10-05-2014, 10:49 AM
Study of DC-AC Measuring Instruments
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Introduction
Measuring instruments are classified according to both the quantity measured by
the instrument and the principle of operation. Three general principles of operation are
available: (i) electromagnetic, which utilizes the magnetic effects of electric currents; (ii)
electrostatic, which utilizes the forces between electrically-charged conductors; (iii)
electro-thermic, which utilizes the heating effect.
Electric measuring instruments and meters are used to indicate directly the value
of current, voltage, power or energy. In this lesson, we will consider an
electromechanical meter (input is as an electrical signal results mechanical force or
torque as an output) that can be connected with additional suitable components in order to
act as an ammeters and a voltmeter. The most common analogue instrument or meter is
the permanent magnet moving coil instrument and it is used for measuring a dc current or
voltage of a electric circuit. On the other hand, the indications of alternating current
ammeters and voltmeters must represent the RMS values of the current, or voltage,
respectively, applied to the instrument.
Various forces/torques required in measuring instruments
Deflecting torque/force: The defection of any instrument is determined by the
combined effect of the deflecting torque/force, control torque/force and damping
torque/force. The value of deflecting torque must depend on the electrical signal
to be measured; this torque/force causes the instrument movement to rotate from
its zero position.
Controlling torque/force:
This torque/force must act in the opposite sense to the
deflecting torque/force, and the movement will take up an equilibrium or definite
position when the deflecting and controlling torque are equal in magnitude. Spiral
springs or gravity usually provides the controlling torque.
Damping torque/force: A damping force is required to act in a direction opposite
to the movement of the moving system. This brings the moving system to rest at
the deflected position reasonably quickly without any oscillation or very small
oscillation. This is provided by i) air friction ii) fluid friction iii) eddy current. It
should be pointed out that any damping force shall not influence the steady state
deflection produced by a given deflecting force or torque.
A multi-range ammeters and voltmeters
An ammeter is required to measure the current in a circuit and it therefore
connected in series with the components carrying the current. If the ammeter resistance is
not very much smaller than the load resistance, the load current can be substantially
altered by the inclusion of the ammeter in the circuit. To operate a moving coil
instrument around a current level 50ma is impractical owing to the bulk and weight of the
coil that would be required. So, it is necessary to extend the meter-range shunts (in case
of ammeters) and multipliers (in case of volt meters) are used in the following manner.
For higher range ammeters a low resistance made up of manganin (low
temperature coefficient of resistance) is connected in parallel to the moving coil (see
Fig.42.2 (a)) and instrument may be calibrated to read directly to the total current.
Ammeter Sensitivity:
Ammeter sensitivity is determined by the amount of current required by the meter coil to
produce full-scale deflection of the pointer. The smaller the amount of current required
producing this deflection, the greater the sensitivity of the meter. A meter movement that
requires only 100 microamperes for full- scale deflection has a greater sensitivity than a
meter movement that requires 1 mA for the same deflection.
Construction of Moving-iron Instruments
The deflecting torque in any moving-iron instrument is due to forces on a small
piece of magnetically ‘soft’ iron that is magnetized by a coil carrying the operating
current. In repulsion (Fig.42.7) type moving–iron instrument consists of two cylindrical
soft iron vanes mounted within a fixed current-carrying coil. One iron vane is held fixed
to the coil frame and other is free to rotate, carrying with it the pointer shaft. Two irons
lie in the magnetic field produced by the coil that consists of only few turns if the
instrument is an ammeter or of many turns if the instrument is a voltmeter. Current in the
coil induces both vanes to become magnetized and repulsion between the similarly
magnetized vanes produces a proportional rotation. The deflecting torque is proportional
to the square of the current in the coil, making the instrument reading is a true ‘RMS’
quantity Rotation is opposed by a hairspring that produces the restoring torque. Only the
fixed coil carries load current, and it is constructed so as to withstand high transient
current. Moving iron instruments having scales that are nonlinear and somewhat crowded
in the lower range of calibration. Another type of instrument that is usually classed with
the attractive types of instrument is shown in Fig.42.8.