×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

3 Phase Power Monitor
2

3 Phase Power Monitor

3 Phase Power Monitor

(OP)
Hello,
I Monitor power of 3 Phase 480V (Delta connection), I get the power in Kw for each phase, how can calculate the total power?
Thanks.

RE: 3 Phase Power Monitor

If you have measured the power in each phase correctly the total power will be the sum of the power in each phase. However if you are asking a question like this there is a strong possibility that you may not have measured the individual phase Wattages correctly.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 3 Phase Power Monitor

Or if you are asking about kVA (total apparent power) then it will be volts X amps, or kW/pf.

If it is only total kW you want, then as Bill says just add the 3 measured values together.

Alan

RE: 3 Phase Power Monitor

(OP)
Bill, Alan thank you so much for replaying.
The data that I get from my monitoring board are: A, V(LL), KVA,PF and KW with accumaltive Kwhr. the reason for my question is that my calculated power,
the sum of (A*V(LL)*PF*3)/1000 per phase does not match the KW from my board.
In addition can I calculate Kw from the Kwhr?
Thanks,
Gadol.
 

RE: 3 Phase Power Monitor

Use sqrt(3) instead of 3. Or use V(LN) instead of V(LL).

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: 3 Phase Power Monitor

Replace the 3 with square root of 3  (or 1.732) in your formula. This is assuming the amps being more or less equal in all three phases.


For single phase KW calcs you need to use per phase voltage which is V(LL)/SQRT(3). This is useful when the currents are not balanced.

Now you if take that single phase value and multiply with 3, you will get the total 3 phase power. This should match what I said in my first paragraph.
 

Rafiq Bulsara
http://www.srengineersct.com

RE: 3 Phase Power Monitor

"can I calculate Kw from the Kwhr?"

(I assume by Kwhr you mean kilowatt-hours and not kw-hours reactive.  Kiloatt hours is usually abbreviated kwh or kWH).

Kwh = average kw x time.  

If your load is relatively constant and you know the length of time that the meter was measuring the load, you can calculate the average KW = KWH/(time in hours).

Read the kwh meter at the beginning of a time period, then read it the end of the period.  The difference in the readings divided by the time is the average kw.

Think of your cars odometer and speedometer.  The speedometer tells how fast you are going (that's kw). The odometer tells how far you have gone (that's kwh).  Using kwh to calculate kw is like using a trip odometer and stop watch to calculate average speed.
 

RE: 3 Phase Power Monitor

(OP)
Thank you so much, it is very helpful info.

When I add the 3 calculated power for each phase I get unrealistic value of total power, but when I avgerage the 3 power values per phase it's matchs my device power rating?

Do I do the right calculation by averaging the power for each phase?
Thanks,
Gadol.

RE: 3 Phase Power Monitor

Maybe your monitor is not configured properly?

Alan

RE: 3 Phase Power Monitor

Did you calculate the values using the "correct" formula as stated in my earlier post and that of Skogsgurra?

Post your calculations and measurements.

Rafiq Bulsara
http://www.srengineersct.com

RE: 3 Phase Power Monitor

(OP)
Yes I have used the correct formula.
For each phase:
Pph=(Aph*Vph(LN)*PF*3)/1000
Pph1=32.77kw Pph2=36.42kw Pph3=36.83kw
the Power monitoring board gives P=38.6kw
So avereging the phases is more like it, is it correct?


Thanks,
Gadol















 

RE: 3 Phase Power Monitor

Still not clear:

Delete 3 from the Pph formula.

Power per phase or Pph=(Aph*Vph(LN)*PF)/1000
Total 3 phase power = 3*Pph

Is the P that meter reads is total power?

What is the Amperage for each phase A, B and C?

Rafiq Bulsara
http://www.srengineersct.com

RE: 3 Phase Power Monitor

(OP)
Hi,
the monitor gives me total power in Kw.
Current per phase: 44.2, 48.3, 49.2 Amps
pf=~1
Thanks,
Gadol.

RE: 3 Phase Power Monitor

The 3 in Pph=(Aph*Vph(LN)*PF*3)/1000 is definitely wrong.  Assuming LN voltage of 277V, your powers are:
A=12.24kW
B=13.38kW
C=13.63kW
Total=39.25kW

Also, it is kW, NOT Kw or kw.

RE: 3 Phase Power Monitor

(OP)
David and all,
thank you so much, now it is making sense.
Happy Holidays,
Gadol

RE: 3 Phase Power Monitor

davidbeach

can you please explain how you came up with the kW values in your last post based on the currents given?

Also I thought that with a Delta connected load the only way you could sum up the individual kW values is is you were measuring them on the phase currents of the delta being inside the delta.  I wansn't sure that you could simpy add the kW values gathered by the the Line currents and L-N voltages for the line currents on a delta configuration as you could with a wye connected load.

RE: 3 Phase Power Monitor

Hi rockman;

Quote:

Gadol (Petroleum)      
21 Dec 09 18:25
Hi,
the monitor gives me total power in Kw.
Current per phase: 44.2, 48.3, 49.2 Amps
pf=~1
Use 277V
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 3 Phase Power Monitor

Waross

O.k. I see what I was doing wrong, I was still throwing in a 1.73 when using the 277 L-N voltage.  I get the correct answers now.

So from this I take it that it does not matter weather the load is connected in wye or delta, when using line currents and L-N voltages to come up with individual line kW's and then add them together for the total.

 

RE: 3 Phase Power Monitor

The only reason I ask about using the individual L-N voltages and Line currents on a detla connected load is because I have a fluke power meter that can be set up to measure a wye or delta network.  Since it will give the same power reading no matter how the load is connected I was wondering what the advantage was for using one over the other.  I guess with using the wye configuration of the meter you can see the individual line kW's for a delta connected load for example?

RE: 3 Phase Power Monitor

(OP)
Hi,
now I'm confused, Delta connection doesn't have N leg, so how come we still have the V(LN) we should only have V(LL)..?
davidbeach, what is the correct answer?
so given the current for each phase, which voltage should we used to calculate the power? and if the total power in the sum of individual phase in Delta connection?

Thanks,
Gadol.

RE: 3 Phase Power Monitor

Measured voltages were never provided so some assumptions needed to be made. We've already had to assume a power factor; assuming a point somewhere inside the delta that has an equal potential to all three phases is less of a stretch and doesn't require further knowledge of phase angles.

Given to amount of data missing you could also try:

((Ia+Ib+Ic)/3)/((Vab+Vbc+Vca)/sqrt(3))

That will provide W, divide by 1000 for kW. I haven't made that calculation but it will give a number very close to my previous answer.  It won't be exact because you can't have a delta connected load, identical phase angles, and different current magnitudes all at the same time. That's also why you wouldn't get the some results if you assumed one of the phases to be the reference voltage for all three phases.  

Neutral doesn't have to exist to be useful. Three wire systems are metered L-G all the time. If you had a neutral conductor you'd have to use it as your neutral voltage or measure the current in the conductor.  

RE: 3 Phase Power Monitor

So what you are saying is if I have a delta connected load I can use the Line currents and L-N voltages to find watts on each phase and then multiply by 3 for total, or go inside the delta and use the phase currents and L-L voltages to find watts on each L-L connection and then multiply by 3 and I will arrive at roughly the same answer?

With the power meter if I have all voltage probes connected including one on ground/neutral then should I get the same results no matter if I tell the meter I have a wye or delta connection?  Only values would be displayed differently.

RE: 3 Phase Power Monitor

(OP)
davidbeach,
 Data is not the problem, I get from my power monitoring board more then enough data.
let's go to the basic:
I get voltage and current for each phase:
I1=47.8A   I2=59.1A    I3=69.7A
V1=244.4V  V2=247.3V   V3=245.9V
PF=~1
What is the Power for Delta connection?
My board gives a power of 43.07kW, does it match the calculate one?

Thanks,
Gadol
 

RE: 3 Phase Power Monitor

If we assume a power factor of 99% the KWs will match.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 3 Phase Power Monitor

(47.8*0.2444)+(59.1*0.2473)+(69.7*0.2459)=43.44kW.  Obviously your PF is merely approximately 1, but not exactly 1.  You're still missing half the data as you only provide magnitudes and no phase angles.  In AC circuits about all you can do with just magnitudes is compare to set points and meter apparent power (VA).  Pretty much everything else requires phase angles.

RE: 3 Phase Power Monitor

gadol:

Calculations using your readings yield 43.44 kW, very close to the meter reading of 43.07.

244.4 *47.8    =11.68
247.3 *59.1    =14.62
245.9 *    69.7    =17.14
    Total    43.44

The load or source connection (wye or delta) does not matter when you are measuring the line quantities. You can consider both of them as black boxes connected with three lines.

You can assume either delta or wye configuration for calculations.

If you assume a wye connection, the line voltage is divided by sqrt 3 and the phase current is same as the line current.

If you assume a delta  connection, the line current is divided by sqrt 3 and the phase voltage is same as the line current.

The final per phase value of kVA or kW will remain the same.

Also for total 3 phase power formula, when you multiply the phase values for either assumption, it will result in the the same final formula of Sqrt(3)*V(LL)*I (LL) for a balanced system

You can also add the three (3) individual phase kVA (or kW if you include power factor), to get the total 3 phase power.


The formula Sqrt(3)*V(LL)*I (LL) only works for balanced system. For unbalanced one you must calculate individual phase values and add them.

Rafiq Bulsara
http://www.srengineersct.com

RE: 3 Phase Power Monitor

A correction to my last post ( ..blame the cut and paste attempt..)

Quote:

If you assume a delta  connection, the line current is divided by sqrt 3 and the phase voltage is same as the line voltage

 

Rafiq Bulsara
http://www.srengineersct.com

RE: 3 Phase Power Monitor

rbulsara

Your example with the "black box" makes perfect sense now.

Why does the power meter then give the option of of setting up the measurement for a wye or delta network?  Is this just for a matter of wheather you want to look at L-N or L-L readings?

For adding the individual phase KVA's then we dont need to worry about phase angles for anything other than determining kW values when measuring current?  Obviously if you were calculating line current from phases currents then I believe you would have to account for phase angles.

RE: 3 Phase Power Monitor

rockman:

I can't comment on on your particular power meter question as there is no accompanying data. It may have to do with how and where in the circuit, you connect the meter. You should consult the meter manufacturer's application engineer.

Think about this, your utility meter does not care if your motor or transformer is wye connected or delta. Does it?

As for the phase angles, do not confuse the phase angle for a power factor of a circuit with the 120 degree phase difference between the three phases of a 3 phase system.

The  SQRT(3) quantity is the result of the 120 degree relationship between the phases and you do not need to consider any other phase angle in a 'balanced' three phase system to calculate kVA.

The phase angle relationship  for determining the power factor is between the voltage and the current of a circuit. That is, between the phase voltage and the current in a given phase.

You could have a different power factor in each phase, but different load (kw) and yet have the same 'apparent' current. For kVA calculation such a circuit will still be balanced. Not for kW calcs.

To your last line in your last post, when calculating the line current from the phase current (if you are measuring at the right spot), only SQRT(3) factor comes or does not come in play depending up on the load connection as explained earlier. No phase angle.

I would suggest you get a basic book on AC polyphase circuit analysis and spend some time with it.

 

Rafiq Bulsara
http://www.srengineersct.com

RE: 3 Phase Power Monitor

rbulsara

My last paragraph dealt with the fact that for multiple L-L single phase circuits tapped off of a 3-phase, 3-wire, bus you would need to factor in the phase angles of all the L-L currents to determine the overall line currents on each of the lines feeding this bus.

It looks like you do not have to do the same with a three phase load, even if the different phases of the load were drawing different loads which would esentially be an unbalanced condition.

RE: 3 Phase Power Monitor

Yes, but that still amounts to the factor of sqrt3.
Phase angle is only important for calculating kW (or power factor).

The best is to look at an example:

Consider a 208Y/120V source and (3) loads of 10kW each at 208V. If they are connected to A-B, B-C and C-A lines respectively, they become a balanced circuit, connected in delta. The total load is 30kW.

Current in each 208V circuit is 10000/208=48A. This is the 'phase' current of the delta created by three loads.

You can calculate the 3 phase line amps using the 3 phase formula kVA= SQRT(3)*V(LL)*I(LL)/1000, which will be 83.2 amps (from A=1000*30/(1.732*208)

This is the same as what you will get by multiplying the 48A of 'phase' current, we calculated before, with Sqrt(3).

48A*Sqrt(3) = 83.2A.

If you have only two load circuits on, one of the line currents will be 83.2A and other two 48A each. Total real load 20kW.

If you have only one of the load circuits on, two of lines will have 48A each, the other will have 0A. Total real load 10kW.

So no phase angle is required to calculate kVA or amps.

However for kW, you need to take into account angle between the current and voltage you are using.

You may want to go through the exercises of considering the "black box" approach and calculate kVA and kW per phase.  (Hint: L-N voltage is 30 degree out of phase with the line-line voltage).  

Rafiq Bulsara
http://www.srengineersct.com

RE: 3 Phase Power Monitor

rbulsara

Great example!  Makes perfect sense now.

I understand that the pahse angle on each line will need to be know for determining the kW on each line.  

I also understand your hint regarding the phase shift.  No matter what the source or load (black boxes) the line current will be in phase with the L-N voltage, and 30deg shifted from the L-L voltage on a given line for three phase loads.

Single phase loads is slighty different for you would need to know weather the current was a L-L or L-N current.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources