Moving Iron Instruments - GATE EE Electrical and Electronic Measurements

Paper Chromatography- Application

• It is specially used for separation of mixtures having polar and non polar compounds.
• It is used for separation of amino acids.
• It can be used to determine organic compound, biochemical in urine, etc.
• Some time used for evolution of inorganic compound like salt and complex.

Characteristics of Moving-Iron Instruments:
Advantages:

• These are less expensive, robust, and simple in construction.
• These can be used for both d.c. and a.c. measurements. However, when used with d.c.,  they are liable to small errors due to residual magnetism.
• These instruments have high operating torque.
• These instruments are reasonably accurate.

Disadvantages:

• These instruments have non-linear scales.
• These instruments are not as sensitive as the permanent-magnet moving coil instruments.
• Errors are introduced due to changes in frequency in the case of a.c. measurements.

Applications: The moving-iron instruments are primarily used for a.c. measurements viz., alternating currents and voltages. They are not used to measure direct currents and voltages because their characteristics are inferior to permanent-magnet moving coil instruments.

Moving Iron Instruments are the most common type of ammeter and voltmeter used at power frequencies in laboratories.
These instruments are very accurate, cheap, and rugged as compared to other AC instruments.

Working Principle of Moving Iron Instruments:

• In Moving Iron Instruments, a plate or van of soft iron or of high permeability steel forms the moving element of the system.
• The iron van is so situated that it can move in the magnetic field produced by a stationary coil.
• The below figure shows a simple moving iron instrument.

• The stationary coil is excited by the current or voltage under measurement.
• When the coil is excited, it becomes an electromagnet, and the iron van moves in direction of offering a low reluctance path.
• Thus the force of attraction is always produced in a direction to increase the inductance of the coil.
• Mind that as the van follows the low reluctance path, the net flux in the air gap will increase which means increased flux linkage of the coil, and hence inductance of coil will increase.
• It shall also be noticed that the inductance of the coil is variable and depends on the position of the iron van.

Torque Equation of Moving Iron Instruments:
Deflecting torque in Moving iron Instruments is given as
Td =  (1/2)I²(dL/dƟ)
In moving iron instruments, the controlling torque is provided by spring. Controlling torque due to spring is given as
Tc = KƟ
Where K = Spring constant
Ɵ = Deflection in the needle
In equilibrium state, deflecting and controlling torque shall be equal as below.
Deflecting Torque = Controlling Torque
⇒ Td = Tc
⇒ (1/2)I²(dL/dƟ) = KƟ
⇒ Ɵ = (1/2)(I²/K)(dL/dƟ)

Concept:
In moving iron instruments, the deflecting torque is unidirectional (acts in the same direction) whatever may be the polarity of the current.
The deflecting torque is given by

Where,
I is operating current
L is self-inductance
θ is deflection
T_d ∝ I²
So that the deflection torque of the moving iron instrument is proportional to the square of the RMS value of the operating current and change in self-inductance. 

Calculation:
When the current through a moving iron instrument is increased by 20%, then deflection torque is

∴ T'_d = 1.44 T_d
Hence deflection torque is increased by 44%

Moving iron (MI) instruments always measures the RMS value of a given quantity.

RMS (Root mean square) value:

• RMS value is based on the heating effect of wave-forms.
• The value at which the heat dissipated in AC circuit is the same as the heat dissipated in DC circuit is called RMS value provided, both the AC and DC circuits have equal value of resistance and are operated at the same time.

Calculation:
Given:
Current = i(t) = (18 + 10 sin ωt + 7 sin 2ωt)
RMS value of current  

Concept:

• Swamping resistor and condenser are used to reduce the frequency error in MI instruments.
• In moving iron instruments, the change in frequency causes an error due to the change in magnitude of eddy currents set up in the metal portion of the instruments.
• The reactance of the operating coil changes by the change in frequency.
• At high frequencies, the meter will give a reading lower than the actual value and at low frequencies, the meter gives a reading higher than the actual.
• This type of error can be eliminated only if the impedance of the coil is independent of the frequency. This is achieved by having a capacitance in shunt with the swamping resistance of the coil as shown in the figure.
  
• In the above figure, C is condenser and r is swamping resistance, whereas L and R represent the reactance and resistance of operating coil in moving iron instrument.

In moving iron instruments, the deflecting torque is unidirectional (acts in the same direction) whatever may be the polarity of the current.
The deflecting torque is given by

Where,
i is operating current
L is self-inductance
θ is deflection
∴ T_d∝i²
So that the deflection torque of the moving iron instrument is proportional to the square of the RMS value of the operating current and change in self-inductance.  

Moving Iron (MI) type instruments:

• Moving Iron (MI) type instruments can be used for both AC & DC measurements. But Moving Iron connected through the current Transformer block DC supply. So that Moving Iron connected through the current Transformer is only used for AC measurements.
• The moving iron meter has a large magnetic reluctance as compared to the PMMC meter. That’s why more power is required to operate the moving iron meter.

Advantages of moving iron:

• It is a universal instrument which can be used for the measurement of AC and DC quantities
• These instruments can withstand large loads and are not damaged even under severe overload conditions
• It is very cheap due to the simple construction

Disadvantages of moving iron:

• These instruments suffer from error due to hysteresis, frequency change, and stray losses
• The reading of the instrument is affected by temperature variation

Impedance in both the parallel branches are zero
Therefore the current from the voltage source will be

i(t) = 200 sin ωt
The impedance on both the parallel branches are equal, so the same amount of current will be flowing through each branch
⇒ I₁ = I₂ = 100 sin ωt
The voltage at node A is
V_A = (- j1) 100 sin ωt
The voltage at node B is
V_B = (j1) 100 sin ωt
Voltage measured by the voltmeter will be 
V = V_A - V_B = - 200j sin ωt

Concept:
PMMC measures dc or average value, hot-wire and moving iron instruments measures RMS value.
If a current i is given as:
i = a₀ + a₁ sin (ω₁t + θ₁) + a₂ sin (ω₂t + θ₂) + a₃ sin (ω₃t + θ₃) +...........
Where,
a₀ = DC value = average value of current
Rms value is:

Calculation:
Given,
V(t) = [100 + 10 sin (ωt) - 5 sin (3ωt)] 
In the above DC value of the signal is 100 V
For moving coil,
V₁ = V_avg = 100 V
For moving iron,

The difference between the two meters is 
V₂ - V₁ = 0.312 volts

Deflecting torque:
The torque needed to move the pointer over a calibrated scale is known as deflecting torque and it can overcome the inertia of the moving system, controlling torque and damping torque.
In moving iron instruments, the deflecting torque is given by

I is the operating current in Ampere
L is the inductance in Henry
θ is the deflection angle in radians
As the operating torque is directly proportional to the square if the operating current. Hence, this instrument possesses high operating torque.

Moving-iron instrument depends on the iron vane's attraction to the current-carrying coil to be measured for proper operation.
Moving-iron instrument:

• Whenever a piece of iron is placed nearer to a magnet it would be attracted by the magnet. The force of this attraction depends upon the strength of the magnetic field. 
•  If the magnet is an electromagnet then the magnetic field strength can easily be increased or decreased by increasing or decreasing the current through its coil.
• Accordingly, the attraction force acting on the piece of iron would also be increased and decreased.
• Depending upon this simple phenomenon, the moving iron instrument was developed.

• The main source of this error is the change in reactance of the meter-coil with frequency.
• It is due to the dependence of the inductive reactance on the frequency of the supply.
• The inductive reactance of the coil increases with an increase in frequency (XL = 2πfL).
• Hence, overall impedance also increases and so the current through the meter reduces.
• Therefore, the deflection will be less.

In order to neutralize this error in the case of MI voltmeters, a properly designed capacitor is connected across the series resistor (i.e., multiplier).