Short Answer
The principle of moving coil instruments is based on the force experienced by a current-carrying conductor placed in a magnetic field. When current flows through the coil, it experiences a force that causes it to move.
This movement is proportional to the current flowing through the coil. Hence, the deflection of the pointer gives the measurement of the electrical quantity.
Detailed Explanation:
Principle of Moving Coil Instruments
Working Principle
Moving coil instruments work on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force. This force causes movement or rotation of the conductor.
In these instruments, a coil is placed between the poles of a permanent magnet. When electric current flows through the coil, a magnetic field is created around it. This magnetic field interacts with the magnetic field of the permanent magnet.
Due to this interaction, a force is produced, which causes the coil to rotate. The amount of rotation or deflection depends on the strength of the current passing through the coil.
A pointer is attached to the coil, which moves over a calibrated scale. The position of the pointer indicates the value of the current or voltage being measured.
The deflecting torque produced in the instrument is directly proportional to the current flowing through the coil. Therefore, the scale of the instrument is uniform and easy to read.
Construction and Operation
A moving coil instrument mainly consists of a rectangular coil, a permanent magnet, a soft iron core, springs, and a pointer.
The coil is made of fine wire and is mounted on a light frame. It is placed between the poles of a strong permanent magnet. A soft iron core is used to produce a uniform magnetic field.
When current flows through the coil, it experiences a force due to the magnetic field, which causes it to rotate. This rotation is controlled by springs, which provide a restoring force.
The springs also help in bringing the coil back to its original position when the current stops flowing. The balance between deflecting torque and controlling torque determines the final position of the pointer.
Damping is also provided in the instrument to stop oscillations and allow the pointer to settle quickly. This ensures stable and accurate readings.
Moving coil instruments are mainly used for measuring direct current (DC). They cannot measure alternating current (AC) directly because the direction of current keeps changing, which causes the pointer to oscillate.
These instruments are widely used as ammeters and voltmeters in DC circuits because of their high accuracy and sensitivity.
They have a linear scale, which makes reading easy. They also consume less power and provide reliable performance.
Conclusion
Moving coil instruments work on the principle of force acting on a current-carrying conductor in a magnetic field. This force causes the coil to move, and the deflection is proportional to the current. These instruments are accurate and widely used for DC measurements in electrical engineering.