Short Note On Electrical Measuring Instrument | ESE, GATE & SSC

Table of Contents

Q.1. When is indicating instrument said to the dead-beat?

Ans. If the degree of damping is adjusted to such a value that the pointer comes up to the correct reading quickly without passing it or oscillating about it, the instrument is said to be deadbeat or critically damped.


Q.2. What do you mean by a linear scale and squared scale?

Ans. 1. The linear scale is not necessarily straight as the name implies but is usually in the form of an arc. It is called linear (or uniform) to denote that it is evenly graduated over the whole length of the arc with equal sub-divisions.

2. When the deflecting torque is directly proportional to the square of the quantity (current or voltage) to be measured and the instrument is spring controlled, we get a squared scale. The characteristic of this scale is that it is crowded in the beginning and spread out near the finish end of the scale.


Q.3. Why should an ammeter have low resistance?

Ans. An ammeter is used to measure current in a circuit. It is thus connected in series with the circuit under test so that the current to be measured or a fraction of it passes through the instrument itself. Its resistance must therefore be as small as possible:
1. Since power wasted in the instrument is given by Im2 Rm where Im is the meter current and Rm is the meter resistance.
2. So that it does not increase the resistance of the circuit into which it is inserted.


Q. 4. Why should a voltmeter have high resistance?

Ans. A voltmeter is used to measure the potential difference between two points of a circuit. It is thus connected in parallel with the circuit or some part of the circuit. A voltmeter must have high resistance so that :
(i) it is not injured by the current that flows through it.
(ii) power wasted is small as the same is given by V2/R.
(iii) it will not materially affect the current in the circuit to which it is connected.


Q. 5. What is D’Arsonval movement?

Ans. This movement was first conceived by French Physicist Arsene d’Arsonval in 1881. This movement is due to force acting on a current-carrying conductor placed in the magnetic field of a permanent magnet. This movement is so sensitive and accurate that engineers always try to devise methods to use this movement in measurements.

Q.6. Why is eddy current damping not provided in dynamometer-type instruments?

Ans. Since the coils of the dynamometer instrument are air-cored, the operating magnetic field is very weak. For this reason, eddy current damping cannot be provided. We provide air friction damping in such instruments.

Q. 7 . Why is a dynamometer-type instrument chiefly used as a wattmeter?

Ans. Dynamometer instruments are very costly. when used as ammeters and voltmeters their performance is inferior to moving coil and moving iron instruments. for this reason, dynameter ammeters and voltmeters are rarely used. however, dynamometer wattmeters are universally used for the measurement of DC as well AC power. The main reason is that the scale is uniform and a high degree of accuracy can be achieved by careful design. keeping in view these features, the extra cost is justified.


Q. 8. Can you list any situation where the dynamometer ammeter or voltmeter is used?

Ans. Dynamometer instruments are common as wattmeters but are seldom used for general purposes voltmeters or ammeters. The principal use for dynamometer ammeters and voltmeters is transfer instruments i.e. when the instrument is required to read both direct and alternating currents as in the case of a.c. potentiometer.


Q. 9. Why are moving-iron instruments not used for DC work?

Ans. The moving-iron instruments are principally used for alternating current and volts. measurements. they can also be used for d.c. work but then they are liable to small errors due to residual magnetism in iron. The residual magnetism can subtractively error. Thus a second reading should be made with leads interchanged when the meter is used on d.c. circuits. The average of the two readings gives the true value. For this reason, moving-iron instruments are primarily used in a.c. circuits at power line frequencies.


Q. 10. What is the advantage of a squared scale over a linear scale?

Ans. In a squared scale, the deflection is proportional to the square of the cod current. lf the full-scale deflection is 100 V, the center scale calibration is 70.7 V instead of 50 V found on a linear scale. This spread-out scale in the upper region is often very useful when the vat of current and voltage tend to fluctuate about particular values. For instance, if line volta fluctuates between 220 V and 230V, it can be monitored more accurately on a squared meter than on a linear scale if both meters have a full-scale deflection of 250 V.


Q. 11. Why is eddy current damping not provided in moving-iron instruments?

Ans. Eddy current damping is not provided in moving-iron instruments because the permanent magnet required for the purpose would affect the magnetic field due to the operating coil of the instrument. This will, in turn, affect the reading of the instrument. Air friction damping is provided in such instruments.


Q. 12. Why are shunts not used to extend the range of moving-iron ammeters?

Ans. Shunts are not used to extend the range of the moving-iron ammeter. it is because the division of current between the operating coil and the shunt varies with frequency(since the reactance of the coil depends upon frequency). In practice, the range of a moving iron ammeter is extended by one of the following two methods:
1. By changing the number of turns of the operating coil.
2. By using a current transformer in conjunction with an a.c. ammeter.


Q. 13. Why are dynamometer instruments insensitive?

Ans. In dynameter type instrument, fixed coils, as well as moving coil, produces a magnetic field. Since energy must be used to create two magnetic fields, such instruments are generally insensitive. Moreover, the power required for a full-scale deflection in such an instrument is much greater than that required for permanent magnet moving-coil instruments.


Q. 14. Why is the pointer of a dynamometer wattmeter in the form of a triangular truss?

Ans. The pointer of a dynamometer is generally a triangular truss with a thin tip mounted at the end. This construction has the following advantages :

1. It makes the pointer strong without making it heavy.
2. The natural frequency of vibration is kept well outside that of the power at any commercial frequency, so steady readings are always obtainable.


Q. 15. What Is the practical application of a hot-wire Instrument?

Ans. Electrostatic instruments are used for high-frequency alternating currents (e.g. in wireless work ) because the inductance of hot wire is very small, Dynamometer or moving-iron instruments are unsuitable at such high frequencies.


Q. 16. Why is an electrostatic instrument not used as an ammeter?

Ans. An electrostatic instrument is based on the principle that an electric force (attraction or repulsion) exists between two charged plates or objects. It is essentially an air condenser; one plate is fixed while the other, which is coupled to the pointer, is free to rotate on the jeweled bearings. When p.d. to be measured is applied across the plates, the electric force between the plates gives rise to a deflecting torque.
When used as an ammeter, there will be a few millivolts of voltage across the instrument. This extremely small p.d. is insufficient to produce any deflecting torque. for this reason, an electrostatic instrument cannot be used as an ammeter. It is only used as a voltmeter.


Q.17. Why does an electrostatic voltmeter give a very accurate reading?

Ans. An electrostatic voltmeter draws negligible current when connected to the circuit. Hence such a voltmeter does not alter the conditions of the circuit to which it is connected. In other words, an electrostatic voltmeter gives very accurate readings.


Q. 18. Why are wattmeters shielded?

Ans. A wattmeter is generally of dynamometer type. Since dynamometer movement employs air-cored coils, the operating magnetic field is quite weak and is easily affected by stray magnetic fields. Stray field errors can be reduced by enclosing the wattmeter in an iron case. The iron case has a magnetic screening effect.


Q.19. Why are dynamometer wattmeters always preferred to induction wattmeters for a.c. power measurements?

Ans. Due to low accuracy and high power consumption, the characteristics of induction wattmeters are inferior to that of dynamometer wattmeters. For this reason, dynamometer wattmeters are almost universally used for the measurement of a.c. as well as d.c. power. However, induction wattmeters have their chief application as panel instruments where the variations in frequency are not too much.


Q.20. What is the chief advantage of hot wire instruments?

Ans. The chief advantage of the hot-wire instruments is that the deflection depends upon the r.m.s. value of current in the hot wire whatever the waveform and frequency. Thus, a hot-wire ammeter can be calibrated with d.c. but used for measuring alternating currents. The same applies to a hot-wire voltmeter provided its series resistance (i.e. multiplier) is non-inductive.


Q. 21. Why are moving-iron and dynamometer instruments not suitable for high applications?

Ans. At high frequencies, the reactance of the operating coil/coils may become so high that the instrument does not draw any current. Consequently, readings cannot be taken.


Q.22. All indicating instruments except permanent-magnet moving coil and dynamometer wattmeter have squared scale. Why?

Ans. Permanent-magnet moving coil instruments have linear scale as they incorporate a Permanent magnet having a constant field which reacts with the current passing through the moving coil and, therefore, the torque or motive power required to rotate the coil varies directly with the value of this current.
(ii) A dynamometer wattmeter has two coils viz fixed coil and a moving coil. The fixed coil is designed to carry the line current or a fraction of it while the moving coil carries current proportional to load voltage. Since one flux is proportional to load current and the other is proportional to load voltage, the torque on the pointer is directly proportional to boa power.
(iii) In all other types of indicating instruments, the scale is non-linear. It is because the meters have no constant auxiliary source of energy, such as the permanent magnet of the moving coil meter, and therefore, the torque at any instant is proportional to the energy in the coil (I2Rt) i.e. it is proportional to the square of current in the coil. Hence, so instruments have a squared scale; being crowded in the beginning and spread out at the finishing end of the scale.

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