Unbalanced 3-Phase Power Calculations

These reading are not that bad. An average value would give you a close approximation of the usage. Your power factor is very bad. You may can some some money by adding capacitors.

considering that there are only two circuits the best way is to calculate the power consumption for each phase of the circuit and add them to obtain the circuit's power consumption. If you do a job do it right.

It *should*, yeah. Unfortunately the meter gave us voltage and current readings for all 3 phases, but only power readings for 2 of the phases and to get the 3-phase power it simply added the 2 phases.

We're going to look into why this happened and hopefully fix it before we take any further readings.

Thank you all for your help, I will use Eddy's method to calculate the power per phase and add them together.

I was thinking along the same lines, but wasn't sure if the same formula would be acceptable for calculating the power in each phase.

Hello,

This is the guest that made the thread. I have a question regarding calculating the power consumption in each phase and adding them together, as shown above.

Say I have a perfectly balanced 3-phase circuit with I=30A, V=600V, and PF=0.8. To calculate the power I would use P=30*600*0.8*1.732=24940W

Now, if I were to use the method above of calculating the power in each phase and adding them together, I would get 3x the power.

So if I use the calculations shown above for each phase and add them together, I expect I would get a result that is much higher than it should be regardless of whether the circuit is unbalanced or balanced. This is the problem I was having before, I don't know the proper, most accurate, way to calculate the power when the 3 phases are unbalanced.

Am I missing something?

Keep in mind the two examples I gave in the first post are just 2 of many measurements we have taken. Some of our measurements have much greater current imbalances and some are close enough to take an average.

Thank you for any help you can give me.

For a Three phase,

Power=Root 3 X Line voltage X Line current XPower factor.

for a Single Phase

Power=Phase voltage X Phase Current X Power factor.

I think in your calculation for single phase loads you are taking Line voltage, this is the reason you get very high figure.Kindly check

You wrote "we have put a power meter on some circuits". If you have used a power meter then that should DIRECTLY give you the power consumption, apart from voltages & current in individual phases. Why do u need to calculate the same all over again?

Thanks and regards

Ashok Toshniwal, Bangalore, India

See my comment (as guest) above. Our power meter returned voltages and currents for all 3 phases but only power for 2 of the phases. It then used the power readings from the 2 phases to calculate the "3-phase" power, which really isn't 3-phase at all because the 3rd phase had power readings of 0.

We measured a number of circuits like this before we caught it and we don't have time to go back and monitor them all again, as we monitor them for days at a time.

Hence the reason I would like to just calculate it myself if I can find an appropriate method.

I realize this is a month later, but I am commenting since I am concerned about the usefulness of your energy audit. It really doesn't matter how you decide to determine the value you use in your audit, since you are obviously not getting accurate audit information.

Your meter was not connected or programmed properly, the voltage and amp information is momentary and does not represent anything of value in an energy audit. The element of time is missing. Even though you have reported power factor values, these too are momentary values. The power formula does not apply to energy, only power, which is momentary and totally invalid data for an energy audit.

It is obvious that your energy audit will be a pure guess based on no data. If you have re-done the data collection and obtained actual energy data, I commend you. If you have gone ahead with the 'energy audit' with the above discussion in mind, I feel bad for your client.

Regards, CJM

No need to feel bad for our client, we have thoroughly gone through our methodology with them and they agree that it is best for their situation. While it would be ideal to set up a power meter for each individual piece of equipment, it is simply not possible given our time-frame or budget. There are thousands of pieces of equipment. I understand your concern and we have voiced the same to the client, however all parties agree that there is only so much depth that we can go into given the budgetary restraints.

Although some assumptions will need to be made, the energy audit will not be pure guess based as most of the run times of individual pieces of equipment are known and we are taking as many measurements as we can, given our budget.

Also, I'm fresh out of school and posting here was more to learn as much as I can. I never took many power courses in school and I like to learn as much as I possibly can on my own (even if it means posting in forums) before I go ask someone like my boss. It makes the information stick better.

I am comforted to hear that you have been open with the client, and that you are expressing honestly about the situation being a learning one.

The time element is the major difference between power and energy. A kiloWatt of load (power) becomes a kiloWattHour (energy) every hour. If the volt and amp values you stated above can be expected to extend for most of the time involved, or can be assigned an amount of time as you say, then the values can be interpolated into an energy audit, but I am still bothered by the amount of 'interpolating' this will require.

The loads must be very steady and regularly scheduled. Also, I am not at all worried about a power factor in the range of .68 for a load of around 2 or 3 amps. This panel obviously is equipment related, whereas the other more significant load is on the panel with the excellent power factor. A load balance difference of 20 amps in the usage ranges you have given also do not represent any significant issue.

After many years of exactness in metering electrical loads, I hope you can understand my reluctance to see power formulas and guesswork serve as an energy audit. I am also a student in many other fields of my interests, and use the same 'ask an expert' method you have expressed. This is the best place I can think of for that.

Regards, CJMcGILL

The power meter readings that you were getting are correct. The power monitoring method used is addressed by Blondel's Theorem, which states that to measure the power delivered by and N wire system, you need N - 1 meters. You can accurately measure three-phase power on a three-wire delta system by referencing one leg as a common, which is why you had power readings on two of the legs and nothing on the third. I have gone through the math and it does work. Google it and see for yourself.

Ah, yes, thank you for the name of the theorem. This is indeed the method that our meter was using. I had found a lot of documentation on the "two watt-meter method" but didn't come across the actual name of the theorem in my travels. After reading about this method a while ago I was able to verify that it is, in fact, the way the meter is calculating power.