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.
- Attraction type MI instrument
- 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:
- Hysteresis error: This error occurs as the value of flux density is different for the same current when ascending and descending.
- Temperature error: The temperature errors are caused due to heating of coil and resistance.
- Stray magnetic field error: The stray magnetic field error are caused due to weak magnetic field and hence can be easily distorted.
- 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.
- 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:
- Less friction error in MI instrument.
- It can be used on A.C. as well as on D.C. supplies.
- Accuracy is within the limits of precision and industrial grades.
- In-expensive.
- 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:
- Scale is cramped at lower end.
- 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 |
Frequently Asked Questions (FAQs) on Moving Iron Instruments
1. What is a Moving Iron (MI) instrument?
A Moving Iron (MI) instrument is an analog electrical measuring device used to measure current, voltage, and power. It works on the principle of electromagnetic induction, where a magnetic field produced by current interacts with a soft iron piece to produce deflection.
2. What are the types of Moving Iron instruments?
Moving Iron instruments are mainly classified into two types:
- Attraction type MI instrument
- Repulsion type MI instrument
3. What is an attraction type MI instrument?
An attraction type MI instrument works on the principle of attraction of a soft iron piece towards a current-carrying coil. The iron moves closer to the coil due to the magnetic field, causing pointer deflection.
4. What is a repulsion type MI instrument?
A repulsion type MI instrument works on the principle of repulsion between two similarly magnetized iron vanes. One vane is fixed and the other is movable, and the repulsive force produces deflection.
5. What is the working principle of MI instruments?
MI instruments operate on the principle that a current flowing through a coil produces a magnetic field, which magnetizes a soft iron piece. The interaction between the magnetic field and iron produces a mechanical force that causes pointer movement.
6. Can MI instruments be used for both AC and DC?
Yes, Moving Iron instruments can be used for both AC and DC measurements because their operation depends on the magnitude of current, not its direction.
7. Why is the scale of MI instruments non-linear?
The scale of MI instruments is non-linear because the deflecting torque is proportional to the square of the current, making the scale cramped at lower values.
8. What are the common applications of MI instruments?
MI instruments are commonly used as ammeters and voltmeters in electrical circuits for both AC and DC measurements.
9. What are the sources of error in MI instruments?
The main sources of error in MI instruments include:
- Hysteresis error
- Temperature error
- Stray magnetic field error
- Frequency error
- Eddy current error
10. What are the advantages of Moving Iron instruments?
- Can be used for both AC and DC
- Simple construction
- Low cost
- Can measure high currents directly
- Less friction error
11. What are the disadvantages of Moving Iron instruments?
- Non-uniform scale
- Low accuracy compared to PMMC
- Affected by external magnetic fields
- Subject to various errors
12. What is the difference between attraction and repulsion type MI instruments?
In attraction type MI instruments, a single iron piece is attracted towards the coil. In repulsion type instruments, two iron vanes are magnetized with similar polarity and repel each other, producing deflection.
13. How does MI compare with PMMC instruments?
MI instruments can measure both AC and DC and are less expensive, whereas PMMC instruments are more accurate, have uniform scale, but are limited to DC only.
14. What type of damping is used in MI instruments?
MI instruments typically use air friction damping to control oscillations and stabilize the pointer.
15. Why are MI instruments widely used in industries?
MI instruments are widely used because of their rugged construction, low cost, ability to measure both AC and DC, and suitability for high current measurements.