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Old 08-07-2018, 01:00 AM   #1
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Default Negative Power Factor in Inductors

I've been meaning to post a question about this since I first came across it 18 months ago and just thought about it again today.

At my previous job we had two massive zinc pots with two inductive heating elements in each one. They melted and maintained the temperature of the zinc we used to galvanize steel and their health was something we monitored very closely. When I got there they had just installed new equipment from which we read line and load side wattage, voltage and amperage and also the power factor for each inductor. One of the inductors always displayed a negative power factor. The engineers said this was normal, in fact, better than normal, ideal!

Now, my understanding of power factor (which is a ratio of phase relationship, yes?) always led me to believe they were either a. full of it or b. trying to cover for a problem they didn't know how to fix. I had been taught PF can't be greater than 1 and it can't be negative, just by virtue of what it is.

My theories are:

a. this particular inductor is somehow actually serving as the load side for the other inductor and is backfeeding voltage, like a step up/step down transformer performing the opposite of how it was intended. In other words it is "receiving" the work of the other inductor and the measurement reflects that or...
b. someone crossed a signal wire and they don't know where or how or
c. someone really messed up and the circuit is somehow capacitive with a giant zinc dialectric
or most likely
d. someone fudged up in the HMI and now there's this weird negative number in there (though how the heck that happened is anyone's guess which is why it was never fixed)

Has anyone ever come across something like this? It would make so much more sense if they miscalculated and put the divisor in the dividend spot but it was always negative somehow between -1 and 0.

I wish I had written down more info or kept some of the company documentation on the system before I left. They installed this monitoring system right before I got there so I do not know if the old analogue circuitry revealed a similar bizarro calculation.

Anyways, just curious. It's kind of always bugged me and as I mentioned, they didn't have a real explanation.
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Old 08-07-2018, 07:32 AM   #2
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Has anyone ever come across something like this? It would make so much more sense if they miscalculated and put the divisor in the dividend spot but it was always negative somehow between -1 and 0.
PF is normally displayed as 0 to 1 with 1 being a perfect power factor. A negative PF would be displayed as .95 or something like that. With out knowing what scale they used its hard to tell what PF you you are talking about.
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Old 08-07-2018, 10:14 AM   #3
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PF is normally displayed as 0 to 1 with 1 being a perfect power factor. A negative PF would be displayed as .95 or something like that. With out knowing what scale they used its hard to tell what PF you you are talking about.

The other three inductor displays in the system were all showing how you would expect a power factor ratio to be displayed, a decimal value between 0 and 1. The inductor in question would display a similar number, but negative. So for example if those three were all reading between say 0.5 and 0.7 the inductor in question would be reading -0.6, literally a negative number.

As I mentioned, likely it's just an engineering mistake in the computer, in other words a mistake in computation. I was curious to know if anyone had any thoughts on other causes, either from equipment or mis-installed system. There is no such thing as a negative PF.
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Old 08-07-2018, 10:57 AM   #4
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You can have negative reactive power depending on whether current leads or lags but power factor is still normally positive. As far as I know it can only go negative when power is coming from the load - a motor backfeeding, solar system, etc. I don't see how that could be the case with just one heater.

I was trying to think of whether this could be a wiring problem or it must be a programming problem. I don't know if this is a typical way to measure power factor:

http://www.giangrandi.ch/electronics...i/cosphi.shtml
Measuring the power factor with the three voltmeters method

If that's the method being used, I think if one of the volt meters had the polarity reversed, you'd get a negative number.
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Old 08-07-2018, 12:47 PM   #5
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Default Yes it can go the other way.

It is quite common in foundries for this to happen. The power factor caps that get switched in don't always fit the correction step. As the metal melts the induction changes due to the shape of the metal ingots.
I believe the POCO don't like it because it does not show up in their bill, and was what we called free energy.


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Old 08-07-2018, 08:28 PM   #6
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In a plc a negative number simply means the highest bit number is turned on. This sometimes happens by accident during programming. There are also a bunch of boxes that you can use in the hmi that will reverse a number and they can accidentally get turned on.

If the power factor moved in a way that you would expect then its probably just a programming mistake. (when its turned off the power factor went back to -1).

Most of the mcc i have been in have the meter mounted on the gear that is networked to the plc. From the panel you can see the raw data including the PF.

Thinking about it ive never payed much attention to the numbers on the panel but for some reason on siemes i think the panel displays the PF as a negative so maybe the other 2 are wrong and all 3 should be negative numbers from a engineering perspective)
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Old 08-08-2018, 12:06 AM   #7
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It is quite common in foundries for this to happen. The power factor caps that get switched in don't always fit the correction step. As the metal melts the induction changes due to the shape of the metal ingots.
I believe the POCO don't like it because it does not show up in their bill, and was what we called free energy.


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If what you're describing is what happened, maybe they just let it ride. I'm not sure how close to unity that mill ran but obviously it was a concern.
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Old 08-08-2018, 12:15 AM   #8
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In a plc a negative number simply means the highest bit number is turned on. This sometimes happens by accident during programming. There are also a bunch of boxes that you can use in the hmi that will reverse a number and they can accidentally get turned on.

If the power factor moved in a way that you would expect then its probably just a programming mistake. (when its turned off the power factor went back to -1).

Most of the mcc i have been in have the meter mounted on the gear that is networked to the plc. From the panel you can see the raw data including the PF.

Thinking about it ive never payed much attention to the numbers on the panel but for some reason on siemes i think the panel displays the PF as a negative so maybe the other 2 are wrong and all 3 should be negative numbers from a engineering perspective)
This particular line was slowly migrating to Allen Bradley from a pretty old GE-FANUC 9070 Genius system (those Genius blocks are tough as hell though) so I'm pretty sure the inductor metering equipment was also AB. It was a point of contention between the on-site contract engineers, many of whom came from GE, and the guys actually on the company payroll. There was a lot of routing back and forth from HMI to the different PLCs though, and no set or consistent protocol on which processor handled what. As a for instance, scaling from probes or sensors was sometimes done in the PLC, sometimes done in the HMI (ie, what exactly does this 4-20 signal represent and how). I wish I had asked this question earlier so I could dig a little deeper.
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Old 08-08-2018, 12:20 AM   #9
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You can have negative reactive power depending on whether current leads or lags but power factor is still normally positive. As far as I know it can only go negative when power is coming from the load - a motor backfeeding, solar system, etc. I don't see how that could be the case with just one heater.

I was trying to think of whether this could be a wiring problem or it must be a programming problem. I don't know if this is a typical way to measure power factor:

http://www.giangrandi.ch/electronics...i/cosphi.shtml
Measuring the power factor with the three voltmeters method

If that's the method being used, I think if one of the volt meters had the polarity reversed, you'd get a negative number.
These were my initial hypotheses, but when I first noticed this issue I had been at the job for 2 weeks and had literally no real-world experience and was way too scared to bring it up to the 30+ year veteran who was showing us around.
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Old 08-08-2018, 11:18 AM   #10
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"Negative" power factor is a capacitive load. "Normal/positive" power factor is an inductive load.


This is normally called lagging (inductive load--current lags voltage), or leading (capacitive load--current leads voltage). Your "negative" power factor is actually a leading load.


Capacitor banks are normally added to correct power factor as near to 1 as possible, ideally at each load (which is why they are referred to as power factor correction capacitors...PFC's). As just the cowboy said, the load is likely changing where the power factor correction is not.


Power factor which is not 1 is "bad" whether "negative" or "positive"...a load too capacitive or inductive. You are still using the same power, but how it affects the utility is where the bill changes. However, if you are running a capacitive load i.e., a negative power factor and you are on the same side of the service as loads which are lagging the leading PF may be helping out in the power factor correction for the distribution by adding a capacitive load. It's not ideal, though, since the service transformer may be affected by overvoltage conditions caused by a leading load similar to how a lagging load can cause overcurrent conditions. Automated reactive power controllers upstream might alleviate the condition, but that's just another band-aid on the problem.


At load power factor correction is normally the best bet since the loads rarely change that much. It sounds like your inductive load is changing either too fast or too much for the PFC to handle, or your PFC isn't matched correctly...like just the cowboy already stated.
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Old 09-25-2018, 07:30 AM   #11
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Default Power Factor Correction

As inductive load increases of a power supply so does the reactive power generated. The higher the reactive ratio of the power generated the lower the power factor will be, on any industrial site you should aim for 0.98 not only for electrical efficiency but the costs related get can get huge and hard to identify on the bills. The inductive load being generated in your case by the equipment used to galvanize a process of which the demand I would expect to be heavily inductive.

Th electrical engineering solution for this is power factor correction which monitors the power factor of an electrical system and reacts proportionally with capacitance. The introduction of the capacitive load counters the inductive loads 'lagging' effect on the power factor of the system with a 'leading' effect.
The aim is to be as close to 1 as possible not leading or lagging both are electrically inefficient. Well specified PFC Equipment will provide an accurate corrected PF reducing electrical faults and lossed and reducing electricity costs
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Old 09-27-2018, 12:56 AM   #12
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PFC correction equipment that uses capacitors is typically done in "steps" of cap banks that are switched in and out of the circuit, so there is only so much granularity that you can attain and sometimes, one part of your system runs leading while other parts are lagging. Since the power is all coming from the same source, all that ultimately matters is the PF for the entire facility being .95 or better. One furnace being leading while the others are lagging is, per your colleague's statement, somewhat "ideal" in that the leading PF of that one furnace was helping out the lagging effects of the others.

When I worked at a steel mill with induction furnaces, what we did was run all of our 600-800HP compressors for the forges with Synchronous motors that were run at a leading PF (negative as you put it) on purpose to correct for the furnaces. There was a continuous analog PF control system so as the induction changed in the furnaces when the ingots melted, the leading PF created by the Synchronous motors was tweaked to that we maintained .98PF almost dead on 24/7/365. That's difficult to get to with banks of capacitors. I've seen some facilities that attained that level of finite control by running a large synchronous motor that is not connected to any load, that's called a "synchronous condenser" when they do that. Used to be more common, now everyone does it with cap banks because they are cheaper and it seems like a waste to let a 600HP motor just sit there running 24/7 with nothing connected to it.
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Last edited by JRaef; 09-27-2018 at 12:59 AM.
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Old 09-27-2018, 06:32 AM   #13
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A P.F above 0.95 for a site is not difficult to achieve with capacitance delivered correctly. We do this for companies who are using injection moulding etc. which frequently switch large inductive loads. We aim to and have never had too much difficulty on any site achieving an average P.F. of over 0.97.
Modern PFC controllers are very capable in a well specified capacitor bank of selecting the required amount of capacitance to react to the systems current P.F.
We analyse all sizes we recommend power factor correction equipment for a week, so capacitance stage sizes are accurately calculated and relevant to each customers demand. I am aware that some PFC companies are recommending and specifying PFC equipment from electricity bills or without monitoring for any length of time, not sure how harmonics or the stage sizes are considered without this.
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Old 09-27-2018, 06:34 AM   #14
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If you require any power factor correction advice or services id be happy to oblige. I hope you have found my post useful
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