04-01-2013, 03:56 PM
Measuring Instruments
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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 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 (Td) is
produced by the effects such as Magnetic,
Electrodynamics, Electromagnetic induction etc.
• 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
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.
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.
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 to the recording mechanism so that
electrical energy consumed in the circuit is directly registered in kWh.