21. What is the effect of the ammeter range on the shunt resistance?
No effect
Varies by a factor of multiplying factor
Varies by a factor of the resistance
Varies by a factor of unity
Answer.2. Varies by a factor of multiplying factor
Explanation:
In order to increase the ammeter range, the shunt resistance Varies by a factor of multiplying factor.
For range extension
Rse = Rm(M – 1)
Rm = Vm/Im
Rsh = Rm/(M – 1)
M = 1 + Rse /Rm
M = Multiplying factor = (Required full scale deflection) / (Initial full scale deflection)
Where,
Rsh = Shunt resistance
Rse = Series resistance
Rm = Meter resistance
Vm = Potential difference across the meter
Im = Meter current
22. The internal resistance of a voltmeter is 20,000 ohms. If this voltmeter is connected in series with a resistance and a 220 volt supply is connected across the combination, the voltmeter reads 200 volts. The value of the resistance is
200 Ω
4000 Ω
2000 Ω
20,000 Ω
Answer.3. 2000 Ω
Explanation:
For range extenstion
Rse = Rm(M – 1)
Rm = Vm/Im
M = Multiplying factor = (Required full scale deflection) / (Initial full scale deflection)
m = V/Vm
Where,
Rsh = Shunt resistance
Rse = Series resistance
Rm = Meter resistance
Vm = Potential difference across the meter
Im = Meter current
Given –
Given,
Rm = 20,000 ohms
V = 220 V
Vm = 200 V
m = V/Vm = 220/200 = 1.1
Rse = Rm (m – 1)
= 20,000 (1.1 – 1) = 2000 ohms
23. For extending range of an instrument a multiplier is ________
Non-inductive
Resistive
Capacitive
Non-capacitive
Answer.1. Non-inductive
Explanation:
Multipliers are non-inductive resistance used in series with the D.C. voltmeter to extend its range. A voltmeter of a lower range can be used for measuring higher voltage by the use of a proper multiplier resistance. The excessive voltage is dropped across the non-inductive resistance. Multiplier Resistance is usually a carbon resistor.
24. A moving coil instrument having a meter resistance of 5 Ω is to be used as a voltmeter of range 0 – 100 V. If the full-scale deflection current is 10 mA, then find the required series resistance.
1999
10000
995
9995
Answer.4. 9995
Explanation:
To increase the range of a voltmeter, we need the series resistance and it is given by
Rse = Rm(M – 1)
Where V is the required voltmeter range
Vm is the voltmeter range
Rm is the meter internal resistance
Calculation:
Given that,
Meter full scale current reading (Im) = 10 mA
Internal resistance (Rm) = 5 Ω
Voltmeter range (Vm) = I m Rm = 10 × 10-3 × 5 = 0.05 V
Required voltmeter range (V) = 100 V
Rse = 5(100/0.05 -1) = 9995 Ω
25. The essential condition for using a multiplier in A.C. voltmeters?
By using ac supply
By maintaining a uniform impedance
By maintaining a uniform frequency
By using a galvanometer
Answer.3. By maintaining a uniform frequency
Explanation:
Frequency error:- If a shunt is to be used with an ammeter operating on a.c., then the inductances of both the ammeter coil and the shunt come into the picture. For the multiplying power ~ to be independent of frequency, the time constants of the ammeter and shunt circuit must be the same.
The condition to be satisfied in the voltmeter is that the total impedance of the voltmeter and the multiplier circuit must be constant for a wide range of frequencies.
26. A moving-coil galvanometer can be used as a DC ammeter by connecting a
High resistance in series with the meter
High resistance across the meter
Low resistance across the meter
Low resistance in series with the meter
Answer.3. Low resistance across the meter
Explanation:
A galvanometer can be converted into an ammeter by connecting a shunt resistance parallel to it
A shunt is basically a wire of very small resistance
A galvanometer can be converted into a voltmeter by connecting a very high resistance in the series
A voltmeter is used to measure the potential difference whereas an ammeter is used to measure the electric current
A galvanometer is used to measure a small electric current in a circuit
27. The error introduce while connecting the shunt in the ammeter is
Temperature
Frequency
Both temperature and frequency
None of the above
Answer.3. Both temperature and frequency
Explanation:
The following are the general errors in ammeters with shunts:
Temperature error
Frequency error
Temperature error is common to both d.c. and a.c. operations, while frequency error occurs only in a.c. operation.
Reasons for temperature error:
(a) The temperatures of the ammeter coil and the shunt are not the same.
(b) The ammeter coil and the shunt are not made of materials with the same temperature coefficient.
Because of the above facts the multiplying power of the shunt changes with the temperature. This error may be eliminated by making the shunt and the ammeter coil of the same material and by maintaining the same temperature for the coil and the shunt.
Frequency error:- If a shunt is to be used with an ammeter operating on a.c., then the inductances of both the ammeter coil and the shunt come into the picture. For the multiplying power ~ to be independent of frequency, the time constants of the ammeter and shunt circuit must be the same.
28. An analog voltmeter uses an external multiplier setting with a multiplier setting of 20 kΩ, it reads 440 V and with a multiplier setting of 80 kΩ, it reads 352 V. For a multiplier setting of 40 kΩ the voltmeter reads______.
371 V
383 V
394 V
406 V
Answer.4. 406 V
Explanation:
Multiplier setting = 20 kΩ
Meter resistance = x
Total resistance = 200 kΩ + x = R1
Voltmeter reading (V1) = 440 V
V1 ∝ I1x
I1 = 1/R1
For multiplier setting = 80 kΩ
Meter resistance = x
Total resistance = 80 KΩ + x = R2
Voltmeter reading (V2) = 352 V
V2 ∝ I2x
I2 = 1/R2
V2/V1 = I2/I1 = R1/R2
352/440 = (20 + x)/(80 + x) = 220 kΩ
For multiplier setting = 40 KΩ
R3 = 260 kΩ
V3/V1 = I3/I1 = R1/R3
= V3/440 = 240/260
V3 = 406 V
29. Which of the following is not a requirement for a DC ammeter’s shunt?
High thermal automotive force
Time-invariant
Low-temperature coefficient
Carry current without excessive temperature rise
Answer.1. High thermal automotive force
Explanation:
The general requirements for shunts are:
The temperature coefficient of the shunt and instrument should be low and should be as nearly as possible the same
The resistance of shunts should not vary with time i.e. time-invariant
They should carry the current without excessive temperature rise
They should have a low thermal electromotive force with copper
Manganin is usually used for shunts of DC instruments as it gives a low value of thermal emf with copper although it is liable to corrosion and is difficult to solder.
Constantan is a useful material for AC circuits since its comparatively high thermal emf, being unidirectional, is ineffective in these circuits.
30. Which device is not used for extending the range of Instrument?
Shunt
Multiplier
Current Transformer
Switchboard
Answer.4. Switchboard
Explanation:
Switchboard is not used for extending the range of Instrument
There are four common devices used for the range extension of ammeter and voltmeter namely; multipliers, shunts, current transformer and potential transformer
An ammeters range can be extended by changing the value of the shunt resistance in the ammeter
A lower resistance would increase the maximum current range of the meter
31. A galvanometer with a full-scale current of 10 mA has a resistance of 1000 Ω. The multiplying power (the ratio of measured current to galvanometer current) of a 100 Ω shunt with this galvanometer is:
100
11
10
110
Answer.2. 11
Explanation:
For range extenstion
Rse = Rm(M – 1)
Rsh = Rm/(M – 1)
Rm = Vm/Im
M = Multiplying factor = (Required full scale deflection) / (Initial full scale deflection)
m = V/Vm
Where,
Rsh = Shunt resistance
Rse = Series resistance
Rm = Meter resistance
Vm = Potential difference across the meter
Im = Meter current
Given-
Im = 10 mA
Rsh = 100 Ω
Rm = 1000 Ω
100 = 1000/(M − 1)
M – 1 = 10
M = 11
32. A (0 a to 50 A) moving coil ammeter has voltage drop of 0.1 V across its terminals at full-scale deflection. The external shunt resistance (in milliohms) needed to extend its range to 0 A to 500 A is.
22.2 mΩ
2.2 mΩ
0.22 mΩ
222.2 mΩ
Answer.3. 0.22 mΩ
Explanation:
For range extenstion
Rse = Rm(M – 1)
Rsh = Rm/(M – 1)
Rm = Vm/Im
M = Multiplying factor = (Required full scale deflection) / (Initial full scale deflection)
m = V/Vm
Where,
Rsh = Shunt resistance
Rse = Series resistance
Rm = Meter resistance
Vm = Potential difference across the meter
Im = Meter current
Given-
Rm = 0.1/50 = 2 × 10-3
M = 500/50 = 10
Rsh = (2 × 10-3)/(10 − 1)
Rsh = 0.22 mΩ
33. A 10 mA meter having an internal resistance of 100 Ohms is to be converted to (0 – 200) mA ammeter. What value of shunt resistance (in ohms) is required?
5.26
5
10.52
10
Answer.1. 5.26
Explanation:
Given that, full scale deflection current (IFSD) = 10 mA
34. A moving coil instrument having a resistance of 10 Ω, gives a full-scale deflection when the current is 10 mA. What should be the value of the series resistance, so that it can be used as a voltmeter for measuring potential differences up to 100 V?
9 Ω
99 Ω
990 Ω
9990 Ω
Answer.4. 9990 Ω
Explanation:
To increase the range of a voltmeter, we need to the series resistance and it is given by
35. A current shunt is in general a ________ resistance.
Two-terminal Device
Four Terminal Device
One Terminal Device
Six Terminal Device
Answer.2. Four Terminal Device
Explanation:
A current shunt is in general a four-terminal resistance the current to be measured being connected to the current terminals. The potential terminals are arranged so that an accurately known resistance and voltage drop exists between them, and situated so that they are unaffected by any heating which may occur at the current terminals.
Two terminals with a large current carrying capacity are called current terminals. These are used for inserting the ammeter in series with the main circuit. The other two terminals are of small size and are called potential terminals. These are used for connecting the basic meter.