What is the total power in a 3-phase circuit using the two wattmeter method?
A
W1 - W2 B
W1 + W2 C
W1 × W2 D
W1 / W2
Analysis & Theory
In the two-wattmeter method, total power = W1 + W2.
What is the power factor when both wattmeters show equal readings?
A
0 B
0.5 C
0.866 (lagging) D
1.0
Analysis & Theory
When W1 = W2, the load power factor is 1 (unity).
In the two-wattmeter method, if one wattmeter reads negative, the power factor is:
A
Less than 0.5 B
Equal to 0.5 C
Greater than 0.5 but less than 1 D
Equal to 1
Analysis & Theory
If one wattmeter reads negative, the power factor is less than 0.5.
How is the power factor (cosφ) calculated using wattmeter readings W1 and W2?
A
cosφ = (W1 + W2) / (W1 - W2) B
cosφ = (W1 - W2) / (W1 + W2) C
cosφ = √3 × (W1 - W2) / (W1 + W2) D
cosφ = (W1 + W2) / √3
Analysis & Theory
cosφ = √3 × (W1 - W2) / (W1 + W2) is used in two-wattmeter method.
When the power factor is 0.5, what is the relation between W1 and W2?
A
W1 = W2 B
W1 = -W2 C
W1 = 0 D
W2 = 0
Analysis & Theory
At pf = 0.5, one wattmeter reads zero and the other reads the total power.
If both wattmeters read equal and opposite values, the total power is:
A
Zero B
Maximum C
Equal to W1 D
Equal to W2
Analysis & Theory
If W1 = -W2, then total power = W1 + W2 = 0.
The two-wattmeter method is applicable for:
A
Only star-connected systems B
Only delta-connected systems C
Both star and delta connections D
Only single-phase systems
Analysis & Theory
This method works for any 3-phase balanced or unbalanced load, star or delta.
What is the angle between the current coil and pressure coil in each wattmeter in this method?
A
90° B
60° C
120° D
Depends on load
Analysis & Theory
In a 3-phase system, the phase angle between wattmeter coils is 120°.
When the power factor is zero (purely reactive load), the two wattmeter readings are:
A
Equal and positive B
Equal and negative C
Equal in magnitude, opposite in sign D
Both zero
Analysis & Theory
For purely reactive loads, W1 = –W2, so total power is zero.
The two-wattmeter method gives accurate results when:
A
Only when load is balanced B
Only for resistive loads C
Both balanced and unbalanced loads D
When only one wattmeter is used
Analysis & Theory
The two-wattmeter method works for both balanced and unbalanced 3-phase loads.