Showing posts with label Measurements. Show all posts
Showing posts with label Measurements. Show all posts

Differences Between Moving Coil (MC) And Moving Iron (MI) Instruments

Differences Between Moving Coil (MC) And Moving Iron (MI) InstrumentsLet us have a comparison between two major type of electrical measuring instruments they are moving coil (MC) and moving iron (MI) instruments.In this article all major differences between moving iron and  moving coil   instruments are discussed.When you go for viva or interview most often question on electrical measurements is what is the difference between mi and mc type instruments?.In this article you will get answer to all differences between MI & MC type instruments.

Differences Between Moving Coil (MC) And Moving Iron (MI) Instruments


M.C Instruments M.I Instruments
1. MC type instruments are more accurate. 1. MI type are less accurate than MC type.
2. Manufacturing cost is high. 2. Cheap in cost.
3. Reading scale is uniformly distributed. 3. Non-uniform scale
(scale cramped at beginning and finishing)
4. Very sensitive in construction & for input. 4. Robust in construction.
5. Low power consumption 5. Slightly high power consumption.
6. Eddy current damping is used. 6. Air friction damping is used.
7. Can be used only for D.C measurements. 7. Can be used for A.C as well as for D.C
measurements.
8. Controlling torqure is provided by spring. 8. Controlling torque is provided by
gravity or spring
9. Deflection proportional to current.( θ α I ). 9. Deflection proportional to square of current.
( θ α I² ).
10. Errors are set due to aging of control
springs,permanent magnet (i.e. No Hysteresis loss)
10. Errors are set due to hysteresis and stray fields.
(i.e. hysteresis loss takes place).

In this comparison between MC and MI Instruments we shared top points.

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difference between moving coil and moving iron instruments
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Torque Equation Of Moving Iron Instruments

In previous tutorial on Moving Iron Instrument Operation construction & working principle was discussed. In this post moving iron instrument torque equation will be derived.


Torque Equation of Moving Iron Instruments


READ HERE Moving Iron Instrument Working Operation CLICK HERE 

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 inductance
θ = 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,

e = d(L*I)/dt
   = I * dL/dt + T * dI/dt     as both I and L are changing.

The electrical energy supplied is given by 

eIdt = { I * dL/dt + T * dI/dt }Idt
       =I²dL + ILdI
The stored energy increases from 1/2*(LI²) to 1/2*[(L+dL)(I+dI)²]
Hence the change in stored energy is given by,
1/2*[(L+dL)(I+dI)²] - 1/2*(LI²)
Neglecting higher order terms,this becomes ILdI +  1/2 * I² dL
The energy supplied in nothing but increase in stored energy plus the energy required for mechanical work done.

I²dL + ILdI = ILdI + 1/2*(I²)dL +Td.
Td.dθ =  1/2( I².dL )

Td = 1/2  I²dL/dθ

While the controlling torque is given by,
Tc = Kθ
where K = spring constant 
Kθ = 1/2  I²dL/dθ
θ = 1/2  I²dL/dθ * 1/K      under equilibrium 






Thus the deflection is proportional to the square of the current through the coil. And the instrument gives square law response.

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Moving Iron Instrument Working Operation

Moving Iron Instruments or MI Instruments

In our previous article we have discussed PMMC Instrument Working Opeartion. In this tutorial on Moving Iron Instrument Working Operation we go through the construction & basic principle of MI type instrument.  

The moving iron instruments are classified as:
i) Moving iron attraction type instruments
ii) Moving iron repulsion type instruments

Moving Iron Attraction Type Instrument Working Operation

Moving Iron Instrument Working Principle : The basic working principle of these instruments is very simple that a soft iron piece if brought near magnet gets attracted by the magnet.

The construction of the attraction type moving iron instrument is shown in the below figure.


lt consists of a fixed coil C and moving iron piece D. The oil is flat and has narrow slot like opening. The moving iron is a flat disc which is eccentrically mounted on the spindle. The spindle is supported between the jewel bearings. The spindle carries 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 large current through the coil only few turns are required.

The controlling torque is provided by the springs but gravity control may also be 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 be provided with the help of vane 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 requires a permanent magnet which would affect or distort the operating field.This is the reason Why why eddy current damping is not used in moving iron instrument.

Moving Iron Repulsion Type Instrument

Moving iron repulsion Type instruments have two vanes inside the coil. the one is fixed and other is movable. When the current flows in the coil, both the vans are magnetized with like polarities induced on the same side. Hence due to the repulsion of like polarities, there is a force of repulsion between the two vanes causing the movement of the moving vane. 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

Radial Vane Emulsion Typo Instrument

Below shows the radial vane repulsion type instrument. Out of the other moving iron mechanisms, this is the moat sensitive and has most linear scale.


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.

Even-though 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 read amperes or volts. The The calibration is accurate only for the frequency for which it is designed because the impedance is different for different frequencies.

Concentric Vane Repulsion Type Instrument

Figure 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 co-axially inside the stationary vane.



Both the vanes are magnetized 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.

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moving iron instrument can be used for measuring
attraction type moving iron instrument
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Permanent Magnet Moving Coil Type (PMMC) Instrument-Working Principle

Permanent Magnet Moving Coil Type (PMMC) Instrument Working Principle 

The basic principle of operation is that when a current carrying conductor is brought in a magnetic field (they should not be parallel to each other) a torque on the conductor is produced. The Permanent Magnet Moving Coil instrument consists of a rectangular coil pivoted so that its sides lie in the air gap between the two poles of a permanent magnet and a soft-iron cylinder. The air gap between the magnet poles and iron core is small and the flux density is uniform and is in a radial direction, so that the flux lines are always at right angle to the current carrying conductor and hence when current passes through the coil, a deflecting torque is produced owing to the interaction between the two fluxes, one due to permanent magnet and the other due to the magnetic field of the coil. This is shown in Fig.1.

constructions of permanent magnet moving coil instruments

Permanent Magnet Moving Coil Type (PMMC) Instrument-Working Principle
Fig.1.
If I is the current flowing in the moving coil in the direction shown, forces F, F will act on the two sides of this coil, the direction of force being determined by Fleming's left-hand rule and is in the direction for this system as shown in Fig.1. The torque causing the coil to rotate is given as F.2r where r is the mean distance of the wires forming the sides of the coil, from the axis of rotation.Now, if N is the no. of turns in the coil, B the magnetic flux density due to permanent magnet, I the current in the coil, l the effective length, the force acting on the coil is given by

Permanent Magnet Moving Coil Type (PMMC) Instrument-Working Principle

F = NBil Newton .. . (4.1)
Hence the torque on the coil is
T = NBil. 2r N-m ... (4.2)

Control torque & Damping torque in  PMMC Instrument



In any measuring (electromagnetic) instrument there are three torques acting on the moving mechanism to which a pointer is connected which moves on the dial of the instrument and indicates the reading of the quantity being measured. One of the three torques is the deflecting torque which we have just studied. The other torques are (i) control torque (ii) Damping torque. In case of PMMC instruments spring made of phosphor bronze provides control torque. The spring also serves as leads to the moving coil. When deflecting torque acts on the coil, both the control torque and damping torques come into action. The control torque restrains the rotation of the coil whereas the deflecting torque tries to rotate the coil. At balance, if the coil has moved through an angle  θ and if k is the spring constant, we have 

as NEZ.2r are constant of a particular meter. Hence equation (4.5) suggests that the current is proportional to θ. Thus the scale is uniformly divided i.e. it is a linear scale i.e. if for I amp the deflection is through an angle θ, for 2I amps the deflection would be 2θ.Now if damping torque were absent, the pointer will keep on oscillating around the mean value and an exact reading cannot be recorded. These oscillations are damped out by the damping torque. The moving coil is wound on an aluminium former which is placed in the magnetic field.As the coil rotates eddy currents are induced in the aluminium former and these eddy currents would try to oppose the cause i.e. the deflecting force and finally when the pointer comes to its actual value being measured, damping is provided by eddy currents and the coil (the pointer) remains stationary at its actual value.

 Torque Equation of PMMC Instrument

       The equation for the developed torque can be obtained from the basic low of the electromagnetic torque. The deflecting torque is given by,

Td   = NBAI
where  Td  = deflecting torque in N-m
 B = flux density in air gap, Wb/m2
N = number of turns of the coil
A = Effective coil area m2
I = Current in the moving coil, amperes
Td  = GI
where G = NBA = constant
The controlling torque is provided by the springs and is proportional to the angular deflection of the pointer.
Tc  = kθ
where  Tc  = controlling torque
K = spring constant, Nm/rad or Nm/deg
θ = angular deflection
for the final steady state position,
Td   = Tc 
GI = Kθ
θ = (G/K) I
I = (K/G)θ

Note: Thus the deflection is directly proportional to the current passing through the coil in Permanent Magnet Moving Coil Type (PMMC) Instrument.

Advantages of Permanent Magnet Moving Coil Type  or PMMC instruments 

(a) Low power consumption
(b) High Torque/weight-ratio
(c) Uniformity of the scale and the possibility of a very long scale
(d) Perfect damping provided by eddy currents induced in the metal former of the moving coil. The metal used is aluminium as it is light in weight
(e) The possibility of a single instrument being used with shunt and resistance to cover a large range of both currents and voltages.
(f) Freedom from errors due to stray magnetic fields.

Tags:PMMC Torque Equation,PMMC instrument wikipedia,permanent magnet moving coil (PMMC),permanent magnet moving coil  (PMMC) mechanism,Working operation
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