Power Measurement in 3-phase star connected circuit with balanced and unbalanced load using two-wattmeter method.

Experiment Number: 16

Aim of the Experiment:

Power measurement in three-phase star connected circuit with balanced and unbalanced load using two Wattmeter method.

Apparatus Required:

Sl. No. NameSpecificationQuantity
1Wattmeter(0-1500)W, 5A/10A, UPF2 Nos.
2Voltmeter(0-250)V, MI1 Nos.
3Ammeter(0-5)A, MI3 Nos.
4Connecting WiresPVC Insulated CopperAs per requirement
5Three-phase Variac(0-500)V, (5A/10A), 1 Nos.
6Rheostat(0-80)Ω, 5A3 Nos.

Circuit Diagram:

Theory:

A watt meter is an instrument for measuring active power directly in a circuit. It has two
coils i.e. current coil & pressure coil. Current coil measures the current through the circuit &pressure coil measures the voltage. Current coil is connected in series & pressure coil in parallel in the circuit. Two wattmeter can be used to measure power in a three phase 3-wire circuit, by making the connections as shown in circuit diagram. The load may be balanced or unbalanced. The current coils are connected in series with two phases and the pressure coils is connected between both phase and the third phase. The total power consumed by the two wattmeter = W1+W2 (algebraic sum).

Here current through the current coil of W1=IR and W2=IB
While potential difference across pressure coil of W1=VRY and W2=VBY

According to the phasor diagram

\[\color\red{W_1}\color\green{ =} \color\red{V_{RY}I_R\;Cos(30+\theta)}\] \[\color\red{W_2 }\color\green{=} \color\red{V_{BY}I_B\;Cos(30-\theta)}\]

Total Power drawn by Three phase load is

\[\color\red{W_1+W_2}\] \[\color\green{ =}\color\red{ V_{RY}I_R\;Cos(30+\theta)\;+\;V_{BY}I_B\;Cos(30-\theta)}\] \[\color\green{here ,}\color\red{Y_{RY} = V_{BY} = V_L}\] \[\color\green{then}\] \[ \color\red{W_1\; +\; W_2 \;= \;\sqrt3V_LI\;Cos\theta}\]
\[\color\red{\phi}\;\color\green{=}\;\color\red{tan^{-1}\sqrt{3}\left(\dfrac{W_2-W_1}{W_1+W_2}\right)}\]

Precautions:

  1. Don’t switch on power supply without concerning respective teachers.
  2. Three phase auto transformer must be kept at minimum potential point before starting.
  3. Resistant value of all rheostats should be kept at maximum position at starting.

Procedures:

(i) Connect all the instruments as per circuit diagram.

(ii) Make sure i. e all instruments are showing zero error.

(iii) After connection, keep the rheostat in maximum position and slowly increase the output voltage of the Variac so that current in each phase is about 1A.

(iv) Then vary the resistance of the rheostats so the load is deliberately unbalanced, i.e. the
current in each phase becomes different.

(v) Calculate the Three-phase power.

Observation Table: for Balance Load

Sl. No.Power in Watts
(W1)		Power in Watts
(W2)		Line Voltage in Volts.
(VL=VRY)		Phase Current in Amp.
(IR)		Phase Current in Amp.
(IY)		 Phase Current in Amp.
(IB)		Total Power in watts.
P=W1+W2
1
2
3
4
5
6
7
Table No: 11.2

Observation Table: for Un-balance Load

Sl. No.Power in Watts
(W1)		Power in Watts
(W2)		Line Voltage in Volts.
(VL=VRY)		Phase Current in Amp.
(IR)		Phase Current in Amp.
(IY)		 Phase Current in Amp.
(IB)		Total Power in watts.
P=W1+W2
1
2
3
4
5
6
7
Table No: 11.3

Conclusion:

To be written by students.

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