Construction, Principle of Operation of MC & MI Meter

 Construction, Principle of Operation of Moving Coil Meter

Principle of Operation: 

The permanent magnet moving coil instruments are the most accurate type for measurements. The action of these instruments is based on the motoring principle. When a current-carrying coil is placed in the magnetic field produced by a permanent magnet, the coil experiences a force and moves.

 As the coil is moving and the magnet is permanent, the instrument is called a permanent magnet-moving coil instrument. This basic principle is called D'Arsonval principle. The amount of force experienced by the coil is proportional to the current passing through the coil.

Construction:

Fig. : Construction of PMMC Instrument

Moving Coil - 

The coil is the current-carrying part of the instrument which is freely moved between the stationary field of the permanent magnet. The coil is mounted on the rectangular former which is made up of aluminium.

Magnet System - 

The PMMC instrument using a permanent magnet for creating the stationary magnets. The Alcomax and Alnico material are used for creating the permanent magnet because this magnet has a high coercive force.

Control  

In the PMMC instruments, the controlling torque is because of the springs. The springs are made up of phosphorous bronze and placed between the two jewel bearings.

Damping 

This damping torque is induced because of the movement of the aluminium core which is moving between the poles of the permanent magnet.

Pointer & Scale 

The pointer is linked with the moving coil. The pointer notices the deflection of the coil, and the magnitude of their deviation is shown on the scale. The pointer is made of lightweight material, and hence it is easily deflected with the movement of the coil

Working: 

The moving coil is either rectangular or circular in shape. It has a number of turns of fine wire. The coil

suspended so that it is free to turn about its vertical axis. The coil is placed in the uniform horizontal and radial magnetic field of a permanent magnet in the shape of a horseshoe. The iron core is spherical if coil is circular and is cylindrical if the coil is rectangular. Due to iron core, the deflecting torque increases, increasing the sensitivity of the instrument.

The controlling torque is provided by two phosphor bronze hairsprings. The damping torque is provided by eddy current damping. It is obtained by the movement of the aluminium former, moving in the magnetic field of the permanent magnet.

The pointer is carried by the spindle and it moves over a graduated scale. The pointer has lightweight so that it can deflect rapidly. The mirror is placed below the pointer to get an accurate reading by removing the parallax. The weight of the instrument is normally counterbalanced by the weights situated diametrically opposite and rigidly connected to it. The scale markings of the basic d.c. PMMC instruments are usually linearly spaced as the deflecting torque and hence the pointer deflection are directly proportional to the current passing through the coil.

Fig. : Top view of PMMC Instrument

In a practical PMMC a Y shaped instrument, member is attached to the fixed end of the front control spring. An eccentric pin through the instrument case engages the Y shaped member so that the zero position of the pointer can be adjusted from outside.

Torque Equation

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

Thus the deflection is directly proportional to the current passing through the coil.

The pointer deflection can therefore be used to measure current. As the direction of the current through to the coil changes, the direction of the deflection of the pointer also changes. Hence such instruments are well suited for the d.c. measurements. In the microammeters and milliammeters upto about 20 mA, the entire current to be measured is passed through the coil. The springs carry current to the coil. Thus the current carrying capacity of the springs, limits the current which can be safely carried. For higher currents, the moving coil is shunted by sufficient resistance. While the voltmeters having high ranges use a moving coil together with sufficient series resistance, to limit the instrument current. Most d.c. voltrneters are designed to produce full scale deflection with a current of 20, 10, 5 or 1 mA.

The power requirement of PMMC instrument is very small, typically of the order of 25 uW to 200 uW. Accuracy is generally of the order of 2 to 5 % of the full scale reading.

Advantage:

1) It has uniform scale.

2) With a powerful magnet, its torque to weight ratio is very high. So operating

current is small.

3) The sensitivity is high.

4) The eddy currents induced in the metallic former over which coil is wound,

provide effective damping.

5) It consumes low power, of the order of 25 W to 200 uW.

6) It has high accuracy.

7) Instrument is free from hysteresis error.

8) Extension of instrument range is possible.

9) Not affected by external magnetic fields called stray magnetic fields.

Disadvantage:

1) Suitable for d.c. measurements only.

2) Ageing of permanent magnet and the control springs introduces the errors.

3) The cost is high due to delicate construction and accurate machining.

4) The friction due to jewel-pivot suspension.

Errors in PMMC Instrument:

    The basic sources of errors in PMMC instruments are friction, temperature and aging of various parts. To reduce the frictional errors ratio of torque to weight is made very high. The most serious errors are produced by the heat generated or by changes in the temperature. This changes the resistance of the working coil, causing large errors. In case of voltneters, a large series resistance of very low temperature coefficient is used. This reduces the temperature errors.

    The ageing of permanent magnet and control springs also cause errors. The weakening of magnet and springs cause opposite errors. The weakening of magnet cause less deflection while weakening of the control springs cause large deflection, for a particular value of current. The proper use of material and preageing during manufacturing can reduce the errors due to weakening of the control springs.

Moving iron Instruments

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

Moving Iron Attraction Type Instruments

Principle:

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.

Construction & Working:

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 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.

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 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 and ii) Co-axial vane type

Radial Vane Repulsion Type Instrument:

This is the most 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.

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 read amperes or volts. 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

 The instrument has 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 is said to have square law 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 law response, the scale of the moving iron instrument is non-uniform.

Torque Equation

Thus the deflection is proportional to the square of the current through the coil. And

the instrument gives square law response.

Advantages:

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 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 severe overload conditions.

7) The range of instruments can be extended.

Disadvantages:

1) Ille scale of the 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) Ille 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. calibrations 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.

Errors in Moving Iron Instrument:

1) Hystersis Error

2) Temperature Error

3) Stray Magnetic Field Error

4) Frequency Error

5) Eddy Current Error