There is no fundamental difference in the operating principles of ammeters and voltmeters. Both are current operated devices (except electrostatic type voltmeters) i.e. deflecting torque is produced when current flows through their operating coils.
In an ammeter, the deflecting torque is produced by the current to be measured or by a definite fraction of it whereas in a voltmeter torque is produced by the current proportional to the voltage to be measured.
Thus, the real difference between the two instruments is in the magnitude of the currents producing the deflecting torque. The essential requirements of a measuring instrument are that its introduction into the circuit, where measurements are to be made, does not alter the circuit conditions and the power consumed by them for their operation is small.
An ammeter is connected in series with the circuit whose current is to be measured. Therefore, it should have a low resistance. On the other hand, a voltmeter is connected in parallel with the circuit whose voltage is to be measured; therefore, it must have high resistance.
Thus we conclude that the difference is only in the resistance of the instrument, in fact, an ammeter can be converted into voltmeter by connecting a high resistance in series with it.
It is already seen that the moving coil instruments can carry the maximum current of about 50 mA safely and the potential drop across the moving coil instrument is about 50 mV. However, in practice, heavy currents and voltages are required to be measured. Therefore, it becomes necessary that the current and voltage being measured be reduced and brought within the range of the instrument.
There are four common devices used for the range extension of ammeter and voltmeter namely; shunts, multipliers and current and potential transformers.
The shunts and multipliers are used to extend the range of moving coil ammeters and voltmeters respectively.
Whereas in the case of moving iron ammeters, for the ranges up to 0 – 250 A, shunts are used and for the ranges higher than that, CTs are used.
And also, in the case of moving iron voltmeters, for the ranges up to 0 – 750 V, multipliers are used and for the ranges higher than that, PTs are used.
Range Extension of Ammeter by Shunt
Shunts are used for the range extension of ammeters. A shunt is a low-value resistance having minimum temperature co-efficient and is connected in parallel with the ammeter whose range is to be extended. The combination is connected in series with the circuit whose current is to be measured.
This shunt provides a bypath for extra current because it is connected across (i.e. in parallel with) the instrument.
These shunted instruments can be used to measure currents many times greater than their normal full-scale deflection currents.
The ratio of maximum current (with shunt) to the full-scale deflection current (without shunt) is known as the ‘multiplying power’ or ‘multiplying factor’ of the shunt.
Emample: A moving coil ammeter reading up to 1 ampere has a resistance of 0.02 ohm. How could this instrument be adopted to read current up to 100 amperes.
Solution: In this case,
Full-scale deflection current of the ammeter, Im = 1 A
Line current to be measured, I = 100 A
Resistance of ammeter, Rm = 0.02 ohm
Let, the required shunt resistance = S
As seen from Figure, the voltage across the instrument coil and the shunt resistance is the same since
both are joined in parallel.
∴ Im*Rm = S*Is = S(I − Im)
or S = Im*Rm/(I – Im)
= 1*0.02/(100 – 1) = 0.02/99 = 0.000202 Ans.
Range Extension of Voltmeter by Multiplier
Multipliers are used for the range extension of voltmeters. The multiplier is a non-inductive high-value resistance connected in series with the instrument whose range is to be extended. The combination is connected across the circuit whose voltage is to be measured.
Example: A moving coil voltmeter reading upto 20 mV has a resistance of 2 ohms. How this instrument can be adopted to read voltage upto 300 volts.
Solution: In this case,
Voltmeter resistance, Rm = 2 ohm
Full-scale voltage of the voltmeter, ν = RmIm = 20 mV = 0.02 V
Full-scale deflection current, Im = v/Rm = 0.02/2 = 0.01 A
Voltage to be measured, V = 300 V
Let the series resistance required = R
Then as seen from figure, the voltage drop across R is V – ν
R *Im = V – ν
or R = (V – v)/Im
or R = (300 – 0.02)/0.01 = 299.98/0.01 = 29998 ohms Ans.
Shunts can not be used to extend the range of moving-iron AC ammeters accurately. It is because the division of current between the operating coil and the shunt varies with frequency (since reactance of the coil depends upon frequency). In practice, the range of moving-iron AC ammeters are extended by one of following methods:
Range Extension of Ammeter by Coil Turns
By changing the number of turns of the operating coil. For example, suppose that full-scale deflection is obtained with 400 ampere-turns. For full-scale reading with 100A, the number of turns required would be = 400 / 100 = 4. Similarly, for full-scale reading with 50A, the number of turns required in = 400/50 = 8.
Thus the ammeter can be arranged to have different ranges by merely having a different number of turns on the coil. Since the coil carries the whole of the current to be measured, it has a few turns of thick wire. The usual ranges obtained by this method are from 0 -250 A.
Range Extension of Ammeter by Current Transformer
For ranges above 0 – 250 A, a current transformer is used in conjunction with 0 – 5 A moving iron AC ammeter as shown in the figure. The current transformer is a step up transformer i.e. number of secondary turns is more than the primary turns.
Usually, the primary winding of the transformer contains a single turn or at the most a few turns. The primary of this transformer is connected in series with the load and carries the load current. The AC ammeter is connected across the secondary of the transformer.
Since in figure, the current transformer ratio is 10:1, it means that line (or load) current is equal to 10 times the reading on the AC meter. Therefore, load current, IL = 3 x 10 = 30 A.
Range Extension of Voltmeter by Potential Transformer
The range of a moving-iron AC voltmeter is extended by connecting a high resistance (multiplier) in series with it.
For ranges higher than 0 – 750 V, where power wasted in the multiplier would be excessive, a 0 – 110 V moving-iron AC voltmeter is used in conjunction with a potential transformer as shown in the figure.
The potential transformer is a step-down transformer i.e. number of primary turns is more than the secondary turns. The primary of the transformer is connected across the load across which voltage is to be measured. The AC voltmeter is connected across the secondary.
Since in figure, the potential transformer ratio is 20:1, the load voltage is equal to 20 times the reading on the AC voltmeter.
Load voltage, VL = 100 x 20 = 2000 V
Note that both secondaries of the instrument transformers are grounded as a safety measure.
Thanks for reading about range extension of ammeter and voltmeter.
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