PMMC Instruments

This presentation includes the operating principle of an ammeter and a voltmeter.

1. Measurement of Current and Voltage : PMMC Instruments

2. Introduction The operating principle of ammeter and voltmeter is same as for both case a deflecting torque is produced by an electric current. In case of ammeter this torque is produced by a current to be measured or by a definite fraction of it. In a voltmeter this torque is produced by a current which is proportional to the voltage to be measured. So, all the analog ammeters and voltmeters are essentially current measuring device.

3. Types of Instruments Types of instruments used as ammeters and voltmeters are: Permanent magnet moving coil (PMMC) Moving iron Electro dynamometer Hot wire Thermocouple Induction Electrostatic Rectifier

4. PMMC Instruments Principle of Operation: When a current carrying conductor is placed in a magnetic field, it experiences a force and tends to move in the direction as per Fleming’s left hand rule. Fleming left hand rule: If the first and the second finger and the thumb of the left hand are held so that they are at right angle to each other, then the thumb shows the direction of the force on the conductor, the first finger points towards the direction of the magnetic field and the second finger shows the direction of the current in the wire. Fig. Flemings left hand rule

5. Construction : Moving coil Magnet system Control Damping Pointer and scale Fig. Construction of PMMC instrument Construction of PMMC Instrument

6. Construction of PMMC Instrument [ Cntd .] Moving Coil: A coil of thin wire is mounted on an aluminum frame (spindle) positioned between the poles of a U shaped permanent. The coil is free to rotate. The current to be measured (in case of ammeter) is fed to the coil. Magnet System: The coil which carries a current, which is to be measured, moves in a strong magnetic field produced by a permanent magnet. The magnet is made up of magnetic alloys like alnico.

7. Construction of PMMC Instrument [ Cntd .] Control: It is that part of the instrument which brings into play a force called controlling force. This force opposes the deflection force and increases with the increase in the deflection of the moving system, to limit its movement. The pointer is brought to rest at a position where the two opposing forces i.e. deflection and controlling forces are equal. Types of control system: Spring control Gravity control In case of spring control system the current is fed to the coil through spiral springs which are two in numbers. Control torque is provided by the spring arrangement .

8. Construction of PMMC Instrument [ Cntd .] Damping: It is that part of the instrument which provides damping force to damp the oscillations of the pointer before come to a rest . Damping torque is produced by movement of the aluminium former moving in the magnetic field of the permanent magnet . Because of the inertia, the pointer of the instrument oscillate about its final deflected position for some time before coming to rest. This causes the waste of time in taking reading, thus damping force act as a brake to prevent the oscillations of the moving system and brings the pointer to it’s final deflected position quickly .

9. Construction of PMMC Instrument [ Cntd .] There are three types of damping instruments: 1 . Critical damp: Pointer rises quickly to its final position without oscillation. 2 . Under damp: oscillations of the system will not be completely prevented . 3 . Over damp: In this the response of the system is slow and lethargic . Fig. Oscillation of the pointer

10. Construction of PMMC Instrument [ Cntd .] Methods of damping: Air friction or pneumatic damping Eddy current or electromagnetic damping Fluid friction damping Pointer and Scale: A pointer attached to the coil over non-magnetic (aluminium) coil former deflects over a calibrated scale indicating the current in coil.

11. Forces for Electro-mechanical Movement Three forces operates in electro-mechanical movement: Deflecting force Controlling force Damping force Fig. Assembled arrangement

12. Forces for E lectro-mechanical Movement [ Cntd .] Deflecting Force: Magnetic field due to current in coil , interacts with field of magnet resulting in deflection (rotation ) of coil. Controlling Force: Spring attached to the pointer is used to balance the deflecting force. The Pointer stops where deflecting force equals to controlling force. Fig. Controlling force of spring balancing the deflecting force.

13. Forces for Electro-mechanical Movement Damping Force: Pointer and coil tend to oscillate before settling. Damping force is provided by eddy current in aluminium coil former . When aluminium coil former moves in the magnetic field of permanent magnet, line of force are cut and eddy current are set up in it. The force that exists between these current and magnetic field is always in the direction opposing the motion and therefore, provide necessary damping. The magnitude of the induce current and therefore of the damping force which is dependent on it, is directly proportional to the velocity of moving system. Fig. Pointer oscillation

14. Torque Equation: Let, l,d = length of respectively vertical and horizontal (width) side of coil, m, N = number of turns in the coil, B = f lux density, Wb /m 2 , I = current through the moving coil, A, K = spring constant, Nm/rad, = final steady state deflection, rad. Deflecting torque, , w here, a constant. Controlling torque, , w here, K spring constant. For final steady deflection, or . Current, . So, the deflection is directly proportional to the current passing through the meter.  

15. Application Fig. Application of PMMC instruments.

16. Galvanometer A Galvanometer is essentially a PMMC instrument designed to be sensitive to a very low level of currents . It has center zero scale for both direction of currents. Galvanometer are often employed to detect zero current or voltage rather than to measure current or voltage. And is referred as NULL DETECTOR. Protection from high current voltage is provided by using adjustable shunt resistor. Fig. Galvanometer

17. DC Ammeter Ammeter is current measuring instrument and is connected in series. To avoid effect on circuit current, ammeter resistance should be low. PMMC is a DC Ammeter but maximum pointer deflection is produced by very small current and its coil can bear only small currents. To measure large currents, a low value shunt resistance is connected to galvanometer, which bypass most of the current. Scale is calibrated to read current range. Fig. Ammeter

18. Swamping Resistance : Moving coil is made of thin Copper wire, whose resistance changes significantly with temperature, resulting in error in measurement. To reduce temperature effect, a resistance ( ) is connected in series to galvanometer , resulting in equivalent coil resistance as .   The resistance, is called swamping resistance. Swamping resistance is made of Manganin or Constantan which has extremely low temperature coefficient. If s wamping resistance is nine times coil resistance, then 1% change in will result in 0.1% change in equivalent coil resistance.   Fig. Swamping resistance

19. DC Voltmeter Coil current of galvanometer is proportional to voltage across the coil and scale can be calibrated to measure voltage making it a Voltmeter. Coil resistance is usually very small therefore coil voltage is very small. Voltage range is increased by connecting a series resistance with galvanometer. The series resistance is termed as multiplier . To avoid loading effect, voltmeter resistance is very high. Fig. Voltmeter

20. Ammeter Shunt Here, internal resistance of the movement resistance of the shunt full scale deflection current shunt current current to be measured Here, Voltage drops across shunt = Voltage drop across movement   Multiplying power, Resistance of shunt,               Fig. Ammeter shunt

21. Voltmeter Multipliers Here, internal resistance of the movement resistance of the multiplier full scale deflection current voltage across the movement full range voltage From the circuit,   Multiplying factor, Resistance of multiplier,     Fig. Voltmeter multipliers          

22. Advantages   The PMMC consumes less power and has great accuracy.  It has uniformly divided scale and can cover arc of 270 degree.  The PMMC has a high torque to weight ratio.  It can be modified as ammeter or voltmeter with suitable resistance. It has efficient damping characteristics and is not affected by stray magnetic field.  It produces no losses due to hysteresis.

23. Drawbacks : The moving coil instrument can only be used on D.C supply as the reversal of current produces reversal of torque on the coil. It’s very delicate and sometimes uses ac circuit with a rectifier. It’s costly as compared to moving coil iron instruments. It may show error due to loss of magnetism of permanent magnet.

24. Thanks   