Electrical Measurements and Instrumentation Full course on slide share

1. Analog Electromechanical
Instruments
Designing a Galvanometer and converting to Ammeter, Voltmeter
And ohmmeter
EE-336 Electrical Measurements and Instrumentation

2. Contents
• Analog Instrument
• Electro-magentic
• Elecro-thermal
• Principle of Operation
• Operating Torques
• Controlling
• Deflecting
• Damping
• Constructional Details
• Suspension
• Taut suspension
• Pivot and Jewel Bearings
• Torque to Weight Ratio
• Controlling systems
• Spring control
• Gravity control
• Damping Systems
• Permanent Magnet Moving Coil
Instruments
• Principle of Operation
• Deflecting Torque equation
• Swamping Resistor
• Advantages and Disadvantages
of PMMCs
• Solved Examples
• Extending the range of PMMC
• Ammeter
• Voltmeter
• Ohmmeter
• Electrodynamometer Type
Instruments

3. Analog Instrument
• An analog device is one in which the output or display is a continuous
function of time and bears a constant relation to its input
1. Electromagnetic, which utilizes the magnetic effects of electric currents
2. Electrostatic, which utilizes the forces between electrically charged
conductors
3. Electro-thermal, which utilizes the heating effect
The effect of the heat produced by a current in a conductor is used in thermocouple and
hotwire instruments. Electrostatic effect is used in electrostatic voltmeters. The
electromagnetic induction effect is used in induction watt-meters and induction energy
meters.

4. Principle of Operation
• Magnetic effect
• Heating effect
• Electrostatic effect
• Electromagnetic effect
• Hall effect

5. Operating Torques
• Any instrument’s deflection is found by the total effect of the deflecting torque/force, control
torque/ force and damping torque/force.
• Three types of torques are needed for satisfactory operation of any indicating instrument.
These are
• Deflecting torque
• Controlling torque
• Damping torque

6. Deflecting Torque
• Responsible to move needles to the reading position
• Responsible to move discs inside rotational counters

7. Controlling Torque
• The act of this torque/force is opposite to the deflecting torque/force.
• The functions of the controlling system are
• To produce a torque equal and opposite to the deflecting torque at the final
steady position of the pointer in order to make the deflection of the pointer
definite for a particular magnitude of current
• To bring the moving system back to its zero position when the force causing
the instrument moving system to deflect is removed

8. Damping Torque
• A damping force generally works in an
opposite direction to the movement of
the moving system.
• It is opposite to both deflecting and
controlling torque

9. Constructional Details in moving system
• The moving system should have the following properties:
• The moving parts should be light
• The frictional force should be minimum
• Because of low power levels, the considerations of various methods
of supporting the moving elements becomes of vital importance.
The moving system can be made light by using Aluminum
as far as possible.
The frictional forces are reduced by using spindle-
mounted jewel bearings and by carefully balancing the
system

10. Suspension
• It consist of a fine, ribbon-shaped metal filament for the upper
suspension and a coil of fine wire for the lower part.
• The ribbon is made of a spring material like beryllium copper
or phosphor bronze. This coiling of lower part of suspension is
done in order to give negligible restrain on the moving system.
• The type of suspension requires careful leveling of the
instrument, so that the moving system hangs in correct
vertical position.
• This construction is, therefore, not suited to field use and is
employed only in those laboratory applications in which very
great sensitivity is required. In order to prevent shocks to the
suspension during transit, etc., a clamping arrangement is
employed for supporting the moving system

11. Taut Suspension
• Suspension and taut suspension are customarily used in
instruments of galvanometer class which requires a low
friction and high sensitivity mechanism.
• A suspension type of instrument can only be used in vertical
position. The taut suspension has a flat ribbon suspension
both above and below the moving element, with suspension
kept under tension by a spring arrangement
• Ribbon suspension, in addition to supporting the moving
element, exerts a controlling torque when twisted.

12. Pivot and Jewel Bearing
• The moving system is mounted on a spindle
made of hardened steel. The two ends of the
spindle are made conical and then polished to
form pivots.
• These ends fit conical holes in jewels located in
the fixed part of instruments.
• These jewels, which are preferably made of
sapphire, form bearings.
It has been found that the frictional torque, for jewel
bearings, is proportional to the area of contact
between the pivot and jewel.
• Thus, the contact area between a pivot and jewel
should be small.
• The pivot is ground to a cone and its tip is rounded to
a hemispherical surface of small area. The jewel is
ground to a cone of somewhat larger angle.

13. Torque/Weight Ratio
• The frictional torque in an instrument depends upon the weight of
moving parts.
• If the weight of the moving parts is large, the frictional torque will
be large.
• Thus, the ratio of deflecting torque to frictional torque is a measure
of reliability of the instrument indications and is the inherent quality
of the design.
• Hence (deflecting) torque/weight ratio of an instrument is an index
of its performance.
• The higher the ratio, the better will be its performance.
• In Other words, the needles and shaft should be light.

14. Controlling System
• The controlling torque is provided by a spring or sometimes by
gravity.

15. Comparison of Spring and Gravity Control
Thus, a gravity-controlled instrument would have a scale which is ‘cramped’ at its lower end instead of being uniformly
divided, though the deflecting torque is directly proportional to the quantity to be measured.

16. Damping System
• Air Friction
• Fluid Friction
• Eddy current Damping

17. Permanent Magnet Moving Coil (PMMC)
instrument
• The principle on which a Permanent Magnet Moving
Coil (PMMC) instrument operates is that a torque is
exerted on a current-carrying coil placed in the field
of a permanent magnet

18. Deflecting Torque of PMMC

19. Controlling Torque of PMMC
The control on the movement of the pointer over the scale
is provided by two spirally wound, phosphor-bronze
springs S1 and S2, one at each end of the spindle S.

20. Swamping Resistor
• The coil of the instrument is made of copper. Its resistance varies with temperature.
• A resistor of low temperature coefficients, called the swamping resistor, is connected in series with the coil.
• Its resistance practically remains constant with temperature.
• Hence the effect of temperature on coil resistance is swamped by this resistor.

21. Advantages and Disadvantages of PMMCs

22. Solved Examples
T=BINA
(A=l x b)

23. Solved Examples

24. Solved Examples

25. Extending the range of PMMC
• A galvanometer or a simple PMMC is used for the DETECTION of current and only its
presence by the help of a fine needle suspension in the vicinity of an Permanent Magnet.
• If we want it to not just detect but also to measures values beyond its range of detection,
one must add some extra resistive components to it.
• An ammeter is used for measuring current therefore the PMMC current should be
reduced by connecting it with a parallel low Resistance.
• This means that the same current will now flow to the same mechanism but its
effectiveness will be reduced by using a parallel low shunt resistance.
• A voltmeter detects the voltage across two terminals so if we connect a galvanometer
across those terminals, it is quiet possible that all the current will be passing through this
galvanometer as it has very minute resistance.
• For this purpose, we use a series resistance with the galvanometer so that when the
whole mechanism is connected in parallel to the terminals across which we want to
calculate voltage, the current should not just flow through this path and a series high
resistance stops it from entry.

26. PMMC to Ammeter

27. PMMC to Voltmeter

28. Sensitivity of PMMC Instrument

29. Solved Examples

30. Solved Examples

31. Solved Examples

32. PMMC to Ohmmeter
• External Voltage is used for excitation as we need I and V for
calculating resistance R and that are possible only if electrical energy
is supplied.
• The scale is always reverse because an open circuit will give 0o
deflection and its value will be ∞Ω
• The Adjustment Resistor is first used to get the full scale reading of
the ohmmeter on a short circuit. And the value is set to 0 Ω (full
scale deflection)
• Equal Rrange and Rout are connected giving half scale deflection which
is properly marked on the scale for the known resistance.
• The resistance is again doubled and another point is marked and the
process continuous till we get the graduated nonlinear scale of ohm
meter.
• The Range switch is used if we want to measure the multiples of the
output resistance connected across + and – terminals as shown.
• Further Description available here.

33. Electro Dynamometer Type Instruments
• The electrodynamometer is a transfer-type instrument.
• A transfer-type instrument is one that may be calibrated with a dc
source and then used without modification to measure ac.
• This requires the transfer type instruments to have same accuracy for
both dc and ac.

34. Thanks