Power Factor calculation

[K.R.Iyer]

Anyone to help me with my question on power factor?

I am using a 3 phase scroll compressor having 8 amp in each of R,Y,B legs consuming total 5,000 watts power input and a condenser fan motor, which is single phase and draws 4 Amps in R leg and total input power of 700 Watts.

What is the Power factor for this condensing unit? What voltage and ampere do I consider for the P.F calculation in the formula, PF = (W)/(1.732 * V * I)? I need to select a P.F correction capacitor & hence need to know where I am now.

[SteinarN]

How do you know the compressor is consuming 5,000W?

V*I*1.73 gives you the kVA which is NOT the same as the power in kW.

To get the true power you need an advanced clamp-on amperemeter measuring the amps in all three legs and the voltage across all three legs simultaneously. The instrument then calculates the true power based on the voltage, amps and the phase angle.
This is always a lower value than the kVA value.

Say the true power is 4.000W and the kVA is 5.000kVA.
This gives a power factor of 4.000/5.000=0.8
The instrument also calculates the kVAr directly or you can calculate it from the kW or kVA and the power factor. Based on the kVAr, HZ and the voltage you can calculate the capacitors.

[wilko123]

Steinar,

KR did give the correct formula for PF not KVA, if you read his comments correctly, my interpretation:

Input Watts
Vl x Il x 1.732

Vl = Line volts
Il = Line Amps

KR Im assuming you have taken the rated KW of the compressor motor, therefore if you use the formula you did already and take 8 amps for the Il and 400 for the Vl ~3

then PF = 5000 / 400 x 8 x 1.732
= 0.90

HOPE THIS HELPS? In terms of the actual line voltage to use i would measure just to make sure across two phases with a multimeter to get the three phase voltage output.On three phase system with no problems electrically/install the PF wont change greatly from the 400 taken as a approx.

[K.R.Iyer]

Steinar,

The total power of my condenisng unit = 5000 + 700 = 5,700 Watts

Taking voltage value (or measurement) is also not a problem. What single ampere value do I use in the formula, because the ampere in the 3 legs are:
R : 8 amp + 4 amp for fan = 12 amp
Y & B : 8 amp each. Should I take average of 12, 8 and 8 in the 3 legs?

[SteinarN]

I'm not an expert in the area, but I think you must use the amp value for the compressor alone, calculate the three capasitors, then take the amp value for the fan, calculate the capasitor for the fan which is to be conected between one phase and neutral. You will then have four capasitors in total.

[THE WOMBAT]

It depens on what the site Power factor is to start with. Most large plants or factorys have large power factor correction units, (basically a box with a speical controller that samples the P.F. and switches in more capacitors as the p.f. drops.

If your P.f. is bad to start with say below 0.8. There will be little effect by adding capacitors, as your then trying to boost the site power factor up. You will only boost the site P.f. if you have a large bank of caps.

If your site is being billed on reactive power (like all large power usage sites), the lower the p.f. the more money you will be charged per unit of energy.

Power factor correction units i have installed i have seen pay back in 2 to 3 years on a site load of 1Mw with a cruddy p.f. of 0.6

If your site has massive inductive loads, you would have to look at installing power factor correction unit.

I used to install ABB P.f. units

hope that helps ya

[chillerman2006]

If your P.f. is bad to start with say below 0.8. There will be little effect by adding capacitors, as your then trying to boost the site power factor up. You will only boost the site P.f. if you have a large bank of caps.

Hi WOMBAT

From your above posting

Would I be right in thinking that if you install a chiller with power factor correction built in this would have no advantage in the way of reducing cost of power bills unless the site also runs power factor correction

[Sridhar1312]

12, 8,& 8 amps in Y,B &R seems to be Unbalanced. First it needs to be set right. Otherwise it is better to take average of the 3 values and work out.

[THE WOMBAT]

Chillerman

the units power factor is corrected for that machine attached, it wont do much to boost it around the chiller.

small commercal installs dont always have reative power meters, so there not billed higher for having large inductive machines running.

if your power has bad p.f. to start with, your chiller would still run slightly better with caps in it.
Your site p.f would still be low ..

a 5kw machine isnt going to drag the p.f. down unless ur living on an extension cord

[gulshan kumar]

thannx great tips

regards

gulshan kumar

[chillerman2006]

Chillerman

the units power factor is corrected for that machine attached, it wont do much to boost it around the chiller.

small commercal installs dont always have reative power meters, so there not billed higher for having large inductive machines running.

if your power has bad p.f. to start with, your chiller would still run slightly better with caps in it.
Your site p.f would still be low ..

a 5kw machine isnt going to drag the p.f. down unless ur living on an extension cord

WOMBAT

Modern chillers i have worked on have power factor correction built into the panel and i was trying to make out if this is beneficial to the customer or do they have to have the whole site pf corrected to lower their power supply bill

cheers chillerman

[THE WOMBAT]

its always best to maintain a heathly site p.f., if the chiller dosn't have p.f. caps in it this will drag it down lower across the site. discharge lighting anything with a choke to restrict current flow. super markets have lots and lots of discharg lighting and compressors.

Most chillers now have caps installed so that it doesn't affect the site'pf. as it stands befre the load of the chiller is added to the power network ,.

i hope that helps a little, too may beers since collage @)