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10-11-2012, 05:25 PM
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Moving Coil Instruments pdf

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.

D.c Ammeter:

An Ammeter is always connected in series with a circuit branch and measures the current
flowing in it. Most d.c ammeters employ a d’Arsonval movement, an ideal ammeter
would be capable of performing the measurement without changing or distributing the
current in the branch but real ammeters would possess some internal resistance.

D.C Voltmeter:

A voltmeter is always connect in parallel with the element being measured, and measures
the voltage between the points across which its’ connected. Most d.c voltmeter employ PMMC
meter with series resistor as shown. The series resistance should be much larger than the
impedance of the circuit being measured, and they are usually much larger than Rm.

Series Ohmmeter:

Rx is the unknown resistor to be measured, R2 is variable adjusted resistance so that
the pointer read zero at short circuit test. The scale of series ohmmeter is nonlinear with
zero at the right and infinity at extreme left. Series ohmmeter is the most generally used
meter for resistance measurement.

Shunt Ohmmeter:

Shunt ohmmeter are used to measure very low resistance values. The unknown
resistance Rx is now shunted across the meter, so portion of current will pass across this
resistor and drop the meter deflection proportionately. The switch is necessary in shunt
ohmmeter to disconnect the battery when the instrument is not used. The scale of shunt
ohmmeter is nonlinear with zero at the left and infinity at extreme right.

A.c Measuring Instrument

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.

Clamp on Meters (Average Responding A.C meter):

One application of average responding a.c meters is the clamp on meter which is used to
measured a.c current, voltage in a wire with out having to break the circuit being measured.
The meter having use the transformer principle to detect the current. That is, the clamp on device
of the meter serves as the core of a transformer. The current carrying wire is the primary winding
of the transformer, while the secondary winding is in the meter. The alternating current in the
primary is coupled to the secondary winding by the core, and after being rectified the current is
sensed by a d’Arsonval meter.
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