# Questions and Answers Electrical Measuring Instruments

### Questions and Answers Electrical Measuring Instruments

1. What do you mean by accuracy?

Ans: –Accuracy is the closeness with which the instrument reading approaches the true value of the variable under measurement.

1. What do you mean by electrical instruments?

Ans: – The instruments which are used to measure electrical quantities (such as current, voltage, power, frequency, etc) are called electrical instruments.

1. What is meant by absolute instruments?

Ans: – The instruments which give the value of the quantity to be measured in terms of the constant of instrument are called absolute instrument.

1. What do you mean by secondary instruments?

Ans: –The deflection of such instruments gives the magnitude of electrical quantity to be measured directly.

1. What do mean by indicating instruments?

Ans: –The instruments which indicate the magnitude of electrical quantity at the time when it is being measured.

1. What do mean by recording instruments?

Ans: – The instruments which give a continuous record of the variations of the electrical quantity to be measured are called recording instruments.

1. What do mean by integrating instruments?

Ans: –The instruments which measure the total quantity of electrical energy in a given time are called integrating instruments.

1. What is deflecting torque?

Ans:- The torque due to which moving system moves from its zero  position in an indicating instrument is called deflecting torque.

1. What do you mean by controlling torque?

Ans:- The torque which brings the pointer to zero position when instrument is disconnected from the circuit or the torque which allows the pointer to deflect in accordance to the magnitude of electrical quantity is called controlling torque.

1. What do you mean by PMMC instrument?

Ans:- The instrument which use the permanent magnet for creating the stationary magnetic flux between which the coil moves is called PMMC instrument .

1. What is the working principle of PMMC instrument?

Ans:- The basic principle of PMMC instrument is that when a current carrying conductor is placed in a magnetic field, a mechanical force is experienced by the  conductor.

1. What device is used to extend the range of DC ammeter

Ans: –Shunt is used to extend the range of DC Ammeter.

1. Which type of instrument is used to measure both AC and DC quantity?

Ans: – Moving iron instrument.

1. Give the application of air friction damping.

Ans: – Air friction damping is used in moving iron and dynamometer type instruments where the working magnetic field is weak and is likely to get distorted with introduction of permanent magnet.

1. Why should an ammeter have low resistance?

Ans: –An ammeter is an electrical instrument which is used for current in the circuit. It is thus connected in series with the circuit under test so that circuit to be measured or a fraction of it passes through the instrument itself. Its resistance must therefore be small as small as possible; (i) since power losses in the instrument is given by I2m Rm where is the meter current and Rm is the meter resistance. (ii) so that is does not increase the resistance of the circuit into which it is inserted.

1. Why should a voltmeter have high resistance?

Ans: – A voltmeter is a instrument which is used to measure potential difference between two points of a circuit. Voltmeter always connected in parallel. 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.

1. Moving iron instrument can be used both d.c. and a.c. why?

Ans: –If current in the coil is reversed the direction of magnetic field produced by the coil will reverse, also the magnetism produced in the soft iron piece will reverse. Hence, the direction of deflecting torque remains unchanged. Thus, these instruments can be used on d.c. as well as a.c. system.

1. PMMC instruments cannot be used on a.c., why?

Ans: – If current in the coil is reversed, the direction of deflecting torque will be reversed because field produced by the permanent magnets does not change. This will give a wrong direction of rotation, thus, the instrument cannot be used on a.c., it can only be used on dc.

1. The scale of the moving iron instruments is non-uniform, why?

Ans: – Since deflection θ = I2, therefore, the scale of such an instrument is non-uniform; being crowded in the beginning.

1. The scale of dynamometer type wattmeter is non-uniform, why?

Ans: –As deflection θ is proportional to the power to be measured which further depends upon the product of I and V , therefore, dynamometer type wattmeters have non-uniform scale being crowded in the beginning.

1. What is the difference between an ammeter and a voltmeter?

Ans: –An ammeter is a low resistance indicating instrument while the voltmeter is a high resistance indicating instrument.

1. Name the two types of moving iron instruments

Ans: -(a) Attraction type moving iron instrument (b) Repulsion type moving iron instrument.

1. What will happen when an ammeter is connected across the circuit?

Ans: –If an ammeter is connected across the circuit like a voltmeter, then a very high current will pass the instrument coil (because of low resistance of an ammeter). This heavy current will produce such as excessive heat, which may damage the insulation of the coil or may destroy the coil. Thus instrument get damaged.

1. How is the controlling torque is provided in permanent magnet moving coil instruments?

Ans: – In permanent moving coil instruments, controlling torque is provided by two phosphor hair spring either helical or spiral coiled in opposite direction, of equal strength.

1. What are the differences between moving coil and moving iron instruments?

Ans: – Moving coil instruments are used only for d.c. measurement. Moving coil instruments have linear uniform scale. Moving coil instruments have comparatively less power consumption. Moving iron instruments are used for a.c. as well as d.c. measurement. Moving iron instruments also have non-uniform scale. Moving iron instruments are comparatively robust due to simple in construction and also cheaper than moving coil instruments. 