Moving Iron instruments working principle and effects of electrical instruments

Effects of electrical instruments:

Type	Effect	Suitable	Instrument
MI	Magnetic effects	DC/AC	(A) (V)
PMMC	Electrodynamic effects	DC	(A) (V)
EMMC	Electrodynamic effects	DC/AC	(A) (V) (W)
Induction type	Electro-magnetic induction effect	AC	(A) (V) (W) (E)
Hotwire	Thermal effect	DC/AC	(A) (V)
Electrolytic meter	Chemical effects	DC	(Ah)
Electrostatic type	Electrostatic effects	DC/AC	(V)

Moving Iron instruments working principle:

Moving iron instruments are a type of analog measuring instrument used to measure electrical quantities such as voltage, current, and power. They operate on the principle of electromagnetic induction, where a magnetic field is generated by the current flowing through a coil, which then interacts with a piece of ferromagnetic material (usually iron) that is free to move. The movement of the iron in response to the magnetic field is then used to indicate the value of the measured quantity on a calibrated scale.

There are two types of MI instruments:

1. Attraction type MI instrument
2. Repulsion type MI instrument

1. Attraction type MI instrument:

• An attraction type moving iron instrument is a type of moving iron instrument where the iron piece is attracted towards the coil, rather than repelled from it as in a repulsion type instrument.
• Construction of this instrument consists of a stationary coil in which the current(I) that is to be measured is passed.
• A piece of unmagnetized soft iron which is of oval shape is mounted rigidly on a spindle.
• This soft iron moves about the spindle and along with the spindle. It is placed close to the stationary coil.
• It moves towards the coil in response to the magnetic field generated by the current flowing through the coil.
• The amount of deflected torque produced will be greater if the magnetic field intensity increases. and control torque is provided either by control spring or gravity control method.
• Most of the instrument we are use spring control. Damping is provided by the air friction damping method.
• The scale is non-linear and avoid parallax errors.
• Attraction-type moving iron instruments are commonly used for measuring AC voltage and current, as well as power.

2.Repulsion type MI instrument:

• A repulsion type moving iron instrument is a type of moving iron instrument where the iron piece is repelled from the coil, rather than attracted towards it as in an attraction type instrument.
• Constructional this instrument consists of two iron vanes. one is attached to a stationary coil and the other is attached to movable spindle. Both vanes are surrounded by the stationary coil through which the current measured is passed.
• This current set up magnetic field which magnetizes both iron vanes with similar polarities. As a result, a force of repulsion is set up between the two vanes. this force produces a deflecting torque 'Td' on the movable vane.
• Thus the spindle & hence the pointer deflects on the scale.
• In other words the iron piece is mounted on a pivot and is free to rotate.
• The magnetic field generated by the current flowing through the coil interacts with the iron piece and causes it to rotate, with the amount of rotation being proportional to the strength of the magnetic field and hence the current being measured.
• On the basis of construction, this type of instruction can be further classified as Radial vane type and co-axial vane type.
• Repulsion type moving iron instruments are commonly used for measuring DC current, as they are capable of measuring both positive and negative currents.

Sources of error in MI instruments:

1. Hysteresis error: This error occurs as the value of flux density is different for the same current when ascending and descending.
2. Temperature error: The temperature errors are caused due to heating of coil and resistance.
3. Stray magnetic field error: The stray magnetic field error are caused due to weak magnetic field and hence can be easily distorted.
4. Frequency error: Frequency error are caused due to changes of reactance of the working coil and also due to changes of magnitude of eddy current set up in metal parts of instrument.
5. Eddy current error: This error is caused due to eddy current induced in metal part of the instrument.

Advantages and Dis-advantages of MI instrument:

Advantages:

1. Less friction error in MI instrument.
2. It can be used on A.C. as well as on D.C. supplies.
3. Accuracy is within the limits of precision and industrial grades.
4. In-expensive.
5. MI instruments can be designed to carry high currents directly without the help of a shunt. because the coil which carries the current is not present on the moving system.

Dis-advantages:

1. Scale is cramped at lower end.
2. subjected to serious errors.

Difference between PMMC, Dynamometer type and MI instruments:

Particular	Moving Coil	Dynamometer type	Moving Iron
Construction	Delicate Construction	Heavy Moving System	Simple
Cost	Very High	High	Low
Power Consumption	Very Low	High	Less than dynamometer type
Scale	Uniform	Non-uniform	Non-uniform
Torque-Weight ratio	High	Small	More than dynamometer type
Suitable for	D.C only	D.C as well as A.C	D.C as well as A.C
Effect of stray magnetic fields	Not affected	Affected	Not affected
Accuracy	High	Poor	Reasonable
Sensitivity	High	Poor	Reasonable
Application	Voltmeters and Ammeters	Generally for wattmeters	Voltmeters and Ammeters

People also ask:

1. Use of MI instruments as ammeter and voltmeter

For use as an ammeter, the fixed coil has comparatively less number of turns and more cross section conductor, whereas for use as a voltmeter, it should be made from a thin conductor(wire) with large number of turns so that its resistance is very high.

2. Use of MI instruments on A.C and D.C supplies

Both types of moving iron instruments can work on A.C as well as D.C supplies. whatever may be the direction of current in the coil, the iron piece gets so magnetized that there is always a force of attraction in attraction type instrument and a force of repulsion in repulsion type instruments. so that we can say MI instrument is also known as unpolarized instruments.