EEE job viva question: Electrical Measuring Instruments part-3
Q: What do you understand by the exciting coil?
Ans: It is the coil in which current flows and produces magnetic flux. In the case of the ammeter, the Number of turns of the thick conductor is less and in the case of the voltmeter, this coil is wound within the conductor this coil is wound with a thin conductor having more number of turns.
Q: What is the function of an aluminum disc?
Ans: It serves two propose:
- It behaves as a short-circuited conductor and produces a circulating current as a result the system is deflected.
- Helps for eddy current damping.
Q: What are the applications of the induction-type instruments?
Ans: As ammeter, as a voltmeter and as a wattmeter etc.
Q: Name the different energy meters generally used?
Ans: Single phase energy meter, three phase energy meter.
Q: What is the main constructional difference between the energy meter and wattmeter?
Ans: The other construction is the same except the control spring and pointer is replaced by the brake magnet, the aluminum disc, and train of the wheel for recording mechanism in energy meters.
Q: How many main coils are there in an energy meter?
Ans: C.C. and P.C. (current coil and pressure coil)
Q: How C.C is connected and wound?
Ans: The connect coil is connected in series with the load and wound with thick conductors having less number of turns.
Q: What is p.c. and how it is wound?
Ans: It is the pressure coil; it is connected across the line. It is wound with a thin conductor having more number of turns.
Q: What is the relationship between the current and voltage in the pressure coil?
Ans: The current lags behind the voltage by approximately 90°.
Q: Name the provision which is made so that the angular displacement between the current and voltage is kept about 90° electrical?
Ans: The shading ring is provided on the central limb of the cores.
Q: How current and voltage relationship in the current coil is determined?
Ans: It depends upon the nature of the load. It will be the same as that of the load current and
inching voltage relationship.
Q: What is the main operating mechanism or systems in induction-type instruments?
Ans: Driving system, moving system, braking system, and registration system.
Q: How many terminals are taken out from the energy meter?
Ans: Four terminals, out of which generally central two terminals are short-circuited inside the meter.
Q: Generally, where two terminals of c.c. are located?
Ans: One and forth extreme two.
Q: How pressure coil is connected?
Ans: Across line and neutral. In the energy meter having four terminals, the coil is connected on the shore-circuited terminal and the small screw near the current coil terminal.
Q: Can you trace out the terminals of c.c. and p.c.?
Ans: By means of the testing lamp. The current coil terminals will give bright light and pressure coil only a small amount of sparking.
Q: What do you understand by the leg adjustment?
Ans: An adjustment of flux and voltage to be in quadrature with the help of a shading bend is known as leg adjustment.
Q: What are the power, factor, quadrature, inductive load adjustment?
Ans: All are the same. Only the different names are given. The same leg adjustment is done.
Q: The shading bend sometimes carries an additional resistance with an adjustable screw, why do we use it?
Ans: To improve the power factor error, so that the flux of p.c. is in quadrature to the voltage fed.
Q: What are the errors generally experienced in energy meters?
Ans: Phase and speed error, friction compensation, temperature and frequency error.
Q: What do you understand by the term creeping?
Ans: Sometimes a slow but continuous rotating is obtained even when the load is removed. This error is known as creeping.
Q: How overloaded compensation is done?
Ans: Wherever the disc rotates in the field of a permanent magnet, the eddy currents are induced causing self-braking. For the suitability of that magnetic force, the magnetic is adjusted by providing a suitable magnetic shunt.
Q: How many elements are there in a three-phase four-wire energy meter?
Q: Where, in case of balanced or unbalanced load, we use the 3-0 4-wire energy meter?
Ans: It can be used even with an unbalanced load.
Q: If a meter is running fast, how will you adjust the energy meter?
Ans: In case of fast running the braking magnetic is adjusted. If the magnet is brought towards the spindle the meter will run fast, if away from the spindle i.e., towards the periphery of the disc the meter will slow down.
Q: What is the principle of operation of the electrostatic type instrument?
Ans: Attraction or repulsion between two charged bodies.
Q: What is the actual formation of this instrument?
Ans: Essentially a voltmeter.
Q: What are the types of these instruments?
Ans: Quadrant type, attraction type.
Q: Where the quadrant-type instruments are used?
Ans: Voltage measurement up to 10 to 25kv.
Q: Where the attraction type instruments are used?
Ans: Voltage measurement above 20 kV.
Q: What are the main parts of the quadrant type voltmeter?
Ans: Four fixed metal double quadrants, circular box, double-ended vane, phosphor bronze wire or silvered quartz thread, damping arrangement, and indicating arrangement.
Q: How many discs are provided in an attraction disc type voltmeter?
Ans: Two, mounted parallel to each other.
Q: What is a megger?
Ans: It is an instrument generally used for measuring the insulation resistance in megaohms.
Q: How does it work?
Ans: It is driven by means of a handle and generates the voltage which is used for measuring the insulation resistance.
Q: For lighting load, which rating of the megger is used?
Ans: 500 V.
Q: What is the general ratio of the voltage rating of the instrument and installation? How much should be the voltage of the instrument be used for power installation testing?
Ans: The rating is generally double the voltage of the supply or appliance. For power installation, it should be of 1000 V megger.
Q: What is the nature of the supply which is used in magger?
Q: What type of generator is used in this instrument?
Ans: Permanent magnet type D.C. generator..
Q: How many coils are there in the ohmmeter used in megger?
Ans: Three, the current coil (deflecting coil) pressure coil and compensating coil.
Q: How the pressure coil is connected?
Ans: It is connected across the generated voltage through a suitable resistance to limit the current drawn.
Q: How the current coil is connected?
Ans: It is connected in series with the terminals, but even then there is a limiting resistance connected in series to limit the current in case of a short circuit.
Q: How many terminals are there in the instrument?
Ans: E and L, two.
Q: What for ligaments are used?
Ans: There are used to supply the current to the moving system.
Q: Where does the pointer of the instrument rest?
Ans: In between the range i.e. maximum and minimum readings.
Q: What is the range of the scale?
Ans: It is from zero to infinity
Q: If the terminals are short-circuited (E and L) what will be the reading of the instrument?
Q: If the terminals E and Lare open, what will be the reading of instruments?
Q: What are the advantages of compensating coil?
Ans: 1. To cancel the effect of the external fields on the moving system,
- To improve upon the division of scale.
Q: What for the guards are provided?
Ans: To shunt leakage current.
Q: Why it cannot be affected?
Ans: Because the instrument deflection is independent of the magnetic strength as the same magnets are used for the generating and motoring actions.
Q: Does the toque developed in the megger depend upon the c.c. and p.c. or not?
Ans: Yes, both coils.
Q: Does the generated voltage depend upon the handle speed?
Ans: Up to certain limits. But if the speed is further fastened, there a clutch is fitted so that the speed and voltage remain constant if driven fast.
Q: What care should be taken for the use of the megger?
- Should in no case be used on the energized line.
- Before use check it first.
- The handle should be rotated at 160 r.p.m. or the instructions supplied.
Q: What is the general range of the instrument?
Ans: 250 V to 2500 V.
Q: Name the instrument used for finding out the earth resistance?
Ans: Earth tester.
Q: What is the frequency?
Ans: It is the number of cycles per second.
Q: What is the unit of frequency?
Ans: c/s Hz.
Q: Name the instrument used for finding out the frequency?
Ans: Frequency meter.
Q: What are the types of frequency meters?
- Electrical resonance type.
- Weston type.
- Ratio meter type.
Q: What is the mechanical resonance type meter?
Ans: In the meter, the frequency says mechanical resonance is used, as every metal has its own natural (mechanical) frequency. So some thin strips called reeds are used. These are placed along with an electromagnet. The electromagnet produces the magnetic field of a certain frequency then the reed having the same natural frequency will start vibrating and the vibration can be noticed by the flag mounted on one side of the reed.
Q: Generally which size of the reeds is used?
Ans: 4 mm width and 0.5 mm thickness.
Q: Do you think that all reeds are identical?
Ans: No, these are not identical.
Q: How these differ?
Ans: Slightly different in dimensions or carry different weights of the flag at their top.
Q: How the vibrations are made to be efficiently noticeable?
Ans: There is flag mounted over every reed which amplify vibrations,
Q: When the mechanical reeds are used how their natural frequency is affected?
Ans: The natural frequency can be affected or say vibrations are affected by the weight, length, and size of the reed.
Q: What is the unique advantage of this instrument?
Ans: It gives reading free from error; due to change in temperature, wave form and the magnitude of the applied voltage provided the voltage is not too low provided appreciable amplitude of the vibration.
Q: What do you know about the emerging coil wound for reed-type instruments?
Ans: It has more number of turns of fine wire and is connected across the supply through a series resistance.
Q: What is the electrical resonance frequency meter?
Ans: The instruments in which the principle of electrical resonance is adopted are named an electrical resonance frequency meter.
Q: How many coils are there in this instrument?
Ans: Two-the fixed coil and moving coil.