Measurement of Poly Phase Power MCQ Quiz in मल्याळम - Objective Question with Answer for Measurement of Poly Phase Power - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക
Last updated on Mar 26, 2025
Latest Measurement of Poly Phase Power MCQ Objective Questions
Top Measurement of Poly Phase Power MCQ Objective Questions
Measurement of Poly Phase Power Question 1:
In the two-wattmeter method of measuring 3-phase power, the wattmeters indicate equal and opposite readings when load power factor is
Answer (Detailed Solution Below)
Measurement of Poly Phase Power Question 1 Detailed Solution
Concept:
In a two-wattmeter method,
The reading of first wattmeter (W1) = VL IL cos (30 + ϕ)
The reading of second wattmeter (W2) = VL IL cos (30 - ϕ)
Total power in the circuit (P) = W1 + W2
Total reactive power in the circuit \(Q=\sqrt{3}\left( {{W}_{1}}-{{W}_{2}} \right)\)
Power factor = cos ϕ
\(ϕ =\text{ta}{{\text{n}}^{-1}}\left( \frac{\sqrt{3}\left( {{W}_{1}}-{{W}_{2}} \right)}{{{W}_{1}}+{{W}_{2}}} \right)\)
Calculation:
The reading of wattmeter 1 is proportional to cos (30 + ϕ)
The reading of wattmeter 2 is proportional to cos (30 - ϕ)
The readings of both the wattmeters will be equal and opposite only when the phase difference is 90°.
The phase difference of 90° is possible when the load is either purely inductive or purely capacitive.
Measurement of Poly Phase Power Question 2:
The power of a n - phase circuit can be measured by using a minimum of
Answer (Detailed Solution Below)
Measurement of Poly Phase Power Question 2 Detailed Solution
According to Blondel’s theorem:
- When a system contains ‘N’ number of phases & ‘N+1’ wires, then ‘N’ number of watt-meters is required to calculate the total power of the system.
- When a system contains ‘N’ number of phases & ‘N’ wires, then ‘N-1’ number of watt-meters is required to calculate the total power of the system.
- In a 3-phase balanced and unbalanced star-connected system, two watt-meters are sufficient to measure the total power.
But if a 3-phase star connected system is balanced then only one wattmeter is sufficient to measure the total power.
Measurement of Poly Phase Power Question 3:
The wattmeter reads
Answer (Detailed Solution Below)
Measurement of Poly Phase Power Question 3 Detailed Solution
Wattmeter:
A wattmeter is an electrical instrument that is used to measure the electric power of any electrical circuit.
The internal construction of a wattmeter is such that it consists of two coils. One of the coils is in series and the other is connected in parallel.
The coil that is connected in series with the circuit is known as the current coil and the one that is connected in parallel with the circuit is known as the voltage coil
Wattmeter measures the average power.
The reading of wattmeter will be,
Average power = Vpc Icc cos ϕ
Where,
Vpc is the voltage across pressure coil
Icc is current flows through the current coil
ϕ is the phase angle between Vpc and Icc
So, wattmeter is a device capable of detecting voltage, current and the angle between the voltage and the current to provide power readings.
Measurement of Poly Phase Power Question 4:
In the measurement of 3 phase power by two watt meter method, for an unbalanced load, the power factor of the load is
Answer (Detailed Solution Below)
Measurement of Poly Phase Power Question 4 Detailed Solution
Concept:
In a two-wattmeter method, for lagging load
The reading of first wattmeter (W1) = VL IL cos (30 + ϕ)
The reading of second wattmeter (W2) = VL IL cos (30 - ϕ)
Total power in the circuit (P) = W1 + W2
Total reactive power in the circuit \(Q = \sqrt 3 \left( {{W_1} - {W_2}} \right)\)
\(\phi = {\rm{ta}}{{\rm{n}}^{ - 1}}\left( {\frac{{\sqrt 3 \left( {{W_1} - {W_2}} \right)}}{{{W_1} + {W_2}}}} \right)\)
Power factor = cos ϕ
Hence, cos ϕ = \(\cos\left[\tan^{-1}\left\{\sqrt{3}\dfrac{(W_2-W_1)}{W_2+W_1}\right\}\right]\)
Note: We can write W2 - W1 or W1 - W2 both, it depends on the nature of the Power factor.
Measurement of Poly Phase Power Question 5:
The power of a n-phase unbalanced circuit can be measured by using a minimum of _________.
Answer (Detailed Solution Below)
Measurement of Poly Phase Power Question 5 Detailed Solution
According to Blondel’s theorem:
- When a system contains ‘N’ number of phases & ‘N+1’ wires, then ‘N’ number of watt-meters is required to calculate the total power of the system.
- When a system contains ‘N’ number of phases & ‘N’ wires, then ‘N-1’ number of watt-meters is required to calculate the total power of the system.
- In a 3-phase balanced and unbalanced star-connected system, two watt-meters are sufficient to measure the total power.
- But if a 3-phase star connected system is balanced then only one wattmeter is sufficient to measure the total power.
Measurement of Poly Phase Power Question 6:
What connections of a wattmeter should be reversed to read an upscale, when wattmeter reading shows negative reading (Pointer reads below Zero)?
Answer (Detailed Solution Below)
Measurement of Poly Phase Power Question 6 Detailed Solution
Concept:
The diagram of 3 phase 2 wattmeter is given below.
As we know that the readings of two wattmeters are
W1 = VRB IR cos ( 30 - ϕ ).
W2 =VYB Iy cos (30 + ϕ ).
Total power consumed by the circuit (Ptotal) = W1 + W2.
Where
W1 = Wattmeter used between R and Y phase
W2 = Wattmeter used between Y and B phase.
VRB = Line-to-line voltage between phase R and B
VYB = Line-to-line voltage between phase Y and B.
IR = Current in phase R.
Iy = Current in phase Y.
The diagram of a wattmeter is shown below.
Explanation:
|
SI No. |
Power factor [pf] (Phase angle) |
Wattmeter readings (W) |
Remarks |
|
|
W1 |
W2 |
|||
|
1. |
Pf = unity [1.0] (ϕ = 0° ) |
+ve |
+ve |
W1 = W2 |
|
2. |
0.5 < p.f.< 1.0 (60° > ϕ > o° ) |
+ve |
+ve |
W1 > W2 |
|
3. |
Pf = 0.5 (ϕ = 60° ) |
+ve |
zero |
Total power = W1 |
|
4. |
0 < Pf < 0.5 (90° > ϕ > 60° ) |
+ve |
-ve |
|W1|> |W2| (Total power is positive) |
|
5. |
Pf = 0 (ϕ =90° ) |
+ve |
-ve |
|W1|= |W2| (Total power = zero) |
From the above table, it is clear that when the p.f. of the circuit is between 0< pf <0.5 or when the power factor angle i.e. ϕ >60° then the reading of wattmeter W2 becomes negative.
To read that value we have to reverse connection of either potential coil or current coil but not both, of wattmeter W2 only. If we reverse both potential coil and current coil simultaneously then also wattmeter W2 shows negative reading only.
- So, because of the above reason option (3) is incorrect.
- Option (1) is incorrect because we can also do that by reversing the current coil also
- Option (2).is incorrect because we can also do that by reversing the potential coil.
- So based on the above explanation option (4) is correct.
Measurement of Poly Phase Power Question 7:
In a three-phase unbalanced load system, the method used to measure power is ______.
Answer (Detailed Solution Below)
Measurement of Poly Phase Power Question 7 Detailed Solution
Measurement of power
According to Blondel's Theorem, for the measurement of power in n-phase, n-wire balanced, or unbalanced systems, (n-1) wattmeters are required.
Thus, for the measurement of power in 3ϕ unbalanced system, 2 wattmeters are required.
The readings of both the wattmeters are as follows:
\(W_1=V_LI_Lcos(30+ϕ)\)
\(W_2=V_LI_Lcos(30-ϕ)\)
The total power is given by:
\(W=W_1+W_2=\sqrt{3}V_LI_L\space cosϕ\)
where, VL and IL are line voltage and line current respectively
cosϕ is the power factor
Additional Information For measurement of power in n-phase, (n+1) wire balanced, or unbalanced systems, (n) wattmeters are required.
For measurement of power in 3ϕ, 4-wire system, 3 wattmeters are required.
Measurement of Poly Phase Power Question 8:
The wattmeter measures -
Answer (Detailed Solution Below)
Measurement of Poly Phase Power Question 8 Detailed Solution
Wattmeter measures the value of active or real power. It detects the average value of power.
It is not used to measure the reactive power and apparent power.
Additional Information
Wattmeter:
A wattmeter is an electrical instrument that is used to measure the electric power of any electrical circuit.
The internal construction of a wattmeter is such that it consists of two coils. One of the coils is in series and the other is connected in parallel.
The coil that is connected in series with the circuit is known as the current coil and the one that is connected in parallel with the circuit is known as the voltage coil
Wattmeter measures the average power.
The reading of wattmeter will be,
Average power = Vpc Icc cos ϕ
Where,
Vpc is the voltage across the pressure coil
Icc is current flows through the current coil
ϕ is the phase angle between Vpc and Icc
So, a wattmeter is a device capable of detecting voltage, current and the angle between the voltage and the current to provide power readings.
Measurement of Poly Phase Power Question 9:
Errors are introduced in the measurement by wattmeter due______ to between current coil & pressure coil.
Answer (Detailed Solution Below)
Measurement of Poly Phase Power Question 9 Detailed Solution
Electrodynamic type wattmeter
- It contains two coils a fixed coil (current coil) and a moving coil ( pressure coil or voltage coil).
- The fixed coil is used to carry the current and is connected in series with the load in any circuit.
- The moving coil carries the current directly proportional to the voltage and is connected across the voltage.
- The moving coil is assumed to be highly resistive in nature.
Causes of errors in wattmeter
1.) Pressure Coil Inductance:
- At lagging power factor loads, due to the effect of inductance, the voltage coil current will become nearly equal in phase with the load current. So, the driving torque increases and makes the wattmeter read high.
- Hence power factor becomes lagging and leads to a high reading.
2.) Pressure Coil Capacitance:
- Apart from inductance, the voltage coil also possesses the effect of capacitance which is due to the inter-turn capacitance of the series resistance.
- So, the wattmeter reads high at lagging power factor loads and low at leading loads.
3.) Mutual Inductance Effect:
- The mutual inductance between the voltage coil and the current coil increases the phase angle.
- Since the error is directly proportional to the phase angle, the mutual inductance between the coils affects the error.
4.) Stray Magnetic Field:
- The stray magnetic fields (i.e., external magnetic fields) may disturb the operating field as it is quite weak in the case of electrodynamometers.
- So, if the external field aids the main field then, the meter reads high and if it opposes the main field then the meter reads low.
5.) Temperature Error:
- Change in the pressure coil resistance is caused due to variations in temperature.
- Due to this temperature variation, the controlling torque produced by the spring movement is also affected.
Measurement of Poly Phase Power Question 10:
A 400 V three phase 50 Hz balanced source is supplying power to a balanced three phase load. Line current flowing through the load is 5A at a power factor angle of 30 degrees lagging. Reading of two wattmeters used to measure the load power are:
Answer (Detailed Solution Below)
Measurement of Poly Phase Power Question 10 Detailed Solution
Concept:
In a two-wattmeter method,
The reading of first wattmeter (W1) = VL IL cos (30 + ϕ)
The reading of second wattmeter (W2) = VL IL cos (30 - ϕ)
Total power in the circuit (P) = W1 + W2 = 3 Vph Iph cos ϕ
CALCULATION:
Given: VL = 400 V, IL = 5 A, ϕ = \(30^\circ\)
W1 = 400 × 5 × cos (30 + 30) = 1000 W
W2 = 400 × 5 × cos (30 - 30) = 2000 W
Additional Information
In two wattmeter method, the power factor is given by
\(cos\phi = \cos \left[ {{{\tan }^{ - 1}}\left( {\frac{{\sqrt 3 \left( {{W_1} - {W_2}} \right)}}{{{W_1} + {W_2}}}} \right)} \right]\)
Here \(\phi = {\tan ^{ - 1}}\left( {\frac{{\sqrt 3 \left( {{W_1} - {W_2}} \right)}}{{{W_1} + {W_2}}}} \right)\)
When both the wattmeter’s show the same positive reading, ϕ becomes zero and hence power factor is unity.