Resettings overloads after Capacitors installation. 1

[EE7860]

I am looking for the formulas or concept of calculating or estimating the new overload settings once power factor correction capacitors are installed at the motor terminals. Remember the capacitors are downstream of the overloads therefore there is a decrease in current seen by the overload. What is that factor? or how can it be calculated or estimated?

 

[jraef]

How I do it:

1) Determine the KVAR you need. I usually just use a chart provided by the capacitor people.

2) Determine the Capacitor current (Ic), using the following formula;

Ic = kVAR x 10³
E (voltage) x 1.732

3) Subtract Ic from Im (motor FLA).


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[jraef]

Alternate method (from Cowern Papers):

Current_new = Motor Full Load (Nameplate) Amps X (Power Factor _Original/Power Factor _Corrected)


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[rbulsara]

jraef's second post is more accurate. Not too sure about the step 3) in the first one.

Motor FLA is combination of real and reactive currents. Assuming 0.8 power factor (or read from the nameplate), the real component is 0.8X FLA and reactive (inductive) is 0.6XFLA. The capacitive reactive current directly subtracts from the inductive reactive current. You take the net reactive current and find out the corrected FLA as square root of (square of real current + the square of the net reactive current).

(0.6 is the sine of the phase angle, if the cosine is 0.8. It will change if the normal pf is different)

 

[waross]

I like jraef's second post.
We don't know how much correction you have used or the original power factor.
A quick guess is to use 80% of the original setting. If the motors start to trip out, go to 90%.

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

 

[opmgr1]

jraef's second post is more accurate. Step 3 of the first post is implying an algebraic subtraction, however these quantities are vector quantities and cannot be add algebraically!

 

[KJLRSA]

Hi EE7860

Abb has done a nice document detailing the calcs for the protection of motors with PF caps. I have attached the document.

http://files.engineering.com/getfil...rrection_of_a_motor_protection_756152_ENa.pdf

 

[EE7860]

I like to thank everyone for the help and suggestions. You all rock..