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.
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 Ins Instruments: Give a continuous record of the variations of the electrical quantity being measured. (example: ECG)
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 (T (T d ) 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. (T c) The Controlling or Restoring Torque (T opposes the Td and increases with the deflection of the moving system. At rest T d = T c
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
Measuring Errors: • Absolute Error (ε0 ): Is the difference between the measured value (A m) and the actual or true value (A) of the unknown quantity. ε0 = δA = Am – A
• Relative Error (εr): Is the ratio of absolute error to the true value of quantity. εr = δA / A = ε 0 / A
Common Errors: 1. Temperature error: The rise in temperature occurs due to heating effect of current flowing through the operating coil. This causes change of resistance of the working coil & resistance of the other resistors connected in the instrument. Consequently, an erroneous reading is obtained while measuring an electrical quantity. This can be reduced by providing sufficient ventilation & cooling. 2. Friction error: When deflecting torque occurs, the spindle of the moving system pivoted in the jewel bearing rotates causing friction which affects the instrument reading. It is minimized by designing a light weight moving system. 3. Observational error: This is usually due to misreading of the scale. It can be minimized by placing parallax mirror with the scale.
Types of Instruments: 1. Movin Moving g Iron Iron Inst Instru rume ment ntss a) Attr Attrac acti tion on Type Type b) Repuls Repulsion ion Type Type
2. PMMC PMMC Ins Instr trum umen ents ts
3. Dynamom Dynamomete eterr Type Type Instrum Instrument entss
4. Inducti Induction on Type Type Instrum Instrument entss
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 (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 H 2 Now, H α I or, F α I 2
so, Td α I 2
Tc is provided by the spring, therefore Tc α θ In steady position of deflection; Tc = Td
Therefore, θ α I 2 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 (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 H 2 Now, H α I or, F α I 2
so, Td α I 2
Tc is provided by the spring, therefore Tc α θ In steady position of deflection; Tc α Td
Therefore, θ α I 2 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 F r 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 (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, T d = 2. F . r r = = 2. B 2. B..l I . I N . N . r Newton-metre
Since all other quantities are constant except I ; Hence, T d α I Tc is provided by the springs, therefore T c α θ In steady position of deflection; T c α T d
Therefore, θ α I
(where θ is angle of deflection)
Advantages of PMMC Instruments: • Uniform scale. • Eddy current damping, so very effective & reliable. • No hysteresis loss. • Low power consumption as the driving power po wer 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 cur rent carrying fixed coil, a mechanical force is exerted on the coil sides s ides of the moving coil and deflection takes place.
In other words, words, when the field produced by the current carrying moving coil ( F r ) tries to come in line with the field produced by the current carrying fixed coil ( F m), a deflecting torque is exerted on the moving system. Used on both DC and AC system
Principle:
Construction:
Working Principle: In Principle: In case of DC system V = Voltage across the load. Let, & I = Load current.
Current through the fixed coil, I 1 α I Current through the moving coil, I 2 α V Since coils are air-cored, B α I 1 Deflecting Torque, T d α I 1 I 2 α I V α Power
α
B I 2
Hence, deflecting torque is proportional to power.
Working Principle: In 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 = V m sin ωt ;
i = I m sin (ωt – Φ)
Instantaneous current through the fixed coil, i 1 α i Instantaneous current through the moving coil, i 2 α e Average deflecting Torque, T d α
In steady position of deflection T c ( α θ ) = T d Hence, θ α Power
Average of (i 2 * i ) 1 α
Average of ( e * i )
α
VI cos Φ (True Power) (where θ is angle of deflection)
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.
Comparison between Dynamometer & Induction Type Wattmeter:
S. No. Dynamometer type Wattmeter
Induction type Wattmeter
1.
Can be used on both DC & AC.
Can be used only on AC.
2.
High degree of accuracy.
Less accurate. It is accurate only at stated frequency & temperature.
3.
Power consumption is lower.
Power consumption is higher.
4.
Has uniform scale.
Has uniform & long scale.
5.
Weaker working torque.
Stronger working torque.
6.
Wei eigh ghtt of of mo mov vin ing g sy syst stem em is lo low wer er..
Weig We igh ht of of mo movi ving ng sy sysste tem m is is hig high her er..
Induction type Single Phase Energy Meter: Principle: When AC current flows through two suitably located coils (current coil & potential coil) , they produce rotating magnetic field which is cut by the metallic disc suspended near to the coils, thus an e.m.f. is induced in the disc which circulates eddy currents in it. By the interaction of rotating magnetic field & eddy currents, electromagnetic torque is developed which causes the disc to rotate. Used on AC system only.
Construction: 1. Driving System i) Series Magnet ii) Shunt Magnet
2. Moving System 3. Breaking System 4. Recording Mechanism
Driving System: Series Magnet: It is placed below the aluminum disc and produces the magnetic field ( Φ sc), which is
proportional to & in phase with line current ( I ). ). Shunt Magnet: It is placed above the aluminum disc. It produces a field (Φ sh) proportional to the applied
voltage. This field is in phase with the current flowing through the pressure coil ( I p) but is in quadrature with the applied voltage.
Moving System: It consists of a light aluminum disc mounted on a vertical spindle. The disc is positioned in the air gap between series & shunt magnet. The spindle is supported by a cup shaped jewelled bearing at the bottom end & has a spring journal bearing at the top end. Since there is no control spring, the disc makes continuous rotation under the action of deflecting torque.
Braking System: A permanent magnet positioned near the edge of the aluminum disc, forms the braking system. When the disc moves in the field of the braking magnet, flux is cut & eddy currents are induced in the disc. The direction of induced currents is such that it opposes the rotation, thus braking torque is produced. Since the induced currents are proportional to the speed of disc ( N ( N ), ), therefore braking torque (T (T b) is proportional to the disc speed. i.e. T b α N
Recording Mechanism: The function of recording or registering mechanism is to record continuously a number on the dial which is proportional to the revolutions made by the moving system. The number of revolutions of the disc is a measure of the electrical energy passing through the meter.
Working Principle: When the energy meter is connected in the circuit, the current coil carries the load current and pressure coil carries the current proportional to the supply voltage. The magnetic field produced by the series magnet is in phase with the line current and the magnetic field produced by the shunt magnet is in quadrature with the applied voltage. Thus, a phase difference exists between the fluxes produced by the two coils. This sets up a rotating field which interacts with the disc and produces a driving torque, thus disc starts rotating. The number of revolutions made by the disc depends upon the energy passing through the meter. The spindle is geared g eared to the recording mechanism so that electrical energy consumed in the circuit is directly registered in kWh kWh..
Difference between Wattmeter & Energy meter: S. No.
Wattmeter
Energy meter
1.
It is an Indicating instrument.
It is an Integrating instrument.
2.
It measures electric power.
It measures electrical energy.
3.
It has graduated scale.
It records the electrical energy.
4.
It has controlling torque.
It has no controlling torque.
5.
It indicates the power consumed It adds-up the energy consumed by by the ckt at a particular p articular instant. the circuit in a given period.
Any Questions ?
Thank You
