Moving Iron Instrument, Construction, Working, Types, Advantages & Disadvantages
Table of Contents
The device that uses a moving iron to measure current or voltage flow is referred to as a moving iron device (Moving Iron Instrument). It operates on the idea that iron that is placed close to a magnet will pull toward it. The magnet field’s strength affects the force of attraction. The strength of the magnetic field that an electromagnet generates depends on how much current flows through it.
Construction of Moving Iron Instrument
The instrument’s moving component is a plate or vane made of soft iron. The vane is positioned so that it can move freely within the stationary coil’s magnetic field. The stationary coil is made by the conductor and is excited by the voltage or current that is being measured. The stationary coil serves as an electromagnet for the moving iron instrument. The electromagnet is a transient magnet whose magnetic field strength changes depending on how much current flows through it.
Working of the Moving Iron Instrument
The stationary coil of copper or aluminum wire used by the moving iron instrument functions as an electromagnet when an electric current is passed through it. The electromagnet’s ability to induce a magnetic field has a direct relationship with the amount of current that flows through it. The inductance of the stationary coil is increased as the iron plates or vane passes through it (the inductance is a characteristic of the conductor that increases electromotive force when varying current passes through it).
The iron vane is drawn to the electromagnet. The vane attempts to occupy the path with the least amount of reluctance as it passes through the coil; reluctance is a magnet property that opposes the flow of current.
The electromagnet produces a force of repulsion as the vane passes through the coil. The coil’s inductance is strengthened by the repulsive force. This occurs as a result of the inverse relationship between the inductance and reluctance.
Classification of the Moving Iron Instrument
The two main categories of moving iron instruments are those that attract and repel. Below is a detailed explanation of them.
1. Attraction Type
The instrument type known as an attraction type instrument is one in which an iron plate attracts from a weaker field to a stronger field.
Construction of Attraction Type Instrument – The attraction-type instrument’s stationary coil is flat and has a small opening. The iron core’s flat disc is the moving component. The magnetic field that draws the iron coil was created by the current flowing through the stationary coil.
The magnitude of the current flow through the iron vane directly correlates to the strength of the deflection as it moves from a low magnetic field to a high magnetic field. In a nutshell, we can state that the iron coil draws inward. The springs used in the attraction-type instruments provided the controlling torque. The aluminum piston, which is attached to the moving coil, lessens the deflection of the coil.
2. Repulsion Type Instruments
The instrument of the repulsion type has two vanes or iron plates. One is stationary, the other is mobile. When current flows through the stationary coil, the vanes become magnetized and begin to repel one another. The moving coil starts to move away from the fixed vane due to an attractive force.
The controlling torque is provided by the spring. The coil’s movement is resisted by the damping torque that the air friction creates. The instrument of the repulsion type is non-polarized, or free from the direction in which current flows through it. It is therefore utilized for both AC and DC.
Advantages of the MI Instruments
The following are the advantages of the moving iron instruments.
- Less Friction Error – Because of their high torque to weight ratio, moving iron instruments exhibit very low levels of friction error. Because the moving parts are lighter in weight and their current carrying part is stationary, the torque weight ratio is high.
- Universal use – The MI instrument can be used for both AC and DC because it is current direction independent.
- Robustness – The instrument’s sturdy design results from its straightforward construction. Furthermore, their current carrying component is stationary.
- Cheapness – Compared to PMMC instruments, MI instruments require fewer turns. So, it is less expensive.
Disadvantages of Moving Iron Instrument
The following are the disadvantages of Moving Iron Instrument.
- Errors – Hysteresis, frequency, and stray magnetic fields cause some serious errors in the instruments.
- Accuracy – It is impossible to obtain an accurate result because the moving iron instruments’ scales are not consistent.
- Difference between AC and DC calibration – Due to the impact of the meter’s inductance and the eddy current used on AC, the calibration of the AC and DC differs. The frequency that they use is used to calibrate the AC.
- Waveform Error – The deflection torque in a MI instrument is not inversely proportional to the current square. This causes the instrument’s waveforms to malfunction.
In MI instruments, there are two types of errors: those that happen on both AC and DC, and those that happen only on AC.