All About Circuits Forum  

Go Back   All About Circuits Forum > Electronics Forums > The Projects Forum

Notices

The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

Reply   Post New Thread
 
Thread Tools Display Modes
  #1  
Old 05-29-2008, 12:44 PM
DanD DanD is offline
New Member
 
Join Date: May 2008
Posts: 2
Question 3 phase power factor calculations, capacitors

I used the power factor information here to calculate the power factor and calculate the capacitor size required to correct the power factor. It cut my electric bill by 20%!

So, now I want to help a friend who has 3 phase on his shop. Can anyone tell me how to practically calculate the capacitor size needed when using 3 phase?

I assume it only requires 2 caps, one between each of the three line wires. Is that correct?
Reply With Quote
  #2  
Old 05-29-2008, 02:19 PM
mik3's Avatar
mik3 mik3 is offline
Senior Member
 
Join Date: Feb 2008
Location: Cyprus, but now in UK (GMT+0)
Posts: 4,846
Blog Entries: 9
Default

Quote:
Originally Posted by DanD View Post
I used the power factor information here to calculate the power factor and calculate the capacitor size required to correct the power factor. It cut my electric bill by 20%!

So, now I want to help a friend who has 3 phase on his shop. Can anyone tell me how to practically calculate the capacitor size needed when using 3 phase?

I assume it only requires 2 caps, one between each of the three line wires. Is that correct?
Well,i am not an expert on three phase circuits but i think you have to use 3 capacitors. Take each pair of phases as a single phase system and calculate the capacitor needed but be aware that the voltage is higher so you have to use capacitors with higher voltage rating.
Reply With Quote
  #3  
Old 05-29-2008, 02:43 PM
gerty gerty is offline
Senior Member
 
Join Date: Aug 2007
Location: Tennessee GMT -5
Posts: 899
Default

mik3 is correct 3 capacitors. phase A-B, phase B-C, and phase A-C.
Reply With Quote
  #4  
Old 05-29-2008, 08:31 PM
DanD DanD is offline
New Member
 
Join Date: May 2008
Posts: 2
Default

Ok, but how do I calculate the capacitor size for 3 phase using a clamp meter and the power meter?
Reply With Quote
  #5  
Old 09-24-2009, 12:56 PM
dhanrajbiradar dhanrajbiradar is offline
New Member
 
Join Date: Mar 2009
Posts: 1
Wink DB

but why in Delta why not star connection of Capacitors?
Reply With Quote
  #6  
Old 09-25-2009, 04:56 AM
GetDeviceInfo's Avatar
GetDeviceInfo GetDeviceInfo is offline
Senior Member
 
Join Date: Jun 2009
Location: Canada
Posts: 1,501
Default

Just out of curiosity, what value of capacitors did you install to save you 20% of your electrical bill, and what loads are you running to get such a poor power factor?
Reply With Quote
  #7  
Old 09-25-2009, 01:06 PM
t_n_k t_n_k is offline
Senior Member
 
Join Date: Mar 2009
Posts: 4,924
Default

Quote:
Originally Posted by DanD View Post
Ok, but how do I calculate the capacitor size for 3 phase using a clamp meter and the power meter?
Firstly, GetDeviceInfo asks a very good question - this is worth knowing.

The three phase system loading may not be balanced across all phases and the individual phase power factors may be quite different. You might have to individually compensate each phase. If the loads vary appreciably then the power factor compensation may have a detrimental effect by actually overcompensating the load.

Remember you can only compensate a lagging power factor with a capacitor - which is usually the case anyway. I'm guessing you knew that anyway.

There are also issues with adding capacitors to your installation which requires due consideration to safety and ciruit protection. Presumably you have allowed for this in your friend's case.

In principle you can compensate each of the 3 phases individually as you did in your single phase installation. You can place the capacitors between each phase line conductor and the neutral - if neutral is connected in the service. You can use lower voltage rating for the capacitors in this mode but the currents will be higher than with a line-to-line connection mode - there's always a trade-off.

Using the power meter and ammeter you can determine the required compensating capacitor (line-to-neutral connection) for each phase as follows.

1. Measure the phase power P
2. Measure the phase current I
3. Calculate the phase reactive power Q = √((V*I)^2-P^2) where V is the known phase-to-neutral voltage.
4. The required compensating (for unity power factor) capacitance C=Q/(ω*V^2) where ω=2**f and C is in Farads
5. Capacitor ratings - Vmin=line-to-neutral voltage and Imin=ωCV Amps

For instance if P=1kW, V=110V, I = 12A and f=60Hz

Q=√((110*12)^2-1000^2) = 861.6VAR

C=861.6/(2**60*110^2)=1.89e-4F or 189uF (rated for 110V and 7.84 A continuous duty). This was a 'bad' case with an original power factor of 0.76 - so you end up with a mighty large capacitor to get back to unity power factor.
Reply With Quote
  #8  
Old 09-25-2009, 01:23 PM
t_n_k t_n_k is offline
Senior Member
 
Join Date: Mar 2009
Posts: 4,924
Default

As a supplementry comment I'm also puzzled that your electric power provider charges you for reactive power. In my country domestic consumers are only charged for the real power they consume - and only fairly substantial customer installations attract a benefit for installing PFC at their mains connection point. Hence my interest in GetDeviceInfo's comment / question. Your reactive power load must have been fairly high for the conductor I^2R losses in your dwelling to have been so significant. I'm assuming (perhaps incorrectly?) that your are a domestic consumer.
Reply With Quote
  #9  
Old 09-25-2009, 01:33 PM
t_n_k t_n_k is offline
Senior Member
 
Join Date: Mar 2009
Posts: 4,924
Default

Quote:
Originally Posted by t_n_k View Post
As a supplementry comment I'm also puzzled that your electric power provider charges you for reactive power. In my country domestic consumers are only charged for the real power they consume
Sorry DanD, this part is not well expressed - ignore that bit. Of course you are only charged for the real power consumed!

The remaining part of my last post is still relevant and your further comments re the details of your installation improvements are certainly of interest.
Reply With Quote
Reply   Post New Thread

Tags
, , , , ,


Related Site Pages
Section Title
Worksheet Oscillator circuits
Textbook Tesla polyphase induction motors : Ac Motors
Worksheet AC power
Worksheet Series-parallel combination AC circuits
Textbook Synchronous condenser : Ac Motors
Textbook Practical power factor correction : Power Factor
Worksheet Capacitance
Textbook Calculating power factor : Power Factor
Textbook Single-phase power systems : Polyphase Ac Circuits
Textbook Wiring Color Codes : Color Codes


Similar Threads
Thread Thread Starter Forum Replies Last Post
Power Factor - Practical Question vg19 General Electronics Chat 8 09-26-2009 12:43 AM
power factor correction for single phase rectifier using matlab jr_mumtaz86 Programmer's Corner 0 09-14-2009 07:20 AM
3 phase - bridge rectifier input/output power Skeebopstop General Electronics Chat 3 03-16-2009 02:51 AM
Three phase diode bridge power loss question fitch General Electronics Chat 7 03-11-2009 02:29 PM
Power Factor Correction 3 phase motor T83 General Electronics Chat 1 11-20-2008 08:39 AM

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT. The time now is 12:44 PM.


User-posted content, unless source quoted, is licensed under a Creative Commons Public Domain License.
Powered by vBulletin
Copyright ©2000 - 2014, vBulletin Solutions, Inc.