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CONSTRUCTION AND BASIC PRINCIPLE OPERATION
OF MOVING-IRON INSTRUMENTS
Moving-iron instruments are generally used to measure alternating voltages and currents. 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:
1. Repulsion (or double iron) type (figure 1)
2. Attraction (or single-iron) type (figure 2)
The brief description of different components of a moving-iron instrument is given below:
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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 aluminum pointer over a graduated scale.
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How it works?
Typical scheme of measuring el. current and voltage
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. In 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. Two irons 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.
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.
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Figure 1 - Repulsion moving iron-instrument
Figure 2 - Attraction moving iron instrument
Measurement of Electric Voltage and Current
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Moving iron instruments are used as Voltmeter and Ammeter only.
Both can work on AC as well as on DC.
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Ammeter
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Instrument used to measure current in the circuit.
Always connected in series with the circuit and carries the current to be measured.
This current flowing through the coil produces the desired deflecting torque.
It should have low resistance as it is to be connected in series.
Voltmeter
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Instrument used to measure voltage between two points in a circuit.
Always connected in parallel.
Current flowing through the operating coil of the meter produces deflecting torque.
It should have high resistance. Thus a high resistance of order of kilo ohms is connected in series
with the coil of the instrument.
Explain how a moving iron instrument
attraction type operates?
Answer
An ‘attraction type’ moving-iron instrument consists of a coil, through which the test current is passed,
and a pivoted soft-iron mass attached to the pointer. The resulting magnetic polarity at the end of the
coil nearest the iron mass then induces the opposite magnetic polarity into the part of the iron mass
nearest the coil, which is then drawn by attraction towards the coil, deflecting the pointer across a scale.
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MOVING COIL –DYNAMOMETER TYPE WATTMETER
Electric power is measured by means of a wattmeter. This instrument is of the electrodynamometer
type. As shown in figure 3-18, it consists of a pair of fixed coils, known as current coils, and a moving
coil, called the voltage (potential) coil. The fixed current coils are wound with a few turns of a relatively
large conductor. The voltage coil is wound with many turns of fine wire. It is mounted on a shaft that is
supported in jeweled bearings so that it can turn inside the stationary coils. The movable coil carries a
needle (pointer) that moves over a suitably graduated scale. Coil springs hold the needle at the zero
position in the absence of a signal.
Wattmeter Connection The current coil of the wattmeter is connected in series with the circuit (load),
and the voltage coil is connected across the line. When line current flows through the current coil of a
wattmeter, a field is set up around the coil. The strength of this field is in phase with and proportional to
the line current. The voltage coil of the wattmeter generally has a high-resistance resistor connected in
series with it. The purpose for this connection is to make the voltage-coil circuit of the meter as purely
resistive as possible. As a result, current in the voltage circuit is practically in phase with line voltage.
Therefore, when voltage is impressed on the voltage circuit, current is proportional to and in phase with
the line voltage. Figure shows the proper way to connect a wattmeter into a circuit
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MOVING COIL –PMMC
CONSTRUCTION:- In a permanent magnet with soft iron pole pieces, a cylindrical iron core is mounted
between the two poles of the magnet giving very narrow air gap in which the sides of a pointed light
rectangular coil lies. The rectangular coil is wound of many turns of coil. The purpose of using core is to
make the field uniform and to reduce the reluctance of the magnetic circuit. A low reluctance helps to
retain permeance of magnet for a longer period.
WORKING OF TORQUE EQUATION:- When the current to be measured is passed through the coil, say in
the direction as shown in fig. (a), deflecting torque is produced. On account of relation between
permanent magnetic field and coil magnetic field, the direction of deflecting torque can be determined
by applying Fleming's left hand rule.
It is the current in amperes flowing through the coil of turns N and length l meters B is flux density in
test as in air gap:Then, deflecting force F = BilN newtons.
If r is the distance in meters in between the centers of the coil and force F.
Then deflecting torque Td = F x r = BilNr Nm
From the above expression it is obvious that if flux density B in the air gap is constant, then
deflecting torque Td ∝ i , Tc= Td
So, θ ∝ i
And since  is directly proportional to current, the scale of the basic dc PMMC instrument are usually
linearly spaced. Hence scale is linear.
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INDUCTION TYPE ENERGY METER
The Electro-Mechanical Energy Meter (electricity meter, kWh meter, etc.) consists of a rotating
aluminum disc mounted between two alternating current (a.c.) electromagnets, M1 and M1.
The disc cuts through the fluxes of these two magnets and therefore two circular eddy currents
are generated in the disc. The rotating torque is produced by the interaction of these eddy
currents and the fluxes induced by the two electromagnets.
M1 is connected in series and it is also known as the series magnet. It produces an alternating
magnetic flux of Φ1, which is proportional to and in-phase with the line current, I.
M2, also known as the shunt magnet, is connected across the supply line and carries a current
proportional to the supply voltage, V. Therefore the magnetic flux, Φ2, it generates is
proportional to the supply voltage, V. However, Φ2 is not in-phase with the V. In fact, it is set to
be 90 ° lagging to V. This is done by having two or three properly adjusted copper rings, C.
Some meters use winding with a series connected lag adjusting resisteror.
The current through the M2 winding is V/ωL.
α = (90 - Φ)
Tdrive = kdrive . ω . (V/ωL) . I sinα
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Tdrive = kdrive V.I cosΦ
Tdrive = P
Where α is the phase angle between Φ1 and Φ2, kdrive is the proportionality constant, P is the
electrical power consumed and Tdrive is the driving torque on the aluminum disc, D.
The breaking torque, is obtained by two permanent magnets mounted in opposite directions.
Tbreak = kbreak . Φbreak . N/Reddy
Where Tbreak is the breaking torque acting on the aluminum disc, Φbreak is the magnetic flux of the
permanent magnets, N is the rotation speed of the aluminum disc, kbreak is the proportionality
constant and Reddy is the resistance of the eddy current path.
Tdrive = Tbreak
⇒N=k.P
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1.
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Voltage coil, C2
Current coil, C1
Stator - concentrates and confines magnetic field
Aluminum disc, D
Brake magnets
Gear mechanism
Display dials
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