Power Measurement In a Three-Phase System

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Wattmeter

Electrical power is measured with a wattmeter. A wattmeter consists of a current coil connected in series with load, while the other potential coil is connected parallel with load.

Depending on the strength of each magnetic field movement, the pointer gets affected. The true or real power is directly shown in a wattmeter. In three-phase systems, power can be measured using several methods. For temporary measurements, a single wattmeter can be used.

However, for permanent measurements, a three-phase wattmeter having two elements is used which indicates both balanced and unbalanced loads.

For an unbalanced load, two wattmeters must be used as shown in the Figure 1.

The total power is calculated by adding the measurement readings given by the two wattmeters. With this method, the power factor can also be obtained.

When using the two-wattmeter method, it is important to note that the reading of one wattmeter should be reversed if the power factor of the system is less than 0.5. In such a case, the leads of one wattmeter may have to be reversed in order to get a positive reading. In the case of a power factor less than 0.5, the readings must be subtracted instead of being added.

The power factor of the three-phase system, using the two-wattmeter method (W1 and W2) can be calculated as follows:

Since the sum and subtraction of readings are done to calculate total true power of a three-phase system, methods shown are not used practically in industry.

Rather three-phase power analyzers are used which are more user-friendly.

Power Factor Meter

It is similar to a wattmeter in principle, only two armature coils are provided with mountings, on a single shaft. They are 90° apart from each other.

Both armature coils rotate as per their magnetic strengths. One coil moves proportional to the restive component of the power, while the other coil moves proportional to the inductive component of the power.

Energy Meter

This shows the amount of power (electric energy) used over a certain period. In a watthour meter, there are two sets of windings.

One is the voltage winding while the other is the current winding. The field developed in the voltage windings causes current to be induced in an aluminum disk. The torque produced is proportional to the voltage and current in the system.

The disk in turn is connected to numeric registers that show electric energy used in terms of kilowatt-hours.

What’s common for Beer Mug and Power Factor? Learn.

Reference: Practical Troubleshooting of Electrical Equipment and Control Circuits – M. Brown

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

1. rajendra
Aug 01, 2016

The formula you mentioned for the PF calculation is wrong .It should be w1-w2 in numerator instead of w1*w2

2. Thys Beeslaar
Sep 02, 2015

I want to summate three or more ct’s (400volt) to one meter. Must the ct’s be the same as say 300/5 in a
5+5+5/5 system? Thus 300/5 & 300/5 & 300/5. If I use say one ct 600/5 my calculation would be wrong.

Regards,

Thys

3. SANTOSHKUMAR
May 04, 2015

HELLO SIR, I HAVE PROBLEM RELATED CORE LOSS MEASUREMENT OF TRANSFORMER CORE ASSEMBLY .WHILE MEASURING CORE LOSS IN YAKOGOWA METER WE ARE GETTING 155 WATTS FOR 63 KVA TRANSFORMER,IF SAME TRANSFORMER WITH CORE COIL ASSEMBLY AND TANKING (IN OPEN CIRCUIT CONDITION) IT MEASURES 185 WATTS WHICH IS MORE THEN THE ALLOWED LOSSES (IT IS MEASURED AT CUSTOMER SITE THEY MEASURING IN 3 WTT METER METHOD).SO WHY THIS HAPPENS SO SIR.

4. alok singh
Nov 29, 2014

hi, you have shared valuable information.

Nov 09, 2014

its good

6. Sriram
Jan 08, 2014

Dear sir,
I have a energy meter which is based on 2 wattmeter method measurement.
when connected to a plant load it showed no reading. it was noticed that the polarities of CT was changed. is it posible to get back the lost readings.

7. Phathakuhle Mnyandu
Dec 11, 2013

Metering on HIGH voltage transmission lines is usually done using the 3phase 4-wire system (63.5V) instead of the two-watt meter method (110V). Any particular reason?

8. davron
Sep 08, 2013

You all provide material which actually should have been known to the seasoned professional , but in attractive and easily digestible snippets! This is very delightful! Thank you for referencing your sources!

9. davron
Sep 08, 2013

Thank you to all contributors ! You all provide material which actually should have been known to the seasoned professional , but in attractive and easily digestible snippets! This is very delightful! Thank you for referencing your sources!

10. sudeep joseph
Jan 20, 2013

and one more thing .. why there is need of only 1 wattmeter under balanced condition and 2 wattmeter under unbalanced condition ?

• arise1000
May 15, 2013

This hardly happens on any power system. But for a balance 3 phase systems. the total power consumed is just three times that of one phase so there is no need to meter all the phases.
The last question would then be self explanatory

11. sudeep joseph
Jan 20, 2013

Good evening Sir !
Just want to ask a question. Why does wattmeter show -ve reading some time ?