Comprehensive viva questions for eee: Electrical Measuring Instruments part-1
Q: What are electrical measuring instruments?
Ans: The instruments which are used to measure the electrical quantities, viz current, voltage, power, frequency, power factor, etc. are known as electrical measuring instruments.
Q: Does it actually measure or compare the quantities?
Q: What are the absolute instruments?
Ans: The instrument which gives the quantity to be measured in terms of instrument constant and its deflection. Such instruments do not require any comparison with any standard instruments. There are rarely used and kept in laboratories for standardization of the other instruments.
Q: What are secondary instruments?
Ans: The instrument in which the deflection gives the magnitude of the electrical quantity to be measured directly.
Q: How the instruments are classified?
Ans: According to the type, working principle, and accuracy.
Q: What are the classifications according to the accuracy?
- Standard instruments.
- Sub-standard instruments.
- Commercial instruments
Q: What is the percentage of error possible in the standard, substandard and commercial instruments?
Ans: 0%, about 0% and \pm 3%, respectively.
Q: Why the standard instruments are made?
Ans: To have a standard and are kept in a research laboratory for standardizing the substandard instruments.
Q: Can you use a standard instrument for commercial purposes?
Q: What are the instruments according to the function?
Ans: Indication type, integrating type, and recording type.
Q: What do you understand by the indicating type instrument?
Ans: The instrument which indicates the reading by means of the deflection of a pointer and can be read on the calibrate scale, is known as indicating type instrument.
Q: What are the recording-type instruments?
Ans: The instrument in which the reading is recorded with the help of a pointer (pen) and on a graph. These instruments can indicate a particular reading at a particular time.
Q: Name the different instruments according to the working?
- Moving coil type instrument.
- Moving iron-type instrument.
- Hot wire-type instrument.
- Dynamometer-type instrument.
- Induction type instrument
- Electrostatic type instrument
Q: What should be the properties of a good/ideal instrument?
Ans: It should have dead beatness, applicability, should not consume more power, accurate, Scale should be such that it may give quick vision to read, free from external atmospheric effects, easy to handle and the pointer should be light enough.
Q: Which is a deadbeat instrument?
Ans: If the instrument rises quickly to its deflecting final position without oscillations the instrument is said to be deadbeat.
Q: What are the different forces employed in an instrument?
Ans: These are three forces- the deflecting force, the controlling force and the damping force.
Q: What do you mean by the deflecting force?
Ans: The force which deflects the deflecting system is known as deflecting force or torque. Generally in all electrical instruments the force is nothing but the electrical quantity.
Q: What is the controlling force?
Ans: The force which controls the deflecting torque is known as controlling force.
Q: Why controlling force is needed?
Ans: To compare the electrical quantity with the calibrated mechanical force.
Q: Will the moving system stop as soon as the controlling force is balanced?
Ans: No, but it oscillates for some time.
Q: What is the name of the force used to overcome the oscillations?
Ans: Damping force.
Q: Suppose the damping mechanism fails, what will happen?
Ans: It will be difficult to note down the final reading quickly or say it will take more time to stabilize the reading because of oscillations.
Q: How many methods are there for obtaining the controlling force?
Ans: Gravity control and spring control.
Q: Why the gravity control system is called so?
Ans: Because the principle of gravitational pull is used in this controlling method.
Q: What is the spring control method of controlling force?
Ans: In this method, the spring is provided on the spindle and the force exerted upon the angle of twist rewound.
Q: Can you obtain a uniform, regular and accurate controlling force by one spring?
Q: What are the drawbacks of one spring?
- The change in temperature causes the change of length of the spring, which in other words affects the final reading.
- The aging factor also changes the stiffness constant of the spring.
- One spring will not give the regular force and changes with the angle of torsion,
Q: What is done to avoid the above errors?
Ans: Double springs are used, wound in opposite direction, so that if one winds the other will unwind and a uniform torque results.
Q: What should be the properties of the spring used for spring control?
Ans: Uniform tension, low or negligible heating effect, non-magnetic and foolproof against mechanical fatigue.
Q: What do you mean by the damping force?
Ans: The force which is used to damp the oscillations is known as damping force.
Q: What are the methods for obtaining the damping force?
Ans: Air damping, fluid damping, and eddy current damping.
Q: Why damping device is essential in the instrument?
Ans: To give a quick and accurate final reading.
Q: What is air damping?
Ans: The damping obtained because of the air thrust.
Q: How air damping is obtained?
Ans: In the air damping, a chamber of aluminum has an aluminum vane, which moves with the deflecting system. The vane experiences a thrust in the opposite direction of the movement because of the oscillations and compression of air damps the oscillation,
Q: What is eddy current damping?
Ans: It is the most effective method. In this aluminum or copper disc moves in the magnetic field so as to be cut by the line of force and eddy currents are set up in the disc. These currents set up a force opposing the cause (oscillation) responsible for the production of these currents and thus the oscillations are damped.
Q: Why it is called a M.C. type instrument?
Ans: Because a coil moves in the magnetic field so it is called a moving coil instrument.
Q: What are the main parts of a moving coil instrument?
Ans: Permanent magnet, coil, aluminum former, hair spring, pivots, spindle, pointer, and calibrated scale.
Q: How the coil is wound?
Ans: Having more number of turns of fine wire.
Q: How this current is fed to the coil?
Ans: Through hair spring.
Q: What is the material generally used for control spring?
Ans: Phosphor bronze.
Q: What type of damping is used in M.C. type instruments?
Ans: Eddy current damping.
Q: How eddy current damping is obtained?
Ans: The aluminum or copper former is used. As this former rotates between the magnetic poles, the force is developed and provides a damping force to the oscillations.
Q: How the deflecting torque and the current in the coil are related?
Ans: The deflection is directly proportional to the current in the coil.
Q: What is the nature of the scale, uniform or uneven?
Q: What are the uses of M.C.-type instruments?
Ans: It can be used as an ammeter and voltmeter.
Q: Can you tell the connection if the m.c. type meter is used as an ammeter?
Ans: Yes, it will be connected in series with the load.
Q: Can a.m.c. type meter be used as a voltmeter? If yes, how?
Ans: Yes, can be used by connecting a resistance of suitable value in series with the instrument.
Q: If an ammeter is connected across the A.C. line what will happen?
Ans: It will be damaged.
Q: Which meter, the ammeter or voltmeter, has high resistance?
Q: Why a shunt is provided with an ammeter?
Ans: To extend the range.
Q: Name the device used for extension of voltmeter range?
Ans: Potential diverter.
Q: Why it is known as the M.I. instrument?
Ans: In these instruments, the moving part is a soft iron piece, so it is known as the M.I. instrument.
Q: What are the types of M.I. type instruments?
Ans: Attraction type and repulsion type instruments.
Q: What are the attraction type instruments?
Ans: The M.I. instrument in which the principle of magnetic attraction is applied is known as attraction type instruments.
Q: Which magnetic property is used in the repulsion type instruments?
Ans: Repulsive, like poles repel each other.
Q: What are the different parts of the M.I. instrument?
Ans: In repulsion type instruments: Fixed coil, moving iron and fixed iron piece, spindle, pointer, calibrated scale, aluminum vane, and chamber, In attraction type instrument: Fixed coil, aluminum vane, and chamber, spindle, pointer, calibrated scale.
Q: Where actually does the repulsion takes place in the case of repulsion type M.I. instrument?
Ans: Between the soft iron pieces (placed in the same magnetic region) generally known as fixed and moving iron pieces.
Q: How the scale of M.I.-type instruments is calibrated?
Ans: Uneven crowded in the beginning and end spreader in between.
Q: Why the scale is uneven?
Ans: Because the deflection is proportional to the square of the current flowing through the magnetic coil.
Q: Why type of damping is used in this type of instruments?
Ans: Air damping.
Q: Can this instrument be used for A.C. and D.C. both?
Q: Up to what scale the reading of the instrument is reliable?
Ans: The reading up to 1/10th of the full scale is not reliable.
Q: Which instrument of M.I. type of M.C. type is robust?
Ans: M.I. type.
Q: Which value of current, r.m.s. or maximum, is measured by M.I. instruments?
Ans: R.M.S. value.
Q: What are the common errors in all instruments?
Ans: Temperature error, frictional error, and observation error,
Q: What do you mean by the observation error?
Ans: The error due to misreading the scale is known as observation error.
Q: What is the frictional error?
Ans: The error is due to the mechanical force action at the pivots.
Q: What is a temperature error?
Ans: The error is mainly due to heating of the working coil and other resistance connected in the instrument, apart from the room temperature, because of the operational current.
Q: How the temperature error can be minimized?
Ans: By using the coil and resistance of very few temperature coefficient materials.