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					Measuring Instruments
Definition:
• The instruments which are used to measure
  electrical quantities are called Electrical
  Instruments.

 Example:      Ammeter
               Voltmeter
               Energy meter
Classification:
  Absolute Instruments: Give the value of the
  quantity to be measured in terms of constants
  of the instrument.
  Example: Tangent Galvanometer

  Secondary Instruments: Determine the
  electrical quantity to be measured directly in
  terms of deflection.
TANGENT GALVONOMETER
 Secondary Instruments:
1. Indicating Instruments: Indicate the magnitude of
   electrical quantity being measured instantaneously.
   (example: ammeter, voltmeter, wattmeter)

2. Integrating Instruments: Add up the electrical quantity
   and measure in a given period of time.
   (example: Energy meter)

3. Recording Instruments: Give a continuous record of
   the variations of the electrical quantity being measured.
   (example: ECG)
Secondary instruments
Essentials of Indicating Instruments:
 Deflecting Torque

 Controlling Torque

 Damping Torque
Essentials of Indicating Instruments:
• Deflecting Torque: Required to move the
  moving system (and hence, the pointer attached
  to it) from zero position when the instrument is
  connected in the circuit to measure the electrical
  quantity.

  The Deflecting or Operating Torque (Td) is
  produced by the effects such as Magnetic,
  Electrodynamics, Electromagnetic induction etc.
Essentials of Indicating Instruments:
• Controlling Torque:
  1. Brings the pointer to zero position when
  disconnected from the circuit.
  2. Allows the pointer to deflect in accordance to
  the magnitude of electrical quantity.

  The Controlling or Restoring Torque (Tc)
  opposes the Td and increases with the deflection
  of the moving system. At rest Td = Tc
Essentials of Indicating Instruments:
• Damping Torque: Suppresses the undue
  oscillations of the pointer and brings the pointer
  to its final position quickly.
  It always acts opposite to motion.

  Damping Methods:
             i) Air Friction Damping
             ii) Fluid Friction Damping
             iii) Eddy Current Damping
Types of Instruments:
1. Moving Iron Instruments
    a) Attraction Type
    b) Repulsion Type




2. Moving coil
   a) PMMC Instruments
   b) Dynamometer Type Instruments
Attraction Type MI Instruments:
 Principle:
 When a soft iron piece or vane is placed in the
 magnetic field of a current carrying coil, it is
 attracted towards the centre of the coil.

 Thus a force is exerted on the soft iron piece
 and deflection in the needle taken place.

 Used on both DC and AC system
Principle: Graphical representation
Construction:
Working Principle:
 Td depends upon the force acting on the iron piece (F).

 Let, H = Field strength produced by the coil
      m = Pole strength of the soft iron piece

 and m α H

 Pulling force acting on the iron piece, F α mH α H2

 Now, H α I              or, F α I2               so, Td α I2
 Tc is provided by the spring, therefore Tc α θ

 In steady position of deflection; Tc = Td

 Therefore, θ α I2

 Hence, scale of the instrument is non-uniform.
Repulsion Type MI Instruments:
Principle:
Repulsive forces will act when two similarly
magnetized iron pieces are placed near to each
other.

Used on both DC and AC system
Construction:
Working Principle:
 Td depends upon the repulsive force between the similarly magnetized iron
 pieces (F).

 Let, H = Field strength produced by the coil
      m1 = Pole strength of the fixed iron piece; (m1 α H)
      m2 = Pole strength of the movable iron piece; (m2 α H)

 Therefore, F α m1 m2 α H2

 Now, H α I                       or, F α I2               so, Td α I2
 Tc is provided by the spring, therefore Tc α θ

 In steady position of deflection; Tc α Td

 Therefore, θ α I2

 Hence, scale of the instrument is non-uniform.
Advantages of MI Instruments:

• Cheaper, robust, simple.

• Used on both AC & DC.

• Reasonably accurate.

• High operating torque.

• Withstand overloads.
 Disadvantages of MI Instruments:

• Can not be calibrated with a high degree of precision
  with DC on account of the effect of hysteresis.

• Non-uniform scale; crowded at the beginning, difficult to
  get accurate readings at this end.

• Not very sensitive.

• Power consumption is quite high.

• Errors due to change in frequency in case of AC.
PMMC Instruments:
 Principle: When a current carrying conductor is
 placed in a magnetic field, a mechanical force is
 exerted on the conductor.
PMMC Instruments:
 Principle (another approach): When a field Fr produced by the
 movable current carrying coil tries to come in line with the main
 field, a deflecting torque is developed. Due to the production of
 deflecting torque, the pointer deflects over the scale.
Construction:
Working Principle:
 Td depends upon the force acting on the coil sides (F).

 Let, B = Flux density in Wb/m2 in the air gap.
      l = Effective length of coil side in metre.
      N = No. of turns of the coil.
      r = Distance in metre between centre of the coil and force.
      I = Current flowing through the coil in ampere.

 Force acting on each coil side, F = B.l.I.N Newton

 Deflecting Torque, Td = 2. F. r = 2. B.l.I.N. r Newton-metre

 Since all other quantities are constant except I; Hence, Td α I

 Tc is provided by the springs, therefore Tc α θ           (where θ is angle of deflection)

 In steady position of deflection; Tc α Td

 Therefore, θ α I

 Hence, scale of the instrument is uniform. It can only be used on DC system.
  Advantages of PMMC Instruments:

• Uniform scale.
• Eddy current damping, so very effective & reliable.
• No hysteresis loss.
• Low power consumption as the driving power is small.
• No effect of stray magnetic field.
• High torque/weight ratio, require small operating current.
• Very accurate & reliable.
  Disadvantages of PMMC Instruments:

• Can not be used for AC measurement.

• Costlier in comparison to moving iron instruments.

• Friction & temperature might introduce some error.

• Errors due to ageing of control springs & permanent
  magnets.
Dynamometer type Wattmeter:
Principle: When a current carrying moving coil is placed
in a magnetic field produced by the current carrying fixed
coil, a mechanical force is exerted on the coil sides of the
moving coil and deflection takes place.

In other words, when the field produced by the current
carrying moving coil (Fr) tries to come in line with the
field produced by the current carrying fixed coil (Fm), a
deflecting torque is exerted on the moving system.

Used on both DC and AC system
Principle:
Construction:
Working Principle: In case of DC system

 Let,        V = Voltage across the load.
 &           I = Load current.

 Current through the fixed coil, I1 α I

 Current through the moving coil, I2 α V

 Since coils are air-cored, B α I1

 Deflecting Torque, Td       α       B I2
                             α       I1 I 2
                             α       IV
                             α       Power

 Hence, deflecting torque is proportional to power.
Working Principle: In case of AC system
   Let, e = Instantaneous voltage across the load.
        i = Instantaneous load current.
        V = RMS value of voltage across the load.
        I = RMS value of load current.
        cos Φ = Power factor (lagging) of the load.
   Now, e = Vm sin ωt;       i = Im sin (ωt – Φ)
   Instantaneous current through the fixed coil, i1 α i
   Instantaneous current through the moving coil, i2 α e
   Average deflecting Torque, Td α                 Average of (i2 * i1)
                                                   α       Average of (e * i)
                                                   α       VI cos Φ (True Power)

In steady position of deflection Tc ( α θ ) = Td           (where θ is angle of deflection)

Hence, θ α Power
Advantages of Dynamometer type Instruments:
• Can be used on both DC & AC circuits.
• Uniform scale.
• High degree of accuracy can be obtained.



Disadvantages of Dynamometer type Instruments:
• At low power factors, the inductances of the potential coil
  causes serious errors.
• The reading of the instrument may be affected by stray fields
  acting on the moving coil.
Any Questions ?
Thank You

				
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