Electrodynamic Instruments
are moving coil instruments and the operating field is produced in it by
another fixed coil. These instruments are generally used as wattmeter
and can also be used as ammeter and voltmeter.
Construction
These instruments consist of a fixed coil
having two equal sections which are parallel to each other. These two fixed
coils are air-cored to avoid from hysteresis losses. When alternating
current passes through the fixed coil, it produces magnetic effect
and moving coil starts moving and a pointer is attached to the moving
coil with a spring.
Deflecting Torque
Electrodynamic Instruments
consist of deflecting torque and its parameters are as follows:
Current passing trough the fixed coil = I1
Current passing through the moving coil = I2
B = KI1 (where K is constant)
Deflecting torque of a coil Td = NBI2lb
= NKI1I2lb N-m (l
x b is the dimension of rectangular coil)
When the instrument is used as an ammeter,
the current remains equal passes through the fixed and moving coil. In this
case:
I=I1=I2
Θ = I2 and I = √Θ
Eddy current damping
occurs while operating of magnetic field. These instruments can be used
as AC and DC measurements. These instruments are very costly for dc
measurements.
Errors
The operating field produced in the
coil is small because of the air - cored coils.
Huge numbers of turns are necessary for the
moving coil.
The control springs produces friction
losses due to limited magnitude.
Resistance
is produced due to the limited current in the coils increases heat.
These instruments are not so sensitive.
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