Power Factor

[eric stromberg]

Wouldn't the voltage peak be to the left of the current peak because it occurs first with respect to time. Another words it occurs first in a shorter amount of time (time increasing from left to right) therefore I would think it would be to the left and before the current peak.

Wow. How embarrassing. That's what i meant to say. I suppose it was all that jumping through the time continuum that messed me up. Yes; on a graph time proceeds from left to right. The future is always on the right. So voltage peaks, and then current shows up later. It is on the right side.

I have a spreadsheet that i built up that shows this. I'll put it on my website and put a link to it here.

Thanks for catching this.

 

[LarryFine]

Just to clarify something, with a lagging power factor where the current lags the voltage, does this mean that the voltage waveform will reach its peak along the time access before the current waveform does?

That is correct.

Looking at a time vs magnitude plot starting from 0s this would mean that the voltage waveform would reach its first peak at 1s and the current waveform would follow reaching its peak sometime later lets say 2s?

Not at 1 or 2s (seconds?), but twice per cycle. A sine wave has a positive peak and a negative peak, with zero crossings between each. So, the lead or lag happens 120 times per second, alternating polarity each time.

A capacitor's charging current is at it's peak while the voltage is changing value at the greatest rate (the zero crossing), and minimizes while the voltage is peaking. That's why capacitive reactive current leads the voltage.

An inductor attempts to resist a change in current flow (by storing, then releasing electrical charge), so it charges (and discharges) more slowly than the applied voltage, so its current peaks lag the voltage peaks.

By the way, only purely capacitive and inductive loads (or those portions of mixed loads) exhibit the complete 90-degree leading or lagging behavior. The phase angle is the result of the proportion of reactance (AC) to resistance (DC).

 

[Smart $]

...So, the lead or lag happens 120 times per second, alternating polarity each time.

...

An example plot of current lagging voltage by 30?.

 

[eric stromberg]

I'm going to attempt this.

Here is a link to a webpage that has a hyperlink to it for my spreadsheet. Clicking the hyperlink should cause the spreadsheet to download to your computer. Try it and let me know if it works.

http://www.ericstromberg.com/Sine Spreadsheet.htm

If it does, there are four tabs on the bottom of the spreadsheet:
1 - Three phase current
2 - Power Factor
3 - Power
4 - Harmonics

If it works, i'll post descriptions

 

[LarryFine]

If it works, i'll post descriptions

It worked for me, but what do I do with it?

 

[eric stromberg]

It worked for me, but what do I do with it?

First tab - 3 phase current

This tab shows how the phase currents affect the neutral current. By playing with the numbers that are highlighted in a somewhat strange green colour, you can see the effect on the neutral.

Try different values for the current in A, B, and C and see what it does. For example, setting C to 0 will show you that the neutral is now equivalent to the other two phases. This is a good way of demonstrating that 20 Amps on phase A and 20 Amps on phase B adds to 20 Amps on the Neutral

Second tab - Power Factor

In addition to varying the voltage and current, you can enter a different value for the power factor and see how it affects the waveforms.

Third tab - Power

Now this one is really neat. Look at the legend on the right. It will show you what the different lines represent. I think the fascinating part of this one is that the overall power for a balanced three phase system is a straight line. I get goose bumps just thinking about it.

Set the voltages for both B and C to 0. Now, you'll see the power of a single phase system. You'll see that the power waveform is a sinusoid. The crest factor of this power sinusoid is 2. In other words, if a three phase motor draws 1000 Watts, it draws it continuously. If a single phase motor draws 1000 Watts, it draws it sinusoidally while varying from 0 to 2000 Watts.

Fourth tab - Harmonics

This simply shows the resultant waveform when harmonics are present.

Have fun. Please let me know if you have any questions.

 

[eric stromberg]

Let me add this, before someone jumps in to "correct" the first tab. The current is shown in what i consider to be the most intuitive fashion. It is written with the assumption that the current is flowing to the load and flowing back on the neutral. From a purely mathematical standpoint, some people like to see the neutral flowing in the same direction as the current in the ungrounded conductors. I understand this perspective, but i think it is counter-intuitive.

 

[Pierre C Belarge]

Eric
I went to open the tab and my NORTON stopped the action, posting that a high threat to access my computer threatened.

Maybe someone should check into this before someone who does not have protection for their computer gets hacked.

 

[eric stromberg]

Eric
I went to open the tab and my NORTON stopped the action, posting that a high threat to access my computer threatened.

Maybe someone should check into this before someone who does not have protection for their computer gets hacked.

Hmmm. Thanks. I don't think about stuff like this becuase i'm on a Mac. No viruses, spyware, or adware. I've transferred this file to Windows machines (with full virus protection) and have never gotten any messages. I'll look into it.

 

[eric stromberg]

As a follow up, i went to a Windows machine that has the latest (Symantec) anti-virus software. I downloaded the spreadsheet and performed a scan of it. There was nothing found.

 

[rattus]

As a follow up, i went to a Windows machine that has the latest (Symantec) anti-virus software. I downloaded the spreadsheet and performed a scan of it. There was nothing found.

Perhaps it is the firewall that is issuing these messages?

 

[mayanees]

Theoretically, one could correct a leading PF, but to my knowledge, this is never a problem.QUOTE]

It ends up that leading power factor can be a problem because the Utility penalizes the customer in the same way as a lagging power factor - even though it helps their distribution system.

That's one of the problems encountered on power factor correction jobs.
You have to take the capacitance off-line when the power factor starts to "lead".
An recent example I was involved in was a hotel that had a 0.75 power factor, so it seemed like an easy fix for the installation of roughly 125 kVAR of capacitance - However, on a moderate temperature day, the A/C units in ecah of the 50 rooms were off, so the facility wasn't pulling drawing much power, and wrose yey, the offensive equipment wasn't running.

So, the recommendation was for a switched-bank cap unit, which came on in increments of 25 kVAR.

And to finish this story, and describe one of the other problems associated with PF correction, this particular Utility had a ratcheting demand clause which looked at your worst demand over a 6-month period, and locked it in for the next 6 months. So, the customer would have to monitor the cap bank to make sure it never tripped out, or their demand charge would ratchet to a higher number for the next 6 months. (low pf increases demand)

JM