Comprehensive viva questions of eee: Electrical Measuring Instruments part-2
Q.Name the instrument in which the heating effect of current is used?
Ans: Hot wire-type instrument.
Q: Where actually heat is produced?
Ans: In the wire which is connected between the fixed and adjustable screws.
Q: What is the material used for this wire?
Ans: Platinum iridium wire.
Q: Is there any pulley used in the hot wire type instruments?
Q: Name the wire used to transfer the sag of the platinum-iridium wire?
Ans: Phosphor bronze wire.
Q: What for the spring is used in this instrument?
Ans: To give motion to the pulley results in the motion of the pointer.
Q: What type of controlling is used?
Ans: Gravity control.
Q: What type of damping is used in this instrument?
Ans: Eddy current damping,
Q: How eddy current damping is used?
Ans: Because the aluminum disc rotates between the magnetic poles of a permanent magnet, induction eddy current and resulting from the damping.
Q: What is the nature of the scale?
Q: Why the scale is uneven?
Ans: Here the deflection is because the increase in length of platinum-iridium is proportional to the square of the current, so deflection to the square of the current, hence uneven scale.
Q: What main precautions are to be observed in the manufacturing of this instrument to avoid any error because of the increase in temperature or the atmospheric changes?
Ans: The base of the instrument is made of the material of which coefficient of expansion is equal to that of hot wire, in order to minimize the error due to uneven expansion between the wire and its fixture.
Q: Why do we use platinum-iridium wire?
Ans: Because the temperature coefficient is fairly constant and the working is safe up to 300°C.
Q: Can you use this instrument on A.C. and D.C. both?
Q: Why does it give connect reading on A.C. and D.C. both?
Ans: Because the heat developed is proportional to the square of current and current on D.C. is equal to the r.m.s. value of current on A.C. side.
Q: Is it advisable to use any safety device for the protection of overloading of this instrument?
Ans: Yes, generally a fuse is provided.
Q: Can you tell the very important utility of this instrument?
Ans: It is not affected by any wave or frequency and stray magnetic effect so it can be used for any frequency.
Q: Does the stray magnetic field affect the reading?
Ans: Not at all.
Q: How is the power consumption of the instrument?
Ans: power consumption is relatively high.
Q: Once adjusted for zero position, will it be suitable for all the time to come?
Ans: No, zero adjustment is required frequently.
Q: Which thread is generally preferred between the spring and phosphor bronze wire?
Ans: Silk thread.
Q: In which position the instrument is kept for use?
Q: What is the working principle of this type of instrument?
Ans: Same as that of motor, wherever a current-currying conductor is placed in the magnetic field, a torque is developed on the conductor which tends to rotate it.
Q: What type of coils is there in a dynamometer-type instrument?
Ans: Fixed coil and moving coil.
Q: What do you know about fixed coil?
Ans: It is generally wound into two halves, with thick conductors having less number of turns.
Q: How the moving coil is wound?
Ans: This coil has more number of turns of fine wire. Generally, it is air-cored.
Q: How moving coil is placed?
Ans: It is fixed on the spindle so that it can move freely.
Q: To avoid eddy current and hysteresis losses how the coils are wound?
Ans: Air cored.
Q: What type of controlling is used?
Ans: Spring controlled.
Q: Which type of damping is used?
Ans: Air damping.
Q: Is the even scale even or uneven?
Q: Why the scale is not even?
Ans: Because the deflection is again proportional to the square of the current.
Q: What is the use of this instrument?
Ans: It can be used as an ammeter, voltmeter and dynamometer type wattmeter, ampere hour meter, etc.
Q: Can it be used for A.C. and D.C. both?
Q: How the coils are connected while using as an ammeter?
Ans: In series for small current but if the current is high then proper shunt is used.
Q: How it is used as a voltmeter?
Ans: The fixed and moving coils are connected in series but a suitable value external resistance is connected in series to drop down the excess voltage.
Q: Can you use this instrument as a wattmeter? If so, how the connections are made?
Ans: Yes, it can be used as a wattmeter. The fixed coil having less resistance is connected in series with the load and the moving coils having high resistance are connected across the line through a series resistance of suitable value.
Q: What is power?
Ans: It is naturally the rate of doing work or the rate at which the energy is transferred. It is the product of voltage and current in D.C. circuit and in A.C. circuits the product of voltage, current, and power factor.
Q: How many current coils are there in a single-phase wattmeter?
Ans: Only one, divided into two halves.
Q: How many p.c. are there in a three-phase wattmeter?
Ans: Three coils or sometimes two of 440 V.
Q: How the p.c. if three is connected?
Ans: In star.
Q: How many minimum c.c. and p.c. are possible for a three-phase wattmeter even for the unbalanced load?
Ans: Two pressure coils and two current coils.
Q: How many common errors are there in a wattmeter?
Ans: Error due to inductance of the pressure coil, due to capacitance of p.c. due eddy current, due to power loss in c.c. and p.c.
Q: What do you mean by the D.C. ampere-hour meter?
Ans: It is D.C. energy meter.
Q: What are the different types of D.C. energy meters?
Ans: These are:
- Ferranti mercury motor meter.
- Elihu Thomson commutator motor meter.
- Wound disc type meter.
Q: What are the different parts of the D.C. Ferranti mercury motor energy řeter?
Ans: Fixed magnets, spindle, copper or aluminum disc, recording mechanism, and Mercury pool, etc.
Q: How many permanent magnets are there in this instrument?
Ans: Two in number.
Q: What are the functions of the two magnets?
Ans: One magnet is used for driving purposes and another magnet as a brake magnet for controlling purposes.
Q: How the damping force is produced?
Ans: By eddy current damping.
Q: If the current flowing through the meter is less a coil is provided to compensate for the driving, what is the name of the coil?
Ans: Compensating coil.
Q: What are the drawbacks of this meter?
Ans: 1. It cannot be used on A.C. and D.C. both.
- If the part of the mercury is less or dirty, the instrument will not work satisfactorily.
Q: How this meter is connected to the load?
Ans: In series with the load.
Q: How the current is lead in the instrument?
Ans: Through one side by the copper brush to the copper disc and taken out from the pivot of the disc.
Q: Can you tell how many purposes does the copper disc solve?
Ans: Driving and controlling (half the disc for driving and another half for controlling).
Q: Why it is called wound disc type meter?
Ans: In these meters, there is a winding in between the discs or say the disc is wound
Q: How this instrument is connected in the circuit?
Ans: In series with the load.
Q: Name the constructional parts of the wound disc type instrument?
Ans: wound disc, spindle, jeweled bearing, and recording, recording mechanism, permanent magnet, commutator and brushes.
Q: What are the drawbacks of the wound disc type instrument?
Ans: 1. cannot be used on A.C.
- Requires special attention for heavy current operation.
Q: How the current is passed through the wound disc?
Ans: Through commutator and brushes.
Q: If it is required to use the instrument on high current what arrangement should be done?
Ans: Use the proper shunt.
Q: Are the wound disc and Ferranti type meters the actual energy meters?
Ans: There are simply the ampere-hour meters.
Q: What are the constructional parts of the elihu Thomson energy meter?
Ans: Commutator, wound armature, current coils, aluminum disc, permanent magnet, spindle, recording mechanism, resistance and compensating winding.
Q: How the magnetic poles are induced for driving purpose?
Ans: By driving the current coil into two halves.
Q: What is the function of the compensating winding?
Ans: If the load current is less and does not produce the sufficient driving torque, the compensating winding produces some flux to help.
Q: How the compensating coil is connected?
Ans: In series with the armature (pressure coil).
Q: Why pressure coil circuit is made a revolving one?
Ans: Because of the light weight. If the current coil is made revolving, the weight will be more and as a result power for turning will be more.
Q: Has the operating torque any relationship with the current coil and armature circuit currents?
Ans: Yes, it is proportional to the product of the currents flowing through the current coil and armature torque is proportional to the flux produced because of the current coil and pressure coil which is armature circuit in this case.
Q: Why these instruments are called induction type meters?
Ans: Because work on the principle of magnetic induction.
Q: Are the instruments suitable for A.C. and D.C. both?
Ans: Suitable for A.C. measurements only.
Q: How much mechanical degree deflection is possible in these instruments?
Ans: Upto 300°.
Q: Name the main constructional parts of A.C. induction meter?
Ans: The magnetic cores which are with are with the ‘shaded’ provision, spindle, controlling arrangement generally spring, aluminum disc, permanent magnet, pointer and calibrated scale,
Q: Which damping method is adopted?
Ans: Eddy current damping.