Electrical and Electronic Measurements mainly deals with study of PMMC i.e. Permanent Magnet Moving Coil Instruments. They are used for measuring DC (direct current) electrical quantities such as voltage, current, and resistance. However, the PMMC instrument cannot be used to measure AC (alternating current) electrical quantities. Let’s see why?
What are PMMC instruments?
The PMMC (Permanent Magnet Moving Coil) instrument is a type of analog instrument that is primarily used for measuring DC (direct current) electrical quantities such as voltage, current, and resistance.
You can watch the quick video below or read along.
The principle of operation of the PMMC instrument is based on the magnetic effect of electric current.
When a DC current flows through the coil, it produces a magnetic field that interacts with the magnetic field of the permanent magnet, causing the coil to rotate.
The amount of rotation is proportional to the strength of the current flowing through the coil, which can be measured and displayed on a calibrated scale.
Two magnets will continuously developing the magnetic field.
Now between these two magnets one coil is placed and that coil is nothingbut the moving coil so here you can see.
When we are giving the DC Supply, current starts flowing through the coil.
This current produces deflecting torque due to which coil starts rotating and the pointer which is attached on that spindle that alsostarts rotating.
This will rotate only in one direction why because we are giving the DC Supply.
This DC supply is having unidirectional in nature.
Due to this, due to this continuous supply, this pointer will rotate in one direction.
When we remove that DC Supply, what is happening the pointer will come to the zero position.
What happens with PMMC on AC?
In this case the EC Supply is given AC means alternating in nature.
So, here this waveform which we are using that is sinusoidal in nature that means it is having alternating in nature alternating means it is having one positive half cycle and the next that is negative half cycle.
So, let us consider first the positive half cycle.
Here we can say the pointer will rotate in one drection direction.
When negative cycle will appear; so automatically these polarities will change the supply polarities will change.
So due to that the torque or you can say this coil will rotate in opposite direction.
The frequency for the AC Supply is 50 hertz.
So the time period for this complete cycle will be 0.2 seconds that means every half cycle will appear for only 0.1 seconds that means for 0.1 seconds.
Before reaching to some value the pointer will try to go in the opposite direction.
The pointer will oscillate at the zero position it will never go in clockwise direction or it will never go in the anti-clockwise direction.
It will be completely oscillating at the zero position only so here it will never show any reading when we give the AC Supply.
So in short, PMMC wonn’t show any reading if we apply the AC. Simple!
Do not forget to check our Awesome GATE courses.