1. CHAPTER:2 Measuring Instruments
PMMC Type Instruments
Moving Type Instruments
Electrodynamometer Type Instruments
Electrostatic Type Instruments
Rectifier Type Instrument
Induction Type Energy Meter

2. Electrodynamometer Type Instruments

3. Electrodynamometer Type Instruments
The necessity for the A.C. calibration of moving iron instruments as well as other types of instruments, which cannot be correctly calibrated, requires the use of a transfer type of instrument.
A transfer instrument is one that may be calibrated with a D.C. Source and then used without modification to measure A.C.
This requires the transfer type instrument to have same accuracy for both D.C. and A.C., which the electrodynamometer instruments have.
These standards are precision resistors and the Weston standard cell (which is a D.C. cell).
It is obvious, therefore, that it would be impossible to calibrate an A.C. instrument directly against the fundamental standards.
Electrodynamics instruments are capable of service as transfer instruments.

4. Electrodynamometer Type Instruments Working:
Electrodynamic type instruments are similar to the PMMC-type elements except that the magnet is replaced by two serially connected fixed coils that produce the magnetic field when energized.
The fixed coils are spaced far enough apart to allow passage of the shaft of the movable coil. The movable coil carries a pointer, which is balanced by counter weights. Its rotation is controlled by springs.
The motor torque is proportional to the product of the currents in the moving and fixed coils.
If the current is reversed, the field polarity and the polarity of the moving coil reverse at the same time, and the turning force continues in the original direction. Since the reversing the current direction does not reverse the turning force, this type of instruments can be used to measure AC or DC current, voltage, or its major application as a wattmeter for power measurement.

5. Electrodynamometer Type Instruments Working Principle:
Electro-dynamometer type instruments are very similar to PMMC type instrument in which the operating field is produced, not by a permanent magnet but by another fixed coil (usually two fixed air cored coils are used).
The PMMC instrument cannot be used on A.C currents or voltages. If A.C supply is given to these instruments, an alternating torque will be developed. Due to moment of inertia of the moving system, the pointer will not follow the rapidly changing alternating torque and will fail to show any reading.
In order that the instrument should be able to read A.C quantities, the magnetic field in the air gap must change along with the change in current. This principle is used in the electro-dynamometer type instrument.

6. Electrodynamometer Type Instruments Construction:
Fixed Coils:
The field is produced by a fixed coil.
This coil is divided into two sections to give a more uniform field near the centre and to allow passage of the instrument shaft.
Fixed coils are usually wound with heavy wire carrying the main current in ammeters and watt meters.
The wire is stranded where necessary to reduce eddy current losses in conductors.
Moving Coil:
A single element instrument has one moving coil.
The moving coil is wound either as a self-sustaining coil or else on a non-metallic former.
A metallic former cannot be used as eddy current would be induced in it by the alternating field.
It should be noted that both fixed and moving coils are air cored.

7. Electrodynamometer Type Instruments Construction:
Control:
The controlling torque is provided by two control springs.
These springs act as leads to the moving coil.
Moving System:
The moving coil is mounted on an aluminum spindle.
The moving system also carries the counter weights and truss type pointer.
Sometimes a suspension may be used in case a high sensitivity is desired.
Damping:
Air friction damping is employed for these instruments and is provided by a pair of aluminum vanes, attached to the spindle at the bottom.
These vanes move in sector shaped chambers.

8. Electrodynamometer Type Instruments Construction:
Damping:
Eddy current damping cannot be used in these instruments as the operating field is very weak (on account of the fact that the coils are air cored) and any introduction of a permanent magnet required for eddy current damping would distort the operating magnetic field of the instrument.
Shielding:
The field produced by the fixed coils is somewhat weaker than in other types of instruments.
It is nearly to Wb/m.
In D.C. Measurements even the earth magnetic field may affect the readings.
Thus it is necessary to shield an electrodynamometer type instrument from the effect of stray magnetic fields.
Air cored electrodynamometer type instruments are protected against external magnetic fields by enclosing them in a casing of high permeability alloy.

9. Electrodynamometer Type Instruments Construction:
Shielding:
This shunts external magnetic fields around the instrument mechanism and minimizes their effects on the indication.
Cases and Scales:
Laboratory standard instruments are usually contained in highly polished wooden cases.
These cases are so constructed as to remain dimensionally stable over long periods of time.
The glass is coated with some conducting material to completely remove the electrostatic effects.
Adjustable leveling screws support the case.
A spirit level is also provided to ensure proper leveling.
The scales are hand drawn, using machine sub-dividing equipment.

10. Electrostatic Type Instruments
Working Principle of Electrostatic Type
Instruments:
As the name suggests the electrostatic type instrument use static electrical field to produce the deflecting torque.
These types of instrument are generally used for the measurement of high voltages but in some cases they can be used in measuring the lower voltages and powers of a given circuit.
Now there are two possible ways in which the electrostatic force can act.

11. Electrostatic Type Instruments Construction of Electrostatic Type Instruments:
When one of the plates is fixed and other plate is free to move, plates are oppositely charged in order to have attractive force between them. Now due this attractive force movable plate will move towards the stationary or fixed plate till the moving plate stored maximum electrostatic energy
In other arrangement there may be force of attraction or repulsion or both, due to some rotary of plate.

12. Electrostatic Type Instruments Force and Torque Equations

13. Electrostatic Type Instruments Force and Torque Equations

14. Electrostatic Type Instruments Force and Torque Equations

15. Rectifier Type Instrument
Rectifier type instrument measures the alternating voltage and current with the help of rectifying elements and permanent magnet moving coil type of instruments
Now one question must arises in our mind why we use rectifier type of instruments widely in the industrial world though we have various other AC voltmeter like electrodynamometer type instruments, thermocouple type instruments etc?
The answer to this question is very simple and is written as follows.
Cost of electrodynamometer type of instruments is quite high than rectifier type of instruments. However rectifier type of instruments as much accurate as electrodynamometer type of instruments. So rectifier type of instruments are preferred over electrodynamometer type instruments.
The thermocouple instruments are more delicate than the rectifier type of instruments. However thermocouple type of instruments is more widely used at very high frequencies.

16. Rectifier Type Instrument Half Wave Rectifier Circuits of Rectifier Type Instruments
Let us consider a circuit given in which the rectifying element is connected in series with sinusoidal voltage source, permanent magnet moving coil instrument and the multiplier resistor.
The function of this multiplier electrical resistance is to limit the current drawn by the permanent magnet moving coil type of instrument. It is very essential to limit the current drawn by the permanent magnet moving coil instrument because if the current exceeds the current rating of PMMC then it destructs the instrument.
Now here we divide our operation in two parts.
In first part we apply constant DC voltage to the above circuit. In the circuit diagram we are assuming the rectifying element as ideal one. Let us mark the resistance of multiplier be R, and that of permanent magnet moving coil instrument be R1.The DC voltage produces a full scale deflection of magnitude I=V/(R+R1) where V is root mean square value of voltage.
Now let us consider second case, in this case we will apply AC sinusoidal AC voltage to the circuit V =Vm × sin(wt) and we will get the output waveform as shown.

17. Rectifier Type Instrument Half Wave Rectifier Circuits of Rectifier Type Instruments
In the positive half cycle the rectifying element will conduct and in the negative half cycle it does not conduct. So we will get a pulse of voltage at moving coil instrument which produces pulsating current thus pulsating current will produce pulsating torque.
The deflection produced will corresponds to the average value of voltage. So let us calculate the average value of electric current, in order to calculate the average value of voltage we have integrate the instantaneous expression of the voltage from 0 to 2 pi.
So the calculated average value of voltage comes out to be 0.45V. Again we have V is root mean square value of current. Thus we conclude that the sensitivity of the ac input is 0.45 times the sensitivity of DC input in case of half wave rectifier.

18. Rectifier Type Instrument Full Wave Rectifier Circuits of Rectifier Type Instruments
We have used here a bridge rectifier circuit as shown. Again we divide our operation into two parts.
In the first we analyze the output by applying the DC voltage and in another we will apply AC voltage to the circuit.
Let us consider first case here we applying DC voltage source to the circuit. Now the value of full scale deflection current in this case is again V/(R+R1), where V is the root mean square value of the applied voltage, R is the resistance of the resistance multiplier and R1 which is the electrical resistance of the instrument.
Now let us consider second case, in this case we will apply ac sinusoidal voltage to the circuit which is given v=Vmsin(wt) where Vm is the peak value of the applied voltage again if we calculate the value of full scale deflection current in this case by applying the similar procedure then we will get an expression of full scale current as 0.9V/(R+R1).
Remember in order to obtain the average value of voltage we should integrate the instantaneous expression of voltage from zero to pi . Thus comparing it DC output we conclude that the sensitivity with ac input voltage source is 0.9 times the as in the case of DC input voltage source.

19. Induction Type Instrument
In all induction meters we have two fluxes which are produced by two different alternating currents on a metallic disc. Due to alternating fluxes there is an induced emf, the emf produced at one point (as shown in the figure given below) interacts with the alternating current of the other side resulting in the production of torque.
Similarly, the emf produced at the point two interacts with the alternating current at point one, resulting in the production of torque again but in opposite direction. Hence due to these two torques which are in different directions, the metallic disc moves.
This is basic principle of working of an induction type meters. Now let us derive the mathematical expression for deflecting torque. Let us take flux produced at point one be equal to F1 and the flux and at point two be equal to F2. Now the instantaneous values of these two flux can written as:

20. Induction Type Instrument

21. Induction Type Instrument
Which is known as the general expression for the deflecting torque in the induction type meters. Now there are two types of induction meters and they are written as follows:
Single Phase Induction type Energy meter
Three phase Induction type Energy meter.

22. Induction Type Instruments Single Phase Induction Type Energy Meter: Working
Here we have assumed that the pressure coil is highly inductive in nature and consists of very large number of turns. The current flow in the pressure coil is Ip which lags behind voltage by an angle of 90 degrees. This current produces flux F. F is divided into two parts Fg and Fp.
Fg:which moves on the small reluctance part across the side gaps. And Fp: It is responsible for the production of driving torque in the aluminium disc.
It moves from high reluctance path and is in phase with the current in the pressure coil. Fp is alternating in nature and thus emf Ep and current Ip.

23. Induction Type Instruments Single Phase Induction Type Energy Meter: Working
The load current which is shown in the above diagram is flowing through the current coil produces flux in the aluminium disc, and due this alternating flux there on the metallic disc, an eddy current is produced which interacts with the flux Fp which results in production of torque.
As we have two poles, thus two torques are produced which are opposite to each other. Hence from the theory of induction meter that, the net torque is the difference of the two torques.

24. Induction Type Instruments Single Phase Induction Type Energy Meter Construction
Single phase induction type energy meter consists of four important systems which are written as follows:
Driving System:
Driving system consists of two electromagnets on which pressure coil and current coils are wounded, as shown above in the diagram.
The coil which consisted of load current is called current coil while coil which is in parallel with the supply voltage (i.e. voltage across the coil is same as the supply voltage) is called pressure coil.
Shading bands are wounded on as shown above in the diagram so as to make angle between the flux and and applied voltage equal to 90 degrees.

25. Induction Type Instruments Single Phase Induction Type Energy Meter Construction
Moving System:
In order to reduce friction to greater extent floating shaft energy meter is used, the friction is reduced to greater extinct because the rotating disc which is made up of very light material like aluminium is not in contact with any of the surface.
It floats in the air. One question must be arise in our mind is that how the aluminium disc float in the air?
To answer this question we need to see the constructional details of this special disc, actually it consists of small magnets on both upper and lower surfaces.
The upper magnet is attracted to an electromagnet in upper bearing while the lower surface magnet also attracts towards the lower bearing magnet, hence due to these opposite forces the light rotating aluminium disc floats.

26. Induction Type Instruments Single Phase Induction Type Energy Meter Construction
Braking System:
A permanent magnet is used to produce breaking torque in single phase induction energy meters which are positioned near the corner of the aluminium disc.
Counting System:
Numbers marked on the meter are proportion to the revolutions made by the aluminium disc, the main function of this system is to record the number of revolutions made by the aluminium disc.

27. Induction Type Instruments Three Phase Induction Type Energy Meter