Friends, today in this article we will learn about the different types of errors in PMMC instruments. I am going to give information about all these in a very good way. So let’s start.
Different types of errors in PMMC instruments
The possible reasons for the occurrence of errors in moving coil (PMMC) instruments are as follows
(a) Weakening of permanent magnets due to aging due to temperature effect.
(b) Weakening of spring due to temperature effect and aging.
(c) Change in resistance of moving coil instruments due to temperature.
(a) Permanent magnet –
In order to make magnetism stable, magnetism fades during heat and vibration treatment. This process damages the initial magnetism.
(b) Springs –
Weakening of springs can be reduced by careful handling of material and pre-ageing during manufacture. However, weakening of springs has an opposite effect on the performance of the device when the magnet is ageing. When the magnetism weakens, the deflection decreases at a given value of current, while when the spring weakens, the deflection increases.
In permanent magnet moving coil devices, on increasing the temperature by 1°C, the strength of the spring decreases by 0.04% and the flux density of the air gap of the magnet decreases by about 0.02%. Thus, on an average, the deflection increases by about 0.02% per °C.
(c) Moving coil –
The moving coil of any measuring instrument is wound with copper wire having a temperature coefficient of 0.004/°C. When the instrument is used as a micro-ammeter or milli-ammeter and the moving coil of the instrument is directly connected to the output terminal, then the deflection of the instrument for constant current decreases with increase in temperature by 0.04% per °C.
In case of using a moving coil instrument as a voltmeter, a high series resistance having a negligible temperature coefficient (made of manganin metal) is used. By using this, the error caused due to temperature is eliminated because the copper coil forms only a very small part of the total resistance of the instrument circuit and hence any change in resistance has a negligible effect on its total resistance.
The situation is different when the current range of the sub-instrument is increased using a shunt. In this case, the main reason for the occurrence of error is due to the large change in the relative resistance of the copper moving coil as compared to the magnin shunt because the temperature coefficient of resistance of copper is higher than that of magnin. To reduce this error, a resistance called swamping resistance, which is made of magnin, is used in series with the moving coil so that the copper coil forms only a part of the total resistance (resistance of the coil and swamping resistance). This swamping resistance is also used for calibration of the ammeter.
What did you learn today:-
Now you must have known what are the different types of errors of P.M.M.C. sub-instrument. You must have got the answer to all these questions very well.
I hope you liked the information given by me, if you have any question/suggestion in your mind, then you can tell me by commenting below, I will definitely reply to your comment. If you liked this post, then you can also share it with your friends and relatives.