearings J 1 and J 2 . Pointer PR is attached to the spindle to move on the scale

4. Deflecting Torque :-

5. Control Torque: The control torque of spring is proportional to the deflection angle  . Thus the scale of PMMC instruments are linear or Uniform

6. Advantages:- Sensitivity is very high Very accurate and reliable Uniform scale up to 270 or more Very effective damping Low power consumption Easily converted into multi range instrument Free from Hysterisis error and not affected by external magnetic field because its permanent magnet shield the coil from external magnetic field.

7. Disadvantages:- It can operate only on direct current because. If the current reverses, deflection also reverses. So for AC supply the deflection alternates and remains at zero position. The moving system is very delicate and can easily be damaged by rough handling. Very fine wire is used in coil so cannot withstand overloading. PMMC instruments are costlier than moving iron instruments

8. Applications:- DC Ammeter DC voltmeter Multimeter Flux meter DC Galvanometer

9. Moving Iron Instrument:-

10.  Scale is not uniform.

11. Moving iron instruments are of two types (a) Attraction type (b) Repulsion type.

12. Attraction type

13. The moving iron is a eccentrically mounted soft iron piece. When the current flows through the coil, a magnetic field is produced Moving iron moves from the weaker field (out side the coil) to stronger field (inside the coil. In other words, the coil attracts moving iron.

14. The controlling torque is provided by the springs. The gravity control can be used for panel type instruments, which are vertically mounted. Generally air friction damping is provided in these type of instruments.

15. Repulsion Type

16. In the repulsion type, there are two vanes inside the coil, one is fixed and other is movable. These vanes are magnetized when the current flows through the coil and there is a force of repulsion between the two vanes resulting in the movement of the moving vane

17. Advantages of moving iron instruments:- It can be used for both AC and DC Robust in construction and relatively cheap Can withstand overload momentarily

18. Disadvantages:- Non uniform scale Affected by stray magnetic field It consumes large amount of power due to I 2 R loss There is magnetic losses in the vanes, so these instruments are not very sensitive

19. Applications:- Voltmeter, Ammeter, Power factor meter. They are suitable for measurement of low frequency high power circuits

20. Electrodynamic Type Instruments

21. The Electrodynamic instrument has a moving coil, but the magnetic field, in which the coil moves is provided by two fixed coils ‘CC’ rather than by permanent magnets. The current in fixed coil produce the uniform magnetic field

22. Control:- The control torque is provided by two springs. Moving System:- The moving coil mounted on an Aluminum spindle. Damping:- Air friction damping Applications:- They are used in Ammeter, Voltmeter, Wattmeter, Power factor meter etc.

23. Thermocouple Type Instruments:-

24. The electrical current flowing in the heating element produces the heat at the junction. The thermocouple produces an emf at its output terminal. This emf can be measured with the help of a PMMC instrument. The emf produced is proportional to the temperature and rms value of the current.

25. Hence the instrument shows a square law response. These instruments can be used for both AC and DC .

26. Advantages:- These instruments indicate the rms value of voltage and current. These instruments are not affected by stray magnetic field. These instruments are free from frequency error and can be used for large frequency range. Very high sensitivity. Disadvantages:- The instrument follow square law Low efficiency due to power loss in thermal conversion

27. Hot Wire Instruments:-

28. When the current is passed through the fine wire, it gets heated and expands. The sag of wire is magnified and the expansion is taken by the spring. This causes the pulley to rotate the pointer to deflect and indicate the value of current. The heating effect is proportional to square of rms value of current.

29. Electrostatic Instruments:-

30. In this type of instruments the deflecting torque is produced by electric field. They are very useful for high voltage measurement in laboratories. Two charged electrodes are used in the instrument.   Linear Motion Type:- One plate ‘A’ is fixed and plate ‘B’ is movable. Plates have opposite charges and restrained by a spring, connected with movable plate.

31. Rectifier Type Instruments:-

32. These instruments are used for measurement of AC voltage and current after converting it into DC. Rectifier is used to convert AC into DC. In these instruments a Permanent Magnet Moving Coil instrument is used as indicating device because they have very high sensitivity.

33. Induction Type Instrument

34. The operation of these instruments is based on the production of torque due to reaction between flux produced by electromagnet and eddy current induced in a metallic disc.