Permanent Magnet Moving Coil Instrument (PMMC)
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A permanent magnet moving coil (PMMC) is a measuring device used to measure electrical current. the dc level is measured very accurately. It has a continuous coil that allows voltage and current to be measured by rotating that coil into a magnetic field. Also called a galvanometer
In a PMMC meter, a coil of wire is placed between two permanent magnets. This arrangement creates a strong magnetic field. According to Faraday's law of electromagnetic induction, when an electric current passes through a wire in the presence of a magnetic field, the wire develops a force and this force causes the coil to move, and Fleming's law of bending governs how it will go away.
Construction of PMMC instrument
- Coil of a wire or Moving Coil
- Permanent Magnets
- Control System
- Pointer & Scale
- Damping Mechanism
- Spring
Coil of a wire or Moving coil
The coil is a current-carrying part of the device that moves freely between two permanent magnets.
Permanent Magnets or Magnet system
There are one or more permanent magnets on either side of the coil. These magnets produce a constant magnetic field that gets stronger over time.
Control System
The coil is attached to a flexible suspension system that allows the electric current in the coil to move freely in response to the interaction between permanent magnets and a magnetic field.
Pointer & Scale
The coil is connected to the pointer, connected by a small pointer shaped like a blade. This symbol is calibrated to indicate the reading on the scale, allowing the measured electrical current to be read. A scale is a device that indicates a reading so that we can note the reading.
Damping Mechanism
To save you the coil from oscillating excessively and settle fast to its equilibrium role, a damping mechanism is often blanketed. This can involve a small magnet moving in a nearby aluminum or copper tube, which generates eddy currents that oppose the coil's motion.
Spring
To ensure that the coil returns to its unique position after the size, small springs are used within the suspension system. These springs provide the restoring force had to carry the coil returned to its equilibrium function.
Working Principal of PMMC
The working principle of a PMMC (Permanent Magnet Moving Coil) is primarily based at the interaction among a magnetic discipline produced by using a everlasting magnet and the magnetic field generated through a cutting-edge-wearing coil. This interaction results in a mechanical deflection of the coil, that is then used to degree modern-day.
Why PMMC used for DC not for AC?
In a PMMC device, there's a magnet that creates a consistent magnetic field and a coil which could pass. With direct modern (DC), the go with the flow of power is always in a single direction, and the coil's motion remains quite steady in response to the solid magnetic subject. This makes it clean to measure and examine the contemporary correctly.
However, with alternating present day (AC), the go with the flow of strength adjustments route unexpectedly, normally many times consistent with second. This rapid alternate in path reasons the coil to vibrate or oscillate, making it difficult to get a solid and accurate studying on the dimensions. The coil's steady lower back-and-forth motion in reaction to the converting magnetic fields of AC makes it fallacious for correctly measuring AC using a PMMC tool.
Torque Equation of PMMC instrument
T=k⋅B⋅I⋅cos(θ)
Where:
- T is the torque experienced by the moving coil (in Nm or dyne-cm).
- k is a constant that depends on the design of the instrument (in Nm/A or dyne-cm/A).
- B is the magnetic field strength in which the coil is placed (in tesla or gauss).
- I is the current passing through the coil (in amperes).
- θ is the angle between the direction of the magnetic field and the plane of the coil.
Error in PMMC instruments:
- Friction and Hysteresis Errors: Friction within the pivot or bearings of the shifting coil can introduce errors main to inaccuracies inside the measurement. Hysteresis that's the phenomenon in which the instrument does not go back to the equal position after reversing the present day direction.
- Temperature Effects: Temperature adjustments can affect the mechanical and electric houses of the instrument leading to versions in its performance.
Advantages of PMMC:
- It consumes low power.
- It has very high accuracy.
- No hysteresis loss.
- Very reliable
Disadvantage of PMMC:
- Cannot measure AC.
- About 50% more expensive than moving-iron instrument because of their accurate design.
- High Cost.
- Some errors are due to the control springs and permanent magnets.
