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Moving iron instruments are a type of electrical measuring device that utilize the principle of electromagnetic attraction and repulsion to measure electrical quantities such as current, voltage and power. These instruments are based on the movement of an iron or iron alloy element, known as the moving iron, which is placed in a magnetic field.
The movement of the iron element is caused by the interaction between the magnetic field and the electrical current being measured. Moving iron instruments are known for their high accuracy, fast response time and low cost, making them a popular choice for use in a wide range of industrial, commercial and laboratory applications.
Moving iron attraction type instruments:
Moving iron attraction type instruments, also known as moving iron ammeters, are specific types of moving iron instruments that measure electrical current. These instruments utilize the principle of electromagnetic attraction to measure the current flowing through a circuit.
The moving iron element, which is typically made of iron or an iron alloy, is placed in a magnetic field created by a permanent magnet or an electromagnet. The movement of the iron element is caused by the interaction between the magnetic field and the electrical current flowing through the circuit.
In a moving iron attraction-type ammeter, the iron element is typically suspended on a pivot or a spring and is connected to a pointer, which is visible on the instrument’s dial. The movement of the iron element causes the pointer to rotate, indicating the magnitude of the current flowing through the circuit.
Attraction-type moving iron instruments are known for their high accuracy, fast response time and low cost. They are widely used in a variety of industrial, commercial and laboratory applications, such as measuring current in power distribution systems, electrical motors, and other electrical equipment.
It’s important to note that moving iron attraction-type instruments are not suitable for measuring direct current (DC) as the moving iron will be in a fixed position, thus not providing any indication of the current.
Moving iron repulsion type instruments:
Moving iron repulsion type instruments, also known as moving iron voltmeters, which are specific type of moving iron instruments that measure electrical voltage. These instruments utilize the principle of electromagnetic repulsion to measure the voltage across a circuit.
The moving iron element, which is typically made of iron or an iron alloy, is placed in a magnetic field created by a permanent magnet or an electromagnet. The movement of the iron element is caused by the interaction between the magnetic field and the electrical voltage across the circuit.
In a moving iron repulsion-type voltmeter, the iron element is typically suspended on a pivot or a spring and is connected to a pointer, which is visible on the instrument’s dial. The movement of the iron element causes the pointer to rotate, indicating the magnitude of the voltage across the circuit.
The moving iron element in a repulsion-type voltmeter is usually a pair of small iron balls or discs, which are placed in the magnetic field and held in position by a spring. When the voltage is applied across the circuit, the balls are repelled away from each other, causing the pointer to rotate.
Repulsion-type moving iron instruments are known for their high accuracy, fast response time and low cost. They are widely used in a variety of industrial, commercial and laboratory applications, such as measuring voltage in power distribution systems, electrical motors, and other electrical equipment.
It’s important to note that moving iron repulsion-type instruments are not suitable for measuring alternating current (AC) as the moving iron will be in a fixed position, thus not providing any indication of the voltage.
Radial vane repulsion type instruments:
Radial vane repulsion type instruments are a specific type of moving iron repulsion instrument that utilizes radial vanes instead of iron balls or discs as the moving element. These instruments are used to measure electrical voltage.
The radial vanes in a radial vane repulsion type voltmeter are typically made of a magnetic material such as iron or an iron alloy. They are suspended in a magnetic field created by a permanent magnet or an electromagnet. The movement of the radial vanes is caused by the interaction between the magnetic field and the electrical voltage across the circuit.
In a radial vane repulsion type voltmeter, the radial vanes are typically arranged in a circular pattern around the centre of the instrument, and each vane is connected to a pointer, which is visible on the instrument’s dial. As the vanes are repelled by the voltage, the pointers rotate, indicating the magnitude of the voltage across the circuit.
Radial vane repulsion-type instruments are known for their high accuracy, fast response time, and low cost. They are widely used in a variety of industrial, commercial, and laboratory applications, such as measuring voltage in power distribution systems, electrical motors, and other electrical equipment.
It’s important to note that radial vane repulsion type instruments are not suitable for measuring alternating current (AC) as the vanes will be in a fixed position, thus not providing any indication of the voltage.
Co-axial vane repulsion type instruments:
Co-axial vane repulsion type instruments are a specific type of moving iron repulsion instrument that utilizes co-axial vanes instead of iron balls or discs as the moving element. These instruments are used to measure electrical voltage.
The co-axial vanes in a co-axial vane repulsion type voltmeter are typically made of a magnetic material such as iron or an iron alloy. They are suspended in a magnetic field created by a permanent magnet or an electromagnet. The movement of the co-axial vanes is caused by the interaction between the magnetic field and the electrical voltage across the circuit.
In a co-axial vane repulsion type voltmeter, the co-axial vanes are typically arranged in a concentric pattern around the centre of the instrument, with one vane inside the other. Each vane is connected to a pointer, which is visible on the instrument’s dial. As the vanes are repelled by the voltage, the pointers rotate, indicating the magnitude of the voltage across the circuit.
Co-axial vane repulsion-type instruments are known for their high accuracy, fast response time, and low cost. They are widely used in a variety of industrial, commercial, and laboratory applications, such as measuring voltage in power distribution systems, electrical motors, and other electrical equipment.
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Advantages of Moving iron instruments:
Moving iron instruments have several advantages over other types of electrical measuring devices:
High accuracy: Moving iron instruments are known for their high accuracy and are capable of measuring electrical quantities with a high degree of precision.
Fast response time: Moving iron instruments have a fast response time, which means they can quickly respond to changes in the electrical current or voltage being measured.
Low cost: Moving iron instruments are relatively inexpensive to manufacture and are widely available, making them a cost-effective option for a wide range of industrial, commercial and laboratory applications.
Durability: Moving iron instruments are sturdy and durable and can withstand harsh environments.
Direct reading: Moving iron instruments provide a direct reading of the electrical current or voltage being measured, which makes them easy to use and understand.
Low power consumption: Moving iron instruments consume less power than other types of instruments, making them more energy-efficient.
Suitable for both AC and DC: Moving iron instruments are suitable for measuring both AC and DC, which makes them versatile and useful in a wide range of applications.
Robustness: Moving iron instruments are robust and can withstand high vibration and shock.
Low maintenance: Moving iron instruments require low maintenance and are easy to repair.
Compatibility: Moving iron instruments are compatible with a wide range of electrical equipment and can be easily integrated into existing systems.
Disadvantages of Moving iron instruments:
Moving iron instruments also have some disadvantages:
Sensitivity to temperature: Moving iron instruments are sensitive to changes in temperature, which can affect their accuracy.
Limited measurement range: Moving iron instruments have a limited measurement range, which means they can only measure a certain range of electrical current or voltage.
Need for calibration: Moving iron instruments require regular calibration to ensure their accuracy.
Not suitable for high-frequency measurements: Moving iron instruments are not suitable for measuring high-frequency signals due to their slow response time.
Limited dynamic range: Moving iron instruments have a limited dynamic range, which means they are not suitable for measuring a wide range of currents or voltages.
Limited accuracy: Moving iron instruments have limited accuracy when compared to digital instruments, which are more accurate
Not suitable for measurement of low currents: Moving iron instruments are not suitable for measuring low currents as the torque developed will be very small and may not be able to move the moving iron element.
Vulnerable to electromagnetic interference: Moving iron instruments are vulnerable to electromagnetic interference, which can affect their accuracy.
Not suitable for high voltage: Moving iron instruments are not suitable for measuring high voltages as the air gap distance between the moving iron element and the magnet will be large and the magnetic field will not be sufficient to move the element.
Not suitable for high temperatures: Moving iron instruments are not suitable for measuring high temperatures as the temperature may affect the properties of the moving iron element and the magnet.
Different errors in Moving iron instruments:
Moving iron instruments, like any other type of measuring device, can be affected by various errors that can impact their accuracy. Some of the common errors found in moving iron instruments are:
Zero error: This occurs when the instrument’s pointer does not align with the zero point on the scale when there is no current or voltage flowing through the circuit.
Sensitivity error: This occurs when the instrument does not respond correctly to changes in the current or voltage being measured.
Hysteresis error: This occurs when the instrument’s pointer does not return to the same position after being moved by a current or voltage.
Temperature error: This occurs when the instrument’s accuracy is affected by changes in temperature.
Magnetization error: This occurs when the instrument’s magnet is not properly magnetized, which can affect the accuracy of the instrument.
Frequency error: This error occurs when the instrument is not able to accurately measure the current or voltage at different frequencies. Moving iron instruments are not suitable for high-frequency measurements as the response time is relatively slow.
Eddy current error: This error occurs when eddy currents are induced in the moving iron element by the alternating magnetic field. These eddy currents produce their own magnetic field that opposes the main magnetic field, which can affect the accuracy of the instrument.
MCQ’s on Moving Iron Instruments:
- Which type of moving iron instrument measures current?
a) Attraction type
b) Repulsion type
c) Radial vane type
d) Co-axial vane type
Answer: a) Attraction type
- Which type of moving iron instrument measures voltage?
a) Attraction type
b) Repulsion type
c) Radial vane type
d) Co-axial vane type
Answer: b) Repulsion type
- What is the principle of operation of moving iron instruments?
a) Electromagnetic attraction
b) Electromagnetic repulsion
c) Electromagnetic induction
d) Both a) and b)
Answer: d) Both a) and b)
- What is the main advantage of moving iron instruments?
a) High accuracy
b) Fast response time
c) Low cost
d) All of the above
Answer: d) All of the above
- What is the main disadvantage of moving iron instruments?
a) Sensitivity to temperature
b) Limited measurement range
c) Need for calibration
d) All of the above
Answer: d) All of the above
- What is the zero error in moving iron instruments?
a) When the instrument’s pointer does not align with the zero point on the scale when there is no current or voltage flowing through the circuit
b) When the instrument does not respond correctly to changes in the current or voltage being measured
c) When the instrument’s pointer drifts over time, even when there is no current or voltage flowing through the circuit
d) When the instrument’s pointer does not return to the same position after being moved by a current or voltage
Answer: a) When the instrument’s pointer does not align with the zero point on the scale when there is no current or voltage flowing through the circuit
- How can eddy current error be minimized in moving iron instruments?
a) Using a moving iron element made of a material with low electrical conductivity
b) Designing the instrument to have a smaller air gap between the moving iron element and the magnet
c) Both a) and b)
d) None of the above
Answer: c) Both a) and b)
- What is the hysteresis error in moving iron instruments?
a) When the instrument’s pointer does not return to the same position after being moved by a current or voltage
b) When the instrument does not respond correctly to changes in the current or voltage being measured
c) When the instrument’s pointer drifts over time, even when there is no current or voltage flowing through the circuit
d) When the instrument’s pointer does not align with the zero point on the scale when there is no current or voltage flowing through the circuit
Answer: a) When the instrument’s pointer does not return to the same position after being moved by a current or voltage
- Moving iron instruments are suitable for measuring which type of current?
a) Alternating current (AC)
b) Direct current (DC)
c) Both a) and b)
d) None of the above
Answer: c) Both a) and b)
FAQ on Moving Iron Instruments:
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Why moving iron is used for both AC and DC?
The moving iron element in an instrument is able to respond quickly to changes in the current or voltage being measured, regardless of whether it is AC or DC. Additionally, the moving iron element can be designed to have a low electrical conductivity, which minimizes the effects of eddy currents and allows for accurate measurement of both AC and DC.
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What are the two types of moving iron instruments?
The two types of moving iron instruments are attraction-type instruments and repulsion-type instruments. Attraction-type instruments are used to measure current, while repulsion-type instruments are used to measure voltage.
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Where is the moving iron instrument used?
Moving iron instruments are widely used in a variety of industrial, commercial and laboratory applications, such as measuring current and voltage in power distribution systems, electrical motors, and other electrical equipment.
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What is the difference between MC and MI instruments?
MC stands for moving coil and MI stands for moving iron. Moving coil instruments use a coil of wire as the moving element while moving iron instruments use a piece of iron or an iron alloy as the moving element. Moving coil instruments are typically more accurate, but moving iron instruments are less expensive and have a faster response time.
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Is moving iron the same as moving magnet?
No, moving iron and moving magnet is not the same. Moving iron instruments use a piece of iron or an iron alloy as the moving element while moving magnet instruments use a magnet as the moving element.
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What is the difference between PMMC and moving iron instruments?
PMMC stands for Permanent Magnet Moving Coil and it uses a permanent magnet to generate a magnetic field and a moving coil as the moving element. Moving iron instruments use a piece of iron or an iron alloy as the moving element, and a magnet (permanent or electromagnetic) to generate the magnetic field.
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Which is best moving magnet or a moving coil?
It depends on the application and the specific requirements. Moving coil instruments are typically more accurate than moving magnet instruments, but they are also more expensive and have a slower response time. Moving magnet instruments are less expensive and have a faster response time, but they are not as accurate as moving coil instruments.