The Reading on the Ammeter in the Figure Below Is 2.50 a
There is no key difference in the operating principles of ammeters and voltmeters. Both are current operated devices (except electrostatic type voltmeters) i.east. deflecting torque is produced when electric 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 divergence between the 2 instruments is in the magnitude of the currents producing the deflecting torque. The essential requirements of a measuring musical instrument are that its introduction into the circuit, where measurements are to exist made, does not modify the excursion weather and the power consumed past them for their operation is small.
An ammeter is connected in serial with the excursion whose current is to exist measured. Therefore, it should accept a low resistance. On the other hand, a voltmeter is continued in parallel with the circuit whose voltage is to exist 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 exist converted into voltmeter past connecting a high resistance in serial with it.
It is already seen that the moving whorl instruments can carry the maximum current of nearly l mA safely and the potential driblet across the moving ringlet musical instrument is nigh 50 mV. However, in do, heavy currents and voltages are required to exist measured. Therefore, it becomes necessary that the current and voltage being measured be reduced and brought within the range of the musical instrument.
At that place 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 atomic number 26 ammeters, for the ranges upward to 0 – 250 A, shunts are used and for the ranges higher than that, CTs are used.
And also, in the example of moving iron voltmeters, for the ranges upwards to 0 – 750 V, multipliers are used and for the ranges higher than that, PTs are used.
Extension of Range of Ammeter by Shunt
It is possible to extend the range of an ammeter by using a shunt. 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 actress electric current considering information technology is connected across (i.due east. in parallel with) the instrument.
These shunted instruments tin can exist used to mensurate 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 ringlet ammeter reading upward to 1 ampere has a resistance of 0.02 ohm. How could this musical instrument be adopted to read current up to 100 amperes.
Solution: In this case,
Full-calibration deflection current of the ammeter, Ithou = 1 A
Line current to be measured, I = 100 A
Resistance of ammeter, Rgrand = 0.02 ohm
Permit, the required shunt resistance = S
As seen from Figure, the voltage beyond the instrument coil and the shunt resistance is the same since
both are joined in parallel.
∴ Im*Rthousand = South*Is = S(I − Im)
orDue south = Im*Rm/(I – Im)
= 1*0.02/(100 – ane) = 0.02/99 = 0.000202 Ans.
Extension of Range of Voltmeter by Multipliers
Multipliers are used for the range extension of voltmeters. The multiplier is a non-inductive loftier-value resistance connected in series with the musical instrument whose range is to be extended. The combination is connected across the circuit whose voltage is to exist measured.
Example: A moving coil voltmeter reading up to 20 mV has a resistance of 2 ohms. How this instrument can be adopted to read voltage up to 300 volts.
Solution: In this example,
Voltmeter resistance, Rg = 2 ohm
Full-calibration voltage of the voltmeter, ν = R1000Im = xx mV = 0.02 V
Full-scale deflection current, Im = 5/Rm = 0.02/2 = 0.01 A
Voltage to be measured, V = 300 V
Let the series resistance required = R
Then every bit seen from figure, the voltage drop across R is V – ν
R *Im = V – ν
or R = (Five – five)/I1000
or R = (300 – 0.02)/0.01 = 299.98/0.01 = 29998 ohms Ans.
Shunts can not exist used to extend the range of moving-fe Air-conditioning ammeters accurately. It is because the division of electric current betwixt the operating coil and the shunt varies with frequency (since reactance of the whorl 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
Past changing the number of turns of the operating scroll. For example, suppose that total-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 accept different ranges by but having a different number of turns on the roll. Since the coil carries the whole of the current to exist 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 past Current Transformer
For ranges higher up 0 – 250 A, a electric current transformer is used in conjunction with 0 – v A moving iron Ac ammeter as shown in the effigy. The current transformer is a step up transformer i.eastward. 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 Air conditioning ammeter is continued across the secondary of the transformer.
Since in effigy, the electric current transformer ratio is 10:ane, it ways that line (or load) current is equal to 10 times the reading on the AC meter. Therefore, load current, I L = 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 serial with it.
For ranges college than 0 – 750 V, where power wasted in the multiplier would exist excessive, a 0 – 110 5 moving-iron AC voltmeter is used in conjunction with a potential transformer as shown in the effigy.
The potential transformer is a step-down transformer i.east. number of primary turns is more than the secondary turns. The principal 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:i, the load voltage is equal to twenty 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 condom measure.
Thank you for reading almost extension of range of ammeter and voltmeter.
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