Voltmeter and Ammeter working principle; Moving iron type instruments working principle
Basic Principle and Construction of Voltmeter and Ammeter
Introduction
Ammeters and Voltmeters are essential instruments used to measure current and voltage in electrical circuits. One of the most common types is the moving-iron (MI) instrument, which works on the principle of the magnetic effect of current. These devices can measure both AC and DC quantities, making them versatile and widely used in power systems.
Principle of Moving-Iron Instruments
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When a current flows through a coil, it produces a magnetic field.
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A soft iron piece placed in this field experiences a force of attraction or repulsion, depending on the instrument type.
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The force causes a deflection of the pointer, which is directly related to the magnitude of the current (in ammeters) or voltage (in voltmeters).
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The deflection is proportional to the square of the current (i²), so the scale is non-linear, compressed at the lower end.
Types of Moving-Iron Instruments
There are two kinds of MI instruments:
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Attraction Type
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A single piece of soft iron is attracted towards the coil when current flows.
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Pointer moves over a scale due to this attraction.
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Repulsion Type
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Two iron pieces are magnetized with the same polarity.
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They repel each other, causing the pointer to deflect.
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Deflection occurs irrespective of current direction (works for both AC & DC).
Construction of Moving-Iron Instruments
The key components are:
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Moving Element
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A soft iron vane or piece that moves when subjected to the magnetic field.
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Coil
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A hollow cylindrical coil through which the current flows, creating a magnetic field.
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Control System
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Provides controlling torque.
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Spring control (modern instruments) or gravity control (older instruments).
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Damping System
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Prevents oscillations of the pointer.
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Air friction damping is used (a piston moving inside an air chamber).
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Eddy current damping is avoided because permanent magnets would distort the weak magnetic field.
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Pointer & Scale
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Lightweight aluminum pointer attached to spindle.
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Scale is non-uniform, crowded at the lower end.
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Working of Ammeter and Voltmeter
Ammeter
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Connected in series with the circuit.
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Entire current to be measured passes through the coil.
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To extend range, a shunt is connected so that only part of the current flows through the meter.
Voltmeter
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Connected in parallel with the circuit.
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A high resistance is connected in series with the coil to limit current.
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This ensures correct proportional deflection without damaging the instrument.
Advantages
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Can measure both AC and DC quantities.
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Rugged and simple construction.
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Low cost compared to moving-coil instruments.
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High torque-to-weight ratio → less friction error.
Limitations / Errors
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Non-linear scale (not suitable for precision measurements).
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Errors due to frequency variation (in AC circuits).
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Sensitive to stray magnetic fields → shielding required.
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Temperature variation affects resistance and accuracy.
Application
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Used as panel meters in laboratories and power stations.
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Suitable where approximate readings are sufficient.
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Common in AC measurement systems.
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