L41 instruments
- 1. Dept of E&E ELE101/102 1 Measuring Instruments
- 2. Dept of E&E ELE101/102 2 Measuring Instruments To monitor the operating variables that define the condition of the supply system. To govern the Quality of electric energy supplied. For the purpose of billing the customers.
- 3. Dept of E&E ELE101/102 3 Classification of Instruments Analog instruments: Mechanical representation is used as the analogue of an electrical quantity. Relatively cheaper Accuracy is less Digital instruments: Used where accuracy is required. Analog input is converted to BCD. Used in Electronic and communication circuits
- 4. Dept of E&E ELE101/102 4 Essential features of measuring Instruments Deflecting system: The deflection produced by the operating torque is proportional to the magnitude of the electrical quantity such as current, voltage, etc. being measured. The deflecting torque causes the moving mechanism to move from its initial zero position. Controlling system: Torque produced by the controlling system is in opposition to the deflecting torque. Pointer comes to rest when deflecting and controlling torques are equal. Damping system: To minimize the oscillations in the deflecting system. Air friction damping, Eddy current damping
- 5. Dept of E&E ELE101/102 5 Principle: A coil which carries the operating current is attached to the moving system. With the movement of the coil, the pointer moves over the scale to indicate the electrical quantity to be measured. Permanent magnet Moving coil (PMMC) Instruments Construction: •Rectangular coils of several turns -aluminum former •Coil is pivoted •Controlling system: two phosphor bronze hair springs •Eddy current damping Counter weight Scale Pointer
- 6. Dept of E&E ELE101/102 6 Working of PMMC Mechanical force experienced by the coil F = NBIL newton Deflecting Torque Td α I Controlling torque Tc α θ At equilibrium Td = Tc Therefore, θ α I Hence, Scale is Uniform (Linear)
- 7. Dept of E&E ELE101/102 7 •High sensitivity •Scale is uniformly divided. •The power consumption is very low. •Not affected by stray magnetic fields. Permanent magnet Moving coil (PMMC) Instruments •Cannot be used for ac. •Cost is higher. Disadvantages: Advantages:
- 8. Dept of E&E ELE101/102 8 Ammeter shunts Shunt is a very low resistance connected across the basic meter. Rm = internal resistance of the basic meter. Rsh = Resistance of the shunt Im = full scale deflection of basic meter. I = Current to be measured. Extension of meter range: I ImIsh Basic metershunt Rm Rsh m mm sh II RI R − = mmshm RIRII =− )(
- 9. Dept of E&E ELE101/102 9 Voltmeter multipliers Multiplier is a very high resistance in series with the basic meter. Rm = internal resistance of the basic meter. Rs = Resistance of the multiplier. Im = full scale deflection of basic meter. v= Voltage across the meter for current Im V = Full range voltage of instrument. V v Rs Rm Im m mm s I RIV R − = Extension of meter range:
- 10. Dept of E&E ELE101/102 10 A permanent magnet moving coil instrument gives full scale reading of 25mA when potential difference across the terminals is 75mV. Show how can it be used as (i)an ammeter for a range of 0-100A (ii)a voltmeter for a range of 0-750V. Ans: 0.00075Ω, 29997Ω Illustration: