3 phase power measurement

5/28/2018 3 Phase Power Measurement

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UNIVERSITY OF TECHNOLOGY, JAMAICA

Faculty of Engineering & Computing

School of Engineering

Electrical Machine

Lab #1: Two Wattmeter Method of Measuring Power in Three

Circuits

In partial fulfilment of the requirement for the

Bachelor of Engineering Degree

Date: February 4, 2010

Ainsloy Richards (Id#:0703146)

Instructor: Nathan Martin


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Objective

To test the theory that the sum of the two wattmeter readings gives the total power in a

three phase balanced load and to compare the experimental value of the power with the

theoretical value.

Theory

General

Balance in a three phase load is achieved when the line voltages are equal and the line

currents are equal. For a balanced three phase system that is connected in wye, the line currents

are equal and line voltages are equivalent to the product of root three and the phase voltage

{( and . On the contrary, the line current is equivalent to the product ofroot three and the phase current and the line voltages are equal to the phase voltages in a delta

connection ( { ( .

Power

The apparent power for each phase in a three system is given by:

, for the delta connection:

Hence, the apparent power in the three phase delta system is given by:

The total active power in a three phase system is three times the product of the phase voltage,

phase current and the power factor ().


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That is;

For the both delta and Wye connection is; For a purely resistive circuit, the power factor is equal to 1 ( ) and the total power isequal to the apparent power.

The power in a three phase system can be measured by using the one wattmeter meter or the two

wattmeter method. The one wattmeter method is limited to measuring power in the star

connection but the two wattmeter method is used for both the star and delta connection. The total

power in the three phase system, when using the two wattmeter method, is given by the algebraic

sum of the power on both meters.

That is; Apparatus

3 Ammeter, 0-5 A 2 Electronic Wattmeter 68 - 200W

3Bulbs, 150 W/ 220V Digital Multimeter (DDM)

Universal Power Supply 60 -105 Three Phase Measurement Panel

Fig. 1 Experimental Circuit


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Procedure

The circuit was setup a shown in figure 1 and the universal power supply switched on.

The variable supply was adjusted to give a reading of 110 V between the red and yellow line.

The line voltages was then measured and recorded. The voltage drop across each phase, the

phase currents, and the line currents were also measured and recorded. The universal power

supply was the switched off and the circuit dissembled.

Results

Table 1a: The necessary data that was collected in the experiment for power calculation

Line

Voltages

(V)

Line

Currents

(A)

Phase

Currents (A)

Phase

Voltages (V)

Measured

Power (W)

Calculated

power (W)

W1 W2

VRY= 110 IR= 0.4375 IRY= 0.24 VRY= 110

50 58

26.4

VYB= 114 IY= 0.45 IYB= 0.25 VYB= 114 28.5

VBR = 113 IB= 0.4625 I BR= 0.2375 VBR= 113 26.8

Total power 108 81.7

Sample Calculations:

Since the circuit was not balanced, the power in each phase must be calculated separately by the

equation;

(Assuming that the loads are purely resistive)For; VRY= 110 V, IRY= 0.24 A

The Power Factor

Taking the average of the line voltages and phase currents and assuming that the system is

balanced.

(

)


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(

)The Total power is by;

The Power factor using the readings from the Wattmeters:

[ ] * +*

+

Discrepancy between the measured and calculated power;

( )

Discussion:

The power in consumed in a three-phase delta or wye connection can be found by the two

wattmeter method. This is achieved by using one of the lines as reference (connected in to the

voltage coil of both wattmeters) and the remaining lines are connected to current coil of the

wattmeters. The experimental value for the total power that was consumed in the three phase

circuit was calculated by the algebraic sum of the power reading on wattmeter 1 and 2. The

dissipated power was found to be 108 W. In theory, the dissipated power in the balanced three-

phase system is three times the power consumed in a single phase. The theoretical power was

calculated to be 81.7 W. When the theoretical and experimental values for the power are

compared, it is revealed that there is a significant 32.2 % discrepancy in the readings and it is

apparent that the experiment did not support the theory. This experiment can be considered as

been unsuccessfully conducted due to the fact that the experimental power is greater that the


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theoretical power. In general, the theoretical values for the power should be greater since it takes

into consideration the ideal situation. It does not take into consideration the resistance of the

wire and the addition resistance associated with the attaching the measuring apparatus to the

system that would have cause the experimental value to be less. The only logical reason for the

having the measured power greater than the theoretical power is that the apparatus used were

faulty. The apparatus must have not been calibrated before the start of the experiment. It must

also be noted that the system was not balanced and this could have contributed to the discrepancy

in the result.

Conclusion:

The experimental power consumed by the system was found to be 108 W and the

theoretical power consumption is 81.7 W. There is a 32.2% deviation of the experimental power

form the theoretical power but due to the source of error encountered in the experiment, it can

still be stated that the total power in the system is the sum of the power reading on both

wattmeters.

Reference

Wildi, T, 2006, Electrical Machines, Drives and Power System, 6th

Edition, Prentice Hall

McPherson, L, 2003, An Introduction to Electrical Machines and Transformers, 2nd

Edition, John

Wiley & son

Milton, R, 2010, lesson_1_Three Phase BEng 3m lecture notes,

http://utechonline.utech.edu.jm/course/view.php?id=413

3 phase power measurement

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