Thursday, 9 November 2017

Ammeter Shunt –Construction and Calculation

Ammeter Shunt is used for the measurement of heavy current using an Ammeter. Ammeter Shunt is basically a low resistance connected in parallel with the moving coil so that most of the current is bypassed by the Shunt and hence only a small current flows through the moving coil.

Why do we need Ammeter Shunt for Measuring High Current?


The basic movement of DC Ammeter is a PMMC Instrument. The coil winding of PMMC Instrument is small and light and is only designed to carry very low current since the construction of an accurate instrument with moving coil to carry current more than 100 mA is not feasible as bulk and weight of coil would be required. Thus if we simply connect the Ammeter for measuring higher current then flow of current through the moving coil will be much more than the for which it is designed and will burn.



Ammeter Shunt Material:


The basic requirement for Ammeter Shunt can be summarized as

1) The resistance of shunt should not vary with time

2) They should carry current without excessive rise in temperature

3) They should have low thermal electromotive force with copper

Well, Manganin is generally used for Shunt of DC Instruments as it gives low value of thermal emf with copper although it is liable to corrosion and difficult to solder. Constantan is used for AC circuit.

Calculation of Ammeter Shunt:


Let us assume that we want to measure a current of I while moving coil of Ammeter is only designed to carry a current of Im (full scale deflection current), therefore we need to use ammeter shunt. We will calculate the value of suitable shunt. Figure below shows the basic circuit of ammeter.

Ammeter-circuit
In the figure above, ammeter shunt resistance Rsh is connected in parallel with the moving coil (Resistance of moving coil is assumed to be Rm).

Therefore, IshxRsh = ImxRm

⇒ Rsh = (Im / Ish)xRm  …………………………….(1)

But Ish = I – Im

Therefore from equation (1), we can write as

Rsh = (Im xRm) /( I – Im)

Dividing Numerator and Denominator by Im,

 ⇒ Rsh = Rm/(I/Im – 1)

⇒ (I/Im – 1) = Rm/Rsh

I/Im = 1+Rm/Rsh

Here, the ratio of total current to be measured to the current in moving coil i.e. (I/Im) is called Multiplying Power of Ammeter Shunt.

Therefore,

Multiplying Power, m = I/Im

                                      = 1+Rm/Rsh

Resistance of Ammeter Shunt,

Rsh = Rm/(m-1)

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