Power Factor Correction Capacitor Sizing

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VoltzElectric

Member
Location
Hudsonville Michigan
Quick question, or maybe not so quick.
I have some pretty poor PF on.....alot of my machines. Most of them are around 60 hp motors with 1200 rpm, 129.7k apparent power, 70k true power and 155A. They are averaging around .55pf.
Here in lies the question.
I have been told that there is a 50/50 rule which is:
?Maximum Capacitor Sizing = 50% of Hp up to 50 Hp'
So, if I have 2 60 hp motors on my machine with a KVAR rating of 107, and im trying to get out of penalty from my POCO (above .8pf), and with this rule, I can only add 2 25KVAR capacitors, that leaves me only at a PF of (rough calculation here) .70 pf. My goal here is not achieved

View attachment 3980
This is the chart I got from the manufacturer of the caps. It only allows 15KVAR, the 50/50 rule allows at least 25 KVAR.......any suggestions? I have a consultant that is coming in who claims they can make it 1.0pf....any thoughts or comments?
 
C

Cold Fusion

Senior Member
Location
way north
Ther are a few things I'm not clear on.

"1200 rpm, 129.7k apparent power, 70k true power and 155A": That must be for two 60 hp motors

"2 60 hp motors on my machine with a KVAR rating of 107", means that the two motors are drawing 107kvars.

If I'm wrong on these two let me know cause none of the rest of this post will make any sense

Couple of questions:

What is the motor nameplate voltage?

What is the power source (at the motor)? Or what is the power source and the feeder length and size.

I'm asking cause the numbers don't sound right.

Additional question
Where did this "50/50 rule" come from?


More later - back towork for me

cf
 
V

VoltzElectric

Member
Location
Hudsonville Michigan
Cold Fusion said:
Ther are a few things I'm not clear on.

"1200 rpm, 129.7k apparent power, 70k true power and 155A": That must be for two 60 hp motors

"2 60 hp motors on my machine with a KVAR rating of 107", means that the two motors are drawing 107kvars.

If I'm wrong on these two let me know cause none of the rest of this post will make any sense

Couple of questions:

What is the motor nameplate voltage?

What is the power source (at the motor)? Or what is the power source and the feeder length and size.

I'm asking cause the numbers don't sound right.

Additional question
Where did this "50/50 rule" come from?


More later - back towork for me

cf
Click to expand...
sorry for the lack of info
480v 3p motors
and yes, the kvar rating is for both motors as well as amp draw and true power.
The feeder source is approx 15ft max from motor to motor starter, through motor breaker to Main machine disconnect
The 50/50 rule came from a rep from the capacitor company (myron/zucker)
 
Besoeker

Besoeker

Senior Member
Location
UK
Cold Fusion said:
Additional question
Where did this "50/50 rule" come from?
Click to expand...
I have come across this, or something similar, before.
With more than a certain value of (PFC) kVAr connected across an induction motor, it can self excite. In practical terms I know and have demonstrated this to be the case.
Some years ago, we were playing around with an idea for a cheap and simple stand-alone generator. The output was from a cage induction motor with self excitation using capacitors. We demonstrated a 30kW unit running successfully in our test bay. That was the simple part. Getting it to market.....
Digression, I know, but it illustrates the point.

In most applications, the self excitation isn't desirable and that's one reason to limit the PFC kVAr. We avoid this problem by having separate contactors for the motor and the PFC.
 
V

VoltzElectric

Member
Location
Hudsonville Michigan
Besoeker said:
I have come across this, or something similar, before.
With more than a certain value of (PFC) kVAr connected across an induction motor, it can self excite. In practical terms I know and have demonstrated this to be the case.
Some years ago, we were playing around with an idea for a cheap and simple stand-alone generator. The output was from a cage induction motor with self excitation using capacitors. We demonstrated a 30kW unit running successfully in our test bay. That was the simple part. Getting it to market.....
Digression, I know, but it illustrates the point.

In most applications, the self excitation isn't desirable and that's one reason to limit the PFC kVAr. We avoid this problem by having separate contactors for the motor and the PFC.
Click to expand...

So your saying that I can go with something closer to what I need to acheive a .90+ PF by running my PFC's on their own contractors? Would I tie them in to energize when the motor is running and shut off when the motor contactor is off then?
 
C

Cold Fusion

Senior Member
Location
way north
Bes is headed where I am. I won't cut in on his answers.

But your numbers still look funny. 155A for two 60 hp motors is right at full loaded on both. But your real power is indicating maybe 42hp each (assuming 90% efficiency).

Normally, motors running at FLA don't have that bad of a power factor (you are showing .55 as I recall). And, the real power is a lot less than I would have expected.

From Nema MG-1:
[FONT=Arial,Bold]
[FONT=Arial,Bold]14.30.3 Effect of Variation in Voltage on Power Factor[/FONT]
​
[/FONT]In a motor of normal characteristics at full rated horsepower load, a 10 percent increase of voltage above
that given on the nameplate would usually result in a decided lowering in power factor. A 10 percent​

decrease of voltage below that given on the nameplate would usually give an increase in power factor.
Click to expand...


That's why I was curious about the voltage at the motor and the nameplate FLA.

cf
 
Mr. Bill

Mr. Bill

Senior Member
Location
Michigan
If your target PF is 0.8 I think you'd need about 35kVAR per motor described.

0.8 * 129.7kVA - 70kW = 33.76kVAR

If the loads swing a lot with nearly all on at some times of the day and none on at other times I'd try to connect them to each motor so they'll disconnect when the motor is off.

I've also seen products like this that can adjust PF based on connected loads measured at the service entrance. While they can keep the PF at 1.0, it costs a lot more for minimal benefit. Tell them you want PF at worst to be 0.81 to avoid the Utility penalty. I don't know if they can program it to allow greater PF when the total loads are less. Something like 0.81 PF with 10 motors running and 1.0 PF with 5 motors running.

http://www.nepsi.com/lvacb.htm
 
C

Cold Fusion

Senior Member
Location
way north
VoltzElectric said:
Also, these machines are OLD. 70's oldest one is a 1971 machine with original motors. My powerfactor is a little better under FLA, but not enough to be remotly close to being where we need it
Click to expand...
Are they old enough to be U frame, 440V? Cause if they are, could the motor terminal voltage be high enough to be pushing them into saturation?

That would push the current up and the pf down.

cf
 
Besoeker

Besoeker

Senior Member
Location
UK
VoltzElectric said:
So your saying that I can go with something closer to what I need to acheive a .90+ PF by running my PFC's on their own contractors? Would I tie them in to energize when the motor is running and shut off when the motor contactor is off then?
Click to expand...
Yes.
Bear in mind that contactors for PFC have to be rated for PFC.
 
Besoeker

Besoeker

Senior Member
Location
UK
Dnkldorf said:
When did they start rating them as such?

We connected them on the load side of the starter, and I doubt the starter ever had a rating for PFC.
Click to expand...
That's a little different to switching them separately.
Here are a couple of links:

http://library.abb.com/global/scot/scot209.nsf/veritydisplay/ff2f11151223e7dec12570850041a49a/$File/1SBC101140C0202.pdf

http://library.abb.com/global/scot/scot209.nsf/veritydisplay/725fba10f5471f66c1256f430042dab9/$File/1SFC101030D0201.pdf
 
weressl

weressl

Esteemed Member
Location
Texas by the Gulf, geat crab, great shrimp
Cold Fusion said:
You may wish to get each motor separately - if you can.

cf
Click to expand...

Not only that but you may wish to size each capacitor for the actual load and power factor of the specific machine. Perhaps one of the reasons why the pf is so low is that your motors are considerably underloaded.

Having said that perhaps buying replacement EPACT motors - instead of capacitors -appropriate for the actual load is money better spent, you improve efficency AND power factor at the same time.
 
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