×
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

Capacitors inject negative vars?

Capacitors inject negative vars?

Capacitors inject negative vars?

(OP)
Inductance is lagging, corresponds to positive Q or vars or reactive power.
Capacitance is leading, corresponds to negative Q.

Inductive machines or components absorb (negative) vars, yes?
Capacitance injects, generates, or produces (negative) vars, yes?

Why is the convention that capacitance injects negative vars, not that capacitance absorbs positive vars and inductance injects positive vars?

It may be that I just don't understand this correctly...


Is my convention wrong? This post states otherwise:
http://www.eng-tips.com/viewthread.cfm?qid=139432: "By convention, reactive power, like real power, is positive when it is "supplied" and negative when it is "consumed.""

RE: Capacitors inject negative vars?

Per the established conventions: a capacitor acts as source of vars, while an inductor acts as a sink of vars.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Capacitors inject negative vars?

(OP)
But a capacitor will create a leading current, therefore negative vars (a negative power triangle). So a capacitor acts as a source of negative vars? Or positive vars?

Much Thanks!

RE: Capacitors inject negative vars?

Vars in a power system are generally assumed to be lagging (inductive) vars.  Under this convention, capacitors produce vars.  

The analogy would be positive watts and negative watts.  Do motors produce negative watts?  I suppose, but the convention is that they consume watts produced by generation sources.  

As has been mentioned many times in other threads - if the flow of watts and vars are in the same direction, the power factor is lagging.  If the watts and vars are in opposite directions, the power factor is leading.  



 

RE: Capacitors inject negative vars?

Good comments already.

Some more comments may or may not help. Here are mine:

There are of course (at least) two equivalent ways to look at the world:
1 - assume vars are a positive quantity, and describe the situation under consideration by the direction in which the vars flow.
2 - establish a reference direction for reactive power flow, and decide whether the situation under consideration represents positive or negative var flow with respect to the reference direction.

Quote:

Inductance is lagging, corresponds to positive Q or vars or reactive power.
Capacitance is leading, corresponds to negative Q.
Your statement about positive Q for inductance and negative Q for reactive follows the 2nd approach, where you have assumed the reference direction of var flow is into the device (load convention for the device).

Quote:

Inductive machines or components absorb (negative) vars, yes?Capacitance injects, generates, or produces (negative) vars, yes?
No.  [positive] Vars flow from power system into the inductive machine. So we say induction machines absorb vars (with the understanding we're talking about positive vars... approach 1).  Likewise positive vars flow from cap into system.   This terminology of contrasting "absorb" and "inject" is more aligned with approach 1 (assuming positive vars) since your adjust your verb (to "absorb" or "inject") to suit the situation (both cases vars assumed positive).
 

=====================================
(2B)+(2B)'  ?

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