In
moving–iron instruments the movable system consists of one or more pieces of specially-shaped
soft iron, which are so pivoted as to
be acted upon by the magnetic field produced by the current in coil. There are
two general types of moving-iron instruments
namely (i) Repulsion (or double iron)
type (ii) Attraction (or
single-iron) type. The brief description of different components of a
moving-iron instrument is given below.
·
Moving
element: a small piece of soft iron in
the form of a vane or rod
·
Coil: to produce the magnetic field due to current flowing
through it and also to magnetize the iron pieces.
·
In
repulsion type, a fixed vane or rod is also used and magnetized with the same
polarity.
·
Control
torque is provided by spring or weight (gravity).
·
Damping
torque is normally pneumatic, the damping device consisting of an air chamber
and a moving vane attached to the instrument spindle.
Deflecting torque
produces a movement on an aluminium pointer over a graduated scale. 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.
Construction
and working of Moving-iron Instruments:
Repulsion Type:
Construction:
Repulsion
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. The two
iron vanes 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.
Working:
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.
Attraction Type:
The moving system
of the instrument is attached to a soft iron piece. The operating current is
passing through a coil placed near it. When current flows in the coil, the
soft-iron piece is attracted towards the coil and the movement causes pointer
to move across the scale.
The pointer will
come to rest at a position where deflecting torque is equal to the controlling
torque.
If current in the
coil is reversed, the direction of magnetic field also reverses and so does the
magnetism produce in the soft iron piece. Hence, the direction of the
deflecting torque remains unchanged. For this reason, such instruments can be
used for both d.c. and a.c. measurements.
Advantages:
- The instruments are suitable for use in a.c and d.c circuits.
- The instruments are robust, owing to the simple construction of the moving parts.
- The stationary parts of the instruments are also simple.
- Instrument is low cost compared to moving coil instrument.
- Torque/weight ratio is high, thus less frictional error.
·
Disadvantages:
- Have non-linear scale.
- Cannot be calibrated with
high degree of precision for d.c. on account of the affect of hysteresis
in the iron vanes.
- This instrument will always
have to be put in the vertical position if it uses gravity control.
Errors:
- Errors due to temperature variation.
- Errors due to friction is quite small as torque-weight ratio is high in moving-iron instruments.
- Stray fields cause relatively low values of magnetizing force produced by the coil. Efficient magnetic screening is essential to reduce this effect.
- Error due to variation of frequency causes change of reactance of the coil and also changes the eddy currents induced in neighboring metal.
- Deflecting torque is not exactly proportional to the square of the current due to non-linear characteristics of iron material.
