The moving iron instruments are classified as:
i) Moving iron attraction type instruments and
ii) Moving iron repulsion type instruments
1.1 Moving iron attraction type instruments
The basic working principle of these instruments is very simple
that a soft iron piece if brought near the magnet gets attracted by the
magnet.
The construction of the attraction type instrument is shown in the Fig.1.
Fig. 1 Moving iron attraction type instruments |
It consists of a fixed coil C and moving iron piece D. The coil
is flat and has a narrow slot like opening. The moving iron is a flat
disc which eccentrically mounted. on the spindle. The spindle is
supported between the jewel bearings. The spindle caries a pointer which
moves over a graduated scale. The number of turns of the fixed coil are
dependent on the range of the instrument. For passing current through
the coil only few turns are required.
The controlling torque is provided by the springs but gravity
control may also ne used for vertically mounted panel type instruments.
The damping torque is provided by the air friction. A light
aluminium piston is attached to the moving system. It moves in a fixed
chamber. The chamber is closed at one end. It can also provided with the
help of van attached to the moving system.
The operating magnetic field in moving iron instruments is very
weak. Hence eddy current damping is not used since it require a
permanent magnet which would affect or distort the operating field.
1.2 Moving Iron Repulsion Type Instrument
These instruments have two vanes inside the coil, the one is
fixed and other is movable. When the current flows in the coil, both the
vanes are magnetised with like polarities induced on the same side.
Hence due to repulsion of like polarities, there is a force of repulsion
between the two vanes causing the movement of the moving van. The
repulsion type instruments are the most commonly used instruments.
The two different designs of repulsion type instruments are:
i) Radial vane type and
ii) Co-axial vane type
1.2.1 Radial Van Repulsion Type Instrument
The Fig. 2 shows the radial vane repulsion type instrument. Out
of the other moving iron mechanism, this is the most sensitive and has
most linear scale.
Fig. 2 Radial Van Repulsion Type Instrument |
The two vanes are radial strips of iron. The fixed vane is
attached to the coil. The movable vane is attached to the spindle and
suspended in the induction field of the coil. The needle of the
instrument is attached to this vane.
Eventhough the current through the coil is alternating, there is
always repulsion between the like poles of the fixed and the movable
vane. Hence the deflection of the pointer is always in the same
direction. The deflection is effectively proportional to the actual
current and hence the scale is calibrated directly to rad amperes or
volts. The calibration is accurate only for the frequency for which it
is designed because the impedance is different for different
frequencies.
1.2.2 Concentric Vane Repulsion Type Instrument
Fig. 3 shows the concentric vane repulsion type instrument. The
instrument has two concentric vanes. One is attached to the coil frame
rigidly while the other can rotate coaxially inside the stationary vane.
Fig. 3 Concentric Vane Repulsion Type Instrument |
Both the vanes are magnetised to the same polarity due to the
current in the coil. Thus the movable vane rotates under the repulsive
force. As the movable vane is attached to the pivoted shaft, the
repulsion results in a rotation of the shaft. The pointer deflection is
proportional to the current in the coil. The concentric vane type
instrument is moderately sensitive and the deflection is proportional to
the square of the current through coil. Thus the instrument said to
have square low response. Thus the scale of the instrument is
non-uniform in nature. Thus whatever may be the direction of the current
in the coil, the deflection in the moving iron instruments is in the
same direction. Hence moving iron instruments can be used for both a.c.
and d.c. measurements. Due to square low response, the scale of the
moving iron instrument is non-uniform.
Consider a small increment in current supplied to the coil of the instrument. due to this current let dθ be the deflection under the deflecting torque Td. Due to such deflection, some mechanical work will be done.
... Mechanical Work = Td dθ
There will be a change in the energy stored i the magnetic field
due to the change in inductance. This is because the vane tries to
occupy the position of minimum reluctance. The inductance is inversely
proportional to the reluctance of the magnetic circuit of coil.
Let I = initial current
L = instrument inductanceθ = deflection
dI = increase in current
dθ = change in deflection
dL = change in inductance
In order to effect an increment dL in the current, there must be an increase in the applied voltage given by,
The electrical energy supplied is given by,
The energy supplied in nothing but increase in stored energy plus the energy required for mechanical work done.
While the controlling torque is given by,
Thus the deflection is proportional to the square of the current
through the coil. And the instrument gives square law response.
1.4 Advantages
The various advantages of moving iron instruments are,1) The instruments can be used for both a.c. and d.c. measurements.
2)As the torque to weight ratio is high, errors due to the friction are very less.
3) A single type of moving element can cover the wide range hence these
instruments are cheaper than other types of if instruments.
4) There are no current carrying parts in the moving system hence these meters are extremely rugged and reliable.
5) These are capable of giving good accuracy. Modern moving iron instruments have a d.c. error of 2% or less.
6) These can withstand large loads and are not damaged even under sever overload conditions.
7) The range of instruments can be extended.1.5 Disadvantages
The various disadvantages of moving iron instruments are,
1) The scale of moving iron instruments is not uniform and is cramped at
the lower end. Hence accurate readings are not possible at this end.
2) There are serious errors due to hysteresis, frequency changes and stray magnetic fields.
3) The increase in temperature increases the resistance of coil,
decreases stiffness of the springs, decreases the permeability and hence
affect the reading severely.
4) Due to the non linearity of B-H curve, the deflecting torque is not exactly proportional to the square of the current.
5) There is a difference between a.c. and d.c. calibration on account of
the effect of inductance of the meter. Hence these meters must always
be calibrated at the frequency at which they are to be used. The usual
commercial moving iron instrument may be used within its specified
accuracy from 25 to 125 HZ frequency range.
6) Power consumption is on higher side.
The various errors in the moving iron instruments are,
1) Hysteresis error: Due to hysteresis effect, the flux density
for the same current while ascending and descending values is different.
While descending, the flux density is higher and while ascending it is
lesser. So meter reads higher for descending values of current or
voltage. So remedy for this is to use smaller iron parts which can
demagnetise quickly or to work with lower flux densities.
2) Temperature error : The temperature error arises due to the
effect of temperature on the temperature coefficient of the spring. This
error is of the order of 0.02 % per oC
change in temperature. Errors can cause due to self heating of the coil
and due to which change in resistance of the coil. So coil and series
resistance must have low temperature coefficient. Hence mangnin is
generally used for the series resistance.
3) Stray magnetic Field Error : The operating magnetic field in
case of moving iron instruments is very low. Hence effect of external
i.e. stray magnetic field can cause error. This effect depends on the
direction of the stray magnetic field with respect to the operating
field of the instrument.
4) Frequency Error : These are related to a.c. operation of the
instrument. The change in frequency affects the reactance of the working
coil and also affects the magnitude of the eddy currents. This cause
error in the instrument.
5) Eddy Current Error : When instrument is used for a.c.
measurements the eddy currents are produced in the iron parts of the
instrument. The eddy current affects the instrument current causing the
change in the deflection torque. This produce the error in the meter
reading. As eddy current are frequency dependent, frequency changes
cause eddy current error.
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