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#1
10-19-2011, 05:37 PM
 Unregistered Guest Posts: n/a
How to calculate power consumption of 3-phase transformer at idle from electrician AM

Hello,
Thank you in advance for all the tips and answers. Recently we moved to a warehouse and we just paid over \$300 bill for the one month from Consumers Energy, which I believe is caused by the transformer that we don't really use. We have some lights and computer only in the warehouse, no machinery. On the bill it stated we used:

2261 kWh - this from using about 25 fluorescent lights (each containing 6x32W tube lights) and 6 computer stations, that's it.

The building has a power line from Consumers Energy at 480V, which was used for welding by previous occupant. The line is connected to a large panel where there is only one breaker one at this moment - for the step down transformer to 240V, which feeds smaller panel with all small (120V) electrical outlets and ceiling lights.

My questions are:

1. How to calculate the power consumption of that transformer when it's in idle (everything in the warehouse is shut off, but the transformer is on and buzzing)?
We have given:
11.3 A
measured by electrician today at the cables that run from large 480V breaker in large panel to the transformer (three cables are there, 4A, 3.3A, 4A, totaling 11.3A)
480V which is the current going to the transformer
Some source also mention Power Factor, do you know if it applies here and if so at what value? I just assumed is is value of 1, so kVa=kW
From what I found this suppose to be:

11.3A x 480V x 1.73 (square root of 3) = 9383.52 VA = 9.38 kVA(kW)
Is it correct?

If so, continuing, what is our power consumption when all the lights and devices are on? Measured amps as well today by electrician (14A, 10A, 9A, totaling 33A)

33A x 480V x 1.73 (square root of 3) = 27434 VA = 27.43 kVA(kW)

Now, if this is correct, our facility runs 40 hours a week with all the lights and computers on and 128 hours a week with the lights off and transformer at idle. That will give us:

40h x 27.43kW = 1097.2 kWh
128h x 9.38kW = 1200.64 kWh

Total 2297.84 kWh per week, assuming 4 weeks per month = 2297.84 x 4 = 9191.36 kWh which is not equal the consumers energy reading (2261 kWh actual) and I am sure this calculation is incorrect because the bill would be \$1011.45 at rate \$0.11/kWh.

Where is an error here?
#2
10-20-2011, 03:23 AM
 JohnS Moderator Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,538 Rep Power: 19
Re: How to calculate power consumption of 3-phase transformer at idle from electricia

Quote:
 Originally Posted by Unregistered Hello, Thank you in advance for all the tips and answers. Recently we moved to a warehouse and we just paid over \$300 bill for the one month from Consumers Energy, which I believe is caused by the transformer that we don't really use. We have some lights and computer only in the warehouse, no machinery. On the bill it stated we used: 2261 kWh - this from using about 25 fluorescent lights (each containing 6x32W tube lights) and 6 computer stations, that's it. The building has a power line from Consumers Energy at 480V, which was used for welding by previous occupant. The line is connected to a large panel where there is only one breaker one at this moment - for the step down transformer to 240V, which feeds smaller panel with all small (120V) electrical outlets and ceiling lights. My questions are: 1. How to calculate the power consumption of that transformer when it's in idle (everything in the warehouse is shut off, but the transformer is on and buzzing)? We have given: 11.3 A measured by electrician today at the cables that run from large 480V breaker in large panel to the transformer (three cables are there, 4A, 3.3A, 4A, totaling 11.3A) 480V which is the current going to the transformer Some source also mention Power Factor, do you know if it applies here and if so at what value? I just assumed is is value of 1, so kVa=kW From what I found this suppose to be: 11.3A x 480V x 1.73 (square root of 3) = 9383.52 VA = 9.38 kVA(kW) Is it correct? If so, continuing, what is our power consumption when all the lights and devices are on? Measured amps as well today by electrician (14A, 10A, 9A, totaling 33A) 33A x 480V x 1.73 (square root of 3) = 27434 VA = 27.43 kVA(kW) Now, if this is correct, our facility runs 40 hours a week with all the lights and computers on and 128 hours a week with the lights off and transformer at idle. That will give us: 40h x 27.43kW = 1097.2 kWh 128h x 9.38kW = 1200.64 kWh Total 2297.84 kWh per week, assuming 4 weeks per month = 2297.84 x 4 = 9191.36 kWh which is not equal the consumers energy reading (2261 kWh actual) and I am sure this calculation is incorrect because the bill would be \$1011.45 at rate \$0.11/kWh. Where is an error here?
The first issue is whether the rated 480 V is measured line to line or line to neutral. Because you have summed the phase currents, you should divide the 480 V by sqrt(3) if line to line, not multiply, and just use 480 V directly if line to neutral. So both calculations are overstated.

The second issue is that the current to the idling transformer is almost entirely reactive (power factor closer to zero than one). Even for the loaded power transformer, the departure of power factor from one may be significant and should be measured. For the "loaded" case, I get 1573 kWh for the month assuming PF = 1 (which I doubt it is). Some residual draw from the idling power transformer may account for the rest.

The ballasts for the lights also draw power so each tube is "eatting" more than its rated 32 W when counting the ballast. I estimate about 8000 W draw from your loads when everything is on. At 40 h/week, 4.3 weeks/month, I estimate 1400 kWh from the loads. If you brought in new wiring or replaced the transformer with a more "right sized" one, you might save a third on your electric bill, but you would have to review investment and payback.
#3
06-20-2012, 12:04 PM
 Unregistered Guest Posts: n/a
Re: How to calculate power consumption of 3-phase transformer at idle from electricia

hi dear , its MUHAMMAD IQBAL ,
to calculate the power consumption , u can use the simple formula
P=(square root of 3) * V * I * CosQ

u cant take ideal bcoz its in runing condition , ideal is limited for books or studies , 1st of all use power factor if u r calculating KW instead of KVA , if u r calculatiing KVA ( real power) so no need to use power factor , 2nd , ur voltage must be stand on 440V instead of 480V , ur voltage is increasing bcoz of FERANTI EFFECT , in this effect , if ur transformer is ON and u r not putting the load in it than ur voltage will increases , and its called losses, losses is also load , more losses consume more power , 3rd , transformer has 99% effecency so include 1% as consumption .
merge all then u will find ur answer.

hope its answer the question .

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