100 Most Important MCQ Of Measurement and Instrumentation with explanation

Answer.2. R.M.S value of the current

Explanation:-

Moving iron instruments are used to measure current and voltage. They are made as ammeters and voltmeters and can measure both ac and dc currents and voltages respectively. Two types of moving iron instruments are manufactured. One type is called attraction type and the other type is called repulsion type.

Repulsion type Ammeter

• In repulsion type, a fixed vane or rod is also used.
• Controlling torque is provided by spring or weight (gravity).
• Damping torque is normally pneumatic, the damping device consisting of an air chamber and a moving vane attached to the instrument spindle.

The fixed coil carries the current or current proportional to the voltage to be measured. On the inner surface of the coil, a specially shaped fixed iron piece is attached.  On the spindle is attached to another piece of iron in the form of a fin. This is the moving iron. The current passing through the coil produces a magnetic ficld. In this magnetic field, both the fixed iron and the moving iron are placed. Both these iron pieces will be similarly magnetized due to the influence of the magnetic field. This will create a force of repulsion among the two pieces of iron. One piece of iron being fixed, the other piece of iron will move which will create on deflecting torque. This deflecting torque has to be opposed by a control torque such that under steady deflection,

T_d = T_c.

If T_d is more than T_C, the pointer will move. Its movement will stop when the deflecting torque, T_d, equals the control torque, T_C. Since the magnitude of deflecting torque depends upon the magnetism of the two pieces of iron and since this magnetism is produced by the current to be measured. we can write

T_d ∝ I²

And Tc ∝ θ  as more the deflection θ,  more is the controlling torque.

Moving iron instruments have their deflection is proportional to the square of the current flowing through the coil. These instruments are thus said to follow a square law response (R.M.S Value) and have non-uniform scale marking. Deflection being proportional to the square of the current, whatever be the polarity of current in the coil, deflection of a moving iron instrument is in the same direction. Hence, moving iron instruments can be used for both DC and AC measurements.

Answer.1. The presence of a permanent magnet required for damping would affect the deflection and hence the reading of the instrument

Explanation:-

The operating magnetic field in moving iron instrument is very weak. Hence eddy current damping is not used since it requires a permanent magnet which would affect or distort the operating field.

Answer.1. Stainless steel

Explanation:-

Error due to Hysteresis:- This source of error is due to hysteresis in the soft Iron part, due to which too high values are recorded by the instrument when the current is increasing and too low readings are liable to be indicated when the current is decreasing.

Hysteresis Loss = K_h × B_M^1.67 × f × v watts

where Kh is the Hysteresis coefficient depends upon the material. The smaller the value of Kh of a magnetic material, the lesser is the hysteresis loss. The armatures of electrical machines and transformer cores are made of magnetic materials having low hysteresis coefficient in order to reduce the hysteresis loss. The steels have Kh values around 130, for cast steel they are around 2500 and for cast iron about 3750.

Errors due to Stray fields:-  External stray magnetic fields are liable to affect adversely the accurate functioning of the instrument. Magnetic shielding of the working parts is obtained by using a covering of cast iron.

Errors with A.C only

Errors may be caused due to change km frequency because the change in frequency produces (i) change in impedance of the coil and (ii) change in magnitude of eddy currents. The error due to the former is negligible in ammeters, as the coil current is determined by the external circuit and the error due to the latter can normally be made small.

Answer.4. The pointer will oscillate about its final deflected position for quite sometime before coming to rest

Explanation:-

Damping Torque: At the final deflected position, when the deflecting and controlling torques are equal, the pointer starts oscillating owing to its inertia and therefore, cannot immediately settle at its final deflected position. If no extra force is provided to damp these oscillations, the moving system will take considerable time before coming to settle to the final deflected position. This is especially undesirable if the number of readings to be taken is quite large.

Air friction, Eddy current damping, Hydraulic damping is used in damping torque.

Answer.1. Dead Beat

Explanation:-

If the instrument is under-damped, the moving system oscillates a lot before it dismally settles down to its steady value.

ln case of critically damped instruments, the pointer reaches its final steady position rapidly and smoothly. Such an instrument is also called dead-beat instrument.

An over-damped instrument produces damping torque more than the required value, as such the pointer moves slowly to its final steady-state value. 

Answer. 3. Slow and lethargic

Explanation:-

The major function of the damping device is to produce a damping force while the moving system is in motion. The damping force should be of such a magnitude that the pointer of the moving system comes to its final steady value quickly without any oscillation.

• This torque is produced only when the instrument is in operation.
• This ensures that the moving system takes just the required time to reach its final deflected position. Then, the instrument is said to be ‘dead-beat’.
• If sufficient damping torque is not produced, the pointer make underdamped oscillations before reaching the steady deflection.
• In over-damped instrument the damping torque is more than the required value, the pointer becomes sluggish and it takes longer than the required time to reach the final deflection.

Answer.3. Induction type

Explanation:-

Induction type instruments

• These instruments works on AC supply only. This is based on the principle of induction motor, therefore called induction type instrument.
• We can use these instruments as an ammeter, voltmeter, wattmeter and an energy meter.
• The scale of induction type instruments is quite uniform and extends over an angle of 300°.
• Currents up to 100 A can be handled by induction type instrument so that their power consumption is fairly large and cost relatively high.

Answer.4. Few turns of thick wire

Explanation:-

Moving iron instruments are of two types: attraction type and repulsion type. In both types, the principle of operation is the force experienced by an iron piece in the presence of a magnetic field. The necessary magnetic field is produced by the ampere-turns of a current-carrying coil. When used as an ammeter the coil has fewer turns of thick wire so that the ammeter has low resistance.

When used as a voltmeter, the coil has the high impedance so as to draw a small current as possible since it is connected in parallel. Since the current through it would be small, it has to have a large number of turns to produce the necessary ampere-turns.

Answer.2. Square of current

Explanation:-

Moving iron instruments are used to measure current and voltage. They are made as ammeters and voltmeters and can measure both ac and dc currents and voltages respectively. Two types of moving iron instruments are manufactured. One type is called attraction type and the other type is called repulsion type.

Repulsion type Ammeter

• In repulsion type, a fixed vane or rod is also used.
• Controlling torque is provided by spring or weight (gravity).
• Damping torque is normally pneumatic, the damping device consisting of an air chamber and a moving vane attached to the instrument spindle.

The fixed coil carries the current or current proportional to the voltage to be measured. On the inner surface of the coil, a specially shaped fixed iron piece is attached.  On the spindle is attached to another piece of iron in the form of a fin. This is the moving iron. The current passing through the coil produces a magnetic field. In this magnetic field, both the fixed iron and the moving iron are placed. Both these iron pieces will be similarly magnetized due to the influence of the magnetic field. This will create a force of repulsion among the two pieces of iron. One piece of iron being fixed, the other piece of iron will move which will create on deflecting torque. This deflecting torque has to be opposed by a control torque such that under steady deflection,

T_d = T_c.

If T_d is more than T_C, the pointer will move. Its movement will stop when the deflecting torque, T_d, equals the control torque, T_C. Since the magnitude of deflecting torque depends upon the magnetism of the two pieces of iron and since this magnetism is produced by the current to be measured. we can write

T_d ∝ I²

And Tc ∝ θ  as more the deflection θ,  more is the controlling torque.

Moving iron instruments have their deflection is proportional to the square of the current flowing through the coil. These instruments are thus said to follow a square law response (R.M.S Value) and have non-uniform scale marking. Deflection being proportional to the square of the current, whatever be the polarity of current in the coil, deflection of a moving iron instrument is in the same direction. Hence, moving iron instruments can be used for both DC and AC measurements.

Answer.1. Higher on descending value but lower on ascending values

Explanation:-

Errors in moving iron meters:

Usually, the following errors occur in a moving iron meters:

1. Friction error
2. Temperature error
3. Error due to stray magnetic fields
4. Hysteresis error
5. Frequency error
6. Waveform error

Hysteresis error

• Due to the hysteresis effect, error in moving-iron instruments, owing to the magnetic hysteresis effect in the parts of the moving system which are made of iron.
• The magnetism lags behind the current resulting higher reading when descending values of current or voltage are measured than when ascending values are measured.
• The position of poles in the moving iron changes with rotation. This is known as position error and is usually small.
• The hysteresis error is reduced considerably by making the iron parts small, and reducing the length of paths of working fluxes to shortest possible and working of iron at a flux density where hysteresis error is small.