24-09-2012, 01:49 PM
Moving Coil Instruments
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There are two types of moving coil instruments namely, permanent magnet moving coil type
which can only be used for direct current, voltage measurements and the dynamometer type
which can be used on either direct or alternating current, voltage measurements.
Permanent Magnet Moving Coil Mechanism (PMMC)
In PMMC meter or (D’Arsonval) meter or galvanometer all are the same instrument, a
coil of fine wire is suspended in a magnetic field produced by permanent magnet. According to
the fundamental law of electromagnetic force, the coil will rotate in the magnetic field when it
carries an electric current by electromagnetic (EM) torque effect. A pointer which attached the
movable coil will deflect according to the amount of current to be measured which applied to the
coil. The (EM) torque is counterbalance by the mechanical torque of control springs attached to
the movable coil also. When the torques are balanced the moving coil will stopped and its angular
deflection represent the amount of electrical current to be measured against a fixed reference,
called a scale. If the permanent magnet field is uniform and the spring linear, then the pointer
deflection is also linear.
Review on Alternating Signal:
The instantaneous values of electrical signals can be graphed as they vary with time. Such
graphs are known as the waveforms of the signal. If the value of waveform remains constant with
time, the signal is referred to as direct (d.c) signal; such as the voltage of a battery. If a signal is
time varying and has positive and negative instantaneous values, the waveform is known as
alternating (a.c) waveform. If the variation of a.c signal is continuously repeated then the signal
is known as periodic waveform.
The frequency of a.c signal is defined as the number of cycles traversed in one second. Thus
the time duration of one cycle per second for a.c signal is known as the period (T). Where the
complete variation of a.c signal before repeated itself is represent one cycle.
Bridges and Their Application
Bridge circuit are extensively used for measuring component values, such as resistance,
inductance, capacitance, and other circuit parameters directly derived from component values
such as frequency, phase angle, and temperature. Bridge accuracy measurements are very high
because their circuit merely compares the value of an unknown component to that of an
accurately known component (a standard).
1- D.c Bridges:
The basic d.c bridges consist of four resistive arms with a source of emf (a battery) and a
null detector usually galvanometer or other sensitive current meter. D.c bridges are generally
used for the measurement of resistance values.
a) Wheatstone Bridge:
This is the best and commonest method of measuring medium resistance values in the
range of 1Ω to the low megohm. The current through the galvanometer depends on potential
difference between point © and (d). The bridge is said to be balance when potential
difference across the galvanometer is zero volts, so there is no current through the
galvanometer (Ig=0).