1. What are the two main classifications of analog instruments?
The two main classifications of instruments are,
a. Absolute instruments. Example tangent galvanometer.
b. Secondary instruments. Example ammeter, voltmeter. Analog instruments are
classified according to their electrical quantity they measure. Example frequency
meter, voltmeter, etc. Principles they work are moving coil, induction.
2. What are three types of secondary instruments?
The three types of secondary instruments are,
a. Indicating type: It only indicates the electrical quantity measured. Example:
Ammeter, Voltmeter, Frequency meter etc.
b. Integrating type: It integrates (sums up) the quantity being measured. Example:
Energy meter.
c. Recording meter: It records as well as indicates the electrical quantity being
measured. Example: 3 pen graphical recorder.
3. Give three operating forces acting on indicating instruments.
a. Deflecting force.
b. Controlling force.
c. Damping force.
4. What are the advantages of digital instruments over analog instruments?
a. Human errors are avoided (comparative error) because the output is displayed in
form of numbers.
b. Power consumption of digital meters are low as compared to analog meters.
5. What is the range of resistances that can be measured using following.
a. Wheatstone bridge – 1 milli Ω to 11 MΩ.
b. Kelvins double bridge – 0.2 micro Ω to 11 Ω.
c. Megger – Insulation resistances more than 100 kΩ
6. What do you understand by tan delta for a insulating material?
Tan delta measurement is done to find the qualities of insulating material. Tan delta
is angle between current due to surface leakage or current due to capacitance and the
capacitive current. That is Tan δ = Ir / Ic.
7. For what purposes transformer ratio meter can be used?
Transformer ratio meter can be used for,
a. To find the ratio of a transformer.
b. To find the phase angle deviation of primary and secondary voltage of
transformer.
c. To find the magnitude of magnetizing currents.
8. Draw connection diagram of ammeter, voltmeter, energy meter, and wattmeter.
9. Illustrate how can you use a single-phase wattmeter to measure 3 phase reactive
power in a circuit?
We can measure reactive power of 3 phase circuit by dingle phase wattmeter by
connecting the current coil in series with a line or load and connecting the pressure
coil across the other two lines.
Reactive power = 3√ V * I * sinϕ Watts.
10. Draw the basic block diagram of digital meter and explain the function of each
block.
Alternator A/D converter BCD counter decoder & LCD display
Vx
Alternator: It reduces the unknown voltage to a small value. Because the reference
voltage is very less and the unknown voltage is maximum.
A/D converter: It converts the analog signal from alternator to digital signals.
BCD counter: It counts the number of pulses (binary counter).
Decoder and display: It decodes the binary code to decimal form and gives a visual
display of it.
11. Draw a neat sketch and explain the use of CT and PT for measurement of power in a
single-phase circuit?
If wattmeter of proper range is not available or if voltage and current ranges are high
we can usr CT and PT of suitable ratio. Connect the CT and PT as shown in figure.
The reading of wattmeter can be multiplied by the ratio to get the actual power.
CT PT
CC
WATT METER
PC
12. Explain the construction and working of moving coil instrument.
Construction: The moving part is a coil wound on a light aluminium frame,
mounted on a shaft made of stainless steel which is pivoted at either ends on jewel
bearing made of sapphire. The coil is placed between poles of a permanent magnet.
Moving system is made light as far as possible to have high torque and weight ratio.
There are two phosphor bronze springs of very less resistance. This is used as
terminals for the current to pass through the coil and out of coil. It is also serves the
purpose of control force.
Aluminium coil former acts as a damping device by eddy current damping. There is
a knife edge pointer and a counter weight to avoid its sagging.
Working: When ever a current carrying conductor is placed in a magnetic field a
force is experiences by the conductor. Moving coil meters work on this principle.
Force = BINL
As the BNL is constant, force (F) is directly proportional to current (I).
That is FαI. Where B is magnetic flux, N is number of turns and L is length of coil.
13. Explain the procedure for measurement of earth resistance.
14. Explain the working principle of wheat stone’s bridge along with equations under
balanced condition. B
P kG Q
I1 I1
A I2 G
I2 D
X S
C
Wheat stone’s bridge works on kirchoff’s law. It is used to measure medium range
resistances. P and Q are fixed standard resistances. S is standard variable resistances.
X is the unknown resistance. G is galvanometer, kG is galvanometer switch and kB
is battery switch. No current will flow through galvanometer if the potentials across
its terminals are equal. So there will be no deflection of galvanometer. This condition
is called the balanced condition.
This is achieved by varying S and also by varying P/Q ratio.
At balanced condition VAB = VAC And VBD = VCD
I1
*
P = I2*X & I1*Q = I2*S
Dividing both I1
*
P = I2*X
I1*Q = I2*S
Question and answers Electrical Maintenance Unit
- 107 -
= P/Q = X/S
Unknown resistance (X) = P/Q * S Ω
15. Explain the working principle of Kelvins double bridge and procedure for the
measurement of terminal resistance.
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