Moving Iron (MI) Instruments

• Generally used to measure AC current or

voltage (but can measure DC current and voltage without any external circuit)

• Pointer is connected to specially designed

soft iron that moves according to the intensity of magnetic field acting on it

• The magnetic field is produced by current

carrying coil

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Classification of MI

1) Repulsion (or double iron) type 2) Attraction (or single iron) type

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Types of torque in MI

• Deflecting torque produces movement on an aluminum pointer over a calibrated scale

• Control torque: provided by spring or weight

• Damping torque: Pneumatic (mechanical damping) aluminum vane attached to the shaft

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Constructional details of

attraction type MI

• Moving element : Soft iron

• Coil: Produces magnetic field & magnetize the iron piece

• Control springs or weights, aluminum vane, aluminum pointer, shaft, calibrated scale, mirror, etc

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Gravity control

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Constructional details of Repulsion type MI

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Constructional details of Repulsion

type MI

• Two concentric iron vanes (Fixed & Movable)

• Solenoid coil (stationary)

• Pointer attached to movable vane

• Movable vane is curved rectangular in shape

• Fixed vane is tampered

• Controls springs, Aluminum pointer, Aluminum vane, calibrated scale, mirror etc

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Constructional details of

Repulsion type MI

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Working principle

• Current flow magnetizes the coil

• The two iron vanes become magnetized with north poles at their upper ends and south poles at their lower ends for one direction of current through the coil

• Due to repulsion, the unbalanced component of force, tangent to the movable element, causes it to turn against the force exerted by the springs

• When no current flows through the coil, the movable vane is

positioned so that it is opposite the larger portion of the tapered fixed vane, and the scale reading is zero

•The amount of magnetization of the vanes depends on the strength of the field, which, in turn, depends on the amount of current flowing through the coil.

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Design for MI Ammeter and MI

Voltmeter

Ammeter: Coil with few turns of large wire

Voltmeter: 1)Solenoid with many turns of small wire 2)Portable size: a) self contained series resistance b)ranges up to 750 V 3) Higher ranges are obtained by the use of additional external multipliers

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Ranges of Ammeters and Voltmeters

For a given moving-iron instrument the ampere-turns necessary to produce full-scale deflection are constant. One can alter the range of ammeters by providing a shunt coil with the moving coil.

Shunts and Multipliers for MI instruments:

For moving-iron ammeters: For the circuit shown in Fig.42.9, let Rm and Lm are respectively the resistance and inductance of the coil and Rsh and Lsh the corresponding values for shunt.

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The ratio of currents in two parallel branches is

It is difficult to design a shunt with the appropriate inductance, and shunts are rarely incorporated in moving iron ammeters. Thus the multiple ranges can effectively be obtained by winding the instrument coil in sections which may be connected in series, parallel or series-parallel combination which in turn changing the total ampere-turns in the magnetizing coil.

For moving-iron voltmeters:

•Voltmeter range may be altered connecting a resistance in series with the coil. Hence the same coil winding specification may be employed for a number of ranges.

•Let us consider a high resistance Rse is connected in series with the moving coil and it is shown below.

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•Note: An ordinary arrangement with a non-inductive resistance in series with the fixed coil – results in error that increases as the frequency increases. The change of impedance of the instrument with change of frequency introduces error in signal measurements.

•In order to compensate the frequency error, the multiplier may be easily shunted by the capacitor.

Advantages: • The instruments are suitable for use in a.c and d.c circuits. • The instruments are robust, owing to the simple construction of the moving parts. • The stationary parts of the instruments are also simple. • Instrument is low cost compared to moving coil instrument. • Torque/weight ration is high, thus less frictional error.

Errors: •Errors due to temperature variation. •Errors due to friction is quite small as torque-weight ratio is high in moving-iron instruments. •Stray/demagnitization fields cause relatively low values of magnetizing force produced by the coil. Efficient magnetic screening/iron-case/thin shield over the working parts is essential to reduce this effect. •Error due to variation of frequency causes change of reactance of the coil and also changes the eddy currents induced in neighboring metal. •Deflecting torque is not exactly proportional to the square of the current due to non-linear characteristics of iron material.

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• Error due to residual magnetism in the vanes • The error can be minimized by reversing the meter connections and averaging the readings

MI for DC purpose

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• Non linear scale

• Draw more power

• Shielding (laminated iron cylinder) should be

provided to protect from external magnetic fields

• Deflecting torque is not exactly proportional to

I2

• Variation of frequency introduce error

Limitations

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Comparison between the scale of

MI and MC

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Types of Errors

1.Gross Errors

2.Systematic Errors

1.Gross Errors

•All the human mistakes while reading and recording.

•Mistakes in calculating the errors also come under this category.

Ex. while taking the reading from the meter of the instrument he may read 21 as 31. All these types of error are come under this category.

Avoided by using suitable measures as:

•A proper care should be taken in reading, recording the data.

•Calculation of error should be done accurately.

•As increasing the number of experimenters we can reduce the gross errors. If each experimenter takes different reading at different points, then by taking average of more readings we can reduce the gross errors.

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2. Systematic Errors

2.1 Instrumental Errors

•Due to wrong construction, calibration of the measuring instruments.

•Arises due to friction or may be due to hysteresis.

•Include the loading effect and misuse of the instruments which results in the failure to the adjust the zero of instruments.

•To minimize this, various correction factors must be applied and in extreme condition instrument must be re-calibrated carefully.

2.2 Environmental Errors

•External condition includes temperature, pressure, humidity or it may include external magnetic field.

To minimize this error:

•Try to maintain the temperature and humidity of the laboratory constant by making some arrangements.

•Ensure that there should not be any external magnetic or electrostatic field around the instrument.

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2.3 Observational Errors •Due to wrong observations. •The wrong observations may be due to PARALLAX. •To minimize the PARALLAX error highly accurate meters are required, provided with mirrored scales.

2.4 Random Errors After calculating all systematic errors, it is found that there are still some errors in measurement are left. These errors are known as random errors. Some of the reasons of the appearance of these errors are known but still some reasons are unknown. Hence we cannot fully eliminate these kinds of error.